abstract
In this study the influence of the petroleum products: diesel fuel and spent engine oil on the sewage
sludge digestion process and biogas production efficiency was investigated. Microbiological, chemical
and enzymatic analyses were applied in the survey. It was revealed that the influence of the petroleum
derivatives on the effectiveness ofthe methane fermentation of sewage sludge depends on the type ofthe
petroleum product. Diesel fuel did not limit the biogas production and the methane concentration in the
biogas, while spent engine oil significantly reduced the process efficacy. The changes in physical–chemical
parameters, excluding COD, did not reflect the effect of the tested substances. The negative influence of
petroleum products on individual bacterial groups was observed after 7 days of the process, while after
14 days probably some adaptive mechanisms appeared. The dehydrogenase activity assessment was the
most relevant parameter to evaluate the effect of petroleum products contamination. Diesel fuel was
probably used as a source of carbon and energy in the process, while the toxic influence was observed in
case of spent engine oil.