USEPA LEAF methods for characterizing phosphorus and potentially toxic elements in raw and thermally treated sewage...

Biologically available phosphorus supports plant growth but can also cause environmental contamination. Sequential extraction methods, such as Hedley fractionation, are the most widely used to assess available phosphorus from solids. However, such methods exhibit numerous deficiencies. The USEPA Leaching Environmental Assessment Framework (LEAF) is a tiered system developed to evaluate releases of Potentially Toxic Elements (PTEs) from solids. This study compared the Hedley fractionation method to the LEAF pH-dependent, parallel batch tests (Method 1313) and dynamic leaching column test (Method 1314) to assess the bioavailability of phosphorus. The three methods were applied to wastewater treatment plant sludge before and after thermal treatment. Both methods revealed similar qualitative trends, namely that thermal treatment transformed phosphorus into less immediately available forms. However, the Hedley and LEAF methods were inconsistent in the forms and amounts of available phosphorus recovered from the solids. The Hedley method left 40% of phosphorus unextracted from sludge and 20% from ash, suggesting that it may be less appropriate for organic materials. Moreover, only 2 of the 6 Hedley phosphorus pools were within environmentally relevant pH conditions. Furthermore, the Hedley method overpredicted the readily available phosphorus. In contrast, the LEAF methods allowed for a more detailed analysis of phosphorus availability - while simultaneously assessing PTEs - across a controlled pH range. Moreover, LEAF used simpler procedures and provided more easily interpreted results. Thus, LEAF facilitates more robust and valuable assessment of organic and inorganic solids being considered for land application.