Phosphorus Concentration and Availability In Septage Samples In Wisconsin.

Due to more stringent phosphorus regulations passed in Wisconsin in 2010, septage haulers are concerned about the impact these regulations will have on their ability to land apply septage and the possibility of increased cost if septage is taken to a wastewater treatment plant.  The purpose of this study was to a) examine the current total and water extractable P concentrations in Wisconsin septage samples, b) investigate the effect of septage application on phosphorus levels in soil, and c) determine if lime treatment of septage impacts phosphorus availability.  Raw septage samples were analyzed for total P and water extractable P, and an incubation was performed on a subset of five septage samples and an inorganic P fertilizer.  Septage samples were added both as untreated and as pH stabilized to the soil at one hydraulic loading rate (~39,000 gal/acre).  Total P, Bray P1, water extractable P, and pH were measured before and after the incubation.  No significant difference in phosphorus concentration of raw septage was observed between categories (p=0.1), and P concentration varied widely within each septage categories (overall ranges = 2-35 ppm and 8-116 ppm for WEP and TP respectively). The lime treatment did not affect soil P level, but did increase the soil pH level by an average of 0.24 pH units.  Septage application to soil did not significantly increase soil P for any septage category (p=0.05). The results of this study indicate that P concentrations in the septage we analyzed are relatively low and did not significantly increase soil test P concentrations with a single application at this hydraulic loading rate. However, any P applied to the soil will accumulate over time if not removed by a crop. These results should be considered preliminary as this study only represents a small fraction of septage in Wisconsin.