Microbial Electrochemical Technologies for Sustainable Nitrogen Removal in Marine and Coastal Environments

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

• High No explicit measurement of N2O or nitrite accumulation during bioelectrochemical denitrification in marine METs. The paper discusses N2O risk in chemical denitrification (Table 1) but does not report N2O yields or nitrite intermediate concentrations in any MET experiment.

  Section 2 notes that chemical denitrification carries 'Risk of ammonia or nitrite production in the nitrate removal process' (Table 1), and Section 3.2 emphasizes the challenge of 'complete metabolization of nitrogen,' yet no MET study in the paper quantifies N2O or NO₂⁻ accumulation. This is critical for marine applications where N2O is a potent greenhouse gas.

  Try. Measure N2O and NO₂⁻ concentrations in cathode effluent across applied voltages (0.2–1.0 V), current densities (0.1–10 A/m²), and HRT (1–24 h) in a two-chamber MEC with marine inoculum and NO₃⁻ feed (10–50 mg N/L).

• High No systematic study of cathode potential (Ecath) or applied voltage windows for nitrogen removal in marine METs. The paper cites applied voltages of 0.2–1.0 V for autotrophic denitrification (Zhan et al. [80]) but does not test this range in a marine context or relate it to nitrite/N2O selectivity.

  Section 3.1 states 'The applied voltage ranges from 0.45 V to 1.0 V, depending on the substrate and the MEC configuration [77,79]' and mentions 'low applied voltage (from 0.2 to 0.4 V) on nitrogen removal due to the enhance of autotrophic denitrification' (Zhan et al. [80]), but no marine MET paper in this review systematically maps voltage vs. nitrogen removal pathway selectivity.

  Try. Operate a two-chamber MEC with seawater catholyte and NO₃⁻ (20 mg N/L) at applied voltages 0.2, 0.4, 0.6, 0.8, 1.0 V; measure NO₃⁻, NO₂⁻, N2O, and N₂ production rates and current density to identify the voltage window for complete denitrification without N2O or NO₂⁻ accumulation.

• Medium Limited data on C/N ratio effects in marine bioelectrochemical systems. The paper discusses organic electron donor requirements for denitrification in conventional systems but does not report how C/N ratio affects nitrogen removal rate or microbial community composition in METs.

  Section 2 notes that 'an electron donor (organic or inorganic) is necessary to ensure nitrate reduction' and that 'Adding an inorganic electron source is significantly less expensive than adding an organic electron source for denitrification' [57], but no MET study in the paper tests C/N ratio or compares heterotrophic vs. autotrophic denitrification performance in marine conditions.

  Try. Compare nitrogen removal rate and current density in a two-chamber MEC with seawater catholyte (NO₃⁻ 20 mg N/L) and acetate-fed anode at C/N molar ratios of 0.5, 1, 2, 4, and 8; measure denitrification rate, current efficiency, and microbial community (16S rRNA) to identify optimal C/N for marine autotrophic denitrification.

• Medium No data on HRT (hydraulic retention time) optimization for nitrogen removal in marine METs. The paper does not report how residence time affects nitrification, denitrification, or N2O selectivity in bioelectrochemical reactors treating marine wastewater.

  Section 3.3 reports TKN removal efficiency (49.4–84.4%) in sediment METs but does not specify HRT or volumetric nitrogen removal rate (kg N/m³·d is given for one inverted SMFC but not for others). HRT is a critical design parameter for scaling.

  Try. Operate a two-chamber MEC with seawater catholyte and NO₃⁻ feed at HRTs of 2, 6, 12, 24, and 48 h; measure nitrogen removal rate (kg N/m³·d), current density, and effluent NO₃⁻, NO₂⁻, N2O concentrations to identify the HRT window for complete denitrification.

• Medium Microbial community composition and nitrifier/denitrifier abundance not reported for any marine MET experiment. The paper emphasizes that 'high physicochemical and biological complexity that characterizes the marine environment makes it impossible to ensure a high abundance of nitrifying and denitrifying microorganisms' (Section 3.2) but provides no 16S rRNA or qPCR data from the SMFC studies.

  Section 3.2 identifies microbial enrichment as a 'considerable challenge' but the SMFC experiments (Sajana et al., Sanders et al.) do not report microbial community structure, nitrifier (AOA, AOB, NOB, COMAMMOX) abundance, or denitrifier (Paracoccus, Pseudomonas, Bacillus) abundance.

  Try. Perform 16S rRNA gene sequencing and qPCR targeting ammonia-oxidizing archaea (AOA), ammonia-oxidizing bacteria (AOB), nitrite-oxidizing bacteria (NOB), and denitrifiers (nirS, nirK, nosZ genes) on biofilm and planktonic samples from SMFC cathode and anode at days 0, 30, 60, 90 to track enrichment dynamics.

• Low No comparison of photosynthetic biocathodes (mentioned in Section 3.3, Sun et al. [123]) with conventional aerated or electrochemical cathodes for nitrogen removal in marine METs. The paper cites improved power output and nitrogen removal but does not quantify the trade-off.

  Section 3.3 states 'The photosynthetic production of dissolved oxygen and dissolved organic matter in a cathode has shown power output improvements and enhanced nitrogen removal efficiency [122]' and references Sun et al. [123] on autonomous day/night operation, but the paper does not provide removal rates, power densities, or cost comparisons.

  Try. Operate parallel two-chamber MECs with (1) aerated cathode, (2) electrochemical cathode at fixed potential, and (3) photosynthetic algal-bacterial cathode under 12 h light/dark cycles; measure nitrogen removal rate, power output, and operating cost (aeration energy, electrode potential) over 90 days.