Chemical attributes of sewage sludges: Relationships to sources and treatments, and implications for sludge usage in agriculture

Abstract Sao Paulo is the most populous and industrialized state of Brazil, which strongly affect amounts and composition of the generated sewage sludges. Their use on agriculture sounds the most feasible disposal strategy since most Brazilian soils have low fertility, but the levels of hazardous substances must be low. This research examined the impacts of sewage source [domestic or mixed (domestic + industrial)] and sewage processing (liming and redox conditions) on the chemical attributes of sludges from 19 wastewater treatment plants of Sao Paulo; as well as their suitability for agricultural use. Sewage source and liming were the main factors affecting sludges’ chemical attributes, while redox conditions played a secondary role. Aerobically treated domestic sewage (C1) generated sludges most suitable for agriculture. Heavy liming in the processing of sewage of mixed origin (C2) produced sludges of much higher pH and Ca, with higher levels of certain hazardous elements (Fe, Zn, Mn, and Cr), but lower levels of OC, N-Kj, and K. Sludges from mixed sewers, not limed (C3) tended to have higher levels of S and hazardous elements (Ba, Mo, Cd, Pb, Ni, Al, and Cu). The inadequately regulated discharge of industrial wastewater into the municipal sewage network accounted for higher levels of hazardous elements in the sludges, but all of them are still suitable for agricultural use since only four sludges showed levels of either Zn (3) or Ni (1) that exceeded the thresholds permitted for direct application to agricultural fields, but both are nutrients to plants and Brazilian soils are often depleted on them. As raw materials for the manufacture of organic fertilizers, the levels of Cd, Cr, or Ni were unacceptable in about half (9) of the samples. It should be revised since hazardous element contents are diluted in the manufacture of the “new compound” and their application rates should be much lower than when sludge is directly applied. In conclusion, “low quality” sludges, such as C2 and mainly C3, have to be either composted or biodigested into “high quality” stabilized sludges; or used in the manufacture of organomineral fertilizers; or treated to remove pathogens and hazardous substances to be amended to agricultural soils, thus avoiding their non-sustainable disposal in landfills.