Impact of the Spreading of Sludge from Wastewater Treatment Plants on the Transfer and Bio-Availability of Trace Metal Elements in the Soil-Plant System

The spreading of sludge from sewage treatment plants increased the production of durum wheat and rapeseed. Their richness in nitrogen, phosphorus, and potassium gives them a beneficial effect on crops. However, the application of the sludge can induce increases in the concentration of metals in plant tissues. This increase can generate disturbances at the level of the cell and organelles, such as mitochondria and chloroplasts, which can be altered. Repeated applications of the sludge on the same site tend to increase the accumulation of heavy metals in the soil, so that an cause toxicities for soil microorganisms, animals, and humans, via the food chain. However, it is important to specify that these nuisances mainly concerned industrial sludge, but the use of this sludge is strictly prohibited. In addition, the high doses used in our field experiments are significantly higher than those authorized in agricultural practice. Finally, the risk assessment by calculating both the level of consumer exposure and the number of years for soil saturation shows that the use of urban sludge is safe, especially in the short and medium-term. Nevertheless, the quality of the sludge to be spread must be constantly monitored