An Attempt at Co-digestion of Sewage Sludge and Soapstock - A By-product of Vegetable Oil Refining

Effectiveness of co-digestion of sewage sludge with soapstock, a by-product of vegetable oil refining, was investigated. Studies lasting 28 days were carried under static conditions in a sludge digestion system consisting of 12 tight, glass flasks of 500 cm(3) each, equipped with kits for biogas collection. 5%, 10%, 15%, 20% and 25% of soapstock was added to the sludge mixture. An increase in biogas production and in percentage share of methane content in the biogas was obtained. The biggest biogas production was recorded for samples with 5% and 10% soapstock content. The biogas production was increased by 51% and 50.5% in comparison to the control sample (no soapstock). In addition, the highest methane content in biogas (74%) was observed for the samples with 5% and 10% soapstock content. The soapstock content of above 10% in the mixture of sludge resulted in the inhibition of biogas production. A fair degree of co-digestion of the tested mixtures was received, namely 42.39% and 42.57%, for 5% and 10% soapstock samples respectively, although it was slightly lower than for the reference sample (47.69%). Dewatering properties of the sludge deteriorated significantly due to the co-digestion process. Capillary suction time (CST) for the reference sample before and after the co-digestion increased from 15 s to 37 s. For the remaining samples, CST increased as well with the soapstock content from 55 s to 435 s.