Summer oxygen sag in a slow river
A town of 1.2 lakh people discharges into a slow-flowing river. The sewage treatment plant is rated 8 MLD but receives about 11.5 MLD; the treated effluent carries BOD 42 mg/L against a consent limit of 20 mg/L, and excess flow bypasses the plant in the monsoon. Upstream dissolved oxygen is 7.2 mg/L. In May, 4 km downstream, DO falls to 3.1 mg/L and fish kills are reported; river flow then is about 2.5 cubic metres per second. Nine kilometres upstream, a sugar mill discharges only during the November-February crushing season, at about 2 MLD with BOD near 900 mg/L, yet river DO in February is 5.8 mg/L. The state pollution board has funds to act on one source this year and proposes to close the sugar mill first, because its effluent is 'ten times stronger'.
1. Calculate the daily BOD load from each source and explain why the larger load is not automatically the larger problem.
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Domestic: 11.5 MLD = 11,500 cubic metres a day; 11,500 x 42 g per cubic metre = 483 kg BOD per day.
Sugar mill: 2,000 x 900 = 1,800 kg BOD per day, about 3.7 times the domestic load. The board's 'ten times stronger' refers only to concentration and ignores both load and dilution.
Impact depends on load divided by the river's capacity to dilute and reaerate. At the May flow of 2.5 cubic metres per second (about 216,000 cubic metres a day), the domestic load alone adds roughly 483/216,000 x 1000, i.e. about 2.2 mg/L of BOD to the fully mixed river. Combined with high May temperature (lower saturation DO, faster BOD exertion) and sediment oxygen demand, that is consistent with the observed fall from 7.2 to 3.1 mg/L.
In February the river carries far more water, so a larger load is diluted and 5.8 mg/L is not contradictory. The binding constraint is the low-flow, high-temperature season - and the domestic discharge flows then, while the mill does not.
2. Suppose the sugar mill is shut for the season. Predict what changes downstream in May and justify the prediction.
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Very little should change, because in May the mill is not crushing and the DO is already 3.1 mg/L. The domestic discharge continues at 483 kg BOD per day into the same 2.5 cubic metres per second, from a plant that is both overloaded (11.5 against 8 MLD) and performing badly (42 against 20 mg/L).
Two lag effects will keep DO depressed. Solids and sludge deposited on the bed during months of overload continue to exert sediment oxygen demand after the water column improves. And a slow river in May has little reaeration capacity, so recovery lags the load reduction by weeks.
Prediction: DO at 4 km may rise only marginally, and fish kills can continue during hot, still spells. Operationally this is diagnostic - if closing the mill changes nothing, the money belongs at the treatment plant; if DO recovers sharply, the mill or a leaking lagoon was contributing more than the seasonal argument suggests, and enforcement should move back to it.
3. Recommend the sequence of actions for the year, and state one condition under which closing the sugar mill first would be justified.
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Sequence: (1) eliminate the bypass - separate storm drains so monsoon flows cannot short-circuit treatment, and enforce the separation; (2) restore the existing plant to its rated 8 MLD, because 42 mg/L indicates operational failure and not only overload, then expand towards 14 MLD with tertiary treatment; (3) provide decentralised treatment or safe septage management for unsewered parts of town; (4) only then enforce the mill's consent with real-time flow and BOD monitoring, and require it to hold effluent in lagoons at the start of crushing.
Closing the mill first would be justified if its discharge entered a reach with effectively no dilution - a tributary or a stagnant pool - or within a short distance upstream of a drinking-water intake, or if the mill extended crushing into March and April when flows fall. In any of those cases the mill becomes the critical-period source and moves to the front of the queue regardless of the seasonal average.