Efficiency of Photosynthetic Microbial Fuel Cells (pMFC) Depending on the Type of Microorganisms Inhabiting the Cathode Chamber

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

• High No measurement of nitrogen speciation (NO₃⁻, NO₂⁻, NH₄⁺, N₂O) in cathode effluent; paper reports only total nitrogen removal % without identifying which N-species accumulated or were removed.

  Abstract states 78% and 69% N-removal but Methods only measured TN and N-NH₄⁺; no mention of NO₃⁻, NO₂⁻, or N₂O. For denitrification systems, N₂O accumulation is a critical environmental concern.

  Try. Measure NO₃⁻, NO₂⁻, NH₄⁺, and N₂O in cathode effluent at each 2-week cycle; quantify N-balance to identify if N is removed as gas, assimilated, or accumulated as nitrite.

• High No cathode potential or anode potential measurement; only voltage across 1 Ω external resistor reported. Electrode potentials relative to Ag/AgCl reference were recorded but not presented.

  Methods state 'Reference electrodes comprising Ag/AgCl...were used to record electrode potentials' but no potentials appear in Results. Without cathode potential, cannot assess whether oxygen reduction is kinetically limited or whether nitrite/N₂O reduction is occurring.

  Try. Report cathode and anode potential vs. SHE throughout light/dark cycles and across the 84-day operation; correlate with power density and N-species to identify electrochemical windows for nitrification vs. denitrification.

• Medium No investigation of current density or HRT effects on nitrogen removal; single HRT (weekly anode replacement) and single current density regime tested.

  Paper does not vary HRT or current density. Don and Babel (cited) showed COD concentration in cathode affects power; no systematic study of how HRT or applied potential affects N-removal pathway or N₂O production.

  Try. Test HRT 2–14 days and cathode potentials −0.2 to +0.4 V vs. SHE; measure N-species removal and N₂O emission to identify operating window for complete denitrification vs. nitrification.

• Medium Anode effluent COD concentration (~221 mg/L) is reported as a single value; no time-series or cycle-by-cycle tracking of COD in anode effluent or cathode medium.

  Section 3.2 mentions 'COD concentration in the anode chamber was about 240 mg/L' but no table or figure shows COD evolution. Without COD profile, cannot assess whether heterotrophic vs. autotrophic conditions in cathode changed over 84 days.

  Try. Track COD, TN, and TP in anode effluent and cathode medium at each 2-week replacement cycle; plot against chlorophyll-a and power density to identify nutrient-limiting phases.

• Medium No measurement of dissolved oxygen gradient between anode and cathode; paper notes oxygen can diffuse from cathode to anode but does not quantify this or measure anode DO.

  Introduction cites Bazdar et al. on oxygen diffusion impairing anode performance, but no anode DO data presented. Cannot assess whether high cathode oxygen (18.65 mg/L with Arthrospira) compromised anode anaerobiosis.

  Try. Measure DO in anode chamber during light and dark phases; if anode DO > 0.5 mg/L, test membrane or separator modifications to prevent oxygen cross-over.

• Low Chlorophyll-a used as proxy for biomass but no direct biomass (dry weight, AFDW) or protein yield reported; cannot assess true productivity or nutrient recovery per unit biomass.

  Figure 4 shows chlorophyll-a only; no correlation to actual biomass or harvest yield. For cost-benefit analysis of using pMFC for nutrient recovery, biomass productivity is essential.

  Try. Measure dry weight and protein content of harvested Arthrospira and Chlorella at each cycle; calculate N and P recovery per kg biomass and compare to synthetic medium cost.