Supervision of the sanitary quality of composting in the Nordic countries

Reduction rates of fecal coliforms, E. coli, Enterococcus, Enterobacteriaceae and Salmonella spp. were measured during composting at 16 Nordic full-scale facilities. The concentrations of the micro-organisms were measured from each of three stages during the composting process: a) the mixed raw material; b) the sanitised compost; c) the finished compost. The facilities treated source-separated biodegradable household waste, municipal sewage sludge, yard waste and/or horse manure. It was very difficult to reduce the concentration of bacteria during the sanitisation phase when household waste or sewage sludge was composted in windrows due to regrowth after turning. In contrast, efficient elimination of bacteria was found at windrow facilities composting yard waste only, i.e. nutrient poor raw material supported only weak regrowth of bacteria. It was therefore concluded that an efficient stabilisation phase is very important to secure an end product with low concentration of fecal bacteria when composting of household waste or sewage sludge using turned windrows. At ½ of the evaluated in-vessel facilities, the concentration of bacteria was reduced very efficiently during the sanitisation phase which indicated a homogenous treatment of the material and heat penetration of the entire mass. At 1/3 of the in-vessel facilities, the reduction of bacteria was small or insignificant as a consequence of general low temperatures (<55?C) during the sanitisation phase or the temperature development in the composting material was heterogeneous. On the basis of the presented results and data available from the literature as well as the legislation practice used in Europe and USA, recommendations for supervision of the sanitary quality of composting using turned windrows and in-vessel technology is outlined. The recommendations are given for the supervision of the sanitary phase securing reduction of pathogens present in the raw material to an acceptable/insignificant level, as well as the stabilisation phase securing biological stability of the end-product thereby minimising the potential for regrowth of pathogenic bacteria and vector attraction.