Heavy metal behavior and phytotoxicity attenuate during sewage sludge composting from multiple sources in Morocco

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Sewage sludge (SS) production is increasing worldwide, inducing a growing need for safe and sustainable management strategies that would enable its agricultural reuse. Composting represents sustainable biotechnology that stabilizes SS, reduces its environmental risks, and enhances its suitability for agricultural application as an organic amendment and nutrient source. This study evaluated the evolution of physicochemical parameters in composts produced from SS collected at three Moroccan wastewater treatment plants (WWTPs) under similar composting conditions. Six windrow mixtures were monitored over 150 days using a multicriteria assessment combining physicochemical analyses, heavy metal (HM) speciation, Mobility Factor (MF), Risk Assessment Code (RAC), and phytotoxicity bioassays. All mixtures exhibited typical composting process evolution, including a rapid thermophilic phase, substantial organic matter (OM) degradation, and decreases in C/N ratios and electrical conductivity (EC). Total concentrations of HMs showed minimal variation during composting; but Community Bureau of Reference (BCR) speciation analysis revealed consistent change from exchangeable and reducible forms toward oxidizable and residual fractions, indicating reduced metal mobility. This stabilization was further confirmed by decreases in MF and RAC, demonstrating lower HM mobility and environmental risk at compost maturity. Phytotoxicity assays using Lepidium sativum and Brassica rapa showed strong initial inhibition (Germination index (GI) < 20%), followed by marked improvements after 45–75 days. At maturity (T150), all mixtures achieved GI > 80%, with several treatments demonstrating stimulatory effects. Significant differences among WWTPs were observed, particularly the elevated chromium concentrations measured in composts derived from Marrakech sludge, highlighting the influence of sludge origin on final compost quality. Overall, the results demonstrate that SS can be transformed into a stable, low-toxicity organic amendment when appropriately composted. These findings support the implementation of source-specific compost quality assessment as a prerequisite for the safe and sustainable agricultural reuse of sludge-based composts.

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