Metal speciation in sludges: a tool to evaluate risks of land application and to track heavy metal contamination in sewage network.

The agricultural spreading of dehydrated sewage sludge from urban sewage treatment plants is economically profitable provided that the soil agronomic quality and the absence of contamination, in particular of heavy metals, are maintained. We evaluated the variability of sludge between five treatment plants in northern Algeria. We determined parameters that account for their agronomic quality and total content of Ag, Cd, Co, Cr, Cu, Ni, Pb, Ti and Zn. The speciation of metals, which determines their bioavailability, was characterized by sequential extraction into five fractions: easily exchangeable, acid-soluble, bound to carbonates and Fe-sulphides, bound to Fe-Mn oxides, bound to organic matter or sulphides, residual. All the sludges analysed showed satisfactory properties for plant growth. High total Ni contents for three of the sludges indicated that they were not landfillable under French or Chinese regulations. Ni, however, was contained in poorly bioavailable fractions and therefore presented a low risk to soils. In contrast, the total Cu was lower than the regulatory limit values, but mainly contained in very bioavailable fractions whose accumulation over time could reach toxic levels for plants over a period of 3 to 11 years depending on the sludges. These results showed that regulations are not adapted and must take into account the bioavailability with regard to the characteristics of the soils on which to spread. The speciation of metals in the sludge has also, on the one hand, made it possible to identify the zone of the sewerage network in which the sources of contamination must be sought and, on the other hand, has given indications on the possible nature of these sources.

Long-term, low technicality sewage sludge amendment and irrigation with treated wastewater under Mediterranean climate: impact on agronomical soil quality.

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.