Passive co-treatment of phosphorus-depleted municipal wastewater with acid mine drainage: Towards sustainable wastewater management systems.

Industrial processes typically produce large wastewater volumes, which, if left untreated, greatly affect receiving ecosystems. However, wastewater treatment can be costly and energy-intensive, with the developing world particularly struggling with wastewater management. As such, simple and cost-effective solutions are urgently required with the passive (no energy or reagents) co-treatment of different wastewater matrices holding great promise. Here, wastewater from a phosphorus recovery system (chemical precipitation) was co-treated with acid mine drainage (AMD). Specifically, phosphorus-rich municipal wastewater was treated with hydrated lime, as to synthesize a wastewater-derived phosphorus product, i.e., calcium phosphate (Ca(PO)), also producing a phosphorous-depleted alkaline effluent. The feasibility of valorising this effluent is examined here by using it for the passive co-treatment of real AMD. Different liquid-to-liquid (v/v) ratios were considered, with the optimum ratio (AMD to phosphate-depleted wastewater) being 1:9. The pH of the co-treated effluent was adjusted to 8.4 (from an initial value of 11.5 in the phosphorus-depleted wastewater and 2.2 in AMD), while metals (∼100% reduction of Fe, Mn, Ni, Cu, Pb, ≥99.5 for Al, Zn, and Mg, 80% for Cr, and 75% for As) and sulphate (89.26% reduction) contained in AMD were greatly removed. This was also the case for the remaining orthophosphate that was contained in the phosphorus-depleted wastewater (93.75% reduction). The electrical conductivity was also reduced in both the AMD (88.75%) and the phosphorus-depleted wastewater (69.21%), suggesting the removal of contaminants from both matrices. Results were underpinned by state-of-the-art analytical techniques, including FE-SEM/FIB/EDX, FTIR, and XRD, along with geochemical modelling (PHREEQC). Contaminants were removed through complexation, (co)adsorption, crystallization, and (co)precipitation. Overall, results suggest that the co-treatment of these wastewater matrices is feasible and could be directly scaled up (e.g., using waste stabilization ponds), while opportunities for the beneficiation of the produced sludge and for water reclamation (e.g., through membrane filtration) could also arise, further promoting the sustainably of this passive co-treatment method.