Evaluating the potential for improving Class A biosolids applications through vivianite recovery
Class A biosolids from water resource recovery facilities (WRRFs) are increasingly used as sustainable alternatives to synthetic fertilizers to replenish macronutrients. However, the high phosphorus to nitrogen ratio in biosolids leads to an accumulation of phosphorus after repeated land applications. Extracting vivianite, an Fe-P mineral, prior to the final dewatering step can reduce the P content in the resulting class A biosolids and achieve a P: N ratio closer to 1:2 of that of synthetic fertilizers. Using ICP-MS, IC, UV-Vis colorimetric method, Mössbauer spectroscopy, and SEM-EDX, a full-scale characterization of vivianite at the Blue Plains Advanced Wastewater Treatment Plant (AWTTP) was surveyed throughout the biosolids treatment line. The vivianite-bound phosphorus and vivianite-bound iron quantified in primary sludge thickening, sludge blend tank 2, after thermal hydrolysis, and after anaerobic digestion, was found to correspond to 8%, 52%, 40%, and 49% of both total phosphorus and total iron that is incoming the plant, respectively. Based on current P: N levels in the Class A biosolids at Blue Plains (BLOOM), a vivianite recovery target of 40% to ideally 70% is required in locations with high vivianite content to reach a P:N ratio of 1:1.3 to 1:2, respectively. Additionally, this study explores the feasibility of using cyclone density separation technology to recover vivianite from the sludge streams. Model simulations were also conducted with Visual Minteq to evaluate the pre-treatment options for maximizing vivianite recovery at different solids treatment train locations. At last, a financial analysis and estimates were performed on recovered vivianite along with potential FeCl3 savings from iron recycling.