Electrochemically driven extraction and recovery of ammonia from human urine

Abstract

Human urine contains high concentrations of nitrogen, contributing about 75% of the nitrogen in municipal wastewaters yet only 1% of the volume. Source separation of urine produces an ideal waste stream for nitrogen and phosphorus recovery, reducing downstream costs of nutrient treatment at wastewater treatment facilities. We examined the efficiency and feasibility of ammonia extraction and recovery from synthetic and undiluted human urine using an electrochemical cell (EC). EC processing of synthetic urine produced an ammonium flux of 384 ± 8 g N m−2 d−1 with a 61 ± 1% current efficiency at an energy input of 12 kWh kg−1 N removed. EC processing of real urine displayed similar performance, with an average ammonium flux of 275 ± 5 g N m−2 d−1 sustained over 10 days with 55 ± 1% current efficiency for ammonia and at an energy input of 13 kWh kg−1 N removed. With the incorporation of an ammonia stripping and absorption unit into the real urine system, 57 ± 0.5% of the total nitrogen was recovered as ammonium sulfate. A system configuration additionally incorporating stripping of the influent headspace increased total nitrogen recovery to 79% but led to reduced performance of the EC as the urine ammonium concentration decrease. Direct stripping of ammonia (NH3) from urine with no chemical addition achieved only 12% total nitrogen recovery at hydraulic retention times comparable with the EC systems. Our results demonstrate that ammonia can be extracted via electrochemical means at reasonable energy inputs of approximately 12 kWh kg−1 N. Considering also that the hydrogen generated is worth 4.3 kWh kg−1 N, the net electrical input for extraction becomes approximately 8 kWh kg−1 N if the hydrogen can be used. Critical for further development will be the inclusion of a passive means for ammonia stripping to reduce additional energy inputs.