Microalgae- and biochar-mediated wastewater treatment: augmentation of bioelectrochemical nitrogen removal and biogas production

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

• Within the paper Biochar simultaneously improves NO3-N removal but impairs NH3-N removal, yet both are fed at a fixed 1.33 ratio.

  One side

  NO3-N removal efficiency in batch C = 99% (highest across all batches)

  “In addition, the mixture of electron donors and acceptors present in the B1 had resulted in rapid NO3-N depletion with a removal efficiency of 99%.”

  Other side

  NH3-N removal efficiency in batch C = 67% (lowest across all batches)

  “In contrast, a decrease to 67% in NH3-N removal efficiency was detected (Fig. 2d).”

  Authors’ reading. Authors attribute this to biochar's inhibition of NH4⁺ migration and suppression of nitrification/ammonification (Section 3.2.2), suggesting biochar alters the microbial community or redox environment to favor denitrification over nitrification. The fixed N ratio means NO3-N is consumed faster than NH3-N is oxidized, creating an imbalance.

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

• High No explicit measurement of cathode potential, anode potential, or applied voltage; operating window for electrode potentials is not defined.

  The paper states 'Electrode spacing and surface area, COD and DO levels, and NH3-N concentration all have a massive effect on the removal efficacy of N' (Section 3.2.4) but does not report the potentials or current density used, making it impossible to replicate or optimize the system.

  Try. Vary cathode potential from −0.5 to −1.0 V vs. Ag/AgCl and anode potential from +0.2 to +0.8 V; measure NH3-N, NO3-N, and N2O at each potential; identify the window that maximizes N2 while minimizing N2O.

• High Dissolved oxygen (DO) in the anode is reported as 0–0.9 mg L⁻¹ but not controlled; no systematic study of DO effect on N removal or N2O selectivity.

  Section 3.2.4 cites Jiang et al. showing N removal efficiency dropped from 87.42% to 75.39% when DO increased from 4.0 to 7.0 mg L⁻¹, yet this paper does not control or vary anode DO, only reports it passively.

  Try. Maintain anode DO at 0, 0.5, 1.0, 2.0, and 4.0 mg L⁻¹ using N2 sparging or air injection; measure NH3-N, NO3-N, N2O, and microbial community composition at each DO level.

• Medium Hydraulic retention time (HRT) is not varied; all batches run for 35 d with continuous NO3-N input at 0.04 mL min⁻¹ but no measurement of actual HRT or effect of HRT on N removal kinetics.

  The paper does not report reactor volume, flow rate, or calculated HRT; batch operation with continuous input makes HRT ambiguous. No comparison of short vs. long HRT.

  Try. Test HRT of 2, 5, 10, and 20 d by varying influent flow rate; measure NH3-N and NO3-N removal efficiency and N2O yield at each HRT.

• Medium Catholyte pH is not controlled or systematically varied; anode pH drops from 8.11–9.31 to 6.57–6.98 over 35 d but cathode pH is not reported.

  Section 3.2.4 shows anode pH dynamics but no cathode pH data; C. vulgaris growth and NH4⁺ assimilation are pH-sensitive, yet no pH control strategy is described.

  Try. Maintain cathode pH at 6.5, 7.0, 7.5, 8.0, and 8.5 using buffer or acid/base dosing; measure C. vulgaris growth rate, NH4⁺ accumulation, and N2O production at each pH.

• Medium C/N ratio is not explicitly controlled or varied; glucose is provided at 500 mg L⁻¹ in artificial medium but no measurement of actual COD or C/N ratio during operation.

  Section 3.2.3 states 'organic compounds must provide adequate electron donors before NO3 removal in WWTPs may be considered practical' but the paper does not test different C/N ratios or measure residual COD.

  Try. Vary initial glucose concentration from 250 to 1000 mg L⁻¹ (C/N ratio ~0.5 to 2.0); measure NH3-N, NO3-N, N2O, and residual COD over time.

• Medium N2O is quantified as a percentage of biogas but not as a mass or molar flux; no measurement of N2O accumulation in headspace or dissolved N2O in liquid.

  Table 5 reports N2O as 6–48% of biogas volume but does not convert to mg N L⁻¹ d⁻¹ or compare to N removal rates; unclear if N2O is a major N loss pathway or minor byproduct.

  Try. Measure N2O concentration in biogas by GC-MS; calculate N2O-N flux in mg N L⁻¹ d⁻¹; compare to NH3-N and NO3-N removal rates to close the N balance.