PLANNING BIOSOLIDS LAND APPLICATION RATES FOR AGRICULTURAL SYSTEMS

Federal regulations restrict land application of all but ''exceptional quality'' biosolids (sewage sludges) to ''agronomic rates.'' When mun icipal biosolids are applied to farmland over time, soil organic nitro gen accumulates, reducing the design application rates in succeeding y ears. This paper describes a simple linear algorithm useful for design ing agronomic rates under such non-steady-state conditions. Designs ar e primarily determined by biosolids nitrogen content, crop nitrogen de mand, and site denitrification rates. Percolating ground water can be incorporated, but the advantages of doing so are generally modest. Thi s paper also describes how the economic sustainability of a site can b e predicted. Examples are described for a Wellsboro, Pennsylvania, cor nfield and for dryland wheat in Hemet, California. Over a 20-year plan ning period, cumulative applications in Wellsboro ranged from 120 to 1 62 Mg/ha dry weight, depending on the design approach. Systems are eco nomically sustainable for from 13 to 16 years. Hemet values ranged fro m 43 to 96 Mg/ha dry weight.