Question 2 of 5: pH and Disinfection Efficiency; Indicator Organisms

Question 2: pH and Disinfection Efficiency; Indicator Organisms (25 marks)

Question text not reproduced: the examination questions are © Engineers and Geoscientists BC. Open the official past paper (linked at the top of this page) to read the question, then follow the worked solution below.

Part (a) — pH and disinfection efficiency (10 marks).

Given. Chlorine gas or hypochlorite applied to a water of variable pH; pK_a of hypochlorous acid = 7.54 at 25 °C (7.58 at 20 °C); hydrolysis constant of Cl₂ in water K_h = 4.5×10⁻⁴ at 25 °C. Find. Why the same applied chlorine dose is far more effective at pH 6 than at pH 8.5, expressed through the governing equilibria and quantified as a speciation and a CT penalty.

Chlorine added to water hydrolyses essentially completely at the pH of any treated supply:

so molecular chlorine is negligible above about pH 4 and the free residual is the pair HOCl / OCl⁻. It is the second equilibrium that governs efficiency:

Hypochlorous acid is a small, electrically neutral molecule and diffuses through the lipid cell wall; the hypochlorite ion carries a negative charge and is repelled by the likewise-negative cell surface, so it must act at the wall rather than inside the cell. The measured germicidal potency of HOCl is 80–100 times that of OCl⁻. The fraction present as the active species follows the Henderson–Hasselbalch form,

which, evaluated across the operating range of a Canadian distribution system, produces the collapse that the question is asking about:

Raising the pH from 6.0 to 8.5 therefore removes about 90 % of the active disinfectant while the analyst still reports the same free residual in mg/L as Cl₂. Disinfection tables respond in the same direction: the CT required for 2-log Giardia inactivation with free chlorine at 10 °C rises from roughly 50 to 110 mg·min/L over that pH range, a factor of 2.2. The factor is smaller than the speciation ratio because a residual measurement re-equilibrates as HOCl is consumed, so OCl⁻ acts as a reservoir.

Two consequences follow directly, and both are worth stating because they are what a plant actually does. First, where lime softening or corrosion control has raised the finished-water pH above 8.5, the CT credit must be earned by more contact time or a higher residual — or by moving the chlorination point ahead of the pH adjustment. Second, chloramination inverts the sensitivity: because

proceeds fastest near pH 8.3 and dichloramine (odorous, and unstable) dominates below pH 7, a chloraminating plant deliberately holds the pH high. The pH that is worst for free-chlorine disinfection is the pH that is best for a stable combined residual, which is precisely why the two strategies are not interchangeable.

Part (b) — Significance and basis of selection of indicator organisms (15 marks).

The significance of indicator organisms is that direct pathogen monitoring is not a workable basis for protecting a water supply. Pathogens are episodic, appear at very low densities in a large volume, are diverse (bacterial, viral, protozoan), and are slow, costly and hazardous to culture. Waiting for a positive Salmonella or Cryptosporidium result means waiting until an outbreak is already underway. An indicator organism reverses the logic: instead of asking "is this specific pathogen present?", it asks "has this water been in contact with the faecal material that pathogens travel in?" That question can be answered every day, on every sample, by a simple culture, and it is the question that operational control actually needs.

The classical basis of selection is a list of criteria that an ideal indicator should satisfy, and the reasoning behind each is what earns the marks:

• Present whenever pathogens are present, and absent otherwise. The indicator must be a member of the normal intestinal flora of warm-blooded animals, so that its presence is diagnostic of faecal contamination rather than of soil or vegetation.
• Present in far greater numbers than the pathogens. Human faeces contain of the order of 10⁹ coliforms per gram against perhaps 10²–10⁶ pathogens, so dilution that reduces the pathogen below detection still leaves a countable indicator. This is the property that gives the test its safety margin.
• At least as persistent as the pathogens, and no more so. An indicator that dies faster than the pathogen gives false reassurance; one that survives indefinitely, or regrows in the distribution system, gives false alarms. This is the criterion on which total coliforms fail.
• Similar resistance to disinfection and to treatment. The indicator must be removed or inactivated at a rate that reflects the pathogen's, or its absence certifies nothing about the barrier.
• Unable to multiply outside the gut. Otherwise the count reports growth in a biofilm rather than contamination at the source.
• Detectable by a simple, rapid, cheap and reproducible method, applicable to all water types by any competent laboratory — membrane filtration or a defined-substrate (Colilert-type) test in 18–24 h.
• Harmless to the analyst, so that routine monitoring carries no occupational risk.

Applied to the candidates, these criteria explain the historical progression. Total coliforms are defined by a biochemical property (lactose fermentation with gas at 35 °C, or β-galactosidase activity) rather than by origin, and the group includes environmental genera such as Enterobacter and Klebsiella that grow on wood, soil and in distribution-system biofilm. They are therefore not faecally specific, and Health Canada now uses them as an indicator of treatment and distribution-system integrity rather than of faecal contamination. Thermotolerant (faecal) coliforms, cultured at 44.5 °C, are a much more specific subset, and Escherichia coli — identified by β-glucuronidase activity — is the only member of the group that is essentially exclusively faecal. That is why the Guidelines for Canadian Drinking Water Quality set the MAC for E. coli in drinking water at none detectable per 100 mL, with total coliforms used as a supplementary operational signal.

No single indicator is sufficient, and the marks are in saying why. E. coli is a vegetative bacterium and is far more sensitive to chlorine than the enteric viruses and the protozoan cysts and oocysts; a chlorinated water may be E. coli-free and still carry infective Cryptosporidium, as the Walkerton and North Battleford experience in Canada made plain. Modern practice therefore supplements the coliform test: enterococci for recreational and marine waters, where they survive salinity better and correlate better with swimmer illness; Clostridium perfringens spores and somatic coliphages as conservative surrogates for protozoa and viruses respectively, because their resistance to disinfection is comparable; and turbidity and particle counts as continuous, real-time surrogates for filtration performance, which is the barrier that actually removes oocysts. The underlying principle in all of this is the multi-barrier approach — the indicator verifies the barriers, and never replaces them.