Determination Of Biosolids Phosphorus Solubility and Its Relationship To Wastewater Treatment.

Biosolids are the largely organic, nutrient-rich, solid or semisolid residuals of wastewater treatment. Sewage sludge becomes biosolids when treated to meet federal and state standards for beneficial reuse via land application as a fertilizer and soil amendment. In North Carolina (NC) and elsewhere, land application is currently governed by biosolids plant-available N and the agronomic N needs of the crop on the receiving soil. Because of the differences between biosolids and crop N:P, N-based biosolids applications typically surpass crop P needs, create excess soil P, and increase surface water pollution potential. USDA Conservation Practice Standards mandate P budgets and in many cases, P-loss risk assessments. Many states have P-based guidelines, and the NC Department of Environment and Natural Resources is considering guidelines for some nutrient-impaired watersheds.  In NC, the P Loss Assessment Tool (PLAT) will likely be used to estimate biosolids P-loss risk. To use PLAT with biosolids, however, the soluble P fraction (soluble P/total P) must be quantified. Forms and quantities of P in biosolids depend on wastewater treatment plant (WWTP) influent and treatment processes. NC WWTPs were surveyed to characterize treatment and biosolids production processes. These can vary significantly, greatly influencing water-soluble and total P. We sampled biosolids from representative WWTPs and analyzed for water-extractable and total P so that PLAT can be used to estimate P-loss risk. We are determining relationships between the P fractions and WWTP processes to ascertain whether the former can be assigned based on the latter. Land application of biosolids based on both P and N will likely decrease application rates, reduce the number of eligible receiving fields, and shorten the time soils can receive biosolids, thus making land application more costly. Hence, widespread adoption of a P-based approach may foster development of alternative beneficial uses of biosolids other than land application.