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

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

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)

Granulation - a unique example of biofilm formation

Granulation - a unique example of biofilm formation

• The UASB reactor was the first reactor design to employ granular anaerobic sludge

• Immobilisation of the bacteria is essential for reactor operation and ww treatment

• Intensively studied - yet incompletely understood

• Although the UASB and related systems have been intensively studied for almost 2 decades, reactor instability due to biomass loss still causes problems - a common cause of reactor instability is poor granulation

• Granules are not formed/retained and biomass loss is not compensated by new growth; Biomass is lost by floatation

• Often biomass is not present as granules, but as poor settling flocs with low volumetric activity

What are granules?What are granules?

• Granules consist of organised aggregates of microbial cells which form a complete methanogenic unit

• A granule contains all the microbial trophic groups necessary for complete breakdown of substrates to CH4

• Dense, well-settling aggregates

• Granule formation occurs spontaneously when the appropriate conditions exist

• The exact requirements are still unclear

• Many factors implicated in granulation including: divalent metals, FeS, carbohydrates, hydraulic regime, ammonium, bacterial appendages, gas loading rate, surface tension of the liquid and extracellular polysaccharides

• Will look in summary at current model for granule formation and some of the factors that influence a successful outcome

The microbial architecture of granules

The microbial architecture of granules

• The bacteria involved in granule formation do not aggregate at random

• With microscopy and micro-electrode measurements, it has been established that granules have an organised distribution of bacterial species and activities

ANAEROBIC DIGESTIONANAEROBIC DIGESTIONCOMPLEX ORGANIC MOLECULES

(e. g. polysaccharides, proteins)

MONOMERS (e. g. glucose, amino acids)

ORGANIC ACIDS, ALCOHOLS, KETONES

ACETATE, CO2, H2

METHANE (CH4)

Hydrolytic/Fermentative bacteria

Fermentative/Acidogenic bacteria

Acetogenic bacteria

Methanogenic bacteria

• Fermentative or acidogenic bacteria are located on the outer edge of the granule (200µm) with the syntrophs and methanogens located in the centre

• The syntrophic acetogens must be located within 1-2 µm of methanogens in order to facilitate interspecies hydrogen transfer

How and why are Granules formed?

How and why are Granules formed?

• Granule formation is complex - can divide the influencing factors into microbiological and environmental

• Microbiological factors include:• EPS• Kinetic parameters• Cell surface characteristics• Trace elements and nutrients• Seed sludge

1. The role of EPS in granule formation

1. The role of EPS in granule formation

• The importance of extra-cellular polymeric substances as a cementing substance is unchallenged - crucial in both the structure and function of biofilms and granules

• EPS is present in biofilms and aggregates and is present in a particularly structured sense in granular aggregates

• It cements cells together, and onto a substratum carrier

• Besides its role in the structural integrity of the granules or biofilms it seems to function as a protecting agent against biocides such as the toxic chemicals which may arise in IWW

• In non-granular biofilms it protects against antibodies, antibiotics, prevent grazing and mediates initial cell attachment to surfaces

• Also acts as a storage material in times of starvation

 

• There is a reported link between the presence of carbohydrates in the wastewater or feed and the maintenance of stable granular sludge

• These high-energy substrates will encourage the production of EPS

• In granules, EPS consists of polymeric carbohydrates, with a large amount of proteins, some lipid and DNA and some other biopolymers

• EPS can be viewed as a highly hydrated gel, which is negatively charged at neutral pH

• This means that it effectively acts as an ion exchanger, strongly binding metal cations e.g. Ca2+, and repelling anions

• This binding capacity could greatly reduce the diffusion of substrates through the granule, but only transiently - sites will become saturated

EPS and FloatationEPS and Floatation

• In practical terms for IWW AD the role of EPS may be vital, as acidogens on the outer layer produce the EPS

• This gives the granule a hydrophilic outer layer around an inner, hydrophobic layer of syntrophs and methanogens

• This hydrophilic layer helps to prevent gas attachment and floatation as well as cementing the granule together

2. The role of kinetic parameters in granule formation

2. The role of kinetic parameters in granule formation

• Growth rates, substrate affinities etc.

• The granule represents a balanced ecosystem, this is not a straightforward outcome kinetically

• The growth rates of fermentative organisms are 5-10 times faster than syntrophs and methanogens - so environmental factors will greatly affect the likelihood of a stable community developing - delicate balance

• The cell yields of methanogens and syntrophs are thus rate limiting for the growth of granules - loading rates and other parameters are based on this

• However, probably the most important kinetic determinant is the outcome of competition for acetate between two methanogens - Methanosaeta (ex. Methanothrix) sp. and Methanosarcina sp.

Acetate Competition and Granulation

Acetate Competition and Granulation

• Methane can be produced via acetate or via H2/CO2 - 70-90% of the biogas produced by anaerobic digesters is via acetate - the key CH4 source

• Surprisingly however only two methanogens have ever been discovered that can utilise acetate - Methanosaeta and Methanosarcina

• Methanosaeta concilli and M. Soehgennii are rod-shaped organisms which tend to grow as filaments

• Methanosarcina barkeri is coccoid and grows 3-6 times faster than Methanosaeta on acetate, but Methanosaeta is the dominant methanogen in granular sludge (up to 90% of methanogenic cells)

• Methanosaeta has a much higher substrate affinity and in a balanced anaerobic digester the concentration of acetate will be very low

• The presence of Methanosaeta is believed to be crucial as the filaments provide a nucleus for granule initiation

The role of cell surface characteristics in granule formation• Hydrophobicity, electrophoretic mobility, and isoelectric

point are used to predict the adhesion of bacteria to surfaces

• Hydrophobicity measured by water contact angle is the most commonly used - allows estimation of the surface energy of bacteria