Organic matter and ammonia removal by a novel integrated process of constructed wetland and microbial fuel cells

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

• High No measurement of N₂O or NO accumulation during denitrification despite high NO₃⁻–N reduction (75.13%)

  The paper reports substantial denitrification but does not quantify whether N₂O is produced as an intermediate or final product. Section 3.2 states 'The NO₃⁻–N decreased from its highest concentration at 70.62 mg/L...to its lowest concentration at 17.56 mg/L' but does not account for the nitrogen mass balance or specify the end product (N₂ vs N₂O)

  Try. Measure N₂O and NO in gas phase above cathode and in effluent using gas chromatography or FTIR; vary cathode potential and dissolved oxygen to determine operating window that maximizes N₂ production and minimizes N₂O

• High No systematic variation of cathode potential or dissolved oxygen concentration to optimize denitrification selectivity

  The paper operates with a fixed external resistance (1000 Ω) and relies on passive aeration via plant roots and air cathode. Section 3.2 mentions 'aerobic denitriers...could use both oxygen and nitrate as terminal electron acceptors' but does not test how cathode potential or DO affects the balance between aerobic and anaerobic denitrification pathways

  Try. Vary external resistance (100–10,000 Ω) or apply controlled cathode potential (−0.2 to +0.2 V vs Ag/AgCl) while measuring NO₃⁻–N, NO₂⁻–N, N₂O, and N₂ production; determine the potential window that maximizes complete denitrification to N₂

• Medium No measurement of nitrite (NO₂⁻–N) accumulation or transient dynamics during nitrification–denitrification coupling

  The paper measures only NH₄⁺–N and NO₃⁻–N, not NO₂⁻–N. Section 3.2 describes nitrification producing NO₃⁻–N but does not report whether NO₂⁻–N accumulates as an intermediate, which could indicate incomplete nitrification or inhibition of nitrite oxidizers

  Try. Add NO₂⁻–N to the sampling protocol at all ports (influent, S1–S8, effluent); measure daily during steady state to detect transient accumulation under varying inlet loads

• Medium No variation of hydraulic retention time (HRT) or organic loading rate to determine the operating window for simultaneous nitrification–denitrification

  The system operates at a fixed HRT of 2 days and inlet COD of 324–708 mg/L. Section 3.1 states 'The MFCs and IVCW-MFCs operate more efficiently under a lower organic matter load' but does not test the trade-off between COD removal, nitrogen removal, and power generation across a range of HRTs (0.5–5 days) or C/N ratios

  Try. Operate parallel reactors at HRT 0.5, 1, 2, 3, 5 days with constant inlet COD or varying C/N ratio (5:1 to 20:1 by mass); measure COD, NH₄⁺–N, NO₃⁻–N, NO₂⁻–N, N₂O, and power density to identify the optimal HRT for nitrogen removal without sacrificing power output

• Medium No measurement of catholyte pH or anode pH dynamics, despite mention of pH gradient effects on power generation

  Section 3.3 states 'a continuous influx was used in this experiment; the wastewater flowed from anode to cathode while transporting protons, shortening the pH gradient between the two points and avoiding the small effect of electricity production caused by excess acidity at the anode or excess alkalinity at the cathode' but no pH data are provided

  Try. Install pH probes at anode inlet, anode outlet, cathode inlet, and cathode outlet; measure pH daily during steady state and correlate with voltage output and nitrogen species concentrations to quantify the pH gradient effect on power generation and nitrification–denitrification balance

• Medium No characterization of the microbial community in the cathode region or comparison of anode vs cathode communities under closed-circuit vs open-circuit operation

  Section 2.3 states 'the substrate-attached biolms from these two regions were sampled for microbial community analysis' and Section 3.3 mentions Geobacter and Desulfuromonas in the anode, but the results section does not present cathode community data or compare the two circuits

  Try. Perform 16S rRNA sequencing on anode and cathode biofilms from both closed-circuit and open-circuit reactors; quantify the relative abundance of nitrifiers (Nitrosomonas, Nitrobacter), denitrifiers (Paracoccus, Pseudomonas), and electrochemically active bacteria (Geobacter, Desulfuromonas) to identify which taxa drive the 14.94% COD and 57.6% NH₄⁺–N improvement in closed-circuit mode