Transcript Granulation - a unique example of biofilm formation

The following slides are provided by
Vincent O’Flaherty.
Dr.
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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
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
• 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 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
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
• 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
• 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?
• 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 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 DIGESTION
COMPLEX ORGANIC MOLECULES
(e. g. polysaccharides, proteins)
Hydrolytic/Fermentative bacteria
MONOMERS
(e. g. glucose, amino acids)
Fermentative/Acidogenic bacteria
ORGANIC ACIDS, ALCOHOLS, KETONES
Acetogenic bacteria
ACETATE, CO2, H2
Methanogenic bacteria
METHANE (CH4)
• 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?
• 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
• 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 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
• 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
• 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 rodshaped 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