Maximizing phosphorus recovery as biofertilizer in an algal wastewater treatment system

Abstract Our previous reports have demonstrated an algal pathway for treating settled sewage and recovering its energy-, nitrogen-, and phosphorous-contents via hydrothermal liquefaction (HTL) of the resulting algal biomass. The first part of the current study was to intensify a critical step in the phosphorous (P) recovery process which entails alkaline leaching of the P-rich byproduct of HTL– biochar, to yield a P-rich eluate. The following parameters driving the efficiency and cost of leaching P from biochar were systematically evaluated: leaching time, concentration of base eluent, mixing during leaching, liquid:solid (L:S) ratio in leaching, and temperature. Optimum leaching time, base concentration, L:S ratio, and temperature were found to be 72 h, 0.5 M, 20:1, and 60 °C, respectively. Under these optimal conditions, >86% of P in the biochar could be extracted at an estimated cost of $4.54/kg P. In the second part of this study, a three-step procedure was developed to recover high-purity biofertilizers from the P-rich eluate of biochar. In the first step, impurities along with 24.6% of P in the eluate were precipitated. In the second step, high-quality struvite (comprising 72.7% of P in the eluate) was precipitated from the supernatant of the first step. In the third step, the precipitate from the first step was dissolved and its P-content was re-precipitated as high-quality calcium phosphate. This three-step procedure resulted in 95.4% P-recovery from biochar eluate at $5.29/kg P. Reclaimed struvite and calcium phosphate were shown to be of high chemical purity, complying with US EPA guidelines for metals.