Digester Feedstock For Biogas Generation

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Practically any natural organic waste can be used to produce biogas. It has been found that pig and poultry manures produce more biogas per unit weight than for example cattle manure. However the carbon to total nitrogen ratio (C: N) is lower than in cattle manure and this increases the risk of digester failure. To overcome this risk, the introduction of agricultural residues (e.g., wheat, barley straw) which are usually readily available, should be considered as these have a high C:N ratio.

Bedding obtained from so called eco-shelters which contain a mixture of straw and manure is an excellent source of biomass for a digester as they provide a good C: N ratio. The fact that some composting of the eco-shelter bedding has begun will assist in the breakdown of the fibrous material.

Physical pre-treatment of solid feedstock by maceration reduces particle size to prevent physical obstruction of pipes and pumps by the fibres, and it also increases surface area available for microbial attack, and thus speeds up the digestion process. Research suggests that lignin cellulose and hemi-cellulose which are almost non-biodegradable in ordinary systems can be degraded to a significant degree after maceration. The introduction of bacterial agents (bioaugmentation) to the feedstock can lead to an increase in the output of methane as well as higher value bio-solids, and assist in the breakdown of the biomass resulting in an overall lower hydraulic retention time (HRT) in the digester. A beneficial “by product” of bioaugmentation is (supplier’s data) the fact that the production of hydrogen sulfide is virtually eliminated.

Manures usually have a total solid (TS) content of about 4 to 22%. In most cases, especially when slurries are not 'homogeneous', this concentration is too high for pumping and piping, and the TS is usually brought down to 4 - 8% by dilution with water, to make a pumpable slurry. Where a combination of liquid waste (slurry) say from the farrowing and gestation sheds is to be combined with “solid” eco-shelter waste a chopper pump/agitator is used to macerate the solids to reduce the particle size going into the digester as well as providing an homogeneous mixture.

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