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Optimization of Subsurface Injection of Municipal Sewage Sludge¹

ABSTRACT

A dynamic programming model was used to determine the optimal yearly application rates for on-land disposal of municipal sewage sludge. The model incorporated economic information and a soil nitrogen balance to find application rates which resulted in minimal cost while maintaining an acceptable level of environmental quality. Based on input conditions for Boulder, Colorado, the model suggests an average optimal annual application rate of 10.5 metric tons/ha at a net cost of $32.33/metric ton.

A sensitivity analysis was performed in which certain input parameters, including nitrogen content of sludge, mineralization rates, allowable nitrate leaching losses, maximum addition of sludge organic nitrogen, land costs, and cash price of crops were identified as most significant.

Key Words: sludge management • dynamic programming • nitrogen balance

¹ This work was supported in part by Natl. Sci. Found. (NSF) Proj. no. GI-42279. Any opinions, findings, conclusions, and recommendations expressed in this paper are those of the authors and do not necessarily reflect the views of NSF.

² Graduate Research Assistant and Associate Professor, respectively, Agric. and Chem. Eng. Dep., Colorado state Univ., Fort Collins, CO 80523.

Received for publication November 7, 1978.