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Agricultural spreading of dewatered sewage sludge from wastewater treatment plants is economically profitable but care must be taken to ensure that there is neither degradation of the agronomic quality of the soils nor contamination of them in the long term, particularly by accumulation of heavy metals. To evaluate the variability of the sludge in a given geographical area, we studied the sludge coming from five treatment plants in northern Algeria. We determined parameters that account for the agronomic quality of sludges and total content of Ag, Cd, Co, Cr, Cu, Ni, Pb, Ti and Zn. We evaluated the bioavailability of theses metals by determining their speciation by sequential extraction, each metal being distributed among five fractions: easily exchangeable, acido-soluble, bound to carbonates and partly to Fe-sulphides, bound to Fe- and Mn-oxides, bound to organic matter or sulphides, contained in poorly soluble minerals. We found that all the analysed sludges had satisfactory properties from an agronomic quality point of view. High total Ni content indicated that three sludges were not spreadable under French or Chinese regulations. Metal speciation, however, showed that Ni was contained in very poorly bioavailable fractions, and therefore presented a low risk in the soils concerned. In contrast, the total Cu was below the regulatory limit values, but contained in very bioavailable fractions with a risk of toxic effects by accumulation over less than 10 years. These results showed that regulations must take into account the bioavailability with regard to the characteristics of the soils on which sludge will be spread. Metal speciation in the sludge also made it possible to identify the zone of the sewerage network on which the sources of contamination must be sought and gave indications on the nature of these sources.

International audience; We evaluated the agronomic quality of loam to clay–loam soils from a Mediterranean area, which have been submitted for more than 10 years to irregular and poorly controlled treated wastewater (TWW) irrigation and sewage sludge (SS) amendment, both TWW and SS issuing from a domestic effluent treatment plant. A soil was submitted to SS amendment only, another to TWW irrigation only, another to both treatments, and another was not submitted to any of these treatments. We found that the agronomic quality of the treated soils has not decreased. In treated soils, it was observed a slight increase of pH, an increase of the cation exchange capacity, and no change of either salinity or electrical conductivity. Sodium adsorption ratio and exchangeable sodium percentage remained satisfactory, below 13 and 15, respectively, and no infiltration problems were observed, indicating that rainwater percolation was sufficient to leach the salt brought by SS or TWW. The soil organic matter (SOM) increased in all SS-amended and/or TWW-irrigated soils, the increase being higher in SS-amended soils. The SOM kept a satisfactory C/N ratio, lower than 15; the SUVA (Specific UV Absorbance) and E2/E3 index showed normal characteristics of the water-extracted organic matter (WEOC). Both SS amendment and TWW irrigation brought high amounts of nutrients in the considered soils and increased the nutrients disponibility, especially regarding K and P, as shown by the nutrient concentrations in the soil water extracts. The pH increase does not seem to be a problem for the bioavailability of trace elements, at least regarding Cu, Zn, Co, and Fe. Complementary studies should be undertaken for Ni. These observations highlight the potential agronomical benefits of application of wastewater and sewage sludge, even without great technicality, the better results being obtained using both SS amendment and TWW irrigation.