The Treatment of PPCP-Containing Sewage in an Anoxic/Aerobic Reactor Coupled with a Novel Design of Solid Plain Graphite-Plates Microbial Fuel Cell

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

• Within the paper Anoxic chamber shows both denitrification and ammonia accumulation simultaneously, suggesting conflicting nitrogen transformations.

  One side

  Anoxic reactor NH₄⁺–N increased to 9.021 mg/L (from 1.767 mg/L influent) due to sludge recycle

  “A significantly increased concentration of NH₄⁺–N was found in the anoxic reactor of 9.02 ± 3.62 mg/L because of the mixing of sewage influent and 100% recycled settlement sludge.”

  Other side

  Anoxic reactor NO₃⁻–N decreased 83.28% (from 1.555 to 0.260 mg/L), indicating active denitrification

  “A removal efficiency of 83.28% for nitrate was measured with a biological reduction from 1.555 ± 0.501 mg/L to 0.260 ± 0.076 mg/L.”

  Authors’ reading. Both processes occur in the same anoxic chamber: ammonia is released from sludge hydrolysis/decay while nitrate is simultaneously reduced by denitrifiers. The authors do not explicitly reconcile this; it reflects the complexity of a mixed anoxic environment with both heterotrophic and autotrophic activity.

• Within the paper Effluent nitrate concentration (1.033 mg/L) is higher than anoxic reactor effluent (0.260 mg/L), contradicting the expectation that aerobic nitrification should only increase nitrate, not exceed it by 4-fold.

  One side

  Anoxic reactor NO₃⁻–N = 0.260 ± 0.076 mg/L (after 83.28% removal)

  “A removal efficiency of 83.28% for nitrate was measured with a biological reduction from 1.555 ± 0.501 mg/L to 0.260 ± 0.076 mg/L.”

  Other side

  Aerobic reactor (cathode) NO₃⁻–N = 0.335 ± 0.124 mg/L; final effluent NO₃⁻–N = 1.033 ± 0.670 mg/L

  “Nitrification was found to occur in aerobic reactor, where the concentration of nitrate was increased from 0.260 mg/L to 1.033 mg/L.”

  Authors’ reading. The authors state nitrification occurs in the aerobic reactor, but the 4-fold increase in nitrate (0.260 to 1.033 mg/L) is larger than expected from ammonia nitrification alone (0.087 mg/L in aerobic reactor). This may reflect measurement variability (high SD: ±0.670 mg/L) or incomplete mixing/sampling, but the paper does not address the discrepancy.

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

• High N₂O emissions are not measured. Given the high nitrification rates (ammonia drops from 9.021 to 0.087 mg/L in aerobic chamber) and the presence of denitrifiers in biofilms, N₂O production risk is unknown.

  Section 3.1 reports near-complete nitrification but provides no N₂O data. The paper states 'Nitrite might be converted to nitrate when the cathodic electrode acts as the electron donor due to denitrification in the MFC' but does not measure N₂O or N₂. N₂O is a potent greenhouse gas and a regulatory concern in nitrogen removal systems.

  Try. Measure N₂O and N₂ in gas headspace of anoxic and aerobic chambers using GC-ECD or membrane inlet mass spectrometry; vary DO (currently 4.22 ± 0.45 mg/L in aerobic chamber) and anoxic HRT (currently 2.04 h) to map N₂O production window.

• High Cathode and anode potentials are not reported. The paper measures voltage across a 1 kΩ external resistance but does not provide individual electrode potentials (vs. Ag/AgCl or SHE), making it impossible to assess whether the cathode is operating in a regime favorable for denitrification or if the anode is limiting.

  Section 2.4 and Figure 4 describe power density and coulombic efficiency but no potentiostat data. The authors note 'Direct electron transfer from microbial cells to electrodes occurs at very low efficiency' but do not measure or control electrode potentials. This is critical for understanding whether the MFC is operating as a true bioelectrochemical system or primarily as a biofilm reactor with incidental electricity generation.

  Try. Install reference electrodes (Ag/AgCl) in both chambers and measure anode and cathode potentials vs. SHE during steady state. Vary external resistance from 1 Ω to 100 kΩ and record potentials to construct a true polarization curve with electrode potentials.

• Medium C/N ratio is not varied. The influent C/N ratio is 257.16:13 (≈19.8:1 by mass), which is higher than the 'best composition' cited (100:5 = 20:1). The paper does not test whether lower C/N ratios (e.g., 10:1 or 5:1) would improve nitrogen removal or reduce residual nitrate in effluent.

  Section 2.3 states the C/N ratio is 'quite close to the best composition (C/N/P = 100:5:1)' but does not explore the effect of C/N on nitrogen removal efficiency or on the balance between nitrification and denitrification. The residual nitrate (1.033 mg/L) and incomplete denitrification (83.28%) suggest carbon may not be limiting, but this is not tested.

  Try. Operate the system at C/N ratios of 5:1, 10:1, 15:1, and 20:1 (by mass) while holding HRT constant at 8 h. Measure NO₃⁻–N, NO₂⁻–N, NH₄⁺–N, and power density to identify the optimal C/N for complete nitrogen removal and electricity generation.

• Medium Anoxic HRT (2.04 h) is not optimized. The paper does not test whether increasing anoxic residence time would improve denitrification (currently 83.28% removal) or reduce the need for sludge recycle.

  Section 2.3 states the anoxic HRT is 2.04 h (out of 8 h total), but provides no justification or sensitivity analysis. The 83.28% nitrate removal and residual 0.260 mg/L NO₃⁻–N suggest the anoxic zone may be undersized. The 100% sludge recycle creates a high ammonia spike (9.021 mg/L) that must be nitrified downstream, increasing oxygen demand.

  Try. Vary anoxic HRT from 1 h to 4 h (by adjusting flow rate or reactor volume) while holding total HRT at 8 h. Measure NO₃⁻–N, NO₂⁻–N, NH₄⁺–N, and denitrification rate to identify the minimum HRT for complete denitrification.

• Medium Dissolved oxygen in the aerobic chamber (4.22 ± 0.45 mg/L) is not controlled or varied. The paper does not test whether lower DO would reduce N₂O production (if any) or whether higher DO would improve nitrification kinetics.

  Table 2 reports DO in the aerobic chamber but does not describe aeration control strategy or test the effect of DO on nitrogen removal or electricity generation. The high nitrification rate (ammonia drops from 9.021 to 0.087 mg/L) suggests DO is not limiting, but the paper does not explore the trade-off between nitrification completeness and N₂O emissions.

  Try. Operate the aerobic chamber at DO setpoints of 1, 2, 4, and 6 mg/L using proportional aeration control. Measure nitrification rate, N₂O emissions, power density, and oxygen transfer efficiency to identify the optimal DO for complete nitrification with minimal N₂O and energy cost.

• Low Catholyte pH (7.48 ± 0.31 in aerobic chamber) is not controlled or varied. The paper does not test whether pH affects nitrification kinetics, denitrification, or electricity generation.

  Table 2 reports pH but does not describe pH control or test the effect of pH on nitrogen removal. Nitrification is known to be pH-sensitive (optimal 7.5–8.5), and the measured pH (7.48) is at the lower end of the optimal range. The anoxic chamber pH (8.07) is higher, which may favor denitrification.

  Try. Vary aerobic chamber pH from 6.5 to 8.5 using acid/base dosing and measure nitrification rate, nitrite accumulation, and power density to identify the optimal pH for the coupled system.