Bioelectrochemical Systems-driven Directional Ion Transport Enables Low-energy Water Purification And Resource Recovery

Bioelectrochemical systems(BESs)are integrated water treatment technologies that generate electricityusing organic matter in wastewater.In situ use of bioelectricity can direct the migration of ionic substances in a BES,thereby enabling sustainable water desalination,resource recovery,and valuable substance production.Recently,much attention has been placed on the microbial desalination cells(MDC)in BESs to drive waterdesalination,while the desalination rate was relatively low and the pH imbalance of electrolyte was serious.Besides desalination,BESs could drive the removal and recovery of ions in the wastewater thus fulfill purification combined with resource recovery.However,the researches in this scope had just emerged and need to be promoted further.In this work,the structure of stacked MDC was optimized to enhance desalination rate;the electrolyte of SMDC was circulated to balance the pH;a microbial nutrient recovery cell was designed and used to purify wastewater and recovery nutrient ions;based on the above works,a self-driven comprehensive wastewater purification system was built with simultenous water desalination and nutrient recovery.The enlarged microbial desalination cell was investigated in this research.The number of MDC desalination cells was optimized between 6 and 14.The configuration of 10-desalination-cell reactor achieved the highest total desalination rate of 206 mg/L and charge transfer efficiencyof 771%,which were the highest among all the present researches.Junction potential increased quickly during the desalination process,and occupied >74% of the potential loss in the MDC.A circulated MDC was built to reconstruct the channel for H+ and OH-to contact and neutralize thus balance the pH of electrolyte.The pH was kept in neutral region without providing buffer solutions.The current output and desalination of the circulated MDC were stable.With the protection of separator,the circulated MDC has been operated for over 60 d with the steady performance of current production,desalination and CODremoval.A microbial nutrient recovery cell(MNRC)was developed to take advantage ofthe energy contained in wastewater,in order to simultaneously purify wastewater and recovernutrient ions.When wastewater was circulated between the anode and cathode chambers of theMNRC,the organics(COD)were removed by bacteria while ammonium and phosphate(NH4+-Nand PO43--P)were recovered by the electrical field that was generated using in situ energy in thewastewater without additional energy input.The removal efficiencies from wastewater were >82%for COD,>96% for NH4+-N,and >64% for PO43--P in all the operational cycles.Simultaneously,the concentrations of NH4+-Nand PO43--P in the recovery chamber increased to more than 1.5 and 2.2 times,respectively,compared with the initial concentrations in wastewater.The self-driven wastewater purification and nutrient recovery system was an integration of the above works.The wastewater was treated sequentially by the anode,cathode and stacked desalination chambers of the system.Driven by the energy contained in wastewater,the simultaneous purification,desalination and nutrient recovery were achieved.The COD,total nitrogen,ammonium nitrogen and total phosphorus in the treated real wastewater all could match the first class standard of wastewater discharge.The ammonium nitrogen and total phosphorus in the recovery solution were concentrated to 83.7 mg/Land 20.8 mg/L,and was used to produce struvite.