Decontaminating Biological Sludge for Agricultural Use

A biological oxidation process called bacterial leaching was used to remove heavy metals from anaerobically digested sewage sludge. The purpose was to decontaminate the sludge so that it could be utilized on agricultural land. The leaching process was found to be affected by pH, aeration and temperature. At α pH of 4, an aeration rate of 100 cm³ of air per litre of sludge per minute and a temperature of 25°C, the following metal removal efficiencies were observed: cadmium, 80 - 85% ; copper, 66 - 80% ; nickel, 70 - 78% and zinc, 84 - 90%. No significant removal was observed for lead. In addition to removing heavy metals, bacterial leaching preserves the soil conditioning and fertilizing properties of the sludge. According to Ontario guidelines, the sludge used in this study was initially not acceptable for use on agricultural lands. However, after bacterial leaching, all criteria for the application of sludge to agricultural lands were easily met.