Decomposition of Anaerobically Digested Sewage Sludge as Affected by Soil Environmental Conditions

Abstract

Microbial oxidation of organic components in anaerobically digested sludge is an important aspect of the fate of sludges in soils. The objectives of this study were to determine the effects of soil characteristics, sludge management procedures, and environmental conditions on the decomposition rate of sewage sludge in soils. Synthetic sewage sludge was prepared by anaerobically digesting a mixture of ¹⁴C‐labelled Candida utilis cells and inorganic salts. The sludge prepared was similar in composition to anaerobically digested municipal sludges. After incorporation into soil, decomposition of sludge was rapid during the first 28 days, but the decomposition rate was low and relatively constant for the remainder of the incubation period. At the end of 336 days of incubation, 46% of synthetic sludge organic ¹⁴C was evolved as CO₂, suggesting that one fraction of anaerobically digested sludge was readily decomposable when added to an aerobic soil. However, the major portion of sludge organic matter was resistant to decomposition. Factors such as soil texture, pH, and moisture content had little effect on sludge decomposition rates. Decomposition was greatest in soil samples receiving surface‐applied sludge and in samples incubated at high temperature (30°C) as compared to samples having sludge incorporated or incubated at 21°C.