Decontamination of sewage sludge produced in animal rendering plants

Sewage sludge orginating from animal rendering plants was composted together with varying quantities of wood chips, farmyard manure and bark. Pouches filled with compost were soaked with suspension of 20 ml E. coli (11.5 x 10(9) cfu/ml) and 20 ml group D streptococci (7.5 x 10(9) cfu/ml) and placed in the middle of each clamp. The inactivation kinetics of the indicator organisms was determined over a period of 24 weeks. The most important factors in the reduction of the test organisms were temperature, pH and the composition of the compost. The fastest reduction of the test organisms (0.3 log/week) was observed in the clamp with sewage sludge and bark, in which maximum temperatures of >67 degrees C were recorded at the beginning of the composting process. After 13 weeks the concentration of D-streptococci in all three clamps were within the international standard values for sanitised compost.