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Granulation - a unique example of biofilm formation


Industrial wastewaters can, in general, be effectively treated using anaerobic ... tetrachloride etc.), xenobiotic products and side products (insecticides, ... – PowerPoint PPT presentation

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Title: Granulation - a unique example of biofilm formation

The following slides are provided by Dr.
Vincent OFlaherty.
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Anaerobic digestion of sulphate-containing
  • 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
    sulphate-reducing rather than a methanogenic
  • 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 high-levels
    of sulphate include
  • Molasses-based fermentation industries - e.g.
    citric acid production, rum distillery
  • Paper and board production
  • 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 non-sporeformers (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,
  • 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,
  • 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 -----gt 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
  • Convert higher carbon-number alcohols, acids,
    ketones etc. to acetate H2 or acetate CO2
  • 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 for energy and reducing equivalents
  • Between SRB and Fermentative bacteria, between
    SRB and OHPA, between SRB and Acetoclastic and
    Hydrogenophilic methanogens

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).
(No Transcript)
Negative effects of competition
  • 1. Reduction of potential methane yield -
    diversion of substrates/reducing equivalents to
  • 2. Sulphide Toxicity - H2S is toxic to all cells
    - toxicity is pH dependent
  • Only the unionised form of H2S to membrane

  • H2S ltgt H HS-
  • HS- ltgt 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 -
  • 4. Dissolved sulphide in the effluent - odour,
    oxygen demand, post-treatment costs
  • 5. Precipitation of Alkali metals - Fe, cobalt
  • 6. Sulphate toxicity - salt effects, not usually

  • 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/BODsulphate 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
  • See review for discussion