Enhanced anaerobic digestion using a sequencing batch reactor

The aim has been to improve the performance and acceptability of anaerobic digestion (AD) by increasing the extent of the biodegradation of organic solids. This should then give a greater biogas yield and a reduced solids residue per unit of feed. Higher gas production means a greater contribution to renewable energy targets and reduced solids output is important because disposal could become more of a problem in future, particularly with domestic waste and sewage sludge. The literature review discussed a number of methods that have been developed under the name of enhanced digestion to attempt to achieve increased solids breakdown. These include feed pre-treatments and temperature staging and acid/methane phased digestion. Another method is to retain solids within the digester and this project investigated the anaerobic sequencing batch reactor (ASBR) as a means of achieving this. Previous work using sewage sludge in ASBRs had reported an increase in gas production and flotation of thickened solids instead of settling, which improved performance. An objective of this project was to replicate this behaviour, but it could not; settlement not flotation took place and the settlement performance quickly deteriorated. This poor settlement led to low supernatant quality, with high levels of suspended solids, and solids losses and decreased mean solids retention time. However at all stages the biogas output of the ASBR was similar to the traditional control CSTR, suggesting that the ASBR should be capable of better performance if the settlement problem could be overcome. The ASBR was also found to give better pathogen destruction and also a lower capillary suction time (implying better dewaterability), than the CSTR. [Continues.]