Digester gas, biogas

Methane is produced as smallest organic molecule during anaerobic decomposition processes in digesters, e.g. on waste water treatment plants (WWTPs). It can be taken from the digester as gaseous mixture with other components, also be produced in the digester. This mixture is called digester gas or sometimes when generated on WWTP sewage gas. Similar anaerobic processes will also produce similar gas mixtures. According to their origin these mixtures are called biogas (from biogas plant), landfill gas (from landfill sites) or mine gas (from coal mines). The composition of these mixtures may vary in a wide field according to the respective sources.

Also digester gas from WWTPs can vary in composition according to the raw waste water quality which is discharged to the WWTP. Mainly industrial discharges may have a strong and characteristic influence. Organic wastes can also be given directly to the anaerobic process in the digester (cofermentation). Obviously, this may influence the composition of digester gas directly. For example, waste from slaughter house like blood will boost the gas production, but it also carries a lot of nitrogen. This will be converted to ammonia or ammonium and will be contained in the sewage sludge and therefore in the process water after sludge dewatering.

Digester gas gives an important contribution to the energy household of the WWTP. Due to the high content of methane (shares of 50 to 70 % are the most published values), it has a high calorific value and can be used for combined heat and power stations (CHP unit) to produce electrical and thermal energy.
According to the composition of the raw waste water specific impurities can be present in the digester gas. Some of them have to be removed by special gas cleaning technologies. Otherwise proteins will produce sulfur compounds (-> sulfur dioxide) or silicon oil containing hair shampoos and conditioners lead to siloxanes (silicon dioxide -> damages CHP motors).

Uncontrolled leakage of digester gas is strictly to avoid. On the one hand this would be a loss of energy. But otherwise as climate killer the main component methane is 23 times more noxious than carbon dioxide, which is the product of methane combustion.

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