Survival of faecal indicators and enteroviruses in soil after land-spreading of municipal sewage sludge

One concern in regard to the spreading of sewage sludge is the potential for contamination of soil and water by pathogens present in sludge. We studied the survival of enteric micro-organisms in sewage sludge following direct land-spreading. The sludge produced by a wastewater treatment plant (capacity equivalent to 2000 inhabitants; sludge storage tank of 700 m3) was spread on a soil, at a rate of 80 m3/ha. The tested micro-organisms included three of specific sanitary interest: faecal indicators, spores of Clostridium perfringens and enteroviruses. The results highlighted three types of behaviour associated with these three groups of micro-organisms. The enteroviruses were not detected 2 weeks after spreading on the soil whereas the concentration of faecal indicators fell slowly with an observed decrease of between 1.2 and 1.8 logarithmic units over 2 months (but without the initial level of the soil being reached). Lastly, the concentrations of C. perfringens remained stable. The different survival times of the studied micro-organisms in soil, confirm the necessity to include several indicators (bacteria and viruses) to estimate reliably the sanitary risk related to sludge spreading.