Biotreatment of Slaughterhouse Wastewater Accompanied with Electrcity Generation and Nutrients Recovery in Microbial Fuel Cell

Tensions the paper itself points to. Each side quotes the passage it comes from.

• Within the paper Nitrate removal efficiency reported as both 100% (maximum) and 90.3–95.5% (steady-state range after day 35)

  One side

  Section 4.3 states 'After 21 days, maximum removal efficiency of nitrate achieved 100%'

  “After 21 days, maximum removal efficiency of nitrate achieved 100%.”

  Other side

  Section 4.3 also states 'The nitrate removal efficiency remained within a range from 90.3% to 95.5% after 35 days'

  “The nitrate removal efficiency remained within a range from 90.3% to 95.5% after 35 days.”

  Authors’ reading. The authors attribute the decline to 'limited supply of COD to the denitrifying bacteria' but do not explain why COD becomes limiting after day 21 when the anode is continuously fed 1000 mg/L COD. This suggests either (1) most COD is consumed in the anode/cathode biofilm and little reaches the external anoxic reactor, or (2) the anoxic reactor HRT or mixing is inadequate. The paper does not clarify which.

• Within the paper Coulombic efficiency (13.37%) is very low despite high COD removal (99%) and stable current (472 mA/m²)

  One side

  Abstract and Section 4.6 report 'Columbic efficiency... 13.37%' and 'current and power densities were stabilized at 472 mA/m² and 165.22 mW/m²'

  “Columbic efficiency... 13.37%”

  Other side

  Section 4.1 states 'maximum and average COD removal efficiencies were 98% and 94%, respectively', implying most substrate is oxidized

  “maximum and average COD removal efficiencies were 98% and 94%, respectively”

  Authors’ reading. If 94–98% of COD is removed, the electrons should be recovered as current. The 13.37% CE indicates that only ~13% of electrons from COD oxidation are transferred to the external circuit; the rest are lost to non-productive pathways (e.g., methanogenesis, sulfate reduction, or cathode side reactions). The paper does not investigate this loss mechanism.

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

• High No measurement of N2O or NO production in the anoxic bioreactor off-gas, despite claiming 100% nitrogen recovery and listing gas chromatography equipment.

  Section 3.1 lists 'Gas Chromatographs' as available equipment, but Section 4.3 reports only NO3− removal without specifying the end product (N2 vs. N2O vs. NO). The abstract claims '100%' nitrogen recovery but does not define the product. If N2O is produced, it is a potent greenhouse gas and the system may not meet environmental standards.

  Try. Measure off-gas composition (N2, N2O, NO, N2/N2O ratio) from the anoxic bioreactor outlet using GC-FID or GC-ECD at steady state (days 25–43). Vary anoxic HRT (15–60 h) and external COD supply (via recirculation) to identify the operating window that maximizes N2 and minimizes N2O.

• High No systematic variation of cathode potential, anode potential, or dissolved oxygen (DO) in the cathode to optimize nitrification stability and Coulombic efficiency.

  Section 2.5 states 'air compressor with a maximum flow rate of 10 mL/min was connected to the cathode compartment' but does not report actual DO levels, cathode potential, or how aeration rate affects NH4+ removal fluctuations (88.5–99.3%). The low CE (13.37%) suggests cathode overpotential or competing reduction reactions (e.g., NO3− reduction at the cathode instead of in the anoxic reactor).

  Try. Control cathode potential potentiostatically (e.g., −0.1 to −0.5 V vs. Ag/AgCl) while monitoring DO (target 2–6 mg/L), NH4+ removal, NO3− accumulation at the cathode outlet, and current. Measure the trade-off between nitrification rate and CE as cathode potential becomes more negative.

• High No investigation of the C/N ratio or COD availability to the anoxic bioreactor; nitrate removal declines from 100% (day 21) to 90.3–95.5% (day 35+) despite constant influent COD.

  Section 4.3 attributes the decline to 'limited supply of COD to the denitrifying bacteria' but the paper does not measure COD in the anoxic bioreactor inlet or outlet, nor does it report the fraction of influent COD that reaches the external reactor. If most COD is consumed in the anode/cathode biofilm, the external reactor is starved.

  Try. Measure COD concentration at the anode outlet, cathode outlet, and anoxic bioreactor inlet/outlet. Calculate the COD mass balance. Vary the recirculation ratio (0–100% of cathode effluent recycled to the anoxic reactor) and measure the effect on NO3− removal efficiency and N2O production.

• Medium No measurement of pH in the cathode or anoxic bioreactor; only anode pH is adjusted to 7–7.2, but nitrification and denitrification have different pH optima.

  Section 2.5 states 'pH of the solution in the MFCs was monitored continuously and adjusted to 7-7.2 using 1M HCl or 1M NaOH solution' but does not specify which compartment(s) were adjusted. Nitrification produces H+ and can lower cathode pH; denitrification consumes H+ and can raise anoxic reactor pH. These shifts may explain the 10.8 percentage-point fluctuation in NH4+ removal.

  Try. Install pH probes in the cathode and anoxic bioreactor. Monitor pH continuously during steady state (days 25–43). Adjust cathode pH to 7.0–8.0 and anoxic pH to 6.5–7.5 independently and measure the effect on NH4+ and NO3− removal stability.

• Medium No characterization of the nitrifying and denitrifying biofilm communities (e.g., 16S rRNA gene sequencing, FISH, or qPCR) to explain the 10.8 percentage-point fluctuation in ammonium removal.

  Section 4.2 describes ammonium removal as 'removal of NH4+ occurred in the biocathodic section by the activity of nitrifying bacteria' but provides no evidence of nitrifier abundance, diversity, or activity. The fluctuation (88.5–99.3%) suggests the nitrifying community is unstable or competing with heterotrophs for oxygen.

  Try. Sample the cathode biofilm on days 10, 21, 35, and 43. Perform 16S rRNA gene sequencing to quantify nitrifier (AOA, AOB, NOB) relative abundance. Correlate nitrifier abundance with NH4+ removal efficiency and DO concentration.

• Medium No measurement of ammonium or nitrate concentration in the anode compartment; the paper only reports anode inlet and cathode outlet, missing the intermediate step where NH4+ diffuses through the cation membrane.

  Section 4.2 states 'The transfer of ammonium ions from the anodic section to the biocathodic section occurred by diffusion through the cation permeable membrane (CMI7000s)' but does not measure the concentration gradient or diffusion rate. This is critical for understanding whether the membrane is a bottleneck.

  Try. Install sampling ports in the anode compartment and measure NH4+ concentration daily. Calculate the diffusion flux across the cation membrane (mg-N/m²·d) and determine whether it limits overall nitrification rate or whether the cathode biofilm is the rate-limiting step.