**4. Conclusion**

The spreading of sludge from wastewater 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 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 like mitochondria and chloroplasts which can be altered. Repeated applications of sludge at the same site tend to increase the accumulation of heavy metals in the soil which can cause toxicities to soil microorganisms, animals and humans, via the food chain. However, it is important to note that these harmful effects mainly concerned industrial sludge, but the use of this sludge is strictly prohibited. In addition, the high doses used in our field experiments are clearly higher than those authorized in agricultural practice. Finally, the risk assessment by calculating both the level of exposure for the consumer and the number of years for a soil to be saturated shows that the use of urban sludge is safe, particularly in the short and medium term. Nevertheless, the quality of the sludge to be spread must be constantly checked. Other metallic trace elements such as mercury, boron brought in by the sludge must be taken into account.
