ANAEROBIC BIOLOGICAL TREATMENT OF INDUSTRIAL WASTEWATERS

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The following slides are provided by Dr.
Vincent OFlaherty.
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Anaerobic Industrial Wastewater Treatment -
Ecology and Technology

• A Short 4 Lecture Course
• Dr. Vincent OFlaherty
• www.nuigalway.ie/microbiology/mel

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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

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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)

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

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• 3 categories based on COD content
• 1. lt 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

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Characteristics of some wastewaters from the
food-processing industrial sector
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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

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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

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• Systems include activated sludge, trickling
  filters - very commonly used for both sewage
  treatment and IWW
• Not covered here - but important!

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• Anaerobic
• Wont 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

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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

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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

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• 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

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• 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

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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)

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• 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

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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
• Reduce HRT
• Consequent decrease in heating costs
• Resultant increase in the net gain of biogas,
  financial and environmental benefit

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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

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Schematic diagram of the anaerobic contact
digester design
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• 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

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• Used mainly for food processing wastewaters with
  a significant content of suspended solids-
• Starch production meat processing abbatoir
  distillery green vegetable canning wastewaters,
  etc.

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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

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• Souse of long HRT is not necessary and is
  obviously very costly
• Alternative designs were developed that allowed
  further reduction of the HRTs 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

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• 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

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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

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• 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

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Schematic diagram of an Anaerobic Filter Reactor
Biogas
Influent/Effluent
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Sludge Bed
Effluent/Influent
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• 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

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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

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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)

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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

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Schematic diagram of an Upflow Anaerobic Sludge
Bed (UASB) reactor
Biogas
Effluent
Sludge Bed
Influent
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EGSB (Expanded Granular Sludge Bed)
Biogas
Effluent
Upflow velocity of 10-15 m/h
Sludge Bed
RECYCLE LINE
Increased sludge-wastewater contact
Influent
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Hybrid Reactor Design
BIOGAS
EFFLUENT
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Matrix - plastic etc.
R E C Y C L E
S L U D G E
INFLUENT
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Scanning electron micrograph of mesophilic sludge
granule at low magnification (Sekiguchi et al.,
1999).
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(No Transcript)
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• Well-settling nature of granules allows them to
  be retained in the reactor

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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.

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• 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.

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• Used mainly at full-scale for treatment of
  wastewaters from the food and drinks sector.
• Growing recent application for more recalcitrant
  wastewaters.

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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

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• Digester specification-
• Upflow, fully-packed anaerobic filter
  random-packed, polypropylene cascade rings
• 7300 m3 volume
• Diameter of 36 m, height of 12.4

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• Operational performance-
• HRT of approximately 1 day
• 52 COD removal
• 81 BOD removal
• 30 m3 biogas/day (66 CH4)
• (corresponds to 18 l/min)
• Biogas is used for steam generation and space
  heating

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North Kerry Milk Processing Plant in Co. Kerry,
Ireland

• Wastewater characteristics-
• 4000 m3/day
• 5000 mg COD/litre
• Digester specification-
• Downflow, random-packed anaerobic filter,
  polypropylene rings
• 4500 m3 volume

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• Operational performance
• HRT of approximately 1 day
• c. 90 COD/BOD removal
• Biogas used for electricity generation (combined
  heat and power plant).

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• Post treatment (activated sludge) prior to
  discharge
• Operated on a seasonal basis (March - October)