Soil degradation of wastewater sludges containing chemical precipitants

Four sludges, three of which contained Ca(OH)₂, FeCl₃ or Al₂(SO₄)₃ for phosphorus removal, were compared as to degradation in soil. All sludges degraded rapidly but rates <25 mg/g approached CO₂ evolution of untreated soil in 15 days while higher rates required about 50 days. Each of the sludges contained lime which, at each addition, increased the soil pH. The Ca-sludge produced the greatest pH increase and evolved larger amounts of CO₂ than the Fe- and Al-sludges. The Ca-sludge evolved 52%, the untreated Al-sludge 20% and the Fe-sludge 5% of the sludge C. Calcium carbonate added to the soil increased the degradation of the Al-sludge and no significant differences amongst sludges were noted in the percentage of sludge C evolved. The degradation of these sludges was more related to soil pH than to chemical treatment of the influent for phosphorus removal.