Abstract
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.
Highlights
Biosolids known as sewage sludge are the treated organic residuals generated from domestic wastewater treatment plants
Physical and chemical parameters included pH, Electrical Conductivity (EC), organic Matter Contents (OM), Total Phosphorus (TP) and metals contents. pH and EC were determined in the supernatant of dissolved sludge sample in reagent water ratio 1/10 (m/v) by the mean of a multi-parameters (HACH, HQ40d), OM was determined by weight ignition at 550°corresponding reference value (Cs) for 5h
The investigated dewatered sludge exhibit a slightly alkaline character and high electrical conductivity denoting high concentration of organic and inorganic ions such as Cl, Na+, K+, NH4+, NO3, SO42, HCO3- in the sludge; which does not favor plant growth despite it is rich in organic matter and phosphorous (Table 3)
Summary
Biosolids known as sewage sludge are the treated organic residuals generated from domestic wastewater treatment plants. With development and population growth, the production of waste and sewage sludge steadily increases from a year to year across countries in the world. In advanced countries like China, over 11.2 million tons of dry sludge is generated (Chu et al, 2009); while Approximately 6×106 metric tons is reported in the U.S each year, of which about 60% is applied to agricultural soil and land application (WEF and NACWA, 2013). It has been reported to be an excellent fertilizer and a potential source of organic substances and essential nutrimental elements such as nitrogen and phosphorus for plants. Sewage sludge can valuably replace synthetic N and P fertilizers (Kelter et al, 1997; Gao et al, 2008; Roca-Pérez et al, 2009).
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