Aerobic Co-Composting of Sewage Sludge with Organic Wastes: Nutrient Dynamics and Sanitization Limitations

Aerobic composting is a cost-effective option for managing sewage sludge and organic waste. The performance of aerobic composting of sludge mixed with locally available organic materials was evaluated under protected, non-temperature-controlled conditions, and potential management options for the resulting products were assessed. Three mixtures were evaluated in two separate runs: sludge and olive mill pomace (1S:1J), sludge with olive mill pomace, manure, and plant waste (3S:3J:2M:2W), and sludge with manure (1S:1M). Pile temperatures reached only 30–40 °C and did not attain the thermophilic range required for reliable sanitization. From the initial to final sampling, organic matter decreased by 5.2–29.8% in the first run and 25.0–42.9% in the second run. Nitrogen changed by +31.1% to +200.8% in the first run and −51.8% to +48.9% in the second run. Phosphorus changed by +80.2% to +217.5% in the first run and −33.0% to +81.9% in the second run. Selected trace elements also showed treatment-specific numerical changes, including a 131.1% increase in Fe and a 72.6% decrease in Zn in 1S:1M during the first run. All measured potentially toxic elements remained below the cited regulatory limits. Microbiological compliance varied by treatment and run, and one second-run mixture contained 9.4 × 103 MPN/g total fecal coliforms, above the reported Class 1 limit. These results show useful nutrient characteristics but do not demonstrate consistent sanitization or unrestricted agricultural suitability. Additional process control and microbiological verification are required before reuse.