Environmental context. Global sewage sludge (biosolids) production is increasing as a result of rapidly growing human population and ensuing industrial activities. Disposal of this waste is becoming a serious environmental issue because the high levels of heavy metals in biosolids can upset soil microbial activity and nutrient balance when the waste is added to forest or agricultural lands. In the present study, a biosolid matrix was spiked with copper and zinc as a model for an environmental scenario. The findings of the present study are applicable to environmental regulations that seek to protect agriculture land, human and animal health, and soil and drinking water quality, in scenarios where biosolids are applied to soil.

Abstract. Biosolids were amended separately with three levels of copper and zinc sulfate, and anaerobically incubated for 117 days. Copper in the unamended-biosolids solid phase was mainly found in the organic and residual fractions (85–95%). Copper addition decreased the percentage of Cu in these fractions and increased the percentage of Cu in the oxide and specifically adsorbed fractions. Zinc in the solid phase was mainly associated with the oxide (35–65%), specifically adsorbed (25–30%), and the exchangeable fractions (10–40%). Relatively, all Cu in the solution phase was complexed with organic matter; Zn was present mainly as Zn²⁺ and as an electrically neutral ion pair, ZnSO₄⁰. Metals were almost completely incorporated into the biosolids matrix by Day 55. Dehydrogenase activity was reduced by 50% (EC₅₀, effective concentration for 50% activity reduction) at the total solution-phase Cu and Zn concentrations of 0.1 and 20 mg L^–1, respectively, and solid-phase exchangeable Cu and Zn concentrations of 410 and 670 mg kg^–1, respectively.