Choosing a Disinfection Strategy
A village of 600 people uses a borehole. Turbidity is normally 0.3 NTU but rises to 8 NTU for 2–3 days after storms, several times a year. Dissolved organic carbon is 3.5 mg/L. Electricity is unreliable. Option A: sodium hypochlorite, low cost, leaves a residual, but forms trihalomethanes, especially with high DOC. Option B: UV, effective against protozoa and no THMs, but needs 5 µm prefiltration and reliable power, and leaves no residual. Option C: chlorination plus point-of-use boiling during outages. The council wants one option and no ongoing monitoring.
1. Evaluate the three options against the stated constraints and recommend a defensible strategy.
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No single option meets every constraint. Option A (chlorine alone) is affordable and gives a residual, but at 8 NTU particulate matter shields microbes, and 3.5 mg/L DOC is enough to produce trihalomethanes above guidelines. Option B (UV alone) avoids THMs and is effective against protozoa, but 8 NTU exceeds the usual <1 NTU requirement for UV, and unreliable power makes it fragile; without a residual, recontamination in the distribution system is possible. Option C adds boiling, which is microbiologically effective during outages, but boiling is fuel-intensive, carries burn risk, and still gives no residual.
A defensible recommendation is a multi-barrier system: filtration/coagulation to keep turbidity low, then chlorine for residual protection, with UV only if power and prefiltration can be guaranteed. During storm events, switch to a documented 'turbidity trigger' protocol: boost filtration, increase chlorine contact time, and advise boiling if turbidity exceeds a set limit.
2. The council says ongoing monitoring is unnecessary. Explain why this is unsafe and outline a minimal monitoring plan.
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Unmonitored disinfection is unsafe because the key variables change: turbidity after storms, DOC, chlorine demand, pH, temperature, and power availability. Without monitoring, the village cannot know whether chlorine residual is adequate or whether THMs are rising.
Minimal plan: daily turbidity and free chlorine residual at the outlet; monthly bacteriological testing (e.g. E. coli / thermotolerant coliforms); quarterly DOC and pH; annual THM testing, with more frequent testing after severe storms. Record power outages and boiling advisories. If free chlorine residual falls below the target, or turbidity exceeds the treatment limit, the operator must act rather than wait for the annual report.
3. What evidence would justify switching from chlorination to UV plus a low-dose residual?
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Switch if three conditions are demonstrated: (1) turbidity can be reliably kept below about 1 NTU by prefiltration, including during storms; (2) power reliability improves, or a backup supply is installed, so UV dose is not interrupted; (3) THM levels approach or exceed the guideline despite precursor control. A low-dose chlorine residual would still be needed downstream.
Conversely, if THMs remain well below the guideline and microbial risk is controlled, chlorine may remain the better choice. The decision should be based on monitoring data, not on a single annual sample.