Evaluating the Potential of Recovering Vivianite for Phosphorus Removal and Improving Class A Biosolids Applications

INTENDED OBJECTIVE: Biosolids from water resource recovery facilities (WRRFs ) are increasingly used for agricultural and recreational land purposes due to their sustainable means to replenish macronutrients. However, the relatively high phosphorous content of biosolids limits certain applications. This study aims to identify optimal locations for recovering vivianite and subsequently phosphorus, to allow class A biosolids reach ideal phosphorus levels. INTRODUCTION: Global consumption rates of synthetic fertilizers per unit of cropland have increased 3 to 8 times over the past half century (Lu et al. 2017). Class A biosolids are increasingly explored as an alternative to commercial fertilizers due to their effective means to replenish carbon and macronutrients (Gilmour et al. 2013). However, the N:P ratio of biosolids is typically lower than commercial fertilizers which can cause an accumulation of phosphorus when applied to soil, increasing risk for eutrophication (Penn et al. 2002). Therefore, it is critical to implement efficient phosphorus-removal technologies/techniques within the biosolids treatment train to improve the N:P ratio and limit the amount of phosphorus that result in the cake. Therefore, this study aims to analyze the P mass balance of the Blue Plains advanced wastewater treatment plant in Washington, DC as well as its vivianite characterization in full-scale. The process sampling, modelling and analysis led to identification of potential locations for vivianite recovery, and initial estimations on how much the P content in our class A biosolids might be reduced by recovering vivianite upfront. MATERIALS AND METHODS: Sampling campaign: Samples were collected in DC Water Blue Plains over a 3-month period (Mid-February to Mid-May 2022). Quantitative analysis: Phosphorus is measured by the EPA 365.1 method. Scanning Electron Microscopy with Energy Dispersive and Mossbauer Spectroscopy: Prepared by method in Prot et al. 2021. Modelling method: Visual Minteq version 3.3 with DOC SHM model RESULTS & DISCUSSION P mass balance at Blue Plains and P content in biosolids product Blue Plains advanced WWRF achieved a 98% incoming TP removal over its primary (enhanced-chemical phosphorus removal) and secondary systems through iron dosing. The removed P is diverted to the solids fraction and results in a Class A biosolids product (BLOOM) with 2.88% P (dry weight basis) and 28-32%TS (Fig.1A). The P:N ratio of our product is 1:1 and thus higher in P content than the target of 1:2 P:N for typical fertilizers. Decreasing P content in BLOOM by 50% or thus recovery 60% of vivianite would increase the value and applicability of BLOOM. Vivianite formation and formation potential along the Solids Treatment Train A total Fe/P molar ratio of 1.5 or greater allows for optimal vivianite precipitation (Cong et al. 2017) which is reflected in DC Water treatment flow diagram (Figu 1). Vivianite was detected in primary thickened sludge, blend of thickened primary (PS) and waste activated sludge (WAS), and digested sludge b ased on sequential extraction method in Wang et al. 2021. This was also further supported by the clear co-location of Fe and P in SEM images (Figure 2A and 2B). These locations are consistent with the vivianite scaling seen in primary sludge gravity thickening lines and centrifuge lines in the facility. Through quantitative analysis, the vivianite mass flow in thickened primary sludge, blended PS/WAS and digested solids accounted for 8%, 52%, and 49% of the incoming P to the plant (Fig. 1). These vivianite ranges are like those reported by iron-dosing plants in Prot et al. 2021 and Wilfert et al. 2016. Estimation of P recovery potential needed to impact Class A biosolids P content Based on the current vivianite-P levels in the three identified locations (Fig. 1), sludge after anaerobic digestion contained the highest level of vivianite-P. This is likely due to higher orthophosphorus levels after thermal hydrolysis treatment which allows for additional vivianite formation with Fe(II). This notion is supported by the low levels of Fe(II) concentrations after anaerobic digestion indicating that most of it is bound (Fig. 1). We estimated how much vivianite recovery efficiency we might need to decrease the P content in the BLOOM with an ideal target of ~0.014 kg P/kg TS and acceptable target of 0.022 kg P/kg TS. Given the highest vivianite levels in digested sludge, a recovery efficiency of at least 40% with ideally 70% is sufficient to meet our targets (Fig. 3). Utilizing vivianite recovery technologies such as magnetic separation (80% of vivianite recovery in digestates according to Wijdeveld et al. 2022), and other alternative options could provide the potential recovery target needed to improve BLOOM. CONCLUSIONS Based on current vivianite-removal technology efficiencies, removing vivianite will significantly impact the phosphorus levels in BLOOM. Results suggest that after anaerobic digestion is a preferred and feasible option to implement recovery technology due to its high vivianite content as well as the opportunity to recover vivianite in a fraction of the digestate to produce low P BLOOM rather than the whole biosolids treatment train. Future testing will explore recovery efficiencies of P and vivianite to confirm feasibility and setup a path towards pilot testing approaches.