Presentation on theme: "Use the left mouse button to move forward through the show Use the right mouse button to view the slides in normal view, edit or print the slides The following."— Presentation transcript:
Use the left mouse button to move forward through the show Use the right mouse button to view the slides in normal view, edit or print the slides The following slides are provided by Dr. Vincent O’Flaherty.
Anaerobic Industrial Wastewater Treatment - Ecology and Technology A Short 4 Lecture Course Dr. Vincent O’Flaherty www.nuigalway.ie/microbiology/mel
Course 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 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
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 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 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 ? 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 CO 2 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 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 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 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
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 2. 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 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 xxxxxxx xxxxxxx xxxxxxx xxxxxxx xxxxxxx xxxxxxx xxxxxxx 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 designs 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 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 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
Well-settling nature of granules allows them to be retained in the reactor
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/m 3 /d - UASB; 100 kg COD/m 3 /d - EGSB Up to 20 m 3 biogas/m 3 /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 ADM citric acid production plant in Co. Cork, Ireland. Wastewater characteristics:- 7000 m 3 /day 12000 mg COD/l 4000 mg sulphate/l
Digester specification:- Upflow, fully-packed anaerobic filter random-packed, polypropylene cascade rings; 7300 m 3 volume Diameter of 36 m, height of 12.4
Operational performance:- HRT of approximately 1 day 52% COD removal 81% BOD removal 30 m 3 biogas/day (66% CH 4 ) (corresponds to 18 l/min) Biogas is used for steam generation and space heating
North Kerry Milk Processing Plant in Co. Kerry, Ireland Wastewater characteristics:- 4000 m 3 /day 5000 mg COD/litre Digester specification:- Downflow, random-packed anaerobic filter, polypropylene rings 4500 m 3 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)