Department of Agricultural Microbiology, University of Life Sciences in Poznań,
Szydłowska 50, 65-656 Poznań, Poland
Abstract: Microbiological characteristics of sewage sludge from a mechanical-and-biological sewage treatment
plant composted in controlled conditions with straw and sawdust are presented. Prepared composts were
placed in four bioreactors with airflow of 4 l air·min-1. In bioreactor K1, K2 and K3 the composted mass consisted
of 65% sewage sludge (K1–sewage sludge 1, K2 – sludge 2, K3 – sludge 3) + 30% sawdust + 5% straw;
while in bioreactor K4 the proportion was: 45% sludge 2 + 50% sawdust + 5% straw. Compost samples were
taken from all chambers at the same time, depending on actual temperature.
Microbiological analyses consisted in the determination (by plate method) on selective media of the
numbers of mesophilic and thermophilic bacteria, fungi and pathogenic bacteria Salmonella sp. Clostridium
perfringens and Enterobacteriaceae. Furthermore, in the experiment, the activity levels of dehydrogenase
were determined using 1% triphenyltetrazole chloride as substratum.
Studies have indicated that the composting process caused a decrease of the number of fungi and pathogenic
bacteria from Enterobacteriaceae family and Clostridium perfringens in all composted matters, as well
as an increase in the number of thermophilic bacteria. Changes in the number of mesophilic bacteria depended
on the compost type. In composts K1 and K2, the composting process caused an increased proliferation of
cells, while in the composts K3 and K4 the number of mesophilic bacteria decreased. On the basis of the
obtained results, it was also found that in the majority of analysis terms, the lowest activity of dehydrogenases
occurred in compost K3, while their level of its activity, in the majority of the studied composts, correlated
most intensively with the number of thermophilic bacteria.