Pre and post digestion biogas hazards: PAPER & PRESENTATION
Tom Goodwin and Joe Mann, Marches Biogas, UK
Steve Bungay, Mott MacDonald, UK
(free)
Anaerobic digestion (AD) is widely used as a method for producing renewable energy via the biological
stabilisation of sewage sludge. Under anaerobic conditions, organic matter in the sewage sludge is broken
down and converted to biogas, which is flammable, comprising of mainly methane (CH4) and carbon dioxide
(CO2).
Anaerobic Digesters are designed to provide the optimal environmental conditions (inoculum, temperature,
residence time) to intensify the breakdown of the organic matter and production of biogas. However,
putrefaction will proceed whenever organic matter is subject to anaerobic conditions. Therefore, although
AD reactors are designed to intensify the process, these processes will occur outside of the AD reactors,
albeit at a slower rate, both prior to and following anaerobic digestion, either due to indigenous
microorganisms’ pre-digestion or residual methanogenic activity post digestion. Due to its flammability, the
production of biogas and CH4 outside of AD reactors has significant safety implications and must be
carefully managed.
This paper investigates the production of CH4 from sewage sludge prior to, during, and following AD using
a combination of field measurements, mathematical modelling, and laboratory testing. In addition to the
modelling, the investigation included measurement and testing at various stages from fifteen AD plants,
including, pre-digestion dewatered sludge, pre-digestion sludge silos, liquid digested sludge, biogas
condensate, post-digestion dewatered sludge, post-digestion dewatering liquors, and post-digestion sludge
silos.
