Enhanced struvite recovery from wastewater using a novel cone-inserted fluidized bed reactor

The feasibility of struvite recovery at low(12.5 mg/L) and high(120 mg/L) phosphorus concentrations was studied by constructing a novel fluidized bed reactor with cones(FBRwc) and without cones(FBRwoc). The crystallization process was continuously operated for 133 days under different hydraulic retention times(HRT = 1–10 hr), pH(7.5–10), and molar ratios of Mg/P(0.75–1.75), N/P(1–10) and Ca/Mg(0–2). The optimum operating conditions of HRT, pH, Mg/P and N/P molar ratios were found to be 2 hr, 9, 1.25, and 7.5, respectively. Under these optimum conditions, the phosphorus precipitation efficiencies of FBRwcwere 93% for low and 98% for high phosphorus influent; however, the efficiencies were 78% and 81% for FBRwoc, respectively. Due to crystal losses at each junction(17%–31%), the crystal recovery efficiency of FBRwocwas relatively low(47%–65%) for both influent concentrations. However, the losses were minimal in FBRwc, which showed 75% and 92% crystal recovery for low and high phosphorus concentrations, respectively. At low calcium concentration, crystal chemical analysis showed the product to be pure struvite(> 99%). The scanning electron microscope and X-ray diffraction results further confirmed that the crystal recovered from FBRwc contained pure struvite, which could be considered a high quality fertilizer. Except HRT, all parameters(pH, Mg/P, N/P and Ca/Mg) were found to be influencing factors for FBRwcperformance. Overall, inserting cones in each part of the reactor played a significant role in enhancing struvite recovery from a wide range of phosphorus-containing wastewater.