Phosphate Recovery from Human Waste via the Formation of Hydroxyapatite during Electrochemical Wastewater Treatment

Electrolysis of toilet wastewater with TiO₂-coated semiconductor anodes and stainless steel cathodes is a potentially viable onsite sanitation solution in parts of the world without infrastructure for centralized wastewater treatment. In addition to treating toilet wastewater, pilot-scale and bench-scale experiments demonstrated that electrolysis can remove phosphate by cathodic precipitation as hydroxyapatite at no additional energy cost. Phosphate removal could be predicted based on initial phosphate and calcium concentrations, and up to 80% total phosphate removal was achieved. While calcium was critical for phosphate removal, magnesium and bicarbonate had only minor impacts on phosphate removal rates at concentrations typical of toilet wastewater. Optimal conditions for phosphate removal were 3 to 4 h treatment at about 5 mA cm^–2 (∼3.4 V), with greater than 20 m² m^–3 electrode surface area to reactor volume ratios. Pilot-scale systems are currently operated under similar conditions, suggesting that phosphate removal can be viewed as an ancillary benefit of electrochemical wastewater treatment, adding utility to the process without requiring additional energy inputs. Further value may be provided by designing reactors to recover precipitated hydroxyapatite for use as a low solubility phosphorus-rich fertilizer

Modular Advanced Oxidation Process Enabled by Cathodic Hydrogen Peroxide Production

Hydrogen peroxide (H₂O₂) is frequently used in combination with ultraviolet (UV) light to treat trace organic contaminants in advanced oxidation processes (AOPs). In small-scale applications, such as wellhead and point-of-entry water treatment systems, the need to maintain a stock solution of concentrated H₂O₂ increases the operational cost and complicates the operation of AOPs. To avoid the need for replenishing a stock solution of H₂O₂, a gas diffusion electrode was used to generate low concentrations of H₂O₂ directly in the water prior to its exposure to UV light. Following the AOP, the solution was passed through an anodic chamber to lower the solution pH and remove the residual H₂O₂. The effectiveness of the technology was evaluated using a suite of trace contaminants that spanned a range of reactivity with UV light and hydroxyl radical (HO^•) in three different types of source waters (i.e., simulated groundwater, simulated surface water, and municipal wastewater effluent) as well as a sodium chloride solution. Irrespective of the source water, the system produced enough H₂O₂ to treat up to 120 L water d^–1. The extent of transformation of trace organic contaminants was affected by the current density and the concentrations of HO^• scavengers in the source water. The electrical energy per order (<i>E</i>_EO) ranged from 1 to 3 kWh m^–3, with the UV lamp accounting for most of the energy consumption. The gas diffusion electrode exhibited high efficiency for H₂O₂ production over extended periods and did not show a diminution in performance in any of the matrices