Microbial monitoring of the influence of the stabilization degree of sludge when applied to soil

The aim of this study was to establish the degree of stabilization of sludge and its influence on the evolution of total aerobic micro-organisms and those of faecal origin when the sludge is applied to soil. Two different sludges were used, from a food industry (D) and from an urban treatment plant, (E). Both sludges were stabilized aerobically in laboratory conditions. The stabilization process was monitored by following the evolution of pathogenic micro-organisms and the content of total solids and volatile matter in suspension, while on the other hand the microbial evolution of the samples of soil plus sludge and control soil (S) were monitored over an incubation period of 80 days. After 20 or 40 days of incubation, in both sludge treatments the populations of pathogenic micro-organisms of faecal origin were below detectable limits, regardless of the degree of stabilization of the sludge. Soil without sludge treatment did not show pathogenic micro-organisms.