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Anaerobic treatment

Series of biological processes in which microorganisms break down organic molecules in absence of oxygen, resulting in the production of a mixture of gases, named biogas, mainly composed of methane and carbon dioxide.

The descriptions of these livestock manure processing technologies were based on 'Flotats, Xavier, Henning Lyngsø Foged, August Bonmati Blasi, Jordi Palatsi, Albert Magri and Karl Martin Schelde. 2011. Manure processing technologies. Technical Report No. II concerning “Manure Processing Activities in Europe” to the European Commission, Directorate-General Environment. 184 pp."

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Short description

Biological anaerobic decomposition of organic matter for biogas production, at a temperature range around 35°C (mesophile) or 55°C (termophile).

Best Available Technique: Not indicated
Objective

The main objective of anaerobic digestion of liquid livestock manure is to produce renewable energy (bio-methane) via biological degradation of organic matter. Other important effects include the reduction of emissions of ammonia after digestate spreading, methane and nitrous oxide, reduced odour and nuisances, increased bio-availability of nitrogen, and sanitation.

Mesophilic plants operate at temperature levels of approximately 37°C, with up to 2°C variation. Most farm scale plants and many regional plants are mesophilic.

Thermophilic plants operate at temperature levels of approximately 52°C, but with accepted temperature variation of only ½°C. The advantages of thermophilic plants are a higher contribution to hygienization and to lower viscosity during the process, facilitating mixing.

Level of complexity

Usual scale

Innovation stage

General diagram

Applied to







Typical technology combinations Anerobic digestion + separation
Pictures

Morsø Bioenergy is an industrial scale biogas plant, which is based on digestion of separation solids that is produced from 375,000 ton slurry annually with a mobile separator on the livestock farms, thus saving 95% of the transport costs by only transporting the separation solids to the plant and not the liquids. The plant likewise separate the digestate and market the separation solids to garden owners and alike, thus not returning it to the livestock farms.

Theroetical fundamentals and process description

Anaerobic digestion is a biological decomposition process following several steps (disintegration, hydrolysis, acidogenesis, acetogenesis, methanogenesis) and with a final conversion of organic matter to biogas, which typically has a methane content of 60-65%.

Usual digesters (reactors where the process is controlled) operates with a maximal dry matter content of 12.5% , and at constant temperature of 30-45°C (mesophile) or 55°C (termophile).

The hydraulic retention time is normally from 15-40 days, and the process happens in one or two stages/reactors, where the first is intended to maximize hydrolysis process and the second the methanogenesis process, giving a slightly higher biogas production in the second.

Propellers are normally installed in the digestion tanks to ensure the digestate remains homogenous and gives a maximal release of biogas.

The biogas production depend much of the type of biomass.

Typically 15% of the energy production from a biogas plant is used to heat up the digester. About 3-4% of the energy is used as electricity consumption for pumping, mixing, transport and other. The remaining energy production can be used for farm purposes or sold.

The regional plants also serve as centres for re-distribution of manure in the region. Both at centralized scale or on-farm scale, often a co-substrate is required to increase biogas production, being easier to manage at large scale.

Manys example plants can be found in Germany, Denmark or Sweden. Anaerobic digestion does not change the overall N/P ratio, and it has only effect on the N availability.

Environmental effects

Effects on air (emissions):

The biogas process contributes positively to the reduction of greenhouse gas emissions in two ways: decreasing methane natural emissions to the atmosphere and decreasing fossil fuels consumption if this is substituted by biogas. Calculations show that the CO2 neutral energy produced by the biogas process contributes with 2 kg CO2-eqv per m3 biogas, if it replaces fossil fuels. Furthermore model calculations show a reduction of naturally developed greenhouse gases (methane and nitrous oxide) of approx. 1.2 kg CO2-eqv per m3 biogas. So, all in all a potential of 3.2 kg CO2-eqv reduction in greenhouse gases/ m3 biogas. A number of odour compounds in the slurry are broken down in the biogas process, but others are formed in their place. The number of odour units (OU) is therefore often just as high above digested slurry as it is above untreated slurry. There is, nevertheless, a marked difference when the slurry is applied. The odour is not as strong and pungent from digested slurry as from raw slurry, and it also disappears faster from a fertilised field, partly because the digested slurry percolates faster into the soil due to its lower DM content, lower particles size and viscosity.

Effects on water/soil (and management):

The digestate is more homogenous, e.g. less lumpy, nutrients more evenly spread out, making the digestate easier to seep evenly into the crop root area, enabling better nutrient uptake from field crop Anaerobic digestion does not change the overall N/P ratio, and it has only effect on the N availability. Field trials performed by Danish Agricultural Advisory Service have proven 17-30% higher field effect (bio-availability) of nitrogen in digested slurry, compared to non-digested slurry; the increase of the field effect is higher for cattle slurry than for pig slurry.

Other effects:

Pathogen reduction and higienization (higher in thermophilic range).

Biosecurity aspects

The sanitating effect of anaerobic digestion is described in "State of Biogas Production in Eurpean Agriculture" (Birkmose et al., 2007) - look at page 27-29. Mainly due to the heating process in the biogas plant a large number of bacteria, virus and parasites are destroyed, but not eradicated.

In line with this, Bagge (2011) clarifies that "To avoid spreading of diseases via biogas plants when digested residues are spread on arable land, a pasteurization stage at 70°C for 60 min before anaerobic digestion gives adequate reduction of most non spore-forming bacteria, such as Salmonella spp., E. coli O157. Some spore-forming bacteria appeared to pass through the biogas process unaffected and thus digested residues should be spread with consideration of the risk of spreading diseases."

Technical indicators

Conversion efficiency:


Calculations for Danish biogas plants shows that these plants in average produces 22 m3 biogas per tonnes of slurry (containing in average 6 % DM)The anaerobic digestion process converts the main part of the organic bounded nitrogen into ammonium, and thereby the concentration of ammonium in digested slurry is increased up to 20 % compared to undigested slurry. The digestate is more homogenous, e.g. less lumpy, nutrients more evenly spread out, making the digestate easier to seep evenly into the crop root area, enabling better nutrient uptake from crops.

  • Net energy consumption - explanation:

    Biogas heat and power production:Power production*: 2.5 kWh per m3 biogasHeat production*: 2.0 kWh per m3 biogas *left after own use of heat and power in the process

  • Reagent 1 - explanation:

    Often no reagents are used for biogas production. Possible reagents/additives are described under process 4.4 (num. 24).

Observations
Economic indicators (Economic figures are rough indications, which cannot be used for individual project planning)
  • Investment cost:

    There are different ways to estimate the investment costs.

    Flotats and Sarquella (2008) propose to use the following equation for the estimation of investment costs for
    biogas plants, where the biogas is converted to electricity:

    Unitary investment [€ /kWh] = 16272*(Electrical power [kW])-0.2114

    The equation shows good correlation to investment prices, and the economy of scale. 

    Gregersen (2002) has made, on basis of regional plants in Denmark, working under co-digestion conditions, the
    following indications of investment sizes and operational costs:

    Foged (2010) proposed using the formula:

    Investment cost, € = 75,000 € + 50 €/ton annual capacity

    This equation also express an economy of scale, and is independent of the use of the biogas. The equation may be
    most applicable for plant sizes up to medium-size regional plants.

    Gregersen (2002) has made, on basis of regional plants in Denmark, working under co-digestion conditions, the following indications of investment sizes and operational costs:

  • Investment cost - basic price, €:

    750000
  • Investment cost - variable price, € per ton:

    50
  • Operational costs - explanation:

    The operational costs indicated here includes costs as internal electricity consumption, labour and insurance. In addition to this it would be expected that

    • maintenance costs of the plant is 2-2½% of the investment cost
    • costs for transport of livestock manure and other in and out of the biogas plant is as indicated under investment costs estimated by Gregersen (2002) to 7-4,7€/m3 
  • Operational costs - € per ton:

    2,1
  • Quantifiable income - text:

    Power production: 4 kWh per m3 of methane sold at app. 0,15 euro per kWh and heat production: 2 kWh of heat per m3 of Methane sold at app. 0,05 euro per kWh. 22 m3 of methane is produced per ton of slurry. Income from selling of power and heat app. 15 to 20 euro per ton of slurry treated. Improved value of treated slurry is not included.

  • Non economically quantifiable benefits:

    Reduction of odour and nuisances, especially during spreading of the digestate as fertiliser on the fields. Sanitation of the slurry.

Literature references
  • Bagge, Elisabeth. 2011. Hygiene Aspects of the Biogas Process. In: Innovative Agro-environmental Tecnologies for Sustainable Food Production in the Baltic Sea Region, 4/2011, p 1-3. http://agro-technology-atlas.eu/docs/bc_wp4_technologies_newsletter_4_august_2011.pdf
  • Birkmose, Torkild S., Henning Lyngsø Foged & Jørgen Hinge. 2007. State of biogas plants in European Agriculture. Prepared for EUROPEAN PARLIAMENT Directorate General Internal Policies of the Union Directorate B – Structural and Cohesion Policies. 70 pages. http://inbiom.net/download/viden_oevrige_emner/state_of_biogas_final_report.pdf 
  • Flotats and Sarquella (2008). Producción de biogás por codigestión anaerobia. Quadern pràctic núm. 1. ICAEN, Spain (www.icaen.net).
  • Foged, Henning Lyngsø (2010). Best Available Technologies for Manure Treatment – for Intensive Rearing of Pigs in Baltic Sea Region EU Member States. Published by Baltic Sea 2020, Stockholm. 102 pp
  • Jørgensen, Peter Jacob (2009): Biogas – green energy, PlanEnergi and Researcher for a Day – Faculty of Agricultural Sciences, Aarhus University, 2nd edition.
  • Hjorth-Gregersen, Kurt (2002): Status for økonomien i biogasfællesanlæg, abstract from report 136, Institute of Food and Ressource Economics, Faculty of Life Sciences, University of Copenhagen, DK
  • Rosager, Frank (2010). Etablering af biogasanlæg I udlandet. PowerPoint. Energinet.dk biogas seminar 8. september 2010.
Real scale installation references

Many plants treating manure across Europe. As example: Morsø Bioenergi, Næssundvej 234, 7970 Redsted Mors

Examples of suppliers