Land application of biosolids under phosphorus-based nutrient management

Phosphorus-based nutrient management will inevitably be required for land application of municipal wastewater biosolids. Water extractable P (WEP) in livestock manures is an indicator of detrimental P loss from agricultural watersheds and initial efforts evaluated its use for biosolids. The WEP to total P percentage (PWEP) in biosolids was compared to manures and triple superphosphate fertilizer. The mean PWEP for conventionally treated and stabilized biosolids was significantly lower than for inorganic fertilizer and manure. Facilities using biological P removal had the highest biosolids PWEP, while heat-dried biosolids had the lowest PWEP. Rainfall simulation experiments found a close correlation between PWEP and runoff dissolved P (RDP) levels. The trend of RDP content followed the pattern: dairy manure >> biosolids without Al or Fe > biosolids with Al > composted biosolids with high Fe ≈ biosolids cake with high Fe. Runoff total P (RTP) levels were greatest for dairy manure, followed by the composted biosolids. Other biosolids treatments showed RTP levels comparable to bare soils. Nominal range sensitivity analysis (NRSA) of the Pennsylvania P-Index showed that manure source factors, namely, P ₂O₅ application rate, application method, and P availability coefficient (PAC) were most sensitive, while fertilizer source factors were the least sensitive input variables. This technique was found to be a powerful tool for evaluating the robustness of the P-Index to various management strategies and recommendations. Research concluded with a case study of the probable effect of the P-Index on an actual land application program in south central Pennsylvania. Under P-Index nutrient management, 71% of fields would not qualify for continued N-based rates using current management practices. Although modifying the biosolids PAC had a powerful influence on the P-loss risk predicted by the P-Index, multiple strategies may be required to justify N-based application rates for biosolids. It is imperative that P-based nutrient management policies reflect different environmental P loss potentials of biosolids and manures. The WEP test should become a routine measure of P-source availability in state-sponsored P site indices. A substantial reduction in land-based biosolids recycling is likely in states where P-source solubility is not considered in nutrient management policies.