Comparative Evaluation of the Potential Noxiousness in Domestic Sludge Used in Agriculture and in Commercial Fertilizers

The noxiousness of actual sludge collected in eight water treatment plants around the city of Toulouse, France, was evaluated using a biological test based on the growth rate of cultured human cells. Results were compared with those obtained from 18 fertilizers and culture supports that are commercially available in gardening shops. Surprisingly, it was found that sludge extracts, at low concentrations (below 5 g of dry material/liter), were improving the cell growth rate, which suggests the presence of useful oligoelements. At higher concentrations, a noxious effect, expressed as inhibition of cell growth, was observed. However, this negative effect was of the same order of magnitude as that obtained, under the same experimental conditions, with commercial garden fertilizers which are available and used without any restriction. It is concluded that discarding the sludge, after submission to the biological test, in controlled amount as an agricultural fertilizer should not be hazardous to the environment.