Mechanisms of Phosphorus Removal in a Pilot-Scale Constructed Wetland/BOF Slag Wastewater Treatment System

Removal of PO4-P, NH3-N, cBOD(5), chemical oxygen demand (COD), Escherichia coli, and total coliforms in a pilot-scale hybrid constructed wetland (CW) wastewater treatment system was evaluated. Wastewater was sequentially passed through a mixing cell, an aerobic treatment cell, a P treatment cell that contained basic oxygen furnace slag (BOFS), and an anaerobic treatment cell. The system operated for 8 months at an average flow rate of 68 L/day. Mean influent concentrations of PO4-P, NH3-N, cBOD(5), and COD were 7, 25, 27, and 60 mg/L, respectively, and mean influent levels of E. coli and total coliforms were 10(4) and 10(5) CFU 100 mL(-1). Effluent from the system had 95-99% lower concentrations of PO4-P, NH3-N, cBOD(5), E. coli, and total coliforms, and a 72% lower concentration of COD, compared to influent values. Backscatter scanning electron imaging coupled with energy dispersive X-ray (EDX) spectra and elemental maps, along with Fourier transform infrared (FTIR) spectra, showed that P was present on the outer layer of the spent BOFS material. Occurrence of calcium-carbonate and phosphate minerals also was indicated through FTIR analysis. X-ray absorption near edge structure (XANES) spectra of samples from the treatment system was consistent with the presence of phosphate adsorbed on calcite, hydroxyapatite, and alpha-tricalcium phosphate. High pH (11.66 +/- 0.70) of pore water in the BOFS cell resulted in inactivation of pathogens. The wastewater pH was subsequently neutralized by sparging CO2(g) before being introduced to the final anaerobic cell.