Optimization of the biogas production at Käppala Wastewater treatment plant

Thickening sludge and biogas potential

measurement in a laboratory scale

Thesis Work by Astrid du Petit Thouars

Supervisor: Torkel Berglund Royal Institute of Technology, Stockholm

Anna Maria Sundin Käppalaförbundet, Stockholm

Examinater: Pål Nyrén

Royal Institute of Technology, Stockholm

Project performed 2007 at Käppala Wastewater treatment plant Lidingö, Stockhom

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ACKNOWLEDGEMENT I would like to thank Käppala association for greeting and allowing me to work within its facilities. More particularly I would like to thank Anna Maria Sundin, my supervisor at Käppala WWTP for her confidence that she accorded me. Anna Maria has followed with attention my work and has contributed by her ideas.

I thank, in other respects, Torkel Berglund who put up as my supervisor at KTH, Kungliga Tekniska Högskolan or Royal Institute of Technology.

Moreover, I would like to thank the personnel at Käppala WWTP especially the personnel in the laboratory, Dan Wilhelmsson for his help in setting up the BlueSens equipment, Dervisa Karat, Maria Gustafsson and Adnan Skurlic to have shown me the analysing methods. I thank also the personnel at the control of operation, Lennart, Matte, Kajsa, Zika, John and Tommy which helped me install the Bellemer Turbodrain into the plant.

During my internship, I participated at different activities, the relay race, the crayfish party, nice dinners; I had a great time during those enjoyable moments after work.

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Abbreviations

COD Chemical Oxygen Demand (mg/l)

GR glödrest, fixed solids

HRT Hydraulic retention time (days)

Nm3 Normal cubic metre (gas volume at standard temperature 0°C and pressure 1 atm)

OLR Organic loading rate (kg VS/(m3.day))

SGP Specific gas production (Nm3biogas/ton VSin)

SRT Solids retention time (days)

SS Suspended solids (SS)

TS Total solids (% of wet weight)

VFA Volatile Fatty acids (mg/l)

VGP Volumetric gas production (Nm3biogas/m3

sludge in)

VS Volatile solids (% of total solids or % of wet weight)

WWTP Wastewater treatment plant

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1. INTRODUCTION............................................................................................................................ 5 1.1. Background ............................................................................................................................. 5 1.2. Advantages of the biogas......................................................................................................... 5 1.3. Aim.......................................................................................................................................... 5 1.4. Questions to be answered ........................................................................................................ 6 1.5. Limitations............................................................................................................................... 6 1.6. Outline of the thesis................................................................................................................. 6

2. PRESENTATION OF KÄPPALA WASTEWATER TREATMENT PLANT ....................... 7 2.1. Käppala wastewater treatment plant........................................................................................ 7 2.2. Käppala association’s organisation ......................................................................................... 8 2.3. Operation of Käppala wastewater treatment pant.................................................................... 9 2.4. Digesting at Käppala wastewater treatment plant ................................................................. 10

3. THEORY STUDY ....................................................................................................................... 11 3.1. Anaerobic digestion............................................................................................................... 11

3.1.1. The anaerobic digesting process ................................................................................... 11 3.1.2. Environmental factors ................................................................................................... 13 3.1.3. Process parameters ........................................................................................................ 14

3.2. Methods for optimizing the biogas production...................................................................... 16 3.2.1. Thickening sludge process ............................................................................................ 16 3.2.2. Biogas production in a laboratory scale ........................................................................ 18

4. SLUDGE THICKENING PROCESS ........................................................................................ 23 4.1. Background ........................................................................................................................... 23 4.2. Aim........................................................................................................................................ 23 4.3. Material and method.............................................................................................................. 23

4.3.1. Equipment ..................................................................................................................... 23 4.3.2. Sampling procedure....................................................................................................... 26 4.3.3. Analysis......................................................................................................................... 26

4.4. Results ................................................................................................................................... 26 4.4.1. Evaluation of the equipment ......................................................................................... 26 4.4.2. Evaluation of the belt thickener .................................................................................... 27 4.4.3. Evaluation of the polymer ............................................................................................. 28 4.4.4. Evaluation of the reject water........................................................................................ 32

4.5. Discussion ............................................................................................................................. 34 5. BIOGAS MEASUREMENT USING A LABORATORY EQUIPMENT .............................. 35

5.1. Introduction ........................................................................................................................... 35 5.2. Material and method.............................................................................................................. 35

5.2.1. BlueSens laboratory equipment .................................................................................... 35 5.2.2. Mapping of the full scale anaerobic digestion at Käppala WWTP ............................... 37 5.2.3. The measuring method .................................................................................................. 38 5.2.4. Calculation of the methane potential............................................................................. 38 5.2.5. Analysis......................................................................................................................... 39

5.3. Results ................................................................................................................................... 39 5.4. Discussion ............................................................................................................................. 42

5.4.1. Calculation of the methane potential............................................................................. 42 5.4.2. Evaluation of the equipment ......................................................................................... 42 5.4.3. Measurement ................................................................................................................. 43

6. CONCLUSION ............................................................................................................................ 45 7. PERSPECTIVES FOR FURTHER STUDIES ......................................................................... 45 REFERENCES..................................................................................................................................... 46

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1. INTRODUCTION

1.1. Background During the past years, the interest of biological treatment of organic waste by anaerobic digestion has increased. Biological treatment with anaerobic digestion is beneficial from both a waste management and an energy-production point of view. In Sweden, a government bill (2004/05:150) written in 2004 says that 35 % of the organic waste from households, restaurants, large-scale kitchens and stores should be recycled by biological methods by 2010.

In anaerobic digestion, microorganisms decompose organic waste into carbon dioxide, water, methane (CH4) and new biomass. The produced gas is called biogas consisted of mainly methane and carbon dioxide. Most of the biogas in Sweden is produced at wastewater treatment plants. If the biogas is collected it, can be used for the production of heat, electricity, or vehicle fuel and therefore replace fossil fuel. Since the transport sector represents a large part of the emissions of greenhouse gases, the use of biogas as vehicle fuel has gained interest in Sweden and in the rest of Europe.

1.2. Advantages of the biogas Since the Kyoto protocol have been adopted by most of the industrial countries and therefore have agreed to reduce their emissions of greenhouse gases, such as carbon dioxide and methane, some alternative energy source need to be adopted. A way to decrease the emissions of greenhouse gases is to replace the actual fossil fuel by renewable vehicle fuel. Biogas as a fuel has several advantages compared to fossil fuels. The emissions of nitrogen oxides and other atmospheric pollutants are lower. Furthermore, the combustion of biogas does not contribute to the enhanced greenhouse effect, and the energy is also renewable. Compared to other renewable energy source, such as solar and wind power, the methane can be stored. Biogas can be mixed with nature gas (85-98% CH4).

Biogas produced directly after digestion contains about 60 to 70% methane. The biogas used as a vehicle gas is consisted of 97% methane (the biogas needs to be upgraded before using it as a vehicle gas). The energetic substance in the produced biogas is methane, which has the energy content of 9,8 kWh/m3.The energy content of one m3 biogas is equivalent to one liter fossil fuel.

The rest product from the anaerobic digestion is a high nutrient sludge, which can be used as a fertiliser in agriculture. However, this has been questioned because of the risk for contamination for instance by heavy metals and pathogens in the case of sludge coming from wastewater treatment.

1.3. Aim Käppala wastewater treatment plant has been running several projects during those last years with the purpose to increase the biogas production with 50% by 2015. This year, Käppala association signed a contract with the bus company of Stockholm city, SL. In 2010 the biogas produced at the plant is going to be bought by SL. The project of selling the biogas as a vehicle gas increases the interest of increasing the biogas production. This could be done by process optimizing and through collecting external organic waste.

The purpose of this thesis is to evaluate the capacity of two different equipments in two projects with the aim to optimize the biogas production.

The first project was to evaluate a sludge thickener used to thicken the sludge at desired dry solids content (total solids TS content) before being pumped to the digester. The thickener is a pilot-scale method which can be used in full scale to improve the anaerobic digestion process by increasing the hydraulic retention time. Käppala WWTP has rented a belt thickener in order to increase the total solids (TS) content of the primary sludge from actual 4% to 10% TS.

The second project was to evaluate a laboratory scale system measuring methane gas potential from different types of sludge. The purpose of evaluating this system is to study if it could be used as a full

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scale optimisation. The first step in the evaluation is to study if the results coincide with actual process at Käppala wastewater treatment plant. Another student will continue the project.

1.4. Questions to be answered In this thesis different questions should be answered:

Questions formulations focused on the belt thickener:

• Will the belt thickener be able to reach a sludge content of 10% total solids? • Would it be possible to use the belt thickener in a continuous full-scale operation with stable

total solid content? • Would the reject water contain enough organic matter to be used as a carbon source for the

biological treatment of the plant? • What would be the operations costs?

Questions formulations focused on the laboratory anaerobic digestion reactors “BCS-CH4 biogas” (called the BlueSens method in the rest of the report):

• By comparing different laboratory methods, which one would be more suitable for this project?

• Will it be possible to use the BlueSens method to simulate biogas and methane production from full-scale operation?

• Will it be possible to measure the methane potential of different substrates in an accurate way?

• Would the results of the laboratory scale coincide with the actual full scale biogas production results?

1.5. Limitations The belt thickener was rented during three weeks to evaluate the capacity of thickening. However the effect on the biogas production was not evaluated in full scale since the thickened sludge wasn’t pumped to the digester.

Due to a limitation of time, the equipment from BlueSens has only been started-up with batch test digestion with only one type of sludge which is primary sludge. Another master student will take over the project and test different combinations of sludge. In the beginning, the thought was to test methane potential capacity of the thickened sludge from the first project.

1.6. Outline of the thesis This thesis consists of five chapters. A presentation of Käppala wastewater treatment plant WWTP is described in the second chapter. The third chapter describes the anaerobic process with the environmental factors and process parameters. It describes also both methods, the thickening sludge process and the measurement of biogas potential in a laboratory scale. The fourth chapter presents the results and method use to thicken the primary sludge. The fifth chapter presents the results of the laboratory scale experiment. The last chapter contains the major conclusions and suggestions for future projects.

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2. PRESENTATION OF KÄPPALA WASTEWATER TREATMENT PLANT 2.1. Käppala wastewater treatment plant

Käppala Association was formed in 1957. Käppala WWTP is one out of three main wastewater treatment plants in Stockholm, the capital of Sweden. The main purpose of the Association is to treat sewage from eleven connected municipalities, see figure 1. This is made by running a 60 km tunnel system for collecting the water, including three pumping stations and a treatment plant. The plant is situated in Käppala, Lidingö, an island east of Stockholm in the inner part of the famous Stockholm Archipelago. The population served are about 410 000 (2005). Sewage from hospitals, schools, offices and industries in the connected areas, equivalent to about an additional 120 000 people, is also treated in the Käppala plant. The average flow through the plant is approximately 1,5 m3/s, equivalent to filling 8 bathtubs every second. The total amount of treated water is about 50 million m3 per year. The treatment efficiency is at least 95% with regard to suspended solids, organic substances and phosphorous compounds. For nitrogen compounds the treatment efficiency is about 80%. Since May 2006 the digested sludge has been treated with the KemiCond process before dewatering in order to increase the concentration of dry solid content. The total amount of sludge is 40 000 tons, or 8 000 tons of dry solids per year. From raw sewage a number of usable products are produced such as sludge containing biosolids used to fertilise farmland, biogas used for central heating of houses in Lidingö and cleaned water which is let out into the sea – a good example of a functioning recycling chain. The water stays a little more than a day in the plant. Käppala Association is since the year of 1999 certified according to ISO 14001, an environmental management system. The Association is also regulated according to EMAS (EMAS= Environmental Management Audit Scheme). The environmental staff at Käppala WWTP ensures that the quality of the sewage and the sludge improves continuously.

Figure 1: The eleven municipalities connected to Käppala WWTP

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2.2. Käppala association’s organisation Käppalaverket is the third biggest wastewater treatment plant of Sweden behind Stockholm water and Rya WWTP in Gothenburg.

• Käppala Association is a municipal association driven by a representative assembly (composed of 33 members) from the eleven municipalities connected to the plant. The assembly’s board is composed of 11 members with one chairman and a vice-chairman. The purpose of this association is, according to the rules from this association order, apprehends and to clean the municipality’s citizens wastewater. The association is responsible of acquire, build, maintain and drive the wastewater treatment plant, the pumps stations, the tunnels and the feed-lines net.

• Each municipality having their water cleaned by Käppala WWTP pay a fee. This fee and the revenue from the energy (biogas) sold is the total revenue for Käppala WWTP.

• The management of the association is managed by a CEO (Chief Executive officer) chosen by the board. Käppala association has 35 employees and the figure 2 represents its organisation. The CEO of Käppala is responsible for the environmental work.

• The administration is composed of the technical management, informatics and administration.

• The technical manager needs to be updated of the new rules regarding the influence on environmental impact. He also needs to inform the industry of what they are allow dumping in the water and writes the environmental report every year.

• The analys manager is in charge of controlling the effluent water pumped into the sea and of the sludge quality. The analyze section is composed of the information section, the laboratory and the environment and development section.

• The operation manager is controlling that the environmental policy is followed and that the use of energy is minimized as much as possible, he is in charge that all the whole operation in the plant is done with the minimum resource possible and that the treatment of the smell is done well.

Figure 2: Käppala association’s organisation

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2.3. Operation of Käppala wastewater treatment pant

Käppala wastewater treatment plant is situated in the rock, see figure 3.

Figure 3: Käppala wastewater treatment plant, Lidingö

When the wastewater comes into the plant, it goes through different step in the treatment process, see figure 3. First the incoming wastewater is pumped to the screens (1) where mainly paper is removed. The removed substances are grinded and led directly into the digester. Then the wastewater goes to the grit chamber (3) where the sand and other heavy particles are removed which otherwise would cause problems in the process. It is after this mechanical treatment that the wastewater is divided in two parts between the old and the new part of the plant (in operation since 1998). Approximately 1/3 of the flow goes through the old part (line 01-06) and 2/3 of the flow is treated in the new part of the plant (line 07-11). In the presedimentation step (4) suspended particles are removed through sedimentation. The sediment, called primary sludge, is pumped to the digester (9). The biological treatment (5) is where naturally occurring bacteria and other microorganisms are used for decomposing organic material, phosphorous and nitrogen compounds in the wastewater. In the final sedimentation (6) the biological sludge settles at the bottom and the main part of the sludge is pumped back to the biological treatment, the excess sludge is treated in the sludge treatment (9). Phosphates, not removed in the biological treatment, are precipitated with ferrous sulphate (FeSO4) (8). Ferrous sulphate is dosed into the biological treatment and before the sand filter (7) where remaining suspended particles are removed. The treated water is led out at in Halvkakssundet between Lidingö and Nacka at 45 meters depth.

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2.4. Digesting at Käppala wastewater treatment plant

Käppalaverket has two digesters of 9000 m3 each, see figure 4. The digesters are in operation since 1969.

Figure 4: Operation of the digesters at Käppala wastewater treatment plant

This figure shows how the operation has been performed since 2004. Primary sludge from the presedimentation is continuously pumped into the first digester, R100. To the second digester, R200, centrifuged excess sludge is pumped continuously from final sedimentation and also outgoing sludge from R100, see figure 4. In order to have an even level in both digesters, the amounts of sludge going in and out are the same. The outgoing sludge from R200 is pumped to the sludge dewatering (10 on figure 3). The digesters are heated at 35°C in a mesophilic mode and are equipped with stirring device and a recirculation flow in order to get a good mixing. The temperature is around 35oC to have an optimal level for the digesting process. The biogas produced in both digesters is going through a gas buffer volume (gasometer) and then to a gas furnace where it gets burned. The energy produced at the burning step is used to heat up the plant and the excess is delivered as district heating to Lidingö.

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3. THEORY STUDY

3.1. Anaerobic digestion 3.1.1. The anaerobic digesting process

The digestion process is an anaerobic process where the organic material is transformed to biogas with the help of microorganisms. The major part of the biogas is composed of methane CH4 (around 60-70%) and carbon dioxide CO2 (30-40%).

Biogas Production

The figure 5 shows how the organic material is transformed to biogas in several steps (Leksell, 2005). The process can be divided into four steps, 1) hydrolysis, 2) fermentation or also called acidogenesis, 3) anaerobic oxidation or also called acetogenesis, 4) Methanization (Davidsson 2007, Leksell 2005).

Figure 5: Transformation of organic material to biogas during the anaerobic digestion (modified

picture Leksell, 2005)

1) Hydrolysis

The first step is the conversion of the complex organic polymers into soluble products such as amino acids, sugars and fatty acids by enzymatic hydrolysis (Davidsson, 2007). Hydrolysis is relatively slow and can be limited by the dissolvation of waste containing large particles with a low surface-to-volume ratio, for instance lignin (Borggren, 2007).

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2) Fermentation/ acidogenesis

Acetate, hydrogen and carbon dioxide as well as volatile fatty acids (VFA) and alcohols, are produced from soluble organic compounds by fermentative bacteria or by anaerobic oxidisers (Davidsson, 2007).

3) Anaerobic oxidation/ acetogenesis

In this step, acetate, hydrogen, and carbon dioxide are produced from degradation of long-chain fatty acids and volatile fatty acids by obligated hydrogen-producing acetogens (Davidsson, 2007).

4), 5), 6) Methanization

This last step is carried out by the methanogens, the largest group of archea microorganisms. The methanogenic bacteria and acetogenic bacteria convert molecular hydrogen and carbon dioxide to methane and acetate, respectively (Starr et al., 1981):

4H2 + 2CO2 → CH3COOH + 2H2O (4)

4H2 + CO2 → CH4 + 2H2O (5)

CH3COOH → CH4 + CO2 (6)

About 2/3 of the methane produced in a digester comes from acetate. The methanogens have slow growth rates and are usually considered rate limiting for the anaerobic process. (Davidsson, 2007). The biogas is made up of 60 to 70 % methane, CH4, and 30 to 40% carbon dioxide CO2. Small quantities of other substances may be present (table I) (Degrémont, 1973).This can be burnt to generate heat and/or electricity.

Table I : composition of the biogas (Degrémont, 1973)

Substances Formula Volume percent

Methane CH4 60-70

Carbon dioxide CO2 25-40

Oxygen O2 0-0,3

Carbon monoxide CO 2-4

Nitrogen N2 1

Hydrogen sulphide H2S 0,05-1,5

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3.1.2. Environmental factors Among the more important environmental factors affecting sludge digestion may be included: temperature, food supply which is influenced by the quality and concentration of solids in the sludge, the pH and alkalinity, pathogens, the volatile acids content of the digesting mixture and the amount and kind of added chemicals if any (Babbitt & Baumann, 1958).

Nutrients

A number of substances are necessary for optimal anaerobic digestion. In particular, the microorganisms require carbon, nitrogen and phosphorus for growth. The amount of carbon has to be in balance with the amount of nitrogen (Davidsson, 2007). Ammonium is the preferred form of nitrogen for bacteria and the carbon should be in a form available to the enzymes. The C:N ratio is an important parameter in estimating nutrient deficiency and ammonia inhibition and the particle size may influence the degradation rate of the waste (Hartmann & Ahring, 2006). The optimum C:N ratio is often suggested to be in the range 20:1 to 30:1. Bacteria also require vitamins, traces of minerals and certain amounts of micronutrients such as sulphur (Davidsson, 2007).

Toxic substances

The most common toxic substances are volatile fatty acids, hydrogen, free ammonia and hydrogen sulphide, but xenobiotics can also be inhibitory (Davidsson, 2007). Retention time and temperature are the most important factors to reduce the pathogen content of the digested sludge (Borggren, 2007)

Temperature of digestion

Three main temperature intervals are suitable for anaerobic bacteria to degrade organic matter, termed the phsychrophilic, mesophilic and thermophilic intervals (see table II).

Table III: The three main temperature intervals used in anaerobic digestion (Davidsson, 2007)

Temperature range Temperature optimum

Phsychrophilic 0-20°C 15°C

Mesophilic 15-45°C 35°C

Thermophilic 45-75°C 55°C

The mesophilic anaerobic digestion is the most common process. It requires the intermediates of each stage of the process (hydrolysis, fermentation, oxidation, methanization) to be in equilibrium between production and consumption or the process will be unstable with low efficiency (Watts and al., 2006). The thermophilic anaerobic process is reported to have an acceleration effect on the biochemical reactions and a higher efficiency in the degradation of organic matter in comparison to the mesophilic process, it requires lower retention time in the digesters. Higher methane production rates are observed under thermophilic conditions but the process has a lack of stability caused by high propionate threshold concentrations.

Psychrophilic digestion is not commonly used for full-scale applications since the microorganisms work slowly (Davidsson, 2007).

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pH and alkalinity

The optimum pH is between 6-8,3 since the acidogens bacteria have an optimal pH at 6 and the methanogens bacteria around 7. If the pH is under 6 it becomes toxic. The formation of fatty acids makes the pH decrease. The relation between the quantity of fatty acid and alkalinity should be <0,1. A high alkalinity 3000-5000mg/l is needed to have a stable process (Borggren, 2007).

3.1.3. Process parameters The process parameters that differ from different processes are in general: temperature, stirring, regularity of feed, retention time, organic loading, and capacity of the reactor. They can be placed in different groups; sludge characteristics, operational parameters and control parameters respectively.

Sludge characteristics

To ensure an efficient biogas production some sludge characteristics are important for instance:

• Total solids (TS) measured in percent and it represents the dry part of the sludge after it has been dried in the oven at 105°C during a day. The measurement is done by dividing the dry part weight by the total weight. The total solids represent the highly concentrated sludge and the sludge fed into the digester should be as thick as possible in order to have a higher concentration of methane-producing bacteria and to get a longer hydraulic retention time (Degrémont, 1973).

100100

105(%) ⋅=Cweightwet

CdryingafterweightTSo

o

• Volatile solids (VS) also called loss by combustion (GF). VS is the content of organic matter in the waste and it’s measured in g VS/L (or in % of TS). GF, loss by combustion is the material in a sample that dries within an hour at a temperature of 550°C. At this temperature most of the organic matter burn up whereas the inorganic matter needs higher temperature to burn up. The substances left in the sample are called calcined residue, fixed solids, GR, and is calculated as below (Borggren, 2007).

100550)(% ⋅−

=weightTS

CatheatingafterweightweightTSTSofGRo

VS is calculated then as:

GRTSofVS −=100)(%

• Chemical oxygen demand (COD): COD and VS are the most important method to decide the quantity of organic matter in the wastewater and sludge. COD is often used for water with low TS content and VS is used to measure the organic content of sludge with higher TS. COD is measured as mg O2/L and describes the quantity of oxygen that is consumed to oxidate the organic matter in the waste.

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Operational parameters

To be able to evaluate the digesting process, it is important to define some parameters.

• Retention time: It includes the hydraulic retention time (HRT), which is the average time where hydraulic is in the digester, and the solid retention time (SRT), which is the average time where solid material is in the digester (Borggren, 2007). The retention time affects the rate at which the microorganisms reproduce, and minimum theoretical digestion time can be put at 3 to 4 days (Degrémont, 1973). The size of the digester or the reactor affects the retention time. The retention time is calculated as below:

)/()()( 3

3

daysludgemdaypersludgeoutgoingofquantitymvolumedigesterdtimeretention =

• Temperature: described in 3.1.2 • Organic loading rate (OLR): it is the measurement of how much organic material is pumped

into the reactor per day and it’s define as:

),()(

),/( 33

daymvolumereactionVSkgdigestertheinmaterialorganic

dayvolumereactionmVSkgOLR inin =

• Vigorous stirring: It increases the chance of bringing the microorganisms into contact with the organic compounds to be broken down and gives the sludge an even consistency and a more even temperature and organic matter concentration (Degremont, 1973).

• The capacity of the reactor: it must be designed to allow the matter to remain long enough to attain the required degree of decomposition (Degremont, 1973). The capacity of the reactor is depending on the retention time.

• Regularity of feed: Both the input of raw sludge and the extraction of digested sludge must be very regular in order to keep the ratio of organic matter to microorganisms as constant as possible and to avoid any sudden variation in the regular development of the microorganisms (Degrémont, 1973).

Control Parameters:

Anaerobic systems must be monitored to ensure successful operation. The most frequently used parameters for process monitoring are:

• Gas production rate measured in two different ways. The specific gas production (SGP) is defined as the biogas produced per unit mass of the total VS in the waste at 0°C and 1 atm.

VS

biogas

MQ

SGP =

SGP specific gas production (Nm3biogas/kg VSin, day)

Q biogas Biogas flow rate at 0°C and 1 atm (Nm3/d)

MVS Mass of organic material (kg VS)

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The volumetric gas production VGP is defined as the biogas produced per unit of the total incoming sludge into the digester.

digester

biogas

VQ

VGP =

VGP volumetric gas production (Nm3biogas/(m3

sludge in,d))

Q biogas Biogas flow rate at 0°C and 1 atm (Nm3/d)

Vdigester Volume of incoming sludge into the digester per day (m3,d)

• Gas composition: the amount of methane gas is the most important. • pH and alkalinity: described in 3.1.2. • The concentration of volatile fatty acid [VFA] used as an indicator since its get formed in the

beginning of the process and are the cause of a pH change.

3.2. Methods for optimizing the biogas production In this thesis two different methods has been used to optimize the biogas production at Käppala WWTP. In this chapter both methods are described and why they have been chosen.

3.2.1. The sludge thickening process

Definition of sludge thickening

Sludge thickening is the first and simplest stage in reducing the volume of sludge by increasing the TS, dry solids content, see figure 6. The other stage in reducing the volume is dewatering and then drying.

Figure 6: thickening, dewatering and drying stages (Svenska Kommunförbundet, 1989)

Thickening has several advantages, it reduces the volume of the sludge pumped into the digester needs to the digesters, and it increases retention time in the storage tanks of anaerobic digester and therefore increases the biogas production. According to T.Buer et al. (1995), wastewater treatment plants

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having a mechanical sludge thickener gets a higher degree of stabilization and a higher gas yield. It will also decrease the cost because less water would need to be heated. The disadvantage of thickening is that, apart from additional cost, it can leave organic sludge with a bad smell (Degrémont, 1973). There are several methods of thickening:

• Thickening by gravity settling: The sludge is fed into a tank (thickener) where it remains a long time so that the sludge can become compacted and then be extracted from the bottom while the interstitial liquid is drawn off the top (Degrémont, 1973). The sedimentation curve representing the variation against time of the separation depth of the mass of sludge is a good guide to the dimensioning of the thickening tank. Thickening tanks should always be built deep enough to allow the sludge to settle under the weight of the solid mass alone. 3,5 m is a minimum for wastewater treatment plant attached to a large city.

• Thickening with elutriation: Elutriation is the process of washing the sludge with clean water

to improve the physical and chemical properties of the sludge. It eliminates fine and colloidal matter and reduces the alkalinity. The elimination of fine particles accelerates thickening of the sludge and increases the output of mechanical thickening or dewatering systems (vacuum filters, filter-presses, centrifuges) (Degrémont, 1973). The lowering of alkalinity should reduce consumption of the reagents. Elutriation tanks are similar to thickeners, with the difference that a large volume of water is added to the sludge at the tank intake.

• Thickening by flotation: The principle is to make the sludge lighter than the water. Flotation is generally defined as a process of separation of materials in suspension in a liquid, making use of the apparent lower density of these materials and causing them to concentrate at the surface where they are then removed. The advantages of this method are smaller surface area and volume of thickening units can be used and sludge of colloidal origin (such as activated sludge) gets more highly concentrated. This method has several disadvantages as well, it is costly and it is not possible to store a large quantity of sludge in the tank (Degrémont, 1973).

• Thickening by gravity filtration: This method is the one used in this thesis and is described in

more details in the chapter 4.

In order to thicken the sludge from 4% dry solids to 10% dry solids with a flow of about 14 m3/h, from all the different method, the most suitable one is the gravity belt thickener and it is also one less costly (WSP förstudie, 2006). This type of equipment can be used continuously in the operation of the water treatment. Comparing with other wastewater treatment plant, the belt thickener is the most common method used to thickened sludge. This type of method, the belt thickener, is also commonly used in the dewatering process. At the dewatering step in a WWTP the content of total solids achieved is around 40%.

Polymer is important in this process in order to flocculate the sludge. A polymer is a substance composed of molecules with large molecular mass, formed by repeating structural units, monomers. The properties vary enormously depending both on what kinds of monomers make up the molecule, and how they are arranged. Polymers are especially designed and accustomed for water treatment processes such as flocculation and sludge conditioning.

The synthetic organic flocculants are classified according to the relative molecular mass, the degree of charge and the ionicity of the polymer:

- Anionic (negative charge) - Nonionic (neutral) - Cationic (positive charge)

Their fields of application are wide, going from the clarification process of surface water and municipal wastewater to industrial in-process water conditioning and effluent treatment. (Kemira, 2007)

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3.2.2. Biogas production in a laboratory scale 3.2.2.1. Determination of biogas production in a laboratory scale

The anaerobic digestion potential, expressed as the amount of biogas produced from a substrate, can be determined as the theoretical methane potential calculated from data on the chemical composition of the waste. It can also be determined by digestion tests, which can be performed on scales from batch laboratory tests to continuous full-scale digestion (Davidsson, 2007). In the end of the experiment, the methane potential is calculated as the accumulated methane produced per amount of volatile solids added to the reactor.

In order to be able to compare and evaluate the biogas potential from the laboratory scale operation with the present full-scale operation it is interesting to study the gas production (Leksell, 2005).

3.2.2.2. Methods for laboratory scale digestion The digestion process is the same for the different methods using different equipments; it’s more the way to sample and measure the out coming gas that differ. In the conventional method, the methane is sample by the use of a syringe and then measured in a gas chromatography. In another method, some sensors called “BlueSens” BCS-CH4 Biogas, can be used placed on the top of bottles in order to measure the produced methane gas by infrared on-line.

Conventional laboratory scale digestion test

The batch laboratory method, described in Hansen et al. study in 2004 uses 2 litre glass bottles with a thick rubber septum allowing many parallel tests to be performed. Before the test 400 ml inoculum (digested sludge taken from the digester) is added in each bottles and the desire substrate to be study (for instance primary sludge), with a total of 2 g VS per 100ml total is recommended (Hansen et al. 2004). Triplicates are used because the method is a biological test method using inoculum from full-scale biogas plant and the test material (waste) is relatively heterogeneous. After set up the reactors are flushed with anaerobic gas containing 80% N2 and 20% CO2 for 2 minutes to ensure anaerobic conditions. For each run, three blanks with only water and inoculum are included to measure the methane production originating from the inoculum. The methane produced is then measured daily during the first week and once a week was sufficient for the rest of the incubation. Typically 80-90 % of the methane potential is produced during the first 8-10 days but some of the organic waste may be slowly degradable and the measurements should therefore be continued for a total of 30-50 days to measure of the maximal methane potential. Gas samples were taken from the headspace of the reactors through the septum with a syringe with pressure lock. The sample is then injected directly into the gas chromatograph where the mass of methane is measured (Hansen et al., 2004) (figure 7).

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Figure 7: The batch reactors with the use of a syringe and the gas chromatograph (modified picture

Leksell, 2005 and JTI, 2007)

The method is suitable for comparison of the methane potential of specific substrates and mixtures of substrates or for testing inhibition effects. The test period required to determine the total methane potential can be rather long (30-50 days).

The biogas production in a laboratory scale can also be analysed using a continuous method. For this method the material required consist of five-liters bottles equipped with some stirring devices and some feeding tubes in order to feed the bottle with sludge every day placed in a water bath at 37°C (figure 8). The gas bags measure the volume of gas produced and it is measured every day. The bags are plugged in a special measurement device and measured by pressing out the gas. The bags are then placed back on the bottles. The experiment is usually done during the same period as the batch method (Leksell, 2005 and JTI, 2007).

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Figure 8: Continuous method set-up (modified picture Leksell, 2005)

BlueSens on-line laboratory scale method

Another alternative is a new method that’s been on the market since 2003. It’s called BCS-CH4 biogas and is distributed by the German company BlueSens. It is a measurement system constituting of microprocessor controlled infrared-gas sensors measuring the concentration of the methane in % volume gas and volumenometers (called milligascounter) measuring the accumulated volume gas produced during the experiment. All the equipment, the sensors and the volumenometers are plugged into a function box which is then plugged to the computer. The on-line measurements are read by software called BACVis. The interval between each measurement is set before starting the experiment (10 second to 120 minutes). The picture (figure 9) shows the BCS-CH4 biogas equipment. During the experiment there isn’t any manipulating require.

Figure 9: BCS-CH4 equipment (BlueSens, 2007)

sensors

Function box

Volumenometers

reactors

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The reactors, compared to the batch method, are consisted of 1 litre-bottles. The amount of inoculum and substrate added into the 1 litre reactors are the same in each bottles. The difference is that the experiment doesn’t need to have triplicates for the reference (inoculum and water) according to the company BlueSens. The bottles are placed in water bath at 36°C and the experiment runs for 30 days approximately. The continuous method could also be adopted with this kind of equipment. The biogas comes out of the bottles generally by a little flush and not by single and constant bubbles. The Milligascounter averages this error coming from such bubble flushes. Therefore this measuring principle offers advantages in comparison with other bubble meters having a photo cell only for bubbles of discrete size (Scherer, 2004).

Comparison between both methods

In order to choose the most suitable equipment for laboratory scale digestion experiments, the different advantages and disadvantages are described in the table III and table IV.

Table III: Advantages and disadvantages of the conventional method

The conventional method

Advantage Disadvantage

Manipulation Use of as many bottles as wanted

Require to use a Gas Chromatograph and the software.

Manipulation requires more work Calibration curve every 20-30 samples.

Material A gas chromatograph is needed to measure the gas

Experiment Different experiment can be done at the same time, a lot of data is obtained

Time The measurement and the manipulation requires a lot more time

Measurement More different data can be measured not only methane can be measured some for

instance CO2

Use of a syringe to analyse the gas in the GC that give about 5 to10 % error

Price For the same price, this method gets more data if running different experiments at the

same time.

GC expensive and includes a lot more material to buy.

Robustness GC may require some reparation.

Syringe and small bottles need to be changed often.

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Table IV: Advantages and disadvantages of the BlueSens biogas methods

BlueSens biogas method

Advantage Disadvantage

Manipulation Automatic, easy to use Use of only 4 to 8 bottles per function box and computer

Material

Only one set of bottles is needed for one type of experiments. Calibration

curve not needed. The material requires calibration with N2 gas only at the beginning of each anaerobic

digestion experiment

Experiment

Only one set of data every month with one set of bottles. Can’t fail an experiment

otherwise a whole month of experiment is lost

Time Fast results because automatic on-line measurements

Measurement

The measurement is done automatically to the computer.

Measure at the time requested and no major risk of errors

Only biogas flow and methane concentration are measured

Price Only the sensor are expensive Expensive for only 4 bottles

Robustness Low maintenance Expensive to change a sensor

After evaluation on these two methods, Käppala association chose to rent a BlueSens system for 5 months to further evaluate it. Due to limited time resources at the laboratory it is wiser to use a system that requires less manipulation. The conventional method would have required more space in the laboratory and required that all the employees working in the laboratory had a study on the use of a Gas Chromatograph. The BlueSens method is easier to set-up and to obtain results. Since Käppala does not need to experiment the biogas potential capacity in different types of sludge all year long, it seems more convenient to order a BlueSens system.

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4. SLUDGE THICKENING PROCESS

4.1. Background During the last couple of years Käppala wastewater treatment plant has focused optimizing their digestion process. They have the aim to increase the biogas production with 50% by the year of 2015. According to a study done in November 2006 by the consulting company WSP, one possible method to increase the production in the digestion tanks is to thicken the primary sludge from the old preliminary sedimentation part (FS01-06).

Sludge thickening reduces the water content and sludge volume and thereby increases hydraulic retention time in the digester and thereby the biogas production. Primary sludge thickening is beneficial to the anaerobic digestion process because it reduces the hydraulic loading to the digester and heating requirements

Actual primary sludge outlet is regulated discontinuously directly to the digester. In order to be able to pump with equal sludge quality continuously to the thickener a new interlayer for the primary sludge from FS01-FS06 is needed. That’s why a gravity belt thickener was chosen as interlayer. Thickening the primary sludge from 4% to 10% TS (total solids) from FS01-06 increases the retention time in the digesters by 6 days in the digester R100 (primary sludge) and by 2 days in the digester R200 (primary sludge from R100 and excess sludge). This will lead to an increase of 900 Nm3/d of the biogas production (WSP förstudie 2006).

4.2. Aim The purpose of this thesis work, is to test the Bellmer Turbodrain belt thickener in order to increase the TS content (total solids) of the primary sludge from 4% to 10% TS. Another purpose is to analyse the content of suspended solids SS, COD and fat acids (VFA) in the rejected water in order to study its possible use as carbon source that is used for the biological treatment.

4.3. Material and method 4.3.1. Equipment

A gravity belt thickener was rented by KICAB AB, an Bellmer Turbodrain TDH 08, see figure 10.

Figure 10: Belt thickener, Bellmer Turbodrain (kicab, 2007)

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The thickener was delivered in a container see figure 11. The container included:

o The belt thickener with a capacity of maximum 14 m3/h o Pump for the incoming sludge o Pump for the outgoing sludge o All the material for liquid polymer o Tank for the thickened sludge

Figure 11: the KICAB container and the picture of the inside with the pump for outgoing sludge and

the polymer material.

The Bellmer Turbodrain works on the principle of gravity filtration. Polymer or flocculants is added to the primary sludge and the flocculated sludge is evenly distributed over the whole width of the continuously circulating belt. The water drains off through the apertures of the belt while the solids are retained. The chicanes create a ploughing action constantly moving the sludge bed and transporting remaining free water close to the belt to drain through the apertures of the belt.

The adjustable ramp multiplies the retention time on the belt, thus allowing more water to drain off and increasing the dry solids content of the thickened sludge. The sludge is discharged from the ramp by a scraper.

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The operation went according to the following:

o Incoming pumping: During the first week of the test period the thickener was connected directly to the presedimentation number 6 (FS06), see figure 12. Once the valve of the presedimentation basin was open, the sludge was pumped in. During the rest of the test period, a different connection was made; the sludge was taken directly from the common feed line that goes to the digester, see figure 13. The flow was adjusted manually on the pump.

Figure 12: Installation of the thickener in the process on the first week

Figure 13: Installation of the thickener in the process the rest of the test period

pump

sampling valve

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o Outgoing pumping: The thickened sludge and the reject water were directly pumped back into the presedimentation basin number 5 (FS05). The flow was adjusted manually on the pump.

o Polymer: Start and stop were run like the incoming pump since the polymer is dosed at the feed line. The dosing is adjusted manually as required after visual judgement of the sludge and the reject water.

4.3.2. Sampling procedure Bellmer Turbodrain was tested during two weeks in April 2007. It was in operation approximately 2 hours a day. During those 2 hours sample on the incoming sludge, outgoing sludge and reject water was taken 5 times each.

During the first 7 days the following parameters were optimized: the belt speed, the type of polymer, the daily consumption of the polymer. During the last 3 day the stability of the operation was evaluated.

Three type of polymer (50% active substance), all anionic polymer with difference in their molecular weight, were tested:

o C1594 from Kemira o XT80 from CIBA o XT60 from CIBA 4.3.3. Analysis

On each sample, the following parameters have been tested; TS (total solids or dry solids) on the incoming and outgoing sludge, COD, suspended solids (SS) and volatile fatty acids on the reject water, see table V.

Table V: parameters tested on the samples

Parameters TS SS COD VFA

Sludge x

Reject water x x x

4.4. Results 4.4.1. Evaluation of the equipment

During the test period the belt thickener has been running without any major complications except for small reparations.

o The polymer dosing in the feed line work well and gave a good mixing at all different concentrations. The polymer is pressured in four different points around the line and then goes directly to the sludge mixer (a weight load eccentric butterfly valve that creates turbidity).

o The outgoing thickened sludge was too thickened and got stuck in the tank and overflowed once. Afterwards the tank was fed with water to facilitate the sludge to be pumped out. Therefore the samples were taken directly from the belt on the ramp. But on a full scale thickener another pump will be installed that is adapted for this type of sludge.

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4.4.2. Evaluation of the belt thickener

The evaluation of the belt capacity was done by testing different parameters. This part includes the method and the results. The method was decided in function of the results, that’s why both of them are described below. The test period was divided in three different parts and in each part the three different polymers were tested:

1. This first part was done during 4 days (18 samples of incoming and outgoing sludge and of the reject water). The test period was done without the ramp and with an incoming sludge flow of about 14 m3/h. The polymer consumption was tested on 9 different values and the belt speed on 7 different values, see table VI. According to this table the TSout in average came up 6,8%. This TS value is too low according to the aim. The next step was to use a different method to improve the % of TSout.

2. This second part was done during 3 days (15 samples each day). This time, in order to get a high % of TSout, the ramp was added on the belt during the operation. By adding the ramp a significant accumulation got formed on the ramp and on the belt, it was almost like the sludge stayed on the belt. Therefore the slam flow got decreased from 14 to 8 m3/h and the belt speed was set at a lower value continuously. This test period was done according the second pump connection, see figure 13. Each day was a polymer tested with 5 different polymer consumption values. Those polymer consumption values were the same for each polymer. According to the results two polymer consumption values (2,2 and 1,6 kg/ton TS) gave results above 10%TS. Therefore the values were chosen for the stability testing. Even with a lower flow without the use of the ramp, it doesn’t increase the percent of TS according to the study done at Tekniska Verken I Linköping AB (Arnell, 2006).

3. This last part was done during 3 days as well (15 samples). The stability of each polymer was tested with the ramp. One polymer was tested each day and 5 different samples were taken under the same conditions. The results were compared to each other and the percent of TSout were observed to see if it stayed approximately the same.

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Table VI: Results focused on the sludge flow, the belt speed and the polymer consumption

Trial period

Day In. sludge flow m3/h

Belt speed Hz

Polymer consumption kg/ton TS

Out. Sludge flow m3/h

Reject water flow m3/h

TSin %

TSout %

14 36,2 1,6 10,8 3,2 5,4 7,3

14 32,7 3,1 8,1 5,9 4,2 7,1

14 40,2 1,7 9,7 4,3 4,4 6,3

14 37 3,2 9 5 4,7 7,3

14 27,1 3,4 9 5 3,1 6,5

1 1,2,3,4

14 30,6 5,2 8,8 5,2 3,3 6,7

7,7 RAMP 17,9 3,1 3,2 4,5 4,1 9,5

7,7 RAMP 17,9 2,2 3,1 4,6 4,1 10,8

7,7 RAMP 17,9 1,6 3 4,7 4,1 10,9

7,7 RAMP 17,9 1,3 3,3 4,4 4 9,1

2 5,6,7

7,7 RAMP 17,9 0,8 3,4 4,3 4,2 8,9

8 7,7 RAMP 17,9 1,7 3,8 3,9 3,9 8,2 3

9, 10 7,7 RAMP 17,9 2,5 3 4,7 4 10,1

Each change of the parameters was determined according to the results obtained in previous operational results. A lower belt speed and lower incoming sludge flow were chosen in the second part because the longer the sludge stayed on the belt the longer it gets to drain the water but it gives more suspended solids in the reject water (see 4.4.4). Another reason of this chosen belt speed was that using the ramp made the retention time on the belt longer and therefore an overload of the sludge. At high belt speed and high incoming flow, the sludge accumulation grew constantly and submerged at last the whole belt. At lower values, the sludge accumulated reasonably. Moreover the increasing of the retention time gave a better thickening.

In a future full scale operation a ramp at the end of the belt (see figure 10) will be used with a pump more adapted to pump the thickened sludge. Therefore the use of the ramp was kept all the rest of the test period in order to get more representative results. According to the results, the ramp gave much higher TS% out, around 10% compared to approximately 7% without the ramp.

4.4.3. Evaluation of the polymer Like mentioned earlier, three different polymers were tested in order to compare the polymer demand (figure 14) and the quality of the flocks. As the polymers had been tested before the test period (the suppliers test and chose the most adopted polymer) in the laboratory with the primary sludge, the efficiency of them and the quality of the flocks were approximately known. The stability of the polymers was tested as well, that is to say if the TS out stayed approximately constant (figure 15). The difference between the flock’s quality for each polymer were undeterminable, they looked the same. It’s because their composition very similar.

It is also interesting to compare the amount of suspended solids in the reject water for each polymer. The following results are an average taken at the period 2 and 3 (17,9 belt speed with Ramp on and 7,7 m3/h incoming sludge flow).

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TS% in TS% outSS mg/l

Figure 14: TS in and TS out and Suspended Solids in Reject water in the second period with different polymer consumption at 17,9 belt speed with ramp on and 7,7 m3/h incoming flow

SS values are a lot higher for C1594 than XT60 and XT80, see figure 14 (this conclusion is based on few values). We can notice as well that between XT60 and XT80 the SS values are approximately the same between 600 and 1200 mg/l. TS out is reaching 10% with the polymer consumption between 2,5 and 1,6 kg/tonTS. This range of polymer consumption was used when analysing the polymer stability.

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TS% in TS% outSS mg/l

Figure 15: TS in and TS out and Suspended Solids in Reject water in polymer stability period (3) at belt speed 17,9 with the ramp on and at 7,7 incoming sludge flow

According to the graphs in figure 15, the XT60 is the most stable polymer but compare to the other polymer the TSin were also stable. C1594 looks like to be the most unstable one, the difference between TSin and TSout varied the most. Once again, the SS values are higher for C1594 than for the other polymers.

The conclusion of the polymer test is that the polymers XT80 and XT60 had a better function than C1594, fewer polymers were needed, and the quality of the reject water was better. The most optimal polymer consumption was an average of 2,5kg/ton TS with 50% active substance in the liquid polymer.

Operation in full-scale

In current operation, the primary sludge is pumped intermittent from presedimentation it represents 260 ton primary sludge/day. The flow of the primary sludge going to the digester when it’s in operation is 11,1 l/s with an average of 3,25 l/s.

The following table VII shows the different flows in full-scale in using the thickener (4%TS to 10%TS).

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Table VII: The incoming and outgoing flows in a full-scale operation using the thickener with and without the ramp

In. sludge flow l/s

In. sludge flow m3/h

Out. Sludge flow m3/h

Reject water flow m3/h

Without the ramp 4%->7% TS

11,1 39,6 26 13,6 Full-scale operation max

flow With the ramp 4% ->10% TS

11,1 39,6 15,2 24,4

Without the ramp 4%->7% TS

3,25 11,7 7,7 4 Full-scale operation with

an average value With the ramp

4% ->10% TS 3,25 11,7 4,5 7,2

The Bellemer turbodrain can not support 39,6 m3/h flow, the maximum capacity is 14 m3/h. In order to pump the primary sludge continuously with an equal sludge quality and a lower incoming flow to the thickener a new interlayer is needed for primary sludge from FS01- FS06. According to the study done by WSP (Förstudie WSP, 2006) there is a solution with the existing gravity thickener could be used as buffer volume.

During the test period when the Bellmer turbodrain thickener was in operation, a lot of aerosol got formed and a high concentration of hydrogen sulfide H2S got measured above the belt. On order to avoid the problem of the smell and to have a better work condition a blower fan need to be installed above the belt.

Operation cost

The TS quantity that need to be thickened is around 260 ton sludge per day (with around 4% TS, it’s equal to 10,4 tonTS/d).

With a polymer consumption of 2,5 kg/ton TS, it leads to need 26 kg polymer/d since 10,4 ton TS is send to the digester per day. In a full scale thickening the polymer will be in a form a powder, it’s less expensive. The powder polymer as 100% active substance and therefore the polymer consumption is less. With a polymer consumption of 2kg/ton TS, the need of polymer per day will be 20,8 kg polymer/d. With a price of 30kr per kilo polymer (powder polymer), the price of polymer per day will be around 500kr/d.

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4.4.4. Evaluation of the reject water

• Reject water as a carbon source

During the test period every day some samples were taken of the reject water to analyse SS, COD, CODfiltered (CODf) and VFA (volatile fatty acids). The purpose of those analyses is to study the possibility to use the reject water as carbon source for the biological treatment in the wastewater treatment. The bacteria involve in the denitrification need a carbon source in order to be able to denitrify nitrates to nitrogen gas. This carbon source must be easy accessible in the form of fatty acid (VFA), ethanol, methanol or such. Incoming COD fractions are described in figure 16. Incoming fraction are important since the different COD-fraction are differently accessible to the process and have different function. The particuled COD fraction is not as assessable for the bacteria as the dissolved COD fraction. The easy biodegradable fraction COD, SS, is divided in two fraction, volatile fatty acids, VFA, and the hydrolysable organic material, SF. The fraction that is a better carbon source is the VFA, that’s why the concentration of VFA in the process is important for the bio process.

If the reject water is pumped in the biological treatment step the quotient COD/tot-N increases and then improves the denitrification. The carbon consumption for the denitrification step is 5 kg COD/kg NO3-N (Nikolic, 2006).

Figure 16: COD fractions (modified picture, Borglund, 2003)

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Presedimen-tation

Biological treatment

thickener

Q=5112 m3/h Q=8280 m3/h

Q reject=7,2 m3/h

0,26 ton COD/d -> 0,052 ton tot-N/d denitrified

5,7 ton tot-N/d 1,55 ton tot-N/d (73% tot-N reduction)

The following table VIII shows the concentration and quantity of CODf and VFA in this operation. Those values have been calculated with quantity of sludge pumps from the presedimentation per day see table VII. VFA is the measurement of the acetate concentration. CODvfa is a good parameter to study since it’s a good carbon source for the denitrifying bacteria.

Table VIII: Analyses on the reject water, mean values

Parameters Reject water flow in full scale m3/h

SS mg/l

COD mg/l

CODf mg/l

VFA mg/l

kg COD/d

kg CODf/d

kg VFA/d

Reject water without the

ramp

4 200 710 510 290 68 48 28

Reject water with the ramp

7,2 1050 1500 760 440 260 130 76

According to the table with the use of the ramp the quantity is 260 kg COD/d, 130 kg CODf/d and 76 kgVFA/d. Since 5 kg COD denitrify 1kg NO3-N, with this capacity around 52 kg NO3-N will be denitrify in the biological treatment. The current quantity of nitrate in the biological treatment is around 5,7 ton Total-N/d.

An overview of a flow and nitrate balance over the presedimentation and the biological treatment is presented in figure 17 and 18. The values are based on the weeks mean values under 2006 from the database WASTE. The values of the amount of nitrates in blue represent nitrate balance with the reject water from the thickening added to the biological treatment process. And the red values represent the actual nitrate balance without the reject water from the thickening.

Figure 17: flow and total nitrate balance at the biological treatment based on the database WASTE

values under 2006

Figure 18: flow and total nitrate balance at the biological treatment with the flow of the reject water added to the process

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Actually 72% of the total amount of nitrates is reduced. In adding to the treatment the flow coming from the sludge thickener Bellmer turbodrain in a full scale operation the reduction of total nitrates will increase of 1 %.

• The influence on the biogas production

The analyses on the reject water evaluated as well how much of the biogas production is lost, due to the fact that some of the COD goes back to the plant with the reject water. COD is a direct source to produce CO2 and CH4. The thickening of the primary sludge implicate that a part of the easy biodegradable part of the organic material in the primary sludge will be used in the biological treatment instead of generating biogas in the digester.

The theoretic gas production per kg degraded COD is 0,62 Nm3/kg CODbiodegradable. (Borglund, 2005)

The quantity of COD lost per days is:

The calculation are based on a reject water flow at 7,2 m3/h.

o In a full-scale operation without the ramp the quantity in a day of COD would be 68 kg COD/d

Gas production of 42,2 Nm3/d

o In a full-scale operation with the ramp the quantity in a day would be 260 kg COD/d

Gas production of 161,2 Nm3/d

The current biogas production at Käppala WWTP is around 15 300 Nm3/d. The consequence in the gas production of the thickening will be an increase of the gas of 900 Nm3/d (WSP förstudie 2006). However the concentration of COD in the reject water represents a decrease of 161 Nm3/d gas and therefore an increase of 739 Nm3/d biogas.

4.5. Discussion The thickener has worked well during this test period without any major problems. The aim of

reaching 10% TS is achieved but with the use of the ramp. With the use of the ramp, the TS value obtained varied a little bit more. This means that in a full scale operation the dosing of the polymer will need to be adjusted continuously. Another consequence of this ramp use is that the quality of the reject water is worst since more particles are load on the belt and increases the SS concentration in the reject water. The polymer XT80 and XT60 gave better results. The COD in the reject water is a possible carbon source for the denitrification step in the biological treatment. The reject water contains a significant concentration of CODf and VFA in order to be a carbon source, in a full-scale operation 0,052 ton total_N/d would be denitrified.

The purpose of thickening the primary sludge is to decrease the hydraulic loading at the digesters. The retention time is therefore increased as well as the gas production and it will decrease the heating cost. The consequence of this increase of thickened sludge in the old part FS01-06 of the plant will lead to an increase of the retention time in the digester of about 6 days for the primary sludge digester R100 and about 2 day for the excess sludge digester R200. The gas production can increase up to 900 Nm3/d (WSP förstudie 2006). The study describes that with a belt speed at 17,9, a incoming sludge flow at around 8 m3/h with the use of the ramp and with a polymer consumption between 2 and 2,5 kg/tonTS, the polymer cost will gets around 500kr/d. In full-scale the polymer consumption will be approximately the same and therefore the cost will be the same 500 kr/d. The total investment represents at about 8,6 Mkr according to the study done by WSP in 2006 to install a Bellemer Turbodrain in a full-scale operation.

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5. BIOGAS MEASUREMENT USING A LABORATORY EQUIPMENT

5.1. Introduction The second part of the thesis only presents a part of the evaluation of the BlueSens system due to the short period of time of one month. This chapter presents the results of the different start-up experiments.

During the last couple of year Käppala has done laboratory scale experiments with the aim to increase the biogas production. The laboratory experiments were done in cooperation with JTI (Institute for agricultural research in Uppsala) with the sludge from Käppala. This institute is using the conventional laboratory method (see chapter 3) and has proven to be a successful way to evaluate biogas potential in different type of substrates. However Käppala has rented the BlueSens equipment due to an easier manipulation. The aim is to evaluate the use of an own laboratory scale equipment by comparing the biogas production in a laboratory scale with the actual full-scale digestion.

5.2. Material and method 5.2.1. BlueSens laboratory equipment

A picture of the BlueSens equipment is presented in chapter 3. However, a quick overview of the system is presented in figure 19. The figure represents one of the four bottles. Each bottle is connected to a cold trap in order to condense the water vapour from the produced gas.

Figure 19: BlueSens system consisted of a reactor, a sensor and a volumenometer

sensor

Reactor = 1 liter flask

Free volume

100 ml substrate

500 ml inoculum

volumenometre

CH4

CH4

CO2

cold trap

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Gas flow through the volumenometer device

Principle of operation of the methane sensor

The sensor is screwed on the top of each bottle (as a cap) and measures the concentration of methane gas in the total volume in the bottle. The functioning of the sensor consist of that infrared radiation is reflected in the measuring cap and the detector measures the attenuated luminous intensity that depends on the amount of gas in the atmosphere of the probe (figure 20). The translucent plane prevents that the sample atmosphere arrives outward. Thereby the sensor does need to be autoclaved. To prevent the measuring cap from condensing moisture the head is heated. That requires about one hour for the first time or after each disconnecting from the power supply (BlueSens instruction manual). The range of error for the sensors is a range of ±3%.

Figure 20: principle of operation of the sensor (BlueSens instruction manual)

Principle of operation of the volumenometer (Milligascounter)

The Milligascounter is set-up as shown in the figure 21.

Figure 21: Volumenometer device (Milligascounter instruction manual)

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The gas to be measured flows in via the gas inlet nozzle (1), through the micro capillary tube (5) located in the base of the volumenometer and up into the liquid casing which is filled a packing liquid (6). The gas rises as small gas bubbles through the packing liquid, up and into the measurement cell (7). The measurement cell consists of two measuring chambers (8), which are filled alternatively by the rising gas bubbles. When a measuring chamber is full, the buoyancy of the filled chamber causes the measurement cell to abruptly tip over into such a position that the second measuring chamber begins to fill and the first empties. The measurement of gas volume therefore occurs in discrete steps by counting the tilts of the measurement cell with a resolution of 1 ml (contains of a measurement chamber). The measured gas escapes through the gas outlet nozzle (Milligascounter instruction manual). The range of error for the volumenometers is a range of ±3%.

5.2.2. Mapping of the full scale anaerobic digestion at Käppala WWTP For the beginning of this project, it is important to point out to compare the biogas measurements obtained by the BlueSens method with the actual values from the full-scale digestion. The table IX presents the mean values for the year 2007 (January to August) of the digestion operation in the digester R100 taken from the database WASTE. The experiments has been done with only primary sludge, therefore a mapping of the biogas production is only necessary for the digester R100. The total volume of the digester R100 is 9000 m3 and the volume of the digesting sludge is 8400 m3. The amount of methane represents approximately 63% of the total gas.

Table IX: Mean values of the digestion parameters of the digestion operation at Käppala WWTP calculated with data from the database WASTE

parameters Digester R100

TS (%) 2,3

VS (%) 65,9

Temperature (°C) 35,6

Organic loading to the digester (ton VSin/day)

24,5

Organic loading rate (kg VSin /m3reaction volume, d)

2,92

Retention time (d) 16,4

Gas production (Nm3/day) 13566

Specific gas production (Nm3/kg VSin, d)

0,55

Specific methane production (Nm3/kg VSin, d)

0,35

Volumetric gas production (Nm3/m3

sludge in, d) 26,44

Volumetric methane production (Nm3/m3

sludge in, d) 16,66

38

5.2.3. The measuring method In order to compare this method with the conventional method this thesis is a development from the study of Hansen et al. 2004 and Leksell 2004.

According to earlier biogas laboratory studies the common volume used for the inoculum is 400 ml in each bottle. The inoculum is a sample of the digested sludge taken from the digester R100. Like in the conventional method the organic loading in principal in each bottle is 40% of the VS from the substrate and 60% of the VS from the inoculum. After calculation the bottles were filled with 400 ml inoculum and 100 ml water for the reference and the other three bottles were filled with 400 ml inoculum and 100 ml substrate (primary sludge). The results will evaluate the difference in the methane concentration and the volume produced from the three bottles containing the same substrate. The bottles are placed in a water bath as shown in the figure 22. The water bath is heated to 36°C as in the full scale digesters where the temperature is at an average of 35.5°C. In each bottle is placed a stirring device controlled by the magnetic stirrers placed under each bottles under the water bath.

Figure 22: The BlueSens equipment set-up for the experiment

5.2.4. Calculation of the methane potential In each experiment using the BlueSens equipment the free volume above the sludge is at least 500 ml (recommended by the instruction). The sensors calculate the concentration of methane contained in the total free volume. In the beginning of each experiment this free volume does not contain only biogas. The produced biogas gets diluted in the free volume and therefore the measurements need to be recalculated to obtain the right concentration of methane in the produced biogas. The appendix 1show the calculation used to obtain the amount of methane from the produced biogas.

39

5.2.5. Analysis Before each experiment some analysis (TS and GR) are necessary to have information about the amount of organic loading put in the bottles. GR is the direct analysis to calculate the amount of organic matter in the form of VS (VS = 100% - % GR). The values on VFA, alkalinity, pH, temperature and VS were taken from the database WASTE.

5.3. Results In the beginning a test experiment was done to see if the equipment responded as expected. Since the respond wasn’t as expected, this paragraph presents only the different way to solve the problems. Due to a limitation of time this thesis doesn’t present results from a whole experiment of a 30 days period.

The following table X describes the three different experiments done during a month and the reason why they were stopped.

Table X: Information about the sludge type, periods and observations for each experiment

Experiment Sludge type Period observations

1 Reference, one bottle: 400 ml inoculum + 100 ml water

Substrate, 3 bottles: 400 ml inoculum + 100 ml primary sludge

7 days The amount of gas and the methane concentration behaved totally differently from each other.

2 500 ml inoculum in each bottle 6 days The bottle number 2 after didn’t produce more gas 2 hours. The production of the gas was too discontinuous.

3 500 ml inoculum in each bottle 9,5 days

The bottle 3 showed a significant decrease of the methane concentration

The table XI gives the information about the organic loading for each experiment.

Table XI: Organic loading for each experiment

Experi- ment

TS%inoculum TS%substrate VS%inoculum VS%substrate Organic load per 100ginoculum (g)

Organic load per 100gsubstrate (g)

1 2,1 6,1 69 86 1,5 (54% of VS) 5,3 (46% of VS)

2 2,3 - 69 - 1,6 -

3 2,4 - 69 - 1,7 -

40

Experiment 1: The table XII present the values obtained.

Table XII: Values obtained for the experiment 1

Experiment 1 Reference Substrate 1 Substrate 2 Substrate 3

Biogas produced (Nml) 98 313 2592 178

Methane produced (Nml) 61 119 1192 17

Methane quantity (vol.%) 62 38 46 9,7

Specific gas produced (Nm3

biogas/kg VSin) 0,0085 0,027 0,23 0,015

Specific methane produced (Nm3

methane/kg VSin) 0,0053 0,010 0,104 0,0015

Volumetric gas produced (Nm3

biogas/m3sludge in)

0,186 0,59 4,94 0,34

Volumetric methane produced (Nm3

biogas/m3sludge in)

0,116 0,22 2,27 0,03

The methane quantity obtained, represent the methane included in the biogas produced. The results from the first experiment are far from the results expected. The experiment was stopped after 8 days since the results were still obviously wrong. In order to make sure the equipment works properly the following experiments were done with digestion only inoculum (digested sludge from the digester R100). Each bottle was filled with the same sludge and with the same amount. In this case the results from each bottle should not different a lot from each other. The following table XIII presents the results from the second experiment.

Table XIII: Values obtained from the second experiment

Experiment 2 Inoculum 1 Inoculum 2 Inoculum 3

Biogas produced (Nml) 192,50 94,6 223,6

Methane produced (Nml) 180,48 92,14 143,6

Methane quantity (vol.%) 93,75 97,5 64,2

Specific gas produced (Nm3

biogas/kg VSin) 0,024 0,012 0,028

Specific methane produced (Nm3

methane/kg VSin) 0,023 0,0116 0,018

Volumetric gas produced (Nm3

biogas/m3sludge in)

0,38 0,19 0,45

Volumetric methane produced (Nm3

biogas/m3sludge in)

0,36 0,18 0,29

41

In the second experiment the bottles were filled with only 500 ml inoculum. The fact that each bottles had the same sludge and the same amount, it was easier to compare the measurement between each bottles. This type of experiment can also explain with more accuracy if the problem comes from the equipment or from the anaerobic digestion.

During the second experiment, the data recorded differ a lot from each bottle (only three bottles were used in this experiment since one of them broke). The stirring was difficult to control since there is a thick layer between the bottles and the magnetic stirrer. The sludge in each bottle was stirred at a high speed because it wasn’t possible to obtain a slower speed. After one day in the experiment the bottle 1 did no longer produced any gas and the decision was to shot down the stirring. Directly after the shot down, some gas were measured in the volumenometer.

According to the data in the bottle 1 and 2, the amount of methane reached 93% and 97%. Since the biogas should contain no more than 70%, the system isn’t recording correct data. Another suggestion was that the system might have a leakage somewhere.

For the next experiment, it was decided to remove the four cold traps from the system and therefore connect the bottles directly to the volumenometer. It was also decided to remove the stirring from the experiment. The table XIV presents the results from the third experiment.

Table XIV: Values obtained from the third experiment

Experiment 1 Inoculum 1 Inoculum 2 Inoculum 3 Inoculum 4

Biogas produced (Nml) 1405 747 672 1178

Methane produced (Nml) 560 257 166 475

Methane quantity (vol.%) 40 34 25 40

Specific gas produced (Nm3

biogas/kg VSin) 0,17 0,09 0,08 0,14

Specific methane produced (Nm3

methane/kg VSin) 0,07 0,03 0,02 0,06

Volumetric gas produced (Nm3

biogas/m3sludge in)

2,81 1,50 1,34 2,36

Volumetric methane produced (Nm3

biogas/m3sludge in)

1,12 0,51 0,33 0,95

The specific gas and methane production in the four bottles differ from each other. The bottle 1 has produced about 40% more gas from the bottle 3. However the amount of methane differs about 20%. The graphs in the figure 23 present the produced gas in each bottle during the experiment period.

42

Experiment 3

0,00

200,00

400,00

600,00

800,00

1000,00

1200,00

1400,00

1600,00

0,0 0,5 1,0 1,5 2,0 2,5 3,0 3,5 4,0 4,5 5,0 5,5 6,0 6,5 7,0 7,5 8,0 8,5 9,0 9,5time (days)

accu

mul

ated

gas

pro

duce

d (N

ml)

STP biogas 1 STP methane 1STP biogas 2STP methane 2STP biogas 3STP methane 3STP biogas 4STP methane 4

Figure 23: Accumulated biogas and the amount of methane in each bottles during the third experiment

5.4. Discussion 5.4.1. Calculation of the methane potential

The methane production could be a little bit higher than the one calculated with the help of the volumenometer and the sensors due to the fact of the assumption that the free volume doesn’t increase. In reality the free volume increases due to evaporation of a small portion of the water volume. The increase is too small to have an impact on the calculation.

5.4.2. Evaluation of the equipment Due to a limitation of time and some assumed gas leakage problems, the experiment hasn’t been done with substrate during 30 days. Therefore it is difficult to compare the results with the actual anaerobic digestion in full-scale. However an evaluation on the manipulation of the equipment has been done during this period.

The advantages of the BlueSens method comparing to the conventional method used at JTI and described in Hansen study are:

- Easy to handle (it takes around 30 min to know how to handle the system)

- It takes only one hour to start an experiment (including calibration of the sensor) and easy to shut-down.

- Takes care of itself since it’s measuring on-line, the only manipulation is to fill up the water-bath regularly

- A lot of data (measurement interval from 30 seconds to 120 minutes)

43

- Relatively robust equipment

- Accuracy of ±3% for the sensors and accuracy ±3% for the volumenometer, ±6% in total.

Disadvantages:

- Only 8 bottles maximum can be use on each function box, per BACVis and computer. At JTI there isn’t a limited number of bottles required.

- The flow counter doesn’t work when the flow is lower than 1 ml per hour, therefore the experiment can’t be running until all organic matter have been consumed.

5.4.3. Measurement Even if some difficulties have come across some observations on the digestion parameters have been possible. The stirring and the temperature are really sensitive to the biogas production. Each time the stirrer had stopped in a bottle and has been starting again, a higher increase was observed on the gas volume curve, see figure 24. However if the sludge is stirred at a too high speed the production seems to be inhibited. At Stockholm water WWTP a similar study have been made using the same equipment and it was observed that the stirring increased the biogas production. A coming study will be more focused on the stirring parameter using a more adapted stirring device.

Figure 24: picture from BACVis showing the produced gas produced from each bottles

The comparison between the bottles

In the experiment 3, the total volume gas produced in each bottles are different. The largest difference is between the bottle 1 and the bottle 3. Since the results are unlike from each other, the experiment isn’t repeatable. However, this conclusion is based on only one experiment which ran only for 9,5 days. Moreover the amount methane obtained of 40% is not as expected (which is around 65%),

44

however only inoculum (digested sludge) was used for this experiment. Further studies will probably give better results.

Stockholm WWTP obtained a good repeatability between each bottle. The suggestion is to run more experiment at Käppala WWTP in order to obtain more data and have a better evaluation.

In future experiments, ammonium, COD, pH and alkalinity will be analysed by the Käppala laboratory (haven’t been analysed for the start-up experiments). Ammonium is analysed to have an overview if the reaction have been inhibited by a too high concentration or too low concentration of ammonium. These analyses are important to evaluate the efficiency of the anaerobic digestion.

45

6. CONCLUSION - The belt thickener was able to reach a sludge content of 10% total solids without major

problems. However the use of the ramp was necessary to reach 10% TS and in this case the total solid content wasn’t stable.

- In a full scale process the belt thickener need to be installed with a pump with greater capacity. Since the content of total solid varied, the dosing of polymer will need some adjustment constantly.

- The reject water could be used as a carbon source for the biological treatment. In a full scale operation the reduction of total nitrate increases with 1%.

- The retention time for the primary sludge in the digesters is increased by 8 days. The thickening also effects the retention time for excess sludge, which increases with two days. The method optimized the biogas production and will increase the production by 900 Nm3/d.

- Compared with the conventional method to study anaerobic digestion in a laboratory scale, the BlueSens method is easier to manipulate. The time resource is a lot less when using the BlueSens equipment since the methane is measured on-line. The conventional method requires the use of a gas chromatograph because the methane isn’t measured on-line. Both equipments can be used either in a batch method or a continuous method. Since the BlueSens is easier to set-up and manipulate, Käppala rented the equipment for five months to evaluate the methane potential of different type of sludge and the equipment itself, and decide if they want to buy the BlueSens equipment at the end.

- In this thesis due to a limitation of time only the primary sludge has been tested. The conclusions with the results obtained from this thesis the repeatability between each bottles is accurate, some more experiments need to done to have better data.

7. PERSPECTIVES FOR FURTHER STUDIES Before deciding whether or not Käppala will invest in a Bellemer Turbodrain, Käppala decided to evaluate the use of an old gravity thickener already built at the WWTP. The primary sludge will be pumped into the gravity thickener were the solids will settle at the bottom. The sludge from the bottom will be thickened and will be pumped into the digester while the water on the top is pumped back in the biological treatment basin. Since the basins from the new part of the plant manage to obtain a primary sludge of 7-8% total solid, another way to thicken the sludge is by rebuilt the actual basin in the old part of the plant with the same configuration as the basins from the new part.

Concerning the BlueSens method a lot of different studies are possible. Since many different experiment have been done using the conventional method, they could used as well using the BlueSens method. A research study at the university of Lund in Sweden done by Åsa Davidsson in 2006, show that it is possible to increase the biogas production at WWTP. Digesting a mixture of food waste from households, sludge from grease separator, fruits and vegetables waste with waste sludge increases the biogas production and the amount of methane.

There are several pre-treatment methods which can be used to increase the biogas production. One approach is to add enzymes either in a separate pre-treatment step or directly into the reactors. The adding of enzyme can increase the methane production of 20 to 60 percent, depending on how much enzymes are added (Eriksson, 2007). The types of enzyme which can be add depend on which composition of sludge is going to be digested. Some other approach to increase the methane production is by pre-treating the sludge with ultra sound or thermal treatment done by heating the sludge at 70 degrees for an hour. A combination of those two methods did not increase the methane production (Eriksson, 2007).

46

REFERENCES

1. Instruction manual for BACVis, sensors and Milligascounter

2. BlueSens – operating instructions BCP-CH4

3. Manual for MilliGascounter®, Ritter

4. Arnell M., 2006. Föravvattning av slam till rötkammare. Tekniska Verken report VATTEN, TEKNIK & PROCESS, Linköping.

5. Babbitt H.E., Baumann E.R., 1958. Sewerage and sewage treatment. 9th edition.

6. Berglund M., 2006. Biogas production from a systems analytical perspective. Academic department, Miljö- och energisystem, Lund University, Sweden.

7. Bernesson S., Hansson K., Robertson M., Thyselius L., 1999. Torrbiogasprocess för lantbruksgröor- studier av aerob förbehandling torrsubstans- och ympningsförutsättningar. JTI report, Kretslopp & Avfall, Nr 19 jordbrukstekniska institutet, Uppsala.

8. Borggren C., 2007. Mätning av metanpotentialen hos slam på Henriksdal och Bromma, metodutveckling och utvärdering av labutrustingen ”BCS-CH4 Biogas”. Thesis work, Stockholm Vatten, Sweden.

9. Borglund A.M., 2006. Förstudie om metoder för förbättrad kväverening på Käppalaverket. Intern report, Käppala wastewater treatment plant, Lidingö Sweden.

10. Borglund A.M., 2003. Evaluation of combined chemical- and biological phosphorus removal at Käppala wastewater treatment plant in Stockholm. Thesis work, Uppsala university school of engineering.

11. Buer T., Ross H-J., Becker T., 1995. Influence of the mechanical pre-thickening of municipal excess sludge on the operation of anaerobic mesophilic sludge treatment. 7th IAWQ International conference on design and operation of large wastewater treatment plants, Technical University of Vienna, Austria.

12. Davidsson Å., 2007. Increase of biogas production at wastewater treatment plant- Addition of urban organic waste and pre-treatment of sludge. Water and environmental engineering, Department of Chemical Engineering, Lund University, Sweden.

13. Degremont, 1973. Water treatment handbook.

14. Eriksson L., 2007. Mer biogas med samrötning. Process Nordic magazine Nr 8, p22.

15. Hansen T.L, Schmidt J.E., Angelidaki I., Marca E., Jansen, J. la cour, Mosbæk H. and Christensen T.H., 2004. Method for determination of methane potentials of solid organic waste. Waste Management 24, 393-400.

47

16. Hartman H., Ahring B.K., 2006. Strategies for the anaerobic digestion of the organic fraction

of municipal solid waste: an overview. Water Science & Technology, Anaerobic digestion of solid wastes IV, vol 53, nr 8, p7-22.

17. Leksell N., 2005. Present and future digestion capacity of Käppala wastewater treatment plant – a study in a laboratory scale. Thesis work, Käppala wastewater treatment plant, Stockholm.

18. Nikolic A., Hultman B., 2006. Förstudie om metoder för förbättrad kväverening på Käppalaverket. Intern Rapport, Käppala wastewater treatment plant, Stockholm.

19. Scherer P.A., Meier D., 2004. Anaerobic digestion of wood after conversion by ablative flash pyrolysis. Proceedings of the 10th World Congress on Anaerobic Digestion, Montreal, volume 4, p. 2141-2146.

20. Starberg K., Haker R., 2006. Slamförtjockning vid Käppalaverket. Förstudie by WSP, Stockholm.

21. Starr M.P., Stolp H., Trüper M.G., Balows A., Schlegel H.G., 1981. The prokaryotes. Volume II, p1416-1420.

22. Svenska kommunförbundet & VAV, 1996. Introduktion till avloppstekniken.

23. Svenskt vatten, 2007. Slamhantering. Avlopsteknik 3.

24. Watts S., Hamilton G. and Keller J., 2006. Two-stage thermophilic-mesophilic anaerobic digestion of waste activated sludge from a biological nutrient removal plant. Water Science & Technology, Anaerobic Digestion of Soild Wastes IV. Volume 53, nr 8, p 149-157.

25. BlueSens, 2007. www.bluesens.de

26. Kicab, 2007. www.kicab.se

27. Kemira, 2007. www.kemira.se

28. Nordberg Å., 2007. Visit at JTI, jordbrukstekniska institutet, Uppsala.

29. Borggren C., 2007. Visit at Stockholm Water wastewater treatment plant.

48

LIST OF THE FIGURES AND TABLES

List of the figures

Figure 1: The eleven municipalities connected to Käppala WWTP ....................................................... 7 Figure 2: Käppala association’s organisation ......................................................................................... 8 Figure 3: Käppala wastewater treatment plant, Lidingö ......................................................................... 9 Figure 4: Operation of the digesters at Käppala wastewater treatment plant........................................ 10 Figure 5: Transformation of organic material to biogas during the anaerobic digestion (modified picture Leksell, 2005) ........................................................................................................................... 11 Figure 6: thickening, dewatering and drying stages (Svenska Kommunförbundet, 1989) ................... 16 Figure 7: The batch reactors with the use of a syringe and the gas chromatograph (modified picture Leksell, 2005 and JTI, 2007) ................................................................................................................ 19 Figure 8: Continuous method set-up (modified picture Leksell, 2005) ................................................ 20 Figure 9: BCS-CH4 equipment (BlueSens, 2007) ................................................................................ 20 Figure 10: Belt thickener, Bellmer Turbodrain (kicab, 2007) .............................................................. 23 Figure 11: the KICAB container and the picture of the inside with the pump for outgoing sludge and the polymer material. ............................................................................................................................ 24 Figure 12: Installation of the thickener in the process on the first week .............................................. 25 Figure 13: Installation of the thickener in the process the rest of the test period.................................. 25 Figure 14: TS in and TS out and Suspended Solids in Reject water in the second period with different polymer consumption at 17,9 belt speed with ramp on and 7,7 m3/h incoming flow .......................... 29 Figure 15: TS in and TS out and Suspended Solids in Reject water in polymer stability period (3) at belt speed 17,9 with the ramp on and at 7,7 incoming sludge flow...................................................... 30 Figure 16: COD fractions (modified picture, Borglund, 2003) ............................................................ 32 Figure 17: flow and total nitrate balance at the biological treatment based on the database WASTE values under 2006 ................................................................................................................................. 33 Figure 18: flow and total nitrate balance at the biological treatment with the flow of the reject water added to the process .............................................................................................................................. 33 Figure 19: BlueSens system consisted of a reactor, a sensor and a volumenometer ............................ 35 Figure 20: principle of operation of the sensor (BlueSens instruction manual).................................... 36 Figure 21: Volumenometer device (Milligascounter instruction manual) ............................................ 36 Figure 22: The BlueSens equipment set-up for the experiment............................................................ 38 Figure 23: Accumulated biogas and the amount of methane in each bottles during the third experiment.............................................................................................................................................................. 42

49

List of the tables

Table I : composition of the biogas (Degrémont, 1973) ....................................................................... 12 Table III: The three main temperature intervals used in anaerobic digestion (Davidsson, 2007) ....... 13 Table III: Advantages and disadvantages of the conventional method ................................................ 21 Table IV: Advantages and disadvantages of the BlueSens biogas methods ......................................... 22 Table V: parameters tested on the samples .......................................................................................... 26 Table VI: Results focused on the sludge flow, the belt speed and the polymer consumption ............... 28 Table VII: The incoming and outgoing flows in a full-scale operation using the thickener with and without the ramp ................................................................................................................................... 31 Table VIII: Analyses on the reject water, mean values......................................................................... 33 Table IX: Mean values of the digestion parameters of the digestion operation at Käppala WWTP calculated with data from the database WASTE................................................................................... 37 Table X: Information about the sludge type, periods and observations for each experiment .............. 39 Table XI: Organic loading for each experiment ................................................................................... 39 Table XII: Values obtained for the experiment 1 .................................................................................. 40 Table XIII: Values obtained from the second experiment..................................................................... 40 Table XIV: Values obtained from the third experiment ........................................................................ 41

50

APPENDIX

Appendix 1: Calculation necessary to come up with the actual amount of methane produced in the bottles from the BlueSens equipment.

Appendix 2: List of the different missions released during the thesis work

Appendix 3: The different relations necessary during this thesis work

Appendix 4: The Gantt diagram presenting the time organisation of this thesis work

51

Appendix 1: Calculation necessary to come up with the actual amount of methane produced in the bottles from the BlueSens equipment.

Since the sensor measure the concentration of methane from the total free volume of around 500ml above the sludge as recommended, the data register on the data need to be recalculated to obtain the actual concentration. This appendix lists the different calculation to obtain the actual amount of methane in the reactors.

The Milligascounter is a volumetric gas meter and therefore measures gas volume in the actual operating state at the time of measurement. The gas volume is dependent on gas temperature, air pressure and water vapour partial pressure. These measurable variables are therefore needed to recalculate to norm conditions. The gas temperature is to be measured at the gas outlet (instructions manual).

Parameters used in the calculation

NV = the produced norm-volume at 0°C and 1 atm (STP); in ml.

)(tVi = the indicated volume at the time t (data from BACVis) measured at room temperature of about 24°C; in ml.

headV = the free volume above the sludge, it’s the total volume in the bottle minus the volume of the sludge; in ml

aP = actual air pressure (data from BACVis); in mbar

Cv

o

P 24 = water vapour partial pressure at 24°C; 29,9 mbar

Cv

o

P36 = water vapour partial pressure at 36°C; 59,5 mbar

LP = pressure of the liquid column silox above the measurement chamber; 2 mbar

NP = normal pressure; 1013,25 mbar

NT = normal temperature; 273,15 Kelvin

aT = room temperature; 24°C

bT = temperature in the bottles; 36°C

measuredCHx ,4= the measured methane concentration (data from BACVis); in %

calculatedCHx ,4= the percent methane calculated in each bottle; in %

52

Calculation of the methane quantity

Biogas volume produced at 0°C and 1 atm(STP):

( ) ( )a

N

N

LC

vaiN T

TP

PPtPtVtV

o

⋅+−⋅

⋅=241000)(

)()(

Biogas volume produced between each measurement Δt:

( ))1()((t)V NN −−=Δ tVtV N

Total Biogas produced at 0°C and 1 atm (STP):

)()(0

tVtV Ni

totN Δ= ∑

Calculation of Vhead at 0°C and 1 atm (STP):

⎟⎟⎠

⎞⎜⎜⎝

⎛ +−=

°

°N

Lva

b

NheadSTPhead P

PPtPTT

tVtV C36

C36,,1000).(

).()(

The recalculated total volume at (t):

)()()( , tVtVtV nSTPheadtot Δ+=

Methane in the produced biogas

Amount of methane between each measurement:

)().()(44 , txtVtV CHNprodSTPCH Δ=Δ

Amount of methane in the free volume Vhead:

)()()(44 ,, txtVtV CHSTPheadheadSTPCH ⋅=

53

Amount of methane in the total volume:

)()()( ,,, 444tVtVtV headSTPCHprodSTPCHCHtot +Δ=

Produced methane between each measurement:

)()1()1( ,,, 444tVtVtV headSTPCHtotCHprodSTPCH −+=+Δ

Total volume of methane produced at each measurement:

∑ Δ=i

prodSTPCHtotSTPCH tVtV0 ,, )()(

44

The percent methane in the total produced gas at each measurement:

100)(

)()(

,

,,

4

4⋅=

tVtV

txSTPtot

totSTPCHcalculatedCH

54

Appendix 2: List of the different assignments done during the thesis work

Different type of assignments given during the thesis

Level of contribution into the project

1. Bibliographic research on operational experiences using the Bellemer turbodrain.

2: work suggested by my supervisor and carried out by myself

2. Bibliographic research on different methods to study anaerobic digestion in a laboratory scale.

2: work initiated by my supervisor and carried out by myself.

3. Planning of the trail period of the primary sludge thickening.

3: work initiated by my supervisor and carried out by myself with methods proposed by my supervisor

4. Take care of the test with the Bellemer turbodrain with different type of polymers and evaluation the most suitable condition for the parameters (speed of the belt, incoming flow, the polymers consumption rate)

2: work demanded for my thesis but carried out with methods suggested and ran by myself

5. Start up the BlueSens method and evaluating the results, making sure the equipment is working properly.

2: work demanded for my thesis but carried out following the manual of instruction and using adopted methods.

55

Appendix 3: The different relations necessary during this thesis work

Relation with different people in Käppala association necessary to realise the thesis work:

- Anna Maria Sundin: guided me during the whole internship and gave advice when needed.

- Laboratory personal: showed me in the beginning who to make the different analysis needed for the project.

- Operation personal: helped me to install and connect the belt thickener to the presedimentation basin.

Relation with different people outside the company necessary to realise the thesis work:

- Bo Lindgren from KICAB: Helped to start up the belt thickener and explain the functioning of it.

- Åke Nordberg from JTI: explain the functioning of the conventional method to study the anaerobic digestion in a laboratory scale.

- Clara Borggren from Stockholm water: explain the functioning of the BlueSens method to study the anaerobic digestion in a laboratory scale.

- Johanna Kullenberg and Annika Garnås from L.A.B. (deliverer of the BlueSens equipment): delivered and helped setting up the BlueSens equipment the first day.

- Holger Mueller from BlueSens: helped me to find the explanation of the different problems which occurred during the experiment with the BlueSens equipment.

56

Abstract

Biogas is a renewable energy source which can replace fossil fuel. The biogas is produced from anaerobic digestion of organic matter. Most of the biogas is produced at wastewater treatment plant. Käppala WWTP situated in Stockholm has the project to sell their biogas as a vehicle gas by 2010. Therefore the increase of the biogas production has gain interest. The objective of this work was to evaluate two different methods that will contribute to optimize the biogas production by: 1) thickening the primary sludge, and 2) measuring the methane potential in a laboratory scale.

The method used to thicken the primary sludge was to use a belt thickener. The sludge is flock by polymer and then drain by the belt thickener. The objective was to thicken the sludge from 4% content total solids (TS) to 10% TS. The experiment has shown that the primary sludge can be thickened by this type of equipment. A thickener primary sludge pumped increases the retention time into the digester by 8 days.

Two different laboratory scale methods exist and they have been compared in this thesis. The conventional method, most used in laboratories, is using a gas chromatograph to measure the methane produced. Since 2003, a new method called “BCS-CH4 biogas” has come on the market sold by a German company called BlueSens. This method is easier to manipulate than the conventional method since its measure the methane on-line. However the evaluation of the methane potential hasn’t been completed due to a limitation of time.

Key words: belt thickener, primary sludge, anaerobic digestion, laboratory scale, biogas

Sammanfattning

Biogas är än förnyelsebar energikälla som kan ersätta fossila fordonsbränslen. Biogasen är en produkt från anaerob rötning av organiskt material. Biogas är till största delen producerad på avloppsreningsverk. Käppalaverket i Stockholm kommer från och med år 2010 sälja biogasen som fordonsgas till SL som driver Stockholms bussar. Detta har medfört att intresset att optimera biogasproduktionen har ökat. Syftet med detta examensarbete var att utvärdera två olika metoder som bidrar till att optimera biogasproduktionen: 1) förtjocka primärslam, och 2) utvärdering av rötning i laboratorieskala.

Metoden som användes för att förtjocka primärslam var en gravimetrisk silbandsförtjockare. Slammet flockas med polymer och avvattnas sedan på bordet. Syftet var att förtjocka slammet från 4% torrsubstans (TS) till 10% TS. Försöket bevisade att det gick bra att förtjocka slammet med den typen av utrustning. Genom att förtjocka primärslammet innan rötning ökar uppehållstiden med 8 dagar.

Två olika metoder för rötning i laboratorieskala har jämförts i detta examensarbete. I den konventionella metoden, används en gaskromatograf för att mäta metanhalten i den producerade biogasen. År 2003 kom en ny typ av analysutrustning ut på markanden, som kallas för ”BCS-CH4 biogas”, denna distribueras av ett tysk företag, BlueSens. Denna metod är lättare att använda än den konventionella metoden, eftersom den mäter metan on-line. På grund av tidsbrist kunde inte metanpotentialen i primärslam bestämmas med Bluesens i detta examensarbete.

Nyckel ord: silbandsförtjockare, primärslam, anaerob rötning, laboratorieskala, biogas.