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INTRODUCTION
ANAEROBIC DIGESTION –
HOW DOES IT WORK?
• Fundamentals of anaerobic treatment – microbiology
and operation
• Advantages and disadvantages of anaerobic treatment
• Engineering design to improve biological function
Stephen Edwards
[email protected]
PhD student in Environmental Engineering,
School of Civil Engineering and
Geosciences, Newcastle University
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DEFINITION
Energetic Constraints
• Biological degradation of organic material in the absence of
Free Energy
kJ/mol glucose
molecular oxygen to a more stable form.
• The main ultimate product for Anaerobic systems is methane
utilising carbon dioxide as the terminal electron acceptor.
C6H12O6 → 3CO2 + 3CH4
• Natural microbiological process that is responsible for most of
the worlds methane production (1 Giga Ton).
Aerobic oxidation
C6H12O6 + 6O2  6CO2 + 6H2O
-2880
Denitrification
+
5C6H12O6 + 24NO3 + 24H  30CO2 + 24H2O + 12N2
-2720
Sulphate reduction
2+
2C6H12O6 + 6SO4 + 9H  12CO2 + 12H2O + 3H2S + 3HS
-492
Methanogenesis
C6H12O6  3CO2 + 3CH4
-428
Ethanol fermentation
C6H12O6  2CO2 + 2CH3CH2OH
-244
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ANAEROBIC DIGESTION PROCESS
ANAEROBIC
DIGESTER
KEY PROCESS
CONTROL/ OPERATING
CONDITIONS
CH4
+
CO2
VOLATILE
COMPLEX SIMPLE
ORGANICS ORGANICS ACIDS,H2,CO2
ACETATE,
CO2, H2
EFFLUENT
HYDROLYTIC
ACIDOGENIC
ACETOGENIC
SUBSTRATE
DEGRADATION/
ORGANIC LOADING
RATE
HYDRAULIC
RETENTION AND
SOLID RETENTION
TIME
TEMPERATURE
TOXIC/INHIBITORY
COMPOUNDS
NUTRIENT BALANCE
MIXING
METHANOGENS
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SUBSTRATE DEGRADABILITY AND
ORGANIC LOADING RATE (OLR)
• Most organic material is biodegradable under anaerobic
conditions. Exception is Lignin.
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HYDRAULIC RETENTION TIME (HRT)
/SOLIDS RETENTION TIME
• Effectively how long the liquid/biomass remains in the reactors.
• HRT doesn’t have to equal SRT.
• When HRT = SRT HRT must be ≥ Biomass generation time
• Typically >15 days.
• Lower HRT higher throughput means better treatment times.
• Longer SRT will result in:
• Higher Solids concentration (more biomass)
• Higher % COD conversion to methane
• Lower daily methane production rate
• Greater stability
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TEMPERATURE
• Influential parameter
• Rise in temperature increase in rate of biochemical and enzymatic reactions –
causing increased growth rates
• Above certain temperature – inhibition and mortality due to denaturing of
structural components and proteins – species dependent
• Thermophillic more efficient than Mesophillic from increased utilisation rates
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pH CONTROL/ALKALINITY/ACIDITY
• pH are a good indicator of reactor performance and stability.
pH, alkalinity and acidity go hand in hand.
• Methanogens show small working pH range (6 – 8)
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ESSENTIAL NUTRIENTS
• C:N:P ratios 150:5:1 produce optimum biogas
• C:N ratios less than 10:1 cause inhibition.
• Ratios greater than 25:1 are inefficient due to nitrogen depletion.
• Lack of acidity and alkalinity will mean pH levels fluctuate meaning gas
production will cease/be reduced.
• High pH levels means more toxic species is present.
• Low pH denatures proteins and enzymes causing gas production to cease
• Anaerobic Facultative Bacteria can operate at larger pH ranges
and potentially thrive within different conditions.
• Controlling pH can limit toxic effects through reduction in undissociated forms.
• Nitrogen required for synthesis of amino acids/proteins etc.
• Insufficient Nitrogen and Phosphorous results in slower growth.
• Ammonia provides a strong base for pH control.
• Solution:
• Trace nutrient supplementation
• Fertilisers (Typically UREA added)
• Co-digestion
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MIXING
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TOXIC/ INHIBITORARY COMPOUNDS
• Small amounts of natural mixing from methane bubbles.
• Contact between the organic matter and microorganisms is improved
through mixing.
• Effects growth rate, distribution, substrate availability, granule formation
and gas production.
• Heavy metals (copper, cadmium, lead, zinc…)
• Volatile Fatty acids (e.g propionate)
• Antibiotics
• Ammonia
• Oxygen
• Achieved through:
• Mechanical mixing (paddles, turbines and propellers)
• Hydraulic shear force
• Gas recirculation
• Other electron acceptors (NO2, NO3, SO4, O2)
• Can cause inefficiency through short circuiting and inhibition.
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WHAT CAN ANAEROBIC TREAT
ADVANTAGES
• Yielding energy from waste instead of using energy
• Municipal sewage sludge
for waste treatment
• Animal residues (Farm Wastes)
• Reduction of nutrient requirements
• Domestic wastewater
• Small land footprint
• Dairy Wastewater
• Removal of pathogens
• Domestic Solid waste
• Reduction/removal of landfill requirements
• Cosmetics Waste
• Biodegradation of aerobic non biodegradables
• Pharmaceuticals
• Production of useful product (Biogas)
• High-strength industrial wastewater – Food processing
• Residues available for fertilizer/soil conditioner
• Renewable energy certificate obligations
industry/ Brewery wastewater
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DISADVANTAGES
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CONVENTIONAL MIXED DIGESTOR
• Long start-up requirement
• Frequently Industrial wastewaters have insufficient alkalinity
• Unacceptable effluent quality of direct discharge
• Insufficient methane generation from dilute wastewaters to
Gas
raise reactor to required temperature
• Sulphide generation
• No nitrification
Northumbrian Water Bran
Sands Digester
Effluent
Influent
• Low kinetic rates at low temperatures
SRT = HRT~ 15d
CMST Reactor
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UASB
ANAEROBIC FILTER
Gas
Laboratory scale UASB treating starch
wastewater
OLR = 6kgCOD/m3.d
Vu = 17m/d
Effluent
35oC
Packing
Packing:
Random or Blocks
UASB cross section
Influent
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MEMBRANE ANAEROBIC REACTOR
SYSTEM (MARS)
REACTOR TECHNOLOGIES
• Completely Mixed (CMSTR)
• Anaerobic Contact
• Upflow Packed Bed
Anaerobic Reactor
• Downflow Packed Bed
UF/MF Unit
Effluent
• Sequencing Batch Reactor (SBR)
• Fluidised Bed (or Expanded Bed)
• Upflow Anaerobic Sludge Blanket(UASB) [or Hybrid
UASB]
Influent
• Baffled Reactor
• Two-Stage Reactors
Recycle
Wastage
• Membrane Reactors (MARS)
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SUMMARY
• Engineering and conditions dictate the structure and
abundance of microorganisms are found within the reactor,
and thus effect performance.
• Biological system always limited by the rate of metabolism and
growth of the microorganisms.
• With good engineering and good operation excellent biogas
production rates can be achieved.
• Continued research into the understanding and operation of
digesters is important to further advance Anaerobic digestion
in the UK.
QUESTIONS
[email protected]
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MOLECULAR TOOLS
• FISH: Fluorescence In situ Hybridisation
Use of fluorescence probes specific to target area that
fluorese under specific wavelengths. Good for identification
and morphology of species.
• qPCR: quantitative polymerase chain reaction to identify
numbers of genes/ml converted to cells per ml.
• Quantify mRNA to understand activity –what bacteria are
doing what
• COMMUNITY PROFILING – DGGE, SEQUENCING
• MICROSCOPY
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