Abstract
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.
Highlights
Discharge of phosphorus-containing wastewater to surface waters can cause algal blooms, leading to growth of toxic cyanobacteria, hypoxia, and disruption of food webs.[1,2] At the same time, phosphorus is a limited resource with an average price that has nearly tripled between 2005 and 2015,3 making the recovery of phosphorus from waste crucial.[4]
Enhanced biological phosphorus removal (EBPR) may provide effective phosphorus recovery in centralized wastewater treatment processes,[8] but in rural communities, small onsite sanitation systems make this technology challenging without engineered processes to maintain the correct microbial population.[9]
Electrolysis of collected toilet wastewater in the pilot-scale onsite treatment system resulted in removal of total phosphate, magnesium, and calcium over the 5 h treatment cycle
Summary
Discharge of phosphorus-containing wastewater to surface waters can cause algal blooms, leading to growth of toxic cyanobacteria, hypoxia, and disruption of food webs.[1,2] At the same time, phosphorus is a limited resource with an average price that has nearly tripled between 2005 and 2015,3 making the recovery of phosphorus from waste crucial.[4]. Phosphorus recovery in rural communities can be accomplished via forced precipitation as struvite (NH4MgPO4·6H2O) or hydroxyapatite (Ca5(PO4)3OH), but these strategies typically require separation of urine and feces, addition of chemicals, or use of sacrificial electrodes that further complicates and increases the cost of existing wastewater treatment strategies.[10−12]
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