We monitored the effect of the application of treated sludge on the behaviour of enterobacteriaceae (mainly faecal coliforms and especially Escherichia coli) in the soil, and studied their evolution over time after application. Three different sludges were used: two from a municipal sewage plant, one of them had been subjected to anaerobic digestion and heat drying, and the other to anaerobic digestion and mechanical dehydration, and one from a dairy waste treatment to aerobic digestion and gravity thickening. Two types of tests were carried out: type O, in the open air, with no possibility of controlling humidity or temperature; and type L, under laboratory conditions, with controlled temperature and humidity. Sludge tests were also run on unscreened soil previously treated with chemical fertilizer. After 80 days of experimentation the populations of faecal coliforms and E. coli had decreased considerably or were undetectable in assays carried out on the soil/sludge mixtures, under both open-air and laboratory conditions, but that, over the same period, in the mixtures containing chemical fertilizer (calcium ammonium nitrate) there had been a considerable increase in the micro-organism populations studied.
Copyright 2003 Elsevier Ltd.