Technologies for Biological and Bioelectrochemical Removal of Inorganic Nitrogen from Wastewater: A Review

Specific gaps in this paper, with an experiment that would close each.

• High Cathode potential and catholyte pH windows for bioelectrochemical denitrification are not specified; N2O accumulation risk is mentioned but no operating thresholds are given.

  Section 3 describes stepwise NO3− reduction to N2 via N2O (Equations 11–15) and notes that Zhao et al. achieved 'successful nitrite and total nitrogen removal' but provides no cathode potential (V vs. SHE), pH range, or current density that suppresses N2O.

  Try. Vary cathode potential from −0.2 to −0.8 V (vs. SHE) and catholyte pH from 6 to 8 in MFC/MEC denitrification; measure N2O, NO2−, and N2 gas production; identify threshold for complete reduction to N2.

• High Bioelectrochemical nitrification in MFC/MEC anodes is not discussed; only denitrification at cathode is covered. Potential for simultaneous nitrification–denitrification in two-chamber MES is unexplored.

  Section 3 focuses entirely on cathodic denitrification; anode reactions (Equations 7, 9) show organic oxidation but no mention of ammonia oxidation or nitrification at anode. Simultaneous nitrification–denitrification (SND) is discussed for conventional reactors (Section 2.2.2) but not adapted to MES.

  Try. Design two-chamber MES with nitrifying biofilm on anode (or separate nitrification chamber) and denitrifying cathode; vary anode potential, DO, and HRT; measure NH4+, NO2−, NO3−, and N2 production; compare energy recovery vs. conventional SND.

• High Current density and hydraulic retention time (HRT) operating windows for bioelectrochemical nitrogen removal are not provided; only conventional processes specify HRT and loading rates.

  Section 3 describes MFC/MEC denitrification but does not report current density (A/m²), applied voltage, or HRT. Conventional processes (Sections 2.1–2.3) mention HRT (4–8 weeks for nitrifier colonization, 5-day SRT for partial nitrification) but bioelectrochemical systems lack equivalent data.

  Try. Operate denitrifying MFC/MEC at current densities 0.5–5 A/m² and HRTs 6–48 h; measure NO3− removal rate (mg N/L/d), N2O selectivity, and bioelectricity recovery; identify optimal trade-off between throughput and complete denitrification.

• Medium Nitrite accumulation thresholds and inhibition mechanisms in bioelectrochemical systems are not quantified; partial nitrification control parameters are given for conventional reactors but not MES.

  Section 2.1 specifies DO ~1.5 mg/L, T >25°C, pH 7.5–8.5, SRT ~5 days for partial nitrification in conventional reactors. Section 3 mentions Zhao et al. used 'nitrite as electron acceptor' but does not report NO2− concentration, accumulation rate, or inhibition threshold in MES cathode.

  Try. Feed MES cathode with varying NO2− concentrations (50–500 mg N/L); measure denitrification rate, N2O production, and biofilm community composition; identify NO2− toxicity threshold and optimal feeding strategy.

• Medium Interaction between C/N ratio and bioelectrochemical nitrogen removal is not explored; conventional denitrification requires external carbon (C/N <5 for SND) but MES anode provides electrons without explicit carbon balance.

  Section 2.2.2 states SND applies to wastewaters with C/N <5; Section 3 shows MEC anode oxidizes acetate (Equation 9) to supply electrons to cathode but does not discuss whether wastewater C/N ratio affects denitrification efficiency or whether acetate supplementation is needed.

  Try. Operate denitrifying MEC with wastewater C/N ratios 2–10 and acetate supplementation 0–500 mg/L; measure NO3− removal rate, electron recovery efficiency, and biomass yield; determine if bioelectrochemical pathway decouples denitrification from external COD.

• Medium N2O selectivity and mitigation strategies in bioelectrochemical systems are not quantified; conventional processes mention N2O as a concern but bioelectrochemical systems lack emission data or control methods.

  Section 2.2 states 'there is always a chance of the generation of the greenhouse gas nitrous oxide, which needs to be monitored' but provides no N2O yield data. Section 3 shows N2O as intermediate (Equations 13–14) but does not report N2O/N2 ratio or methods to suppress N2O in MFC/MEC.

  Try. Operate denitrifying MFC/MEC under varying cathode potentials, pH, and electron supply rates; measure N2O and N2 gas production; correlate N2O selectivity with operating parameters; test pH buffering or potential control to minimize N2O.