A Simulation Model for Sludge Decomposition in Soil

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Abstract
A computer simulation model which describes the rate of sludge decomposition in soil has been developed. The model, SLUDGE, uses first‐order kinetics. The rate constant was modified as the type of C species undergoing decomposition, temperature, and water content varied. The model computes decomposition on a daily basis during the rapid decomposition phase and on a monthly basis during the slow decomposition phase. Tests of the model showed that the model adequately describes sludge decomposition in soil. Model output for municipal sludge showed that half the sludge was decomposed over a 32‐ to 57‐month period for the soil temperature and moisture regimes considered. The time (month) of sludge addition had a large effect on the decomposition of the rapidly decomposable sludge fraction, but only a small effect on the half‐life of sludge. When yearly average soil temperature and water content data replaced monthly data in the model, predictions were changed in some cases suggesting use of yearly data could lead to erroneous results when computing sludge turnover. When “optimum” soil temperature and water content were used in the model, the half‐life of sludge was reduced by 37 to 66%; thus, laboratory studies conducted under “optimum” conditions would probably underestimate half‐lives under field conditions. In a modification of the model where annual June additions of sludge were made, the ratio of accumulated sludge to an annual addition was from 4.3 to 7.9 after 50 years of sludge addition. These ratios were slightly larger than those calculated using equations developed to compute such ratios for a relatively constant soil environment.