Transcript Granulation - a unique example of biofilm formation

The following slides are provided by
Vincent O’Flaherty.
Dr.
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Anaerobic digestion of sulphatecontaining wastewaters
• Industrial wastewaters can, in general, be
effectively treated using anaerobic digestion produces large quantities of methane which can
be burned to generate electricity or for heating use of combined heat and power plants allows for
generation of electricity and heat recovery
• Normally less than 10% of the biogas produced is
required to operate the plant - also produces far
less waste biomass than aerobic system = less
disposal costs
• Modern digester design makes the process more
attractive - can operate at high rates and
therefore smaller, cheaper digester can be used
• Usual procedure is to have first stage anaerobic
and then small activated sludge plant to “polish”
the effluent = achieve discharge standards. May
need some nutrient removal or other tertiary
steps depending on the fate of the effluent
Problematic industrial
wastewaters
• Application to industrial wastewaters can
occasionally be complicated by microbiological
problems - typical example is treatment of
sulphate containing wastewaters - examples of
the type of negative microbial interactions
which can occur in an engineered ecosystem
• Industrial wastewaters can contain high levels
of recalcitrant organic chemicals (e.g.
chloroform,
carbon
tetrachloride
etc.),
xenobiotic products and side products
(insecticides, herbicides, detergents etc.)
• Can also contain significant quantities of
inorganics some of which may be highly toxic
e.g. cadmium in tannery wastewater
• In anaerobic systems the presence of alternative
external oxidising agents ( e.g. sulphate SO42-)
can promote the development of a sulphatereducing rather than a methanogenic population
• This will result in the channelling of electrons
towards the formation of H2S not methane
SRB and anaerobic digestion
• Very complex systems - absolute need to fully
understand the microbiology in order to control
treatment plant operation - i.e on one hand a
useable fuel is generated and on the other hand a
malodourous atmospheric pollutant is produced
under sulphidogenic conditions
• Presents a challenge to microbiologists because
of the complexity of the systems and the
technical difficulty in studying them
•
Examples of wastewaters that contain highlevels of sulphate include:
1. Molasses-based fermentation industries - e.g.
citric acid production, rum distillery
2. Paper and board production
3. Edible oil refinery
•
Many other industries use sulphuric acid in their
processes - leads to sulphate in the ww
So what?
• In the absence of external oxidising agents
(sulphate, nitrate, etc.) anaerobic ecosystems are
methanogenic - flow of reducing equivalents is
directed towards the reduction of CO2 to CH4
• In the presence of sulphate - the flow may be
redirected towards the reduction of sulphate to
sulphide by sulphate reducing bacteria (SRB)
• In other words there is a competition between
different microorganisms for substrate
• What will determine the outcome of
competition?
• Very important to know as on one hand a
useable fuel is produced, while on the
other hand a toxic, corrosive malodourus
compound is produced
• Bacteria that reduce sulphate to H2S are either
assimilatory or dissimilatory:
• 1. Assimilatory Sulphate Reduction: Carried out
by many different bacteria - purpose is to reduce
sulphate to sulphide prior to uptake of S for
assimilation into S-containing proteins etc.
• No major environmental effect only amount of
sulphate needed for bacterial growth is reduced
• e.g Klebsiella sp. - only reduce 1 mg sulphate for
every 200 mg (d.wt.) of cells produced
• 2. Dissimilatory Sulphate Reduction: Totally
different process only carried out by a unique
group of bacteria carrying out anaerobic
respiration using sulphate as electron acceptor
• Consequently transform large amounts of
sulphate to H2S during growth
• e.g. Desulphovibrio sp. - for every 1 mg of
sulphate reduced, only 0.5- 1.0 mg (d.wt) of cells
are produced
• SRB exhibit enormous ecological,
morphological and nutritional diversity grouped together only on the basis of
carrying out dissimilatory sulphate
reduction - 1 common property
• Widely distributed in the natural
environment - include both sporeformers
(Desulfomaculum sp.) and nonsporeformers (Desulfovibrio sp.)
• 13 eubacterial and 1 archaeal genera
• Can be divided into two broad categories based
on their metabolism:
• 1. Incomplete Oxidisers: carry out incomplete
oxidation of organic compounds to acetate, CO2
and H2S - can use a very wide range of starting
organics e.g. aliphatic mono- and dicarboxylic
acids, alcohols, amino acids, sugars, aromatic
compounds etc.
• Desulfomicrobium, Desulfobulbus,
Desulfoboyulus, Thermodesulfobacterium,
Desulfovibrio*, Desulfomaculatum*
• * Most common species
• 2. Complete oxidisers: Complete oxidation of
starting organic substrates to CO2 and H2S same wide range of substrates, but can also grow
on acetate, breaking it down completely to CO2
• Desulfobacter, Desulfococcus, Desulfosarcina,
Desulfomonile, Desulfonema, Desulfoarculus,
Archaeoglobus
• Chemolithotrophic species also common - grow
on H2/CO2 or on CO very common ability to grow
on H2 - very important in certain ecosystems
• Basically use H2 as energy source and fix
CO2 (autotrophic as carbon source)
• 4H2 + SO42- + H+ -----> 4 H2O +HSG˚´ = -150KJ/mole
• Very favourable reaction energetically
• SRB very versatile metabolically - in the
absence of sulphate in their environment,
they can switch from anaerobic
respiration to chemoorganotrophic
fermentation - energy gain by substrate
level phosphorylation only
• V. important as allows maintainence of
SRB in the absence of sulphate
• 2 types of fermentation possible:
1. Fermentation (independent of H2 conc.)
• Grow fermentatively on sugars, carboxylic
acids, alcohols etc.
• Incomplete degradation to l.mwt acids and
alcohols and CO2
2. Growth as syntrophic OHPA species (H2
dependent)
• Convert higher carbon-number alcohols,
acids, ketones etc. to acetate + H2 or
acetate + CO2 + H2
• Same restrictions as syntrophs
What happens during anaerobic
treatment of sulphate containing
wastewaters?
• Competition between SRB and other anaerobes
for common organic and inorganic substrates
• Competition
equivalents
for
energy
and
reducing
• Between SRB and Fermentative bacteria,
between SRB and OHPA, between SRB and
Acetoclastic and Hydrogenophilic methanogens
ACETATE
4a
1
COMPLEX
ORGANIC
MOLECULES
1
1
5
FERMENTATION
INTERMEDIATES
2
CH , CO 2
3
4
5
4b
H /CO
2
2
Carbon flow in anaerobic digesters: 1 = Hydrolytic/fermentative bacteria; 2 = Obligate hydrogen
producing acetogens; 3 = Homoacetogenic bacteria; 4a = Acetoclastic methanogens; 4b =
Hydrogenotrophic methanogens; 5 = Fatty acid synthesising bacteria (O'Flaherty, 1997).
ACETATE
1
4a
5
H S + CO2
2
COMPLEX
ORGANIC
MOLECULES
1
FERMENTATION
INTERMEDIATES
2
3
CH4+ CO2
5
1
H /CO
2
2-
SO4
Sulphate Reduction
2
4b
Negative effects of competition
• 1. Reduction of potential methane yield diversion of substrates/reducing
equivalents to H2S
• 2. Sulphide Toxicity - H2S is toxic to all
cells - toxicity is pH dependent
• Only the unionised form of H2S to
membrane permeable
• H2S <====> H+ + HS• HS- <====> H+ + S• At neutral pH approximately 20-50% of the
H2S is present in the unionised form
• At pH 8-9 virtually all the H2S is
undissociated - toxicity increases with
increasing pH
• With respect to AD, fermentatives are far
less susceptible to H2S toxicity than
syntrophs, methanogens or even SRB
• IC50 of 50-400 mg/l H2S for methanogens
• 3. H2S in the biogas - H2S is very volatile,
so will appear in the biogas causing
problems of odour, corrosion, release of
SO2 during burning
• May well have to be stripped from the biogas costly
• 4. Dissolved sulphide in the effluent - odour,
oxygen demand, post-treatment costs
• 5. Precipitation of Alkali metals - Fe, cobalt etc.
• 6. Sulphate toxicity - salt effects, not usually
significant
HOW TO SOLVE THE PROBLEMS
• Eliminate sulphate from the process,
occaisonally possible using chemical
precipitation, often not
• Engineer the ww treatment process, need to
understand the microbial ecology
• Outcome of competition is determined by
the following factors:
• COD/BOD concentration
• Chemical composition of the ww
• Sulphate conc.
• COD/BOD:sulphate ratio
• Bacterial population of the sludge
• pH of reactor operation
• Mass transfer limitations
• Very complex microbiological problem:
• Theoretical predications can be made based on
kinetic and thermodynamic considerations
• However, these do not correspond to what is
measured in practical situations - especially for
conversion of the key (70% of biogas) substrate
acetate
• See: review for discussion