Application of Vermifiltration for Domestic Sewage Treatment

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

• Within the paper Nitrate is expected to decrease via nitrification–denitrification but instead increased dramatically in both filters.

  One side

  'Generally, the nitrates in raw sewage are supposed to decrease during the treatment as they are supposed to be converted from nitrate and further decomposed to nitrogen gas and water through the nitrification and denitrification processes.'

  “Generally, the nitrates in raw sewage are supposed to decrease during the treatment as they are supposed to be converted from nitrate and further decomposed to nitrogen gas and water through the nitrification and denitrification processes.”

  Other side

  'An increase was observed in nitrates in the VF of 96–98% and CF 62–82%.' Raw sewage NO₃ = 9 mg/L; VF effluent = 376 mg/L; CF effluent = 256 mg/L.

  “An increase was observed in nitrates in the VF of 96–98% and CF 62–82%.”

  Authors’ reading. Authors attribute the increase to 'removal of ammonia through adsorption which leads to the [text cuts off]' but do not complete the explanation or reconcile why denitrification did not occur. The system appears to be aerobic throughout, favoring nitrification over denitrification, but this is not explicitly stated or measured (no dissolved oxygen data provided).

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

• High Dissolved oxygen profile through the media depth is not measured; authors claim 'anoxic conditions' drive phosphate removal but provide no evidence.

  Section 4.6.3 attributes phosphate removal to 'anoxic conditions' but Table 3 and methodology do not include DO measurements. Without DO data, it is impossible to determine whether the system is aerobic (explaining nitrate accumulation) or has anoxic zones (which would be needed for denitrification).

  Try. Measure dissolved oxygen at 5 cm depth intervals through the media column (top gravel, sand, soil, bottom) at steady state; correlate DO profile with nitrate and phosphate removal; test whether increasing HRT or reducing aeration creates anoxic zones that suppress nitrification and promote denitrification.

• High Ammonia (NH₄⁺ or NH₃-N) concentration in influent and effluent is not reported; authors mention 'removal of ammonia through adsorption' as the cause of nitrate increase but provide no data.

  Section 4.6.4 states 'removal of ammonia through adsorption which leads to the [incomplete]' but no ammonia measurements are given. Without ammonia data, the nitrification pathway cannot be confirmed, and the mechanism of nitrate accumulation remains speculative.

  Try. Measure NH₄⁺-N, NO₂⁻-N, and NO₃⁻-N (specify basis: -N or total) in influent and effluent at steady state; calculate nitrogen mass balance to determine whether ammonia is being oxidized to nitrate or lost to volatilization/adsorption; test whether increasing HRT allows denitrification to consume the accumulated nitrate.

• Medium Nitrite (NO₂⁻) accumulation is not monitored; incomplete nitrification or inhibition of nitrite oxidation could explain high nitrate levels or indicate operational instability.

  Only nitrate (NO₃⁻) is measured; nitrite is not reported. If the system is accumulating nitrite, it suggests incomplete nitrification or inhibition of Nitrobacter, which could lead to N₂O production or nitrite toxicity.

  Try. Measure NO₂⁻-N in influent and effluent; if NO₂⁻ > 0.1 mg/L, investigate whether pH, DO, or ammonia concentration is limiting nitrite oxidation; test whether increasing HRT or adjusting pH suppresses nitrite accumulation.

• High N₂O production is not measured; denitrification under short HRT and high loading may produce N₂O rather than N₂, representing both a loss of treatment efficiency and a greenhouse gas emission.

  The paper does not measure N₂O or any gaseous nitrogen products. If denitrification is occurring (which is not confirmed), it may be incomplete, producing N₂O instead of N₂. This is critical for environmental impact and nitrogen recovery.

  Try. Measure N₂O in the gas phase above the biofilter using gas chromatography or FTIR; measure N₂ using isotope tracing (¹⁵N) or headspace analysis; determine the N₂O/(N₂+N₂O) ratio to assess denitrification completeness; test whether increasing HRT or adding a separate anoxic chamber reduces N₂O and increases N₂.

• Medium Earthworm survival, activity, and biomass are not quantified; it is unclear whether worms are thriving, dormant, or dying under the high loading and short HRT.

  The paper does not report worm counts, biomass, or viability after the experiment. Without this data, it is impossible to determine whether the 20 g/L density is appropriate or whether worms are contributing to treatment.

  Try. Count and weigh earthworms at the start and end of the experiment; measure worm respiration or activity (e.g., burrowing depth, casting production); correlate worm biomass with treatment efficiency; test whether increasing HRT or reducing loading improves worm survival and treatment performance.

• Medium C/N ratio of the influent is not reported; this is critical for predicting whether denitrification will be carbon-limited or whether heterotrophic nitrification will dominate.

  Domestic sewage typically has a C/N ratio of 5–10, but this paper does not specify the influent BOD:NO₃ ratio or discuss carbon limitation. Without this, it is unclear whether the system has sufficient organic carbon to drive denitrification.

  Try. Calculate influent C/N ratio from BOD and nitrogen species data; if C/N < 5, test whether adding external carbon (e.g., acetate) or increasing HRT improves denitrification; measure dissolved organic carbon (DOC) in influent and effluent to assess carbon availability.