Utilization of Sewage Sludge in Agricultural Soil as Fertilizer in the Republic of Benin (West Africa): What are the Risks of Heavy Metals Contamination and Spreading?

Biosolids are the treated organic residuals, also known as sewage sludge, which are generated from domestic wastewater treatment plants. It is an organic matter and nutrients [phosphorous (P) and nitrogen (N)]-rich material. Due to these properties, it is has been reported to be a suitable fertilizer or soil amendment which can valuably replace synthesized NPK. Unfortunately, this matter is heavily loaded with inorganic pollutants, namely heavy metals which limit its land application. Here, we evaluated the quality of treated sewage sludge from Wastewater Treatment Plants (WWTP) in the republic of Benin; and assessed the eco-toxicological risks of heavy metals contamination and spreading related to the use of sludge as fertilizer in agriculture. Results revealed a slightly alkaline (pH ≥6.5) and high electrical conductivity (EC >4 mS cm^-1) of the sludge. Heavy metals including Cd, Co, Cu, Zn, Ni, Cr, Pb, Fe and Mn contents were at high level: 27.1, 18.6, 777.1, 1271.7, 165.5, 297.0, 131.2, 3184.0 and 394.2 mg kg^-1 DM, respectively. Further fractionation of metals showed high mobility of metals classified as follow: Cu > Cd > Pb > Cr > Ni > Fe > Mn > Co > Zn. As a consequence, although the sludge is rich in organic matter and phosphorous, its high value of EC and heavy metals contents, with high mobility do not favor its used in agriculture soils. In addition, metals were enriched in the sludge; meaning that anthropogenic sources, human activities greatly contributed to the accumulation of metals in the sludge.