Sustainable Management of Chlorine Consumption in an Outdoor Swimming Pool: A Case Study of the Silpakorn University Swimming Pool

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

• High No measurement of dissolved oxygen (DO) or redox potential (ORP) during the chlorine reduction phase; these would clarify whether ammonia oxidation and nitrite conversion are oxygen-limited or chlorine-limited.

  The paper shows ammonia declined and nitrite disappeared post-intervention, but does not measure DO or ORP to distinguish whether lower chlorine reduced oxidant availability or whether microbial nitrification/denitrification was the driver. Section 3.3 states 'NO₂⁻ does not tend to accumulate in water but is rather converted into nitrate rapidly' but provides no mechanistic evidence.

  Try. Measure DO (mg/L), ORP (mV), and chlorine residual simultaneously on days 1, 14, 21, 28, 35; correlate ammonia and nitrite trends with DO/ORP to identify the rate-limiting oxidant.

• High No characterization of microbial community (nitrifiers, denitrifiers, heterotrophs) before and after chlorine reduction; the decline in ammonia and nitrite may be due to microbial activity rather than chemical oxidation.

  The paper attributes nitrite conversion to rapid chemical oxidation ('typical because NO₂⁻ does not tend to accumulate') but does not rule out biological nitrification or denitrification. Chlorine reduction may have allowed nitrifying bacteria to establish, explaining the ammonia decline and nitrite removal.

  Try. 16S rRNA gene sequencing or qPCR for ammonia-oxidizing bacteria (AOB), nitrite-oxidizing bacteria (NOB), and denitrifiers on water samples from days 1 and 35; correlate abundance with nitrogen species changes.

• Medium No investigation of N₂O or N₂ production; the paper measures NO₃⁻, NO₂⁻, and NH₃ but does not close the nitrogen mass balance or detect gaseous nitrogen loss.

  If ammonia declined 79% and nitrite was removed, the nitrogen must go somewhere. The paper shows nitrate only declined 10.7%, leaving ~70% of the ammonia unaccounted for. This could be N₂O or N₂ emission, but is not measured.

  Try. Measure headspace N₂O (gas chromatography) and estimate N₂ loss via isotope pairing or acetylene inhibition assay on days 1, 14, 28, 35 to quantify denitrification and N₂O yield.

• Medium No variation of chlorine dose below 1 kg/day or above 2 kg/day to establish a dose–response curve for ammonia oxidation and nitrite formation kinetics.

  The paper tested only two chlorine doses (2 kg and 1 kg daily) and found that 1 kg was sufficient for disinfection. However, no intermediate doses (1.5 kg, 0.75 kg) were tested to map the relationship between chlorine availability and nitrogen species speciation.

  Try. Conduct a 4-week trial with chlorine doses of 0.5, 0.75, 1.0, 1.5, 2.0 kg/day; measure ammonia, nitrite, nitrate, and residual chlorine daily to establish a dose–response curve and identify the threshold for ammonia oxidation.

• Low No measurement of pH-dependent speciation of ammonia (NH₃ vs NH₄⁺) or hypochlorous acid (HOCl vs OCl⁻); the paper measures pH but does not calculate the fraction of each species.

  Section 3.1 discusses how pH affects HOCl⁻ vs OCl⁻ disinfectant efficiency, but does not calculate the speciation. Similarly, ammonia speciation depends on pH and temperature; the paper does not report whether the 0.34 mg/L is NH₃ or NH₄⁺ or a mixture.

  Try. Calculate ammonia and hypochlorous acid speciation using Henderson–Hasselbalch equations for each day; plot speciation against residual chlorine and ammonia concentration to identify whether HOCl⁻ availability or pH-dependent ammonia form drives oxidation kinetics.

• Medium No long-term (>5 weeks) monitoring of nitrogen species or microbial regrowth after chlorine reduction; the paper does not assess whether the system reaches a new steady state or whether ammonia/nitrite accumulation resumes.

  The intervention lasted 5 weeks (days 1–35). The paper states in Section 4 (Conclusion) that 'the long-term microbial risks due to the reduced amount of chlorine added should be monitored' as a future recommendation, indicating this is a known gap.

  Try. Extend monitoring to 12–24 weeks post-intervention; measure ammonia, nitrite, nitrate, and total/fecal coliform bacteria weekly to confirm that the system does not revert to higher ammonia or nitrite levels and that microbial regrowth does not occur.