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  • Anaerobic digestion

    Andrea Lanzini

    Biofuels Summer School



  • • Anaerobic digestion

    • Biogas resource

    • Applications

    • Fuel clean-up

    • Case study: biogas CHP plant


  • Anaerobic digestion

  • Anaerobic digestion - Fundamentals

    In anaerobic processes, microorganisms (enzymes and bacteria), that work

    in absence of free oxygen or linked in the form of nitrates, sulphates, etc.,

    degrade via biological pathways the organic substances (e.g. biowaste).

    The organic compound is converted through subsequent oxidation and

    reduction reactions in its most oxidized state, CO2, and in most reduced one,

    CH4. This two gaseous compounds represents the most important metabolites

    and are the main constituents of biogas.

  • 1. HYDROLYSIS: in this phase, large organic polymers such as carbohydrates, fats and proteins are broken into smaller constituents, like simple sugars, aminoacids, fatty acids and water.

    3. ACETOGENESIS: the organic acids are formed. They are the raw material for the eventual methanogenesis. The bacteria responsible for the third phase, the acetogens, are highly sensitive to temperature fluctuations. The methanogenesis itself also slowly starts during this phase.

    2. ACIDOGENESIS: the further breakdown of remaining components. It is done by the acidogenic bacteria which convert the organic matter in short-chained fatty acids, alcohols, CO2, H2 and ammonia.

    4. METHANOGENESIS: methane (biogas) is formed from acetate (about 70%) and 30% from CO2 and H2 in this phase. Also CO2 is relased and, in small proportions, also water, H2S, and N2. The content of methane in biogas typically varies between 50 and 70% depending on the substrate charateristcs used.

    AD steps

  • From organic waste to biogas

    CH3COOH → CH4 + CO2

    Source: Girard M. et al., 2013, “Biodegradation in Animal Manure Management”, Research and

    Development Institute for the Agri-Environment (IRDA), Québec, Canada

  • Microorganism at work

  • Temperature ranges for AD

    Psychrophilic temperature in a range of 4-25°C (optimal values 15-20°C);

    Mesophilic temperature in a range of 10-40 °C (optimal value 35°C);

    Thermophilic temperature in a range of 45-70°C (optimal value 50°C).

  • Bacteria

    The temperature of the digester has to be maintained within a certain temperature range in order to prevent the bacteria from being killed.

    Thermophilic condition is the most critical (unstable) to maintain and leads to the higest yield (degradation of organic matter is maximized).

    In most cases, methane-forming bacteria control the process. Methane formers are

    very sensitive to environmental factors (high ammonia concentrations, low phosphorus

    concentrations, low pH, temperature, and the presence of toxic substances), and reproduce

    very slowly. Consequently, methane formers are difficult to grow and are easily

    inhibited. Therefore, process design and the operation of conventional anaerobic

    digestion are tailored to satisfy the needs of the methane-forming bacteria.

  • Factors influencing AD

    Total solids

    Carbon to nitrogen (C/N) ratio


    Retention time

  • The amount of solids (% w/w) of the organic waste feeding

    the digester is a relevant operating parameter.

    Total solids

    Wet digestion TS < 10% w/w Sludge from wastewater treatment plant

    Semi-wet digestion 10 < TS < 20% w/w Manure, codigestion plants (organic waste + sludge)

    Dry digestion TS > 20% w/w Organic fraction of municipal solid waste (OFMSW)

  • Mass balance of an anaerobic digestor

    t.s. 100 kg

    v.s. 75 kg

    Ash/FC 25 kg

    Biogas = 37.5 kg (non-degraded v.s. = 50% of total v.s.)

    Feed v.s./t.s.=75%


    t.s. = total solid v.s. =volatile solid

    Non-degraded v.s. = 37.5 kg

    Ash/FC = 25 kg

    Digester effluent (digestate) = 62.5 kg

    The efficiency of the process is 50% (in terms of organic matter degradation)

  • Final disposal of the digestate

    Courtesy of Marcopolo S.p.a.

  • Anaerobic digestion of sludge

    Biogas gasometer


    From prethickner

    To thickner

    Courtesy of Gruppo SMAT

  • Sludge heating and stirring

    Courtesy of Marcopolo S.p.a.

    Mechanical stirring is adopted in 90% of the digesters

  • Tank shape

  • Anaerobic digestion of animal waste

    a a

    a a

    Cow manure Cow manure (separated solid) Cow manure (solid)

    Chicken manure Chicken litter (solid) Swine manure

    Source: Special Issue of «L’Informatore Agrario» (3/2013)

  • Biogas and biomethane exploitation pathways

    Manure Sewage sludge

    Biowaste OFMSW

    Agricultural residues


    Organic feedstock

    Anaerobic digestion

    Engine / Fuel Cell


    Upgrade Injection in

    distribution grid

    Compression & storage

    Biogas Biomethane

    Transportation fuel

    Biogas clean-up from contaminants (e.g., H2S, siloxanes, halocarbons, etc.) is required.

    Sludge Electricity

  • GHG emission reduction cost

  • Biogas resource

    EU current production and potential

  • Anaerobic digestion

    Anaerobic digestion (AD) is a proved technology to reduce the putrescible

    matter of organic waste while turning part of it into useful energy (i.e.,


    The scale of AD plants ranges from the relatively small and local (farm –

    rural environment) to larger plants in municipalities (urban environment) or

    intensive animal farming settlements.

  • Ancient roots

    The collection of urban organic waste is a practice with ancient roots

  • Biogas production in EU

    Source: own elaboration of Eur’ObservER data

  • Biogas production in EU

    Source: own elaboration of Eurostat data

  • EU biogas production by source and country

    Source: 2016, Optimal use of biogas from waste streams. An assessment of the potential of biogas from

    digestion in the EU beyond 2020

  • World biogas production

    The total primary energy supply was 573 EJ globally in 2014

  • Applications


    Why is Anaerobic Digestion useful?

    (1) prevention

    (2) preparing for re-use

    (3) recycling

    (4) other recovery, e.g. energy recovery

    (5) disposal.

  • Is there a large biogas potential?

    The biogas potential is related to the following sources:

    Human beings (sewage sluge)

    Livestock (manure)

    Biowaste (organic fraction of municipal solid waste, OFMSW)

    Industries with organic effluents/waste

    Residual Municipal Solid Waste (MSW)

    Arable land (energy crops)

  • Biogas from manure: energy potential





    Total dry solid

    waste (kg


    Biogas production rate

    (m3 biogas/day/head)


    power rate



    Buffaloes 194 2.74 0.73 54 92

    Camels 27 4.11 1.37 244 58

    Cattle 1,468 3.15 0.84 158 2025

    Chickens 20,887 0.03 0.01 2 390

    Goats 976 0.27 0.09 19 160

    Horses 60 4.11 1.37 244 128

    Mules 10 4.11 1.37 244 22

    Pigs 977 0.68 0.35 75 646

    Sheep 1,163 0.41 0.14 26 265

    A farm of 1,000 pigs can produce up to 750 kW of biogas

  • Biomethane as transportation fuel

    Q: How far can you drive a car with the daily manure

    of 1 pig?

    A: About 2 km…


  • Issues/opportunities for biogas use

    Daily and seasonal

    fluctuation in biogas


    Contaminants (some biogases

    are heavily contaminated;

    e.g., landfill biogas)

    Thermal load of the digester (can reduce sthe net power output of

    the plant)

    Combined heat and power (CHP)

    electric production with internal thermal


    Fuel cell technology for modular and

    high-efficiency power



  • From waste treatment to resource recovery plants

    Primary + secondary sludge (C-H-N-O substrate)


    Biogas (CH4,CO2, N2 traces)



  • From waste treatment to resource recovery plants

    Primary + secondary sludge (C-H-N-O substrate)


    ANODE Biogas (CH4,CO2, N2 traces)

    Anode off-gas (H2O, CO2 ,H2, CO, N2 traces)

    P o

    w er

    H e




    Fresh air

    Vitiated air





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