Predicting long-term decomposition of biosolids with a seven-day test

Decomposition of biosolids (municipal sewage sludge) in soil Has been characterized by long-term (>60 d) studies. This study was conducted to determine if short-term incubations could be used to estimate long-term decomposition behavior, Twenty-four biosolid/soil combinations were incubated at 25 degrees C for a period of about 60 d. Decomposition followed first order kinetics with two fractions, rapid and slow, based on decomposability, The percentage of biosolid C in the rapid fraction (%Rapid), and the rapid and slow fraction rate constants (k(r) and k(s), respectively) were determined using linear regression of log transformed data (sequential decomposition model) and nonlinear regression of the original data (simultaneous decomposition model). Both models adequately estimated the %Rapid, k(r), and k(s). The sequential model provided the best relationship between %Rapid and the percent decomposition at 7 d, %D-7, and the only relationship between k(r) and %Rapid. The slow fraction rate constants used in this study were compared to those estimated using data from other studies. The slow fraction rate constant used in prediction of long-term decomposition was from one of these other studies. Predictions of long-term decomposition agreed well with observed values (r(2) = 0.76). It is proposed that a single measurement, %D-7, could be used io assess long-term decomposition for biosolids similar to those reported here. The equation should not be applied to biosolids with small rapid fractions (<10%) such as those stored for extended periods or composted or to biosolids not originating from municipalities.