Energy From Solid and Liquid Wastes - VIII

8/14/2019 Energy From Solid and Liquid Wastes - VIII

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Lecture No: 18

Urban Waste to Energy from Landfill Biogas Projects and by Pyrolysis Plants

18.1. Introduction

The urban waste is usually dumped in so called municipal landfills or municipalrefuse dumps. These landfills are usually away from the city and occupy substantial land

areas. Urban waste is transported by road trucks and is dumped into the landfills. Landfill

waste gets fermented by natural bacterial decay (by anaerobic fermentation) and releases

methane rich fuel gas. This gas is called landfill gas or refuse-tip-gas. Obtaining the

methane rich fuel gas from landfills is the most economical and environmentally

attractive method of obtaining energy from urban waste. Landfill Gas is being used as a

renewable energy source in several countries in the world (Table 18.1).

Table 18.1 Landfill Gas Project Sites (1998)

Outlet

Country Boilers/heating

Kilnsfurnaces

Electricity

generation/

chp*

Purification(pipeline)vehicle fuel

Otherknownapplication

Totalschemes

Trials/schemeplanned

United statesWest GermanyUKSwedenItaly

HollandDenmarkCanadaFranceNorwaySwitzerlandAustraliaBrazilIndiaChile

7

14+

5

2

---

1

---

1

1

---

1

---

---

---

1

5

7

---

---

---

---

2

---

---

---

---

---

---

22*

17

7**

4++

2

---

3

---

---

----

----

1

---

1

10

---

---

---

1

---

---

---

---

----

----

---

1

---

14

7

2

1

3

3

---

---

---

1

---

---

---

---

54

43+

19

7

6

4

3

3

1

1

1

1

1

1

13

1

6

---

---

8

---

---

---

1

---

1

---

1


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--- --- --- 1 --- 1 1

Total 33 15 57 13 31 146 32* one scheme generates electricity and sells gas.

+ includes one research project.

** two schemes generates electricity and also gas for heating.

++ Four schemes are recorded as Boiler, CHP.

CHP = combined heat and power.

Pyrolysis was tried for converting biomass from urban waste to energy. However,

the pyrolysis is used mainly for making wood-charcoal.

18.2. Applications of Landfill Gas

Landfill gas contains predominantly methane (54% by volume). The landfill gas

is used in following applications directly:

--- As a fuel for burning in boilers (without purification)

--- As a fuel for Kilns, Furnaces.

The purified methane obtained from landfill gas is used in following applications

---- As a vehicle fuel.

---As a fuel for diesel engines.

--- As a fuel for Diesel Engine, to produce electrical energy

---After upgrading, supplied as fuel gas to domestic consumers.


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Fig 18.1 Application of Landfill gas (LFG).

12.3. Composition of Landfill Gas

The land-fill gas is generated by the fermentation of organic matter dumped in the

landfill. The process sis called anaerobic fermentation i.e. decomposition caused by

(anaerobe, the microorganisms) without need of oxygen. This process is suitable for

municipal manure. The process takes place at low temperatures up to 60C and requires

moisture. The gases produced vary in composition with time taken by the process

(Fig.18.2). After a period of 2 months from starting, the landfill gas has mainly methane

(52%) and carbon dioxide (46%). During initial periods other gases like oxygen,

hydrogen, nitrogen etc. are released in different proportions.

URBAN WASTE

LANDFILL

GAS

RAW

LANDFILL

FILTERS &

PURIFIERS

FUEL FOR IC

ENGINE

ELECTRIC

POWER

FURNACES

DOMESTI

ENERGY FOR

URBAN

CONSUMERS


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Fig.18.2 Composition of the landfill gas changing with time

During the decomposition, the temperature of the upper portion of the land-fill

rises to about 60C. Landfill gas is not a pure methane carbon dioxide mix. It has several

other gases including some corrosive gases. For simple burning applications such as

furnaces and kilns, the landfill gas is used without separation of methane and other

constituents.

For domestic cooking gas, the landfill gas is converted to compressed Natural GAS

(CNG) or Liquid Natural Gas (LNG) by intermediate process. For use in vehicles as a

fuel, the methane gas is separated form the total landfill gas and is purified to pure

methane.


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Lecture No: 19

Gas production- factors- Composition of landfill gas

19.1. Gas production

If landfill gas is to be collected and managed either for energy production, flaring

or for other reasons it is important to know the quantities and composition of the gas. In

many cases predictions of future gas production and composition is desired. Of course the

large variations in process conditions between landfills means that there is a great deal of

uncertainty involved with estimating gas production, however, some general tendencies

and tools will be presented in the following sections.

19.1.1 Gas quantities

The total gas production from a typical enhanced bioreactor landfill will vary between

60-400 m3 per ton of waste with an average of 230 m 3per ton (Gendebien, 1992). The

annual gas production from a landfill is also very variable both as a function of time and

between landfills. Gas production is in general controlled by:

Landfill temperature

Waste water content

Waste composition

Waste age

Landfill top cover ( entrance of atmospheric air )

Different materials have different biogas potentials and waste composition

therefore has a very marked effect on gas production. It is possible to use Eq.5.2 to

estimate the theoretical biogas potential, however, it is in general not a very good

estimate of landfill gas production due to the mixed nature of the wastes and the

variability in landfill process conditions. Gas potentials may be estimated if the waste

composition is known using the data in Table 19.1. Gas potentials are typically lower in

landfills than what would be expected if the materials were processed in a biogas reactor

due to the intrusion of atmospheric oxygen into parts of the landfill.


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Table 19.1. Gas production in Nm3 per ton dry matter for different biodegradable

wastes. Data from Ehring (1984)

Waste fraction Gas potential (55%methane)

Food waste 191 344Food waste with high bread content 135Grass clippings 176

Leaves 60

Food and yard waste with high wood content 45 60

Newspapers 120Magazines 100 225

Cardboard 317

Mixed paper 65 242Sawdust 30

MSW 160

Waste age has influence on gas production because it takes some time before

newly deposited waste enters the methane phases. The gas production rate therefore will

vary through time depending on the succession of the phases. If the gas production as a

function of waste age is known for a particular landfill it is possible to estimate the

landfill gas production from the landfill as a function of time. Here the data in Fig 19.1

will be used as an example, however it is noted that it is best to use data from landfills

located in the same geographical area and are receiving wastes similar to the landfill inquestion. Also the fitted function in Fig. 19.1 may not fully represent gas production rate

as a function waste age as data are only available for waste ages between 5 and 21 years.

But for the sake of illustration the data is adequate. The total annual gas production rate at

a given time T for a landfill receiving a waste quantity M each year can be estimated as:

T T

B(T) = Mt . rT-tdt = Mi . rT-i

1 i=0

Where T is the landfill age (years) at the time of estimation, t is time (years), Mt

is waste mass received at the landfill in year t and rT-t is the annual gas production rate

per ton of waste with waste age T t, i.e.., waste that was deposited in year t. considering

a landfill that receives 50,000 tons of wastes each year and assuming that the annual gas

production follows that in Fig. 7.8 ( with production rate equal to zero for waste ages


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above 30 years) the total gas production will follow the curves shown in Fig 19.2 each

curve represents different time spans for deposition of waste at the landfill. It is seen that

the maximum gas production rate increases with the duration of the deposition period

until about 30 years. If wastes are deposited at the landfill for more than 30 consecutive

years the gas production will reach a maximum rate that will remain constant for some

time depending on the length of the deposition period. The shapes of the curves depend

upon the relation between the waste age and gas production rate (Fig.19.1). As discussed

earlier the shape of the curves in Fig. 19.2 is a function of the shape of the gas production

curve fitted in Fig. 19.1 and the results should therefore not be regarded as generally

applicable to landfills.

Fig 19.1. Annual methane production in m3 per ton waste for 86 landfills. Bars

indicate one standard deviation and the curve is fitted polynomial. In the cases of nostandard deviation only one measurement was available


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Fig 19.2. Calculated landfill gas production rate at a landfill receiving 50,000 tons of

waste per year for different time periods

19.1.2. Gas composition

Landfill gas is mainly composed of methane and carbon dioxide. The gas may

also contain smaller amounts of other gases. When the gas is extracted from the landfill

atmospheric air can be sucked into the waste and may be mixed with the gas. The gas can

therefore contain smaller amounts of nitrogen and oxygen. Table 19.2 lists average

values for typical landfill gas component concentrations. Landfill gas can also contain

several other organic and inorganic trace components. Some of these compounds can be

toxic or are carcinogens.

Table 19.2. Typical landfill gas component concentrations

Gas Range AverageCH4

CO2

N2

H2

O2

30 65

25 30

5 301 3

0-5

48

38

121

1

The concentrations of these trace gases depends upon the composition of the

waste deposited at the landfill. The most important trace gases are vinyl chloride,

benzene, toluene, chloroform, and dichloromethane. These and other trace compounds


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have been detected at European landfills. Typical concentration ranges of several trace

gases found in landfill gases are shown in Table 19.3. Not all trace compounds found in

landfill gas have been deposited at the landfill with the wastes. Several compounds are

formed as a result of the microbial degradation of other compounds in the landfill.

Table 19.3. Concentrations of common trace components found in landfill gas

(Willumsen 1988).

Compound Concentration range (ppm)

Vinyl chloride (VC) 0.03 44

Benzene 0.6 - 32

Chloroform 0.2 2

Dichloromethane 0.9 490

Toluene 4 197

Xylene 2.3 139

Ethylbenzene 3.6 49

Chlorodifluoromethane 6 602

Dichlorodifluoromethane 10 486

Trichloroethylene (TCE) 1.2 116

Perchloroethylene (PCE) 0.3 110

Ethanol 16 1450

Propane 4.1 630

Butane 20. 626

Carbondisulfide 0.5 22

Methanediol 0.1 430

Hydrogensulfide 2.8 27.5

Chlorine 1 10

Many chlorinated compounds such as solvents are degraded under anaerobic

conditions by sequential dechlorination resulting in the formation of less chlorinated

compounds. An example is perchloroethylene or tetrachloroethylene (PCE) that has been


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used widely as a degreasing agent in the metal industry. This compound is transformed

under anaerobic conditions to trichloroethylene (TCE) by microbial removal of one

chlorine atom as illustrated in Fig. 19.3. TCE can under anaerobic conditions be

transformed by further dechlorination into dichloroethylene (DCE), which can be

degraded both anaerobically or aerobically. Under anaerobic conditions DCE is

dechlorinated into vinyl chloride VC whereas it under aerobic conditions will be

mineralized all the way to CO2, H2O and HCl. Vinyl chloride is not very degradable

under anaerobic conditions and it will therefore disappear slower than parent compounds

PCE, TCE and DCE, PCE and TCE have no proven toxic or carcinogenic effects in

humans but VC has been shown to have both types of effects for humans and animals.

Under anaerobic conditions VC can be transformed into ethane, however, it is degraded

faster aerobically into CO2 and H2O. The products of anaerobic dehalogenation of PCE

are all on gas form at normal temperature and pressure. They will therefore appear

sequentially in both the landfill gas and the percolate if dehalogenation takes place in the

landfill. Figure 19.4shows measurements of PCE and it dehalogenation products in

landfill. It is evident in case (a) that no transformation takes place, as no intermediate

products appear. This is likely because no PCE degrading microorganisms are present in

the percolate. The slow decrease in PCE concentrations may be caused by evaporation of

PCE from the percolate. In case (b) degradation starts some lag period whereas in cases

(c) and (d) degradation proceeds rapidly. In case (b) Microorganisms capable of

degrading PCE are likely present in the percolate, however, they have not been exposed

to PCE prior to the experiment and therefore require some time to adapt to the new

substrate. In cases (c) and (d) microorganisms seem to be well adapted to PCE and

degradation of PCE and its intermediates is complete within 2 months. Most of the trace

compounds listed in Table 19.3 will be present both in percolate, in the gas phase and

adsorbed to the solids in the landfill. Some compounds adsorb very strongly and this will

slow the degradation and transport of these compounds within the landfill. Strongly

adsorbing compounds will therefore be more persistent in the landfill than non-sorbing

compounds. Volatile compounds will be present in the landfill gas in high concentrations

in the initial phases of the landfill life and concentrations will decrease as the compounds

are removed with the gas or percolate.


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Fig 19.3. Sequential dechlorination and transformation of PCE to ethane under

anaerobic conditions

Fig 19.4. Sequential dehalogenation of PCE in four different landfill percolates


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19.1.3. Gas utilization

If the landfill gas is collected it can be used as a source of energy by conversion to

electricity and heat. The most widespread method of energy production is to use the gas

to drive a gas engine that is coupled to a generator that in turn produces electricity. Some

plants also use part of the waste heat in the engine cooling water for heating nearby

homes and business. This is mostly used in Europe where as it is not seen very much in

USA. The waste heat normally amounts for at least 50% of the total energy contained in

the input gas. Normal plants based on gas engines are from 350 to 1200 kW in size. At

larger plants the energy conversion is based on gas or steam turbines. The gas is

sometimes also used in boilers for heating water only. This can be used for heating. One

reason that this approach is not widespread is that the price of electricity usually is higher

than that of heat. Also electricity is normally easy to sell via the power grid. Landfill gascan also be used directly for instance in place of natural gas. The gas may be used in

brick or cement production or in local water heaters. The gas can also be cleaned to

match the quality of natural gas (mostly methane) by removal of primarily CO 2. The gas

can then be distributed via existing natural gas network. This means that new energy

conversion plants are not needed. On the other-hand gas-cleaning plants are required. The

technology is not widespread mostly due to economic reasons. At certain locations the

gas is used in compactors, garbage trucks, buses or even cars. The economy in this type

of gas utilization depends upon costs and taxes on other fuels in the region as well as the

cost of constructing the gas collection and purification plant. The gas can also be used in

fuel cells where it is converted to electricity. This technology is still in the development

phase but it is expected that it will be used in the transport sector in the future.

In 1997 there were 550 landfill gas collection plants with energy production

worldwide (Willumsen, 1997). In Denmark there was 18 plants (1998) with a total

production of 110.000 Nm3 landfill gas per day, equivalent to 5.1 MW energy (heat and

power). This amounts to 0.1% of the total energy consumption in Denmark.

19.2. Percolate cleaning

Percolate contains many different organic and inorganic compounds. The

concentrations of these compounds often require treatment of the percolate before


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discharge. Percolate from enhanced bioreactor landfills has typically much higher

concentrations of organic materials and salts than regular wastewater. Concentrations

may change rapidly following variations in precipitation and waste deposition patterns.

The percolate flow will typically follow the precipitation pattern with a smoother

variation. Old percolate is normally difficult to treat biologically as most of the

degradable material has disappeared. Below is listed different methods for cleaning

percolate. Cleaning in existing wastewater treatment plants can be done provided the

percolate is diluted well with wastewater. There should not be more than 5% percolate in

the mixture as there is a risk for overload with respect to organic matter, nitrogen or salts.

Physical chemical cleaning can be used for instance for pretreatment prior to biological

treatment or after biological treatment has been completed. Addition of precipitating

agents can be used for removing inorganic compounds (salts, heavy metals, turbidity and

color). pH adjustment and aeration can remove dissolved ammonia and aeration can also

remove methane that can be an explosion hazard if the percolate is discharged into the

sewer. Chemical oxidation with hydrogen peroxide has also been used for removal of iron

and it has also been used to oxidize sulfide to remove odor and reduce corrosion

problems in treatment facilities. Biological treatment of percolate is equivalent to

wastewater treatment where organic matter, nitrogen and phosphorous can be removed.

In old percolate a large fraction of the organic matter is difficult to degrade biologically

and more advanced methods are necessary. Methods such as activated carbon filters,

reverse osmosis and treatment with UV light can be used. Terrestrial methods such as

irrigation,. Root-zone filters and infiltration plants have proven effective as final

treatment methods (Christensen 1998) and are in some cases also useful as pretreatment

methods. The advantage of terrestrial methods is their low installation and maintenance

costs. There are however, problems with their efficiency in periods with low temperatures

or high flow rates.


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Lecture No: 20

Landfill Gas Collection System

Landfill sit is usually a void, valley or a former quarry in which the urban waste is

dumped. For example, a landfill site near Bedford, UK is a former brick quarry having

original void volume of 10 million m3 and covering 100 km2 land area. The site receives

about 1000 t of urban waste per day by rail, road from London.

The gas collection system consist of Wells comprising vertical pipes of 80 to 120

mm diameter with holes in the cylindrical body. The wells are driven in the landfill. The

well-pipes and collection pipes are of polythetene. Knockout drums are installed in the

pipelines for removal of water.

A typical landfill site has 20 to 40 wells and the collection pipe system. The wells

are connected to manifolds and the gas is collected from the manifolds and piping

system.

1. Gas Compression Equipment.

The gas if filtered before and after compressor. The compressor increases the

pressure required by the consumer device (e.g. diesel engine or a furnace). A single vane

type gas compressor may be provided. A typical rating is 690m3/h of gas at discharge

pressure of 1.3 bar. After compression the gas is passed through after cooler, bafflewater

separator, fine filter etc, before feeding to the gas consumer device.

2. Gas Purification.

The landfill gas contains methane, carbon dioxide and many other impurities.

Purification of this gas is complex and expensive. The cheaper and effective methods of

purification have been developed recently (1990). These methods employ semi-

permeable membrane and molecular sieves. The purified methane can be used as a fuel

for transport vehicles.

3. Energy Conversion Equipment.The landfill gas can be purified to pure methane and then used as a fuel for

internal combustion engine. The Internal Combustion Engine can drive pumping sets.

Alternatively, the Internal Combustion Engine can drive generator to produce electrical

energy. For examples, a typical land-fill gas site may have one to four spark ignition type

four stroke internal combustion engines. The engine drives a 350 kVA, 415 V generator.


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The generator electric power is supplied to the distribution system at 6.6 kV via a step-up

transformer in the substation.

Technically, producing electricity from landfill gas project is more complex and

demanding than use of gas directly as a fuel. Before admitting into the IC Engine, the gas

should be cleaned, filtered and purified.

The plant demands

More operational controls.

Greater security at outlet of Landfill Gas System.

Better operation and management of the total plant.

For producing electricity sufficient gas should be available continuously to operate

the plant throughout the year with a good plant load factor.


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Lecture No: 21

Anaerobic Digestion

The purpose of sludge digestion is to convert bulky, odorous sludges to arelatively inert material that can be rapidly dewatered without obnoxious odors. The

bacterial process as summarized in Eq. 21.1,consist of two successive processes that

occur simultaneously in digesting sludge. The first stage consists of breaking down large

organic compounds and converting them to organic acids along with gaseous by-products

of carbon dioxide, methane, and trace amounts of hydrogen sulfide. This step is

performed by a variety of facultative bacteria operating in an environment devoid of

oxygen, if the process were to stop there, the accumulated acids would lower the pH and

would inhibit further decomposition by pickling the remaining raw sludge. For

digestion to occur, second-stage gasification is needed to convert the organic acids to

methane and carbon dioxide.

Acid-splitting methane-forming bacteria are strict anaerobes and are very

sensitive to environmental conditions of temperature, pH, and anaerobiosis. In addition,

methane bacteria have a slower growth rate than the acid formers, and are very specific in

food supply requirements. For example, each species is restricted to the metabolism of

only a few compounds, mainly alcohols and organic acids, while carbohydrates, fats and

proteins are not available as energy sources.

CO2, CH4

H2S CH4

Organic matter Organic acids and ----- (21.1)

Acid-forming Acid-splitting CO2

bacteria methane-forming

bacteria

Stability of the digestion process relies on proper balance of the two biological

stages. Buildup of organic loading or a sharp rise in operating temperature. In either case,


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the supply of organic acids exceeds the assimilative capacity of the methane-forming

bacteria. This imbalance results in decreased gas production and eventual drop of pH,

unless the organic loading is reduced to allow recovery of the second-stage reaction.

Digesters may generate foam as a result of over-feeding. Accumulation of toxic

substances from industrial wastes, such as heavy metals, may also inhibit the digestion

problems is often difficult to determine. Monitoring volatile solids loading, total gas

production, volatile acids concentration in the digesting sludge, and percentage of carbon

dioxide in the head gases are the methods most frequently employed to give advance

warning of pending failure. These measurements can also indicate the most probable

cause of difficulties. Gas production should vary in proportion to organic loading.

Volatile acids content is normally stable at a given loading rate and operating

temperature. The percentage of carbon dioxide should also remain relatively constant.

Monitoring digestion by pH, measurements is not recommended, since a drop in pH does

not precede failure but announces that it has occurred.

Table 21.1 lists the general operating and loading conditions for anaerobic

digestion.

Single-Stage Digestion

A photo of a single-stage fixed-cover anaerobic digester is shown in Figure 21.1.

the photo also shows ancillary equipment associated with digester heating: boiler, heat

exchanger, and sludge recirculation piping. Raw sludge is pumped into the tank through

feed pipes. Mixing pumps discharge at nozzles within the digester to keep the contents

from stratifying. Without mixing, sludge separates, with a scum layer on top, a middle

zone of supernatant water of separation underlain by actively digesting sludge, and a

bottom layer of digested concentrate. A limited amount of mixing is also provided by

withdrawing digesting sludge, passing it through a sludge heater, and returning it through

the inlet piping. Supernatant is withdrawn from anyone of a series of pipes extended from

the supernatant box. Digested sludge is taken from the tank bottom for dewatering. High-rate digesters are completely mixed the contents do not tend to separate or develop a clear

supernatant, and the entire contents of the digester must be dewatered.

For digesters designed with floating covers, the cover floats on the sludge surface,

and liquid extending up the sides provides a seal between the tank wall and the side of the

cover.


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Table 21.1.General Operating and Loading Conditions for Anaerobic Sludge Digestion

Temperature: Optimum

General operating range

98oF (36.7oC)

85o-99oF (29o 37oC)PH: Optimum

General limits

7.0 to 7.1

6.7 to 7.4

Gas production

Per pound of volatile solids added

Per pound of volatile solids destroyed

8-12 cu ft (230- 340 litres)

16-18 cu ft (450-510 litres)

Gas composition: Methane

Carbon dioxide

Hydrogen sulfide

65 to 69 percent

31 to 35 percent

trace to 80 mg/l

Volatile acids concentration

General operating range 200 to 800 mg/lAlkalinity concentration

Normal operation 2000 to 3500 mg/l

Volatile solids loading

Conventional single stage

First-stage high rate

0.02-0.05 lb VS/cu ft/daya

0.05-0.15 lb VS/cu ft/day

Volatile solids reduction



50 to 70 percent

50 percent

Solids retention time

Conventional single stage 30 to 90 daysFirst-stage high rate 15 to 20 days

Digester capacity based on design

equivalent population



4 to 6 cu ft/PEb

0.7 to 1.5 cu ft/PE

a1.01b/cu ft/day = 16,000 g/m3 .d

b1.0 cu ft = 0.0283m3


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Fig 21.1. Photo of a single-stage fixed-cover anaerobic digester

Gas rising out of the digesting sludge is collected in the gas dome and is burned as

a fuel in the sludge heater; often the excess is wasted to gas burner. The cover can rise

vertically from the landing brackets to near the top of the tank wall guided by rollers

around the circumference of keep it from binding. The volume between the landing

brackets and the fully raised cover position is the amount of storage available for digested

sludge; this is approximately one-third of the total volume.

Digestion in a single-stage floating-cover tank performs the functions of volatile

solids digestion, gravity thickening, and storage of digested sludge. When sludge is

pumped into the digester from the primary settling tanks, the floating cover rises, making

room for the sludge. Unmixed operation permits daily drainage of supernatant equal to

approximately two-thirds of the raw sludge feed. Being high in both BOD and suspended

solids, the withdrawn water is returned to the inlet of the treatment plant. Periodically,

digested sludge is removed for dewatering and disposal. In large plants, digested sludge

may be dewatered mechanically, however, in small installations it is frequently spread in

liquid form on farmland or is dried on sand beds and hauled to land burial. Weather often

dictates the schedule for land disposal, and, consequently, substantial digester storage

volume is required in northern climates.


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Typical operation lowers the cover to the landing brackets in the fall of the year to

provide maximum storage volume for the winter. Fixed-cover digesters, where sludge is

withdrawn as the digested sludge is displaced by the raw feed sludge, maintain a constant

volume. Fixed-cover digesters require holding tanks, sludge lagoons, or other locations

where displaced digested sludge can drain. Because the volume is constant and the cover

is fixed, these digesters can be mixed by roof-mounted turbine mixers.

The digester contents can be mixed using turbine mixers, externally mounted

pumps, and gas mixing in draft tubes. Turbine, roof-mounted mixers are very efficient at

mixing the entire tank contents. Rags can be removed by reversing the mixing direction.

External mixing pumps can be mounted in draft tubes inside or outside of the digester, or

in a pump piped to the digester. Figure 21.1 shows mixing pumps mounted outside of the

digester tank. Pump mixing is also very effective, but may require multiple dischargepoints for large digesters. Gas mixing induces a flow within the draft tube to provide

mixing. Mixing requirements may be expressed in terms to power input or turnover time.

Typical values for power are 0.2 to 0.3-hp/1000 cu ft (0.005 to 0.008 kW/m 3). No

allowance is made for the efficiency of converting power into mixing. Turnover time is

calculated by taking the volume of the digester divided by the mixing flow rate. Typical

designs are based on turnover rates of 30 to 60 min.

Two-Stage Digestion

In this process, two digesters in series separate the functions of biological

stabilization from gravity thickening and storage shown in Figure 21.2. The first-stage

high-rate unit is completely mixed and heated for optimum bacterial decomposition.

These systems are available for installation in either fixed or floating-cover tanks. By

using a floating cover digested sludge does not have to be displaced simultaneously with

raw sludge feed as is required with a fixed-cover tank. In either case, however, the sludge

cannot be thickened in a high-rate process because continuous mixing does not permit

formation of supernatant.

Actually, the discharged sludge has a lower solids concentration than the raw feed

because of the conversion of volatile solids to gaseous end products. The second-stage

digester must be provided with either a floating cover or gas dome and have provisions

for withdrawing supernatant. The unit is often unheated, depending on the local climate

and degree of stabilization accomplished in the first stage. By minimizing hydraulic


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disturbances in the tank, the density of the digested sludge and clarity of the supernatant

are both increased. Two-stage digestion may be advantageous in some plants, while

conventional operation may be better in others. The determining factors include the size

of the treatment plant, flexibility of sludge handling processes, method of ultimate solids

disposal, storage capacity needed, and interrelated element of climatic conditions. For

large plants with a number of digesters, series operation provides better utilization of

digester capacity, but for small plants with limited supervision the conventional operation

is frequently more feasible.

Fig 21.2. Two-stage anaerobic digestion is performed by two tanks in series.

(a) The first stage tank on the left is completely mixed for optimum digestion.The second stage with a gas dome cover is for gravity thickening and storage

of digested sludge

(b) Photo of two-stage digesters at the Northeast wastewater treatment facility in

Lincoln, Nebraska.


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Sizing of Digesters

Historically, conventional single-stage tanks have been sized on the basis of

population equivalent load on the treatment plant. Heated digester capacity for a trickling

filter plant processing domestic wastewater was established at 4 cu ft (0.11 m3) per capita

of design load. For primary plus secondary activated sludge, the total tank volume

requirement was increased to 6 cu ft (0.17 m3) per capita. These values are still used as

guidelines of sizing conventional digesters for small treatment works.

Total digestion capacity can be calculated for conventional single-stage operation

using Eq. 11-42. Application of this formula requires knowing the characteristics of both

the raw and digested sludges.

V1 + V2

V = T1 + V2 x T2 --------- (21.2)

2

where V = total digester capacity, gallons (cubic meters)

V1 = volume of daily raw sludge applied, gallons per day (cubic meters per day)

T1 = period required for digestion, days (approximately 30 days at a temperature

of 85 to 90o F or 30 to 35oC)

T2 = period of digested sludge storage, days

The volume needed for the high-rate unit in a two-stage digestion system is based

on a maximum volatile solids loading and minimum detention time. For new designs the

generally adopted maximum allowable loading is 0.08 1b VS/cu ft/ day (1300 g/m3 .d)

and the minimum liquid detention time is ten days. At these loadings and a temperature of

95oF, volatile solids reduction should be 50 percent or greater. No specific design criteria

are established for second-stage tanks in high-rate systems, since thickening and digested

sludge requirements depend on local sludge disposal procedures.

Start-up of Digesters

Anaerobic digestion is a difficult process to start because of the slow growth rate

and sensitivity of acid-splitting methane-forming bacteria. Furthermore, the number of

these microorganisms is very low in raw sludge compared with acid-forming bacteria.

The normal procedure for start-up is to fill the tank with wastewater and to apply raw


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sludge feed at about one-tenth of the design rate. If several thousand gallons of digesting

sludge from an operating digester are used as seed, the new process cab be operational in

a few weeks. However, if only raw sludge is available, developing the biological process

may take months. Careful additions of lime added with raw sludge are helpful in

maintaining the pH near 7.0, but erratic dosage can result in sharp pH changes

detrimental to the bacteria. After gas production and volatile acids concentration have

stabilized, the feed rate is gradually increased by small increments to full loading. Daily

monitoring of this process involves plotting the daily gas production per unit of raw

sludge fed, percentage of carbon dioxide in the head gases, and concentration of volatile

acids in the digesting sludge.


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Lecture No:22

Energy Recovery from sago effluent design and application

Physico-chemical characteristics of CSFE :The CSFE characteristics ere analysed throughout the period of study for different

parameters and are shown in Table 22.1. Since the period of investigation extended from

off season to peak season, a wide variation was observed in all the parameters.

Table 22.1. Characteristics of CSFE

SI.No. Parameters Off-season Season1. Total solids (TS),

mg/I

2100 3620 3900 4650

2. Volatile solids (VS),

mg/I

1700 2880 3320 3740

3. Biochemical

Oxygen Deman

(BOD), mg/I

556 2510 2650 4025

4. Chemical Oxygen

Deman (COD) mg/I

1110 4880 5515 7060

5. Total Kjeldahi

Nitrogen, mg/I

58 75 80 94

6. Total organic

carbon, mg/I

2550 2890 2900 3560

7. Cyanide content,

mg/I

0.3 0.55 0.65 0.9

8. Threshold Odour

Number (TON)

65 95 110 140

9. PH 4.5 4.9 4.9 5.3

10. BOD : COD ratio 0.5 0.51 0.48 0.5711. C : N ratio 44 38.5 36.3 37.9

Design and fabrication of anaerobic high rate reactors

The methodology adopted for the design and fabrication of the anaerobic high rate

reactors are outlined in the following section.


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Selection of reactor configuration

In consideration of the recommendations of Guiot and Vander Berg (1984) and

Kennedy and Guiot (1986) that, a hybrid reactor can combine the advantages of upflow

anaerobic filter and UASB, it was decided to design and fabricate an Upflow Anaerobic

Hybrid reactor (UAHR). The expected advantages were an easy start-up and avoidance of

possible complications with sludge granulation. Single phase operation was selected

considering the recommendations of Lettinga and Hulshoff Pol (1980) that there is no

reason to go for a two phase system in the case of soluble wastes.

Media placement and selection:

It was decided to place the media on the upper 55 per cent of the reactor, height,

leaving 10 cm at the top from the liquid surface (Young, 1991).

Coconut shell was selected as the media to be compared with conventional PVC

pall rings in the UAHR. The media size was selected considering the recommendations of

Young (1991). The specific surface area around 100 m2 / m3 with a porosity over 85 per

cent was the consideration for selection. The PVC pall rings selected had dimensions of

25 mm (diameter) x 22 mm (length). Coconut shells were broken into pieces such that it

has an approximate specific surface area near to the recommendation. Coconut shells

were available as half pieces and each half piece was broken into 2 to 3 pieces and

screened to get a size 5 cm 10 cm.

Estimation of media characteristics

The procedures adopted for the estimation of specific surface area, porosity and

bulk density for both media were as follows:

The PVC pall rings were filled in a cylindrical vessel of 30 cm diameter and the

bulk volume was measured for a known number of pall rings. The surface area of one pall

ring was physically determined by linear measurements.

Surface area of one pall ring(s), m

2

x No. of pall rings (N)Specific surface area (Ss), = ----------------------------------------------------------------------

m2 / m3 Bulk volume occupied by N number of pall rings, m3


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To determine the specific surface area of coconut shells, 100 numbers of half

shells were selected randomly and the surface area was linearly measured. The mean

surface area of one half shell was then obtained. These shells were then broken into

required size and were randomly filled in a cylindrical vessel of diameter 30 cm and the

bulk volume occupied was found out.

Surface area of one pall ring(s), m2 x No. of pall rings (N)

Specific surface area (Ss), = ----------------------------------------------------------------------

m2 / m3 Bulk volume occupied by N number of pall rings, m3

To determine the porosity (P), the PVC pall rings as well as coconut shell media

were filled in a cylindrical vessel with a predetermined volume of water. The media were

filled in the vessel so that they are submerged and filled up to the water level. The new

volume was noted down.

Initial volume of water

Porosity of media (P) % = --------------------------------------- x 100

Volume after filling with media

The bulk density was estimated by finding the weight of a known volume for both

types of media.

Dimensions of UAHRs

The procedure adopted for arriving at the dimensions of the pilot scale UAHRs

are given below.

Design daily feed = 50 l / day

Design HRT = 4 day

Reactor liquid volume = 50 x 4 = 200


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The reactor height was selected considering the previous studies and ease for

fabrication. Ozturk et al. (1993) used a 140 cm high UAHR with 60 per cent media. A

height of 1.9 m was selected for the pilot scale UAHR. A cylindrical cross section was

adopted since this is the most widely used one due to the enhanced uniformity in mixing

and flow.

Design media height, as percentage

Of reactor height = 55 per cent (Young, 1991)

Clearance between top liquid

Surface and media top level = 10 cm

1.9 x 55

Total height of media section = ----------- = 1.045 m ~ 105 cm

100

The bottom 40 cm height of the reactor was designed in a conical shaped to

enable mixing of feed and also easy sludge withdrawal (if necessary).

Total liquid volume, V = 0.2 m3

= Vol. Of media filled cylindrical portion

+ Vol. of no-media cylindrical portion

+ Vol. of conical portion

P 1

0.2 = ( --- D2x 1.05 x --- ) + (0.4 x --- x --- D2 )

4 100 3 4

+ ( 0.45 x --- D2 )

4


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0.25465

D = -------------------------

0.0105 P + 1.5833

Where.

D = Diameter of the reactor, m

P = Porosity of media, per cent

But, a uniform diameter should be selected for both reactors, so as to get uniform

hydraulic parameters. Hence average porosity (P) was taken for the design purpose.

Gas holder

The Volume of gas holder was selected considering a biogas productivity of 3 lll

feed. A gas volume measurement schedule of once daily was assumed at HRTs upto 8 day

and as required thereafter.

200

Daily feed at 8 day HRT = ----- = 251

8

Gas production = 25 x 3 = 751

Hence, a gas holder volume of 80 l was selected. The gas holder was to be

designed in such a way that it provides a water seal like arrangement with provision for

up and down movement. Hence, a clearance of 5 cm was provided in between the outer

water jacket wall and the inner digester wall.

Diameter of gas holder = D + 0.05 m


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Diameter of water jacket = D + 0.05 + 0.05 = D + 0.1 m

Volume of gas hbolder, V (g) = 0.075 = ( (D+0.05)2/4) x H(g)

Where, H (g) is the effective gas holder

Gas production

Gas production rates are the most important indicators of reactor performance for

anaerobic reactors. Table 4.25 shows the gas production data of the UAHRs at PSS

periods of various HRTs. The mean daily gas production of reactor 1 increased from 58.6

l(15 day HRT) to 458.5 l (1 day HRT) showing 7.8 times increase, while reactor 2 had an

increase from 58.1 lto 474 l(8.2 times). At the same time specific gas production (l/kg

TS) decreased from 908.5 l(at 15 day HRT) to 574 l(at 1 day HRT) for reactor 1. For

reactor 2, the corresponding figures were 844.5 and 556 l/kg TS. The per cent decrease

over initial values were 6.8 and 34.1, respectively for reactors 1 and 2. The trend of

variation over different HRTs are illustrated in Fig.4.27. The maximum specific gas

productions obtained in this study were 3.5 and 3.3 fold higher than the highest value of

434.8 l / kg TS reduced obtained in batch digestion experiment for reactors 1 and 2,

respectively.

Specific gas production

A maximum specific gas production of 1108 l/kg VS and 1030 l/kg VS were

obtained for reactors 1 and 2, respectively at the longest HRT of 15 day. The

corresponding minimum values were 725 l/kg VS and 703 l/kg VS at the shortest HRT of

1 day. Chawla (1986) reported that a maximum gas production of 1000 l/kg VS is

achievable, assuming a VS reduction of 50 per cent. In the present study, the VS

reduction corresponding to the maximum specific gas production (reactor 1 at 15 day

HRT) was 76.2 per cent. Hence the maximum value of specific gas production was much

higher than the aove reported value. However, the maximum specific gas production

expressed as l/kg VS destroyed (1454 l) was lower than the maximum value of 2000 l/kg

VSdestroyed reported byChawla (1986). Lo and Liao (1986) also could get a biogas yield of

1048 l/kg VS for a mixture of screened dairy manure and winery waste which is similar

to the results of the present study.


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Specific gas productions in terms of BOD and COD also exhibited similar pattern

of steady decrease wich is depicted in Fig.4.27. The maximum values (at 15 day HRT)

were 1121.4 l/kg BOD and 604 l/kg COD for reactor 1. The corresponding values were,

1125 l/kg BOD and 561.7 l/kg COD for reactor 2. The minimum values obtained at


31/34

Table 22.2. Gas production at different HRTs

ParametersHRT

15 11 8 6 4 2.5 1.67 1

Mean daily gas production, 1 Reactor 1 58.6 80 118.6 153.7 194.3 272.2 354.7 458.5Reactor 2 58.1 78 125 162 198.3 271.7 356.5 475

Specific gas production, 1/kg TS Reactor 1 908.5 903.4 919.4 881.3 822 775 723 574Reactor 2 844.5 826.3 901.9 871 787 726 680 556

Specific gas production, 1/kg VS Reactor 1 1108 1123.6 1140.4 1101.8 1028 993 909 725

Reactor 2 1030 1028.2 1113.1 1088.7 981 930 857 703Specific gas production, 1/kg BOD Reactor 1 1121.4 1197.2 1064.9 1019.6 1053.1 1050.2 1042.6 825.0

Reactor 2 1125.0 1094.3 1052.0 1007.5 1007.6 982.7 982.4 799.6

Specific gas production, 1/kg COD Reactor 1 604.0 599.6 600.4 583.9 583.5 548.4 536.2 449.8Reactor 2 561.7 548.1 593.2 576.9 558.3 513.2 505.2 436.0

Specific gas production, 1/1 feed Reactor 1 3.9 3.9 4.24 4.1 3.45 3.02 2.62 2.04Reactor 2 3.63 3.6 4.16 4.05 3.3 2.83 2.47 1.98

Specific gas production, 1/m3 reactor Reactor 1 260.4 356 527 683 863 1210 1576 2038Reactor 2 242.1 325 520.8 675 826 1132 1485 1975


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Volumetric biogas production

The maximum biogas productivity obtained per litre of CSFE was 4.24 l/land 4.16 l/l

(at 8 day HRT) for reactors 1 and 2, respectively (Table 22.2.). The corresponding minimum

values were 2.04 l/l and 1.98 l/l at 1 day HRT. From 15 to 8 day HRT period, the biogas

productivity increased due to the increased TS content of 4620 mg/las against 4300 mg/lat

15 day HRT. There after the values showed a decreasing trend as depicted in Fig.4.28. This

could be attributed to the decrease in reactor performance at increased loading rates as well as

the lowering in strength of the feed. Fernandez (1999) could get 6 m3 CH4/m3 of wastewater

wile treating citric acid factory effluent and this high volumetric productivity compared to the

present study might be due to the high strength of the wastewater.

The volumetric gas production (l / m3 of reactor volume) steadily increased from

260.4 at 15 day HRT to the maximum values of 2038 and 1975 l/m3 (1 day HRT) for reactors

1 and 2, respectively. The increase of volumetric biogas production was gradual upto 4 day

HRT and drastic from 4 day to 1 day HRT due to the sudden increase of HLR and OLR.

TS and VS reductions

The TS and VS reduction as per cent of influent concentration is shown in Table 4.26.

The maximum TS reduction of 60 per cent and 59.3 per cent occurred at 15 day HRT, for

reactors 1 and 2, respectively. The minimum TS reductions were 33.8 per cent and 32.4. The

TS reduction was almost steady upto 8 day HRT and there after it showed a steady decreasing

trend. The VS reduction also showed a similar trend. The maximum VS reductions were 76.2

and 75.9 per cents at 15 day HRT for reactors 1 and 2. The minimum values were 49.5 and 48

per cent at 1 day HRT. 1 day HRT were 825 l/kg BOD and 449.8 l/kg COD for reactor 1 and

799.6 l/kg BOD and 436 l/kg COD for reactor 2. It became evident from these parameters

that reactor 1 was superior to reactor 2 at PSS of all HRTs with respect to specific gas

productions expressed in terms of TS, VS, BOD and COD.

Dararatana (1991) got a very high specific biogas production of 0.98 m3/kg CODremoved

from cassava alcohol slop. The maximum specific biogas production of 0.604 m3 / kg COD

obtained in this study (reactor 1 at 15 day HRT) is equivalent to 0.628 m3/kg COD removed

which is much lower than the above reported value.

According to the reports of FAO (1983), beef cattle wastes can produce 0.56-0.71m3

CH4/kg VS added. Hashimoto (1983) reported a value of 0.49 m3 CH4/kg VS added for swine

manure. Sarada and Nand (1989) could get 0.60 m3 CH4/kg VS from tomato processing waste

while Viswanath and Nand (1994) got 0.53 m

3

CH4/kg VS from silk industry wastes. In thisstudy, a methane productivity of 0.78 and 0.74 m3 CH4/kg VS were obtained for reactor 1 and


33/34

2, respectively at 15 day HRT. These values decreased to 0.47 and 0.45 m3 CH4/kg VS for

reactors 1 and 2, respectively at 1 day HRT.

The values of specific gas production obtained at the longest HRT of 15 day were

higher than the values reported by these workers and the values obtained at the shortest HRT

of 1 day were lower or nearer to the ranges reported by other workers. This is due to the

difference in substrate and operating conditions. CSFE was a highly biodegradable material

with readily hydrolysable constituents like starch. The very dilute nature (maximum TS, 4650

mg/l) of the feed stock also helped in the well mixing of substrate solids which favoured easy

digestion. 15 day was a rather long HRT at which very high biodegradation efficiency could

be achieved.

BOD and COD reduction

The BOD and COD reduction of the UAHRs at PSS periods various HRTs are shown

in Table 22.3. A very high BOD reduction of 99 per cent and 98.9 per cent for reactors 1 and

2 were obtained at the longest HRT of 15 days. The maximum COD reduction were 96.2 and

96 per cents for reactor 1 and 2, respectively. Upto 6 day HRT, both the reactors exhibited

steady performance irrespective of the influent concentrations. There after a decreasing trend

was observed due to the increased loading rated. Ths change was sharp between 4 day HRT

and 2.5 day HRT because of the drastic change in HLR (60 per cent increase). The lowest

reduction of 78.9 and 77.4 per cent BOD and 77.4 and 76 per cent COD occurred at 1 day

HRT. Reactor 1 was found superior to reactor 2 in BOD and COD reduction at all HRTs.


34/34

Reactor performance at PSS of different HRTs

ParametersHRT

15 11 8 6 4 2.5 1.67 1TS

Reduction,

%

Reactor

1

60 57.2 56.7 55.5 50.5 46.1 40.3 33.8

Reactor

2

59.3 56.8 55.6 55.1 49.5 44.9 38.1 32.4

VS

Reduction,

%

Reactor

1

76.2 75.3 75.9 70.6 65.7 59.9 53.8 49.5

Reactor

2

75.9 74.7 75.4 69.9 64.5 58.6 52.4 48

BOD

Reduction,

%

Reactor

1

99 98.6 98.8 98.6 95.6 87.5 83 78.9

Reactor

2

98.9 98.3 98.3 98.2 94.4 85.8 82.4 77.4

COD

Reduction,

%

Reactor

1

96.2 96.4 96.2 96.1 93.2 86.3 81.8 77.4

Reactor

2

96 96.3 95.8 95.2 92 85 80.5 76

TVA

Reduction,

%

Reactor

1

97.5 96.7 96 95.5 94.5 92.5 91.8 90.3

Reactor

2

96.9 96.1 95.6 95.5 93.9 91.9 91.1 89

TON

Reduction,

%

Reactor

1

97.1 96.1 95.2 91.7 89.2 80 66.3 50

Reactor

2

96.4 94.8 94.5 93.1 88.5 79.1 63.2 37.8

CH4

content of

biogas, %

Reactor

1

70 71 72 72 70.5 68 66 65

Reactor

2

72 73 74 73 71 68.5 65 64

Documents

Energy From Solid and Liquid Wastes - VIII