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The following slides are provided by Dr. Vincent O’Flaherty.

Anaerobic Industrial Wastewater Treatment -

Ecology and Technology

Anaerobic Industrial Wastewater Treatment -

Ecology and Technology

A Short 4 Lecture Course

Dr. Vincent O’Flaherty

www.nuigalway.ie/microbiology/mel

Course OutlineCourse Outline

• Anaerobic biological treatment of industrial wastewaters

• The phenomenon of granulation of anaerobic sludge - an example of co-operative interaction between different trophic groups of microbes

• The anaerobic treatment of sulphate-containing wastewaters - an example of competitive interactions between different groups of microbes

INDUSTRIAL WASTEWATERSINDUSTRIAL WASTEWATERS

• Very different from sewage sludge, animal manures, MSW, etc.

• Usually produced in large volume; low content of suspended solids; BOD/COD contributed mainly by dissolved organics; varied chemical composition

• Generally readily biodegradable (with the exception of some pharmaceutical/fine chemical wastewaters)

• Very variable range with respect to the organic matter content (BOD/COD), the solids content, the chemical composition, the biodegradability of the chemicals and the C:N:P ratio

• e.g. from food processing (abattoirs, dairy, cannery etc.), brewing, distillery, pharmaceutical, fine chemical, tannery, etc.

3 categories based on COD content:

1. < 2000 mg/l COD

2. 2000 - 10000 mg/l COD

3. 10000 - 100000 mg/l COD

Raw domestic sewage has a COD of 400 - 600 mg/l

Characteristics of some wastewaters from the food-processing industrial

sector

Characteristics of some wastewaters from the food-processing industrial

sectorIndustry BOD

(mg/l)COD(mg/l)

Suspendedsolids(mg/l)

pH

Brewery 850 17,000 90 4 - 6

CitrusCannery

2,000 - 7,000 Acid

Dairy 600 -1,000

2 -4,000

200 - 400 Acid

Potatoprocessing

2,000 3,500 2,500 11 -13

Sugar beet 450 -2,000

600 -3,000

800 - 1,000 7 - 8

Slaughterhouse 1,500-

2,500

4,000-

6,000

800 7

Silage 50,000 70 -80,000

low Acid

Options available for treatment of IWW

Options available for treatment of IWW

• Principal components are soluble pollutants

• The removal of soluble organic matter from wastewaters is always a biological process - the most widely applied biotechnological process

• Essentially, the choice is between aerobic and anaerobic processes

ADVANTAGES AND DISADVANTAGES OF AEROBIC AND ANAEROBIC TREATMENTADVANTAGES AND DISADVANTAGES OF AEROBIC AND ANAEROBIC TREATMENT

• Aerobic

• generally achieves full BOD removal

• occurs at ambient temperature

• doesn't need enclosure

• produces large quantities of waste biomass requiring safe disposal

• Requires high energy consumption for aeration purposes

• Systems include activated sludge, trickling filters - very commonly used for both sewage treatment and IWW

• Not covered here - but important!

• Anaerobic

• Won’t achieve complete BOD removal

• Must be heated* and enclosed

• Achieves a high rate of pathogen kill and reduces odours

• Produces much smaller amounts of waste biomass

* Uses up to 30% of the biogas - latest work is on use of low-temperature systems

Main AdvantageMain Advantage

• Between 70-80% of the energy content of the waste constituents is conserved in the methane product - net production of a usable fuel, renewable energy

Why Anaerobic Treatment for IWW ?

Why Anaerobic Treatment for IWW ?

• Increasingly used for the treatment as:

• It produces biogas. This energy source is used by industries for heat and power generation or steam production - net producer of fuels whereas aerobic systems are heavy fossil fuel-utilisers, net reduction in CO2 emissions/greenhouse effect

• It produces less waste sludge (biomass) than aerobic systems, less to dispose of (expensive)

• Used as an alternative to or in conjunction with aerobic treatment systems - depending on the fate of the treated effluent

• Used to remove COD/BOD prior to discharge to a municipal sewer

• Used with aerobic plant - first stage anaerobic followed by aerobic treatment to discharge standard (also other treatments if required)

• AD is increasingly applied because high-rate reactor designs overcame some problems

Historical DifficultiesHistorical Difficulties

• CSTR designs originally used, same as for manuries and sewage sludge

• In these systems the hydraulic retention time (HRT) is equal to the solids retention time (SRT) - necessary to allow hydrolysis of solid organics

• BUT also required because of the very slow growth rate of methanogens and syntrophs (5-9 day dt in some cases)

• Risk of washout of bacteria is HRT is less than 10 days

• CSTR initially used for IWW with high levels of particulates - e.g. abbatoir, vegetable processing etc.

• As a result of v. long HRT need a very large digester volume - capital and running costs are high, so not often feasible

Development of AD designs specifically for IWW

Development of AD designs specifically for IWW

Aim was to get benefits of AD, but reduce the disadvantages - i.e. costs, digester volume

Logic is :

1. Reduce HRT

2. Consequent decrease in heating costs

3. Resultant increase in the net gain of biogas, financial and environmental benefit

TWO MAIN STRATEGIES DEVELOPEDTWO MAIN STRATEGIES DEVELOPED

• 1. Biomass Recycle (Anaerobic Contact)

• Analogous to aerobic activated sludge systems

• Biomass washed out of the system is separated and returned to the digester

• Separate SRT from HRT - biomass retention time becomes longer

Schematic diagram of the anaerobic contact digester design

Schematic diagram of the anaerobic contact digester design

BIOGAS

SLUDGE RECYCLE

INFLUENT

EFFLUENT

• Allows operation at higher organic loading rates - smaller digester volumes required lower capital costs for construction

• Used mainly for the kinds of IWW treated previously by CSTR

• Allows reduction of the HRT to 6-12 days (1/2 to 1/4 of digester volume) - 60-95% COD removal

• Used mainly for food processing wastewaters with a significant content of suspended solids:-

• Starch production; meat processing; abbatoir; distillery; green vegetable canning wastewaters, etc.

Retention of the Biomass within the Reactor Independent of the Wastewater

Flow

Retention of the Biomass within the Reactor Independent of the Wastewater

Flow2. Retained Biomass Systems

• Second generation of IWW AD designs

• AC systems rarely operated below 6 day HRT - because ww being treated usually contains insoluble organic polymers -i.e. hydrolysis is the rate limiting step

• But most IWW have very low ss content, BOD or COD is contributed by soluble, low Mwt organics that are readily biodegradable

• So…use of long HRT is not necessary and is obviously very costly

• Alternative designs were developed that allowed further reduction of the HRT’s and these 2nd generation digesters are the most important in terms of modern IWW treatment

• Idea is to retain biomass inside the digester independent of the ww flow - allows HRT to be much reduced

• HRT in these retained biomass digesters can be reduced to as low as several hours depending on the wastewater and the digester design and mode of operation.

• Significant reduction in reactor volume achieved

2 Main Types of Retained-Biomass Digesters

2 Main Types of Retained-Biomass Digesters

• 1. Fixed-Film Systems

• 2. Granular Sludge-based Systems

Anaerobic filter/fixed film systems

• Strategy is to provide an inert surface for bacterial adhesion - biofilm formation

• Supports include plastic, sand etc. - depending on the physical arrangement of the support, biomass may also be retained as flocs or aggregates in the interstitial spaces

• Either fixed-bed or fluidised-bed designs

• Fixed-bed Systems are packed with support media with large surface area for biofilm development

Schematic diagram of an Anaerobic Filter Reactor

Effluent/Influent

Influent/Effluent

Biogas

Sludge Bed

xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx

xxx

• WW is passed over the biofilm - either in upflow or down flow direction - biogas is collected at the top of the digester

• Fluidised-bed Systems use very small particles of sand or activated carbon

• Very fast upflow velocity is applied so that the bed is fluidised - HRT is in hours not days, but expensive to operate and not very stable

High-rate reactor designsHigh-rate reactor designs

Biogas Biogas Biogas

Effluent Effluent

Influent

Influent

EffluentInfluent Effluent Influent

A B

Biogas Biogas

Effluent Effluent

Influent Influent

D E

C

• Anaerobic digester designs based on biomass retention:

• (a) anaerobic filter/fixed bed reactor;

• (b) downflow stationary fixed-film reactor;

• (c) expanded bed/fluidised bed reactor;

• (d) upflow anaerobic sludge blanket reactor; Expanded granular Sludge Bed

• (e) hybrid sludge bed/fixed bed reactor

2. Granular Systems2. Granular Systems

• Biomass self-aggregates into dense well-settling granules

• Thus it is retained within the digester even during upflow operation (not washed out)

Granular Sludge Bed (UASB/EGSB/Hybrid) systems

Granular Sludge Bed (UASB/EGSB/Hybrid) systems

• e.g. UASB reactor, most commonly applied worldwide

• Very high biomass density in the reactor - allows very high organic loading rates

• Optimal spatial organisation of different trophic groups within the granules

Schematic diagram of an Upflow Anaerobic Sludge Bed (UASB) reactor

Influent

Effluent

Biogas

Sludge Bed

EGSB (Expanded Granular Sludge Bed)

Influent

Effluent

Biogas

Sludge Bed RECYCLE LINE

Increased sludge-wastewater contact

Upflow velocity of 10-15 m/h

Hybrid Reactor DesignHybrid Reactor Design

RECYCLE

SLUDGE

BIOGAS

INFLUENT

EFFLUENTxxxxxxxxxxxxxxxxxx Matrix -

plastic etc.

Scanning electron micrograph of mesophilic sludge granule at low magnification (Sekiguchi et al., 1999).

• Well-settling nature of granules allows them to be retained in the reactor

USE OF ANAEROBIC DIGESTION FOR INDUSTRIAL WASTEWATER

TREATMENT

USE OF ANAEROBIC DIGESTION FOR INDUSTRIAL WASTEWATER

TREATMENT• Installation of anaerobic digesters for

industrial wastewaters has grown very rapidly over the past 15-20 years.

• UASB design is the most widely used, EGSB becoming more common.

• Very high loading rates and biogas productivity; HRT typically 1 day or less.

• Up to 30 kg COD/m3/d - UASB; 100 kg COD/m3/d - EGSB

• Up to 20 m3 biogas/m3/d

• Typically achieve 80-99% COD removal.

• A.D. treated wastewater is either discharged to the municipal sewer for final treatment prior to discharge or subjected to aerobic polishing, NPK removal, etc. by the industry prior to discharge to the receiving waterbody.

• Used mainly at full-scale for treatment of wastewaters from the food and drinks sector.

• Growing recent application for more recalcitrant wastewaters.

EXAMPLE OF FULL-SCALE ANAEROBIC DIGESTER FOR INDUSTRIAL WASTEWATER

TREATMENT

EXAMPLE OF FULL-SCALE ANAEROBIC DIGESTER FOR INDUSTRIAL WASTEWATER

TREATMENT

• ADM citric acid production plant in Co. Cork, Ireland.

• Wastewater characteristics:-

7000 m3/day

12000 mg COD/l

4000 mg sulphate/l

• Digester specification:-

Upflow, fully-packed anaerobic filter random-packed, polypropylene cascade rings;

7300 m3 volume

Diameter of 36 m, height of 12.4

• Operational performance:-HRT of approximately 1 day52% COD removal81% BOD removal

30 m3 biogas/day (66% CH4)(corresponds to 18 l/min)

• Biogas is used for steam generation and space heating

North Kerry Milk Processing Plant in Co. Kerry, Ireland

North Kerry Milk Processing Plant in Co. Kerry, Ireland

• Wastewater characteristics:-

4000 m3/day5000 mg COD/litre

• Digester specification:-

Downflow, random-packed anaerobic filter, polypropylene rings

4500 m3 volume

• Operational performance

• HRT of approximately 1 day• c. 90% COD/BOD removal

• Biogas used for electricity generation (combined heat and power plant).

• Post treatment (activated sludge) prior to discharge

• Operated on a seasonal basis (March - October)