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 municipal biosolids are applied to farmland over time, soil organic nitrogen accumulates, reducing the design application rates in succeeding years. This paper describes a simple linear algorithm useful for designing agronomic rates under such non-steady-state conditions. Designs are primarily determined by biosolids nitrogen content, crop nitrogen demand, and site denitrification rates. Percolating ground water can be incorporated, but the advantages of doing so are generally modest. This paper also describes how the economic sustainability of a site can be predicted. Examples are described for a Wellsboro, Pennsylvania, cornfield and for dryland wheat in Hemet, California. Over a 20-year planning period, cumulative applications in Wellsboro ranged from 120 to 162 Mg ha dry weight, depending on the design approach. Systems are economically sustainable for from 13 to 16 years. Hemet values ranged from 43 to 96 Mg ha dry weight.