Optimised operational parameters for improved nutrient recovery from hydrolysed urine by bio-electroconcentration

Abstract

• Optimised bio-electroconcentration recovered N from urine at 56.1 ± 2.4 %. • Implementation of pH-activated catholyte recirculation can boost the N recovery. • The resulting fertiliser contained 21.2–23.5 g N L −1 , 1.1 g P L −1 and 5.4–8.9 g K L −1 . • The specific energy consumption remained stable at 4.1 ± 0.1 kWh per kg N recovered. • NH 4 + and HCO 3 − were responsible for 66.0 % and 50.0 % of the charge transport. Anthropogenic activity is responsible for an imbalance in the world’s nutrient cycles. New methods to remove and recover nutrients from waste streams are necessary to meet the increasing global nutrient demand without further depleting finite resources. Bio-electroconcentration is a process combining electrodialysis and microbial electrolysis, where an electroactive microbial community at the anode facilitates the electrodialytic recovery of nutrients into a liquid fertiliser concentrate. This study addresses the optimisation of this process towards the implementation of nutrient bio-electroconcentration from source-separated urine, a resource collected via urine-diverting technologies which are becoming increasingly adopted worldwide. The nitrogen recovery efficiency of triplicate bio-electroconcentration cells (BECs) was optimised by varying different operating parameters, including the hydraulic retention time (HRT) and the recirculation ratio of the catholyte into the anodic compartment. At a low HRT of 1.7 h, each BEC could recover an average equivalent to 4.9 ± 0.1 kg of nitrogen per m 3 per day, with an average nitrogen recovery efficiency of 14.0 ± 0.4 %. At a high HRT and with pH-activated catholyte recirculation, the N recovery efficiency reached a maximum of 69.6 %, the highest reported to date for BECs and any other microbial electrochemical technologies for nitrogen recovery, while recovering an average of 3.1 ± 0.1 kg of nitrogen per m 3 per day. In both scenarios, the BECs produced concentrated liquid fertilisers (21.2 ± 0.3 g N L −1 , 1.1 g P L –1 , and 5.4 ± 0.2 g K L −1 ) at an average specific power consumption of 4.1 ± 0.1 kWh per kg of nitrogen, which is significantly lower than required for the combination of N fertiliser production via the Haber-Bosch process and conventional wastewater treatment via nitrification/denitrification (∼ 23.5 kWh kg N −1 ). The BECs also had the ability to recover potassium and bicarbonate ions, which accounted for 8.2 ± 1.0 and 50.0 ± 0.9 % of the charge transport through the cation- and the anion-exchange membranes, respectively. Due to its inherent low ratio in urine (2.6 ± 0.1 % of the total anions as HPO 4 2− ), P only accounted for 2.3 ± 0.2 % of the total transport across the anion-exchange membrane.