Abstract
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•89% total nitrogen and 97% total phosphorous recovered from municipal wastewater.•Electrochemical recovery as 47% (NH4)2(SO4) solution and 21% amorphous Ca3(PO4)2.•No electrode fouling observed during 2 years of experiments with polarity reversal.•Total energy projected at 1.21 kWh/m3 for municipal wastewater.•Economical particularly when nutrient pollution control costs are considered.
The manufacture and use of chemical fertilizers has led to surface water eutrophication, mining-related environmental effects, and consumption of 1–2% of global energy supplies for commercial production of nitrogen (N) fertilizer by the Haber-Bosch process. However, recovery of nutrients from wastewater can help to close the nutrient cycle, from farm to food to municipal wastewater. In this work, we demonstrate simultaneous N and P recovery from settled sewage in a continuous-flow reactor, by precipitation of Ca3(PO4)2 and stripping of NH3(g) following electrochemical pH shifting, termed electrohydromodulation (EHM). pH was dropped anodically to ~6 for stripping of CO2, to address subsequent buffering and calcite precipitation, then raised cathodically to ~11 for Ca3(PO4)2 precipitation and NH3(g) stripping, and neutralized prior to discharge. Ammonia recovery by stripping under vacuum using 0.3 m (1 ft) of glass Raschig Rings packing material was found most efficient. Recovery of 89% and 97% of average total N & P from municipal primary effluent was achieved in the continuous-flow bench reactor, at an electrochemical energy demand of 0.623 kWh/m3. Total energy demand, including energy for EHM, filtration, ammonia stripping and absorption was projected at 1.21 kWh/m3. Precipitates were found to be amorphous with a Ca/P ratio of ~3.66, with the ratio depending somewhat on flow rate at a constant voltage. Results suggest that 0.91 L (0.24 Gal) of 5.8 M H2SO4 can recover 90% of ammonia in 3.785 m3 (1000 Gal) of wastewater containing 25 mg-N/L. Overall, the process appears economical relative to competing recovery processes and commercial fertilizer production.
