Thirty MLD Plant Receiving Thirty-Eight: Dilution, Load and Compensation on a Class B River
A river classified for outdoor bathing (Class B) leaves a hill town and runs 22 km through a city before reaching a festival bathing ghat. A 30 MLD sewage treatment plant serves the city but receives about 38 MLD; the excess partly spills into a storm drain that joins the river 600 m below the plant outfall. In a pre-monsoon fortnight the State Board laboratory records: plant outlet BOD 52 mg/L, SS 78 mg/L, faecal coliform 2 × 10⁵ MPN/100 mL; river 500 m upstream of the outfall BOD 2.2, DO 6.4 mg/L, FC 180; river 1 km downstream BOD 9.8, DO 3.9 mg/L, FC 4.6 × 10⁴. Upstream low-flow discharge is about 120 MLD. The municipal corporation proposes to 'increase river flow by releasing dam water' and asks for three months before doing anything else. The plant's consent requires BOD 30 mg/L or better and carries a condition that the design capacity shall not be exceeded.
1. Quantify the pollution load and judge compliance of the downstream stretch against its designated best use. Show the load arithmetic.
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Load arithmetic. For a concentration in mg/L and a flow in MLD, load in kg/day = flow (MLD) × concentration (mg/L), because 1 MLD × 1 mg/L = 1 kg/day.
- BOD load discharged ≈ 38 MLD × 52 mg/L = 1,976 kg/day, against a permitted 38 × 30 = 1,140 kg/day at the consent limit - an exceedance factor of about 1.7 on BOD and about 1.3 on hydraulic load (38/30).
- The plant is hydraulically overloaded by roughly 27%, which explains the solids washout (SS 78) and the poor disinfection (FC 2 × 10⁵): chlorine contact time and settling are both short at that flow.
Designated best use test (Class B: DO ≥ 5 mg/L, BOD ≤ 3 mg/L, faecal coliform ≤ 500 MPN/100 mL). Downstream values are DO 3.9, BOD 9.8 and FC 4.6 × 10⁴ - all three criteria fail. Upstream, the same stretch is DO 6.4 and BOD 2.2 (coliform 180), i.e. it meets the class. The conclusion is that the 600 m reach below the outfall is a failing stretch and the bathing ghat is unsafe for its designated use; the cause is identifiable and local.
Point for the answer: the DO collapse from 6.4 to 3.9 over 1 km is a stronger indicator than BOD alone - it shows an active oxygen demand plus ammonia load, so the answer should not stop at 'BOD is high'.
2. The corporation argues that dam release will restore the river. Test that proposal numerically and state what it implies for the remedial plan.
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Mixing check for BOD. For the reach to meet BOD 3 mg/L: (2.2 × Qr + 52 × 38) / (Qr + 38) ≤ 3 => 52 × 38 − 3 × 38 ≤ (3 − 2.2) × Qr => 49 × 38 ≤ 0.8 Qr => Qr ≥ 2,328 MLD, i.e. about 61 times the effluent flow and roughly 19 times the present 120 MLD low flow.
Coliform is worse. (180 Qr + 2 × 10⁵ × 38)/(Qr + 38) ≤ 500 => about 23,700 MLD of dilution water. No reservoir release can supply this.
Reasoning and implication. Dilution is not treatment: it converts a point-source failure into a river-wide failure while the pathogen load stays the same, and it fails at exactly the time it is needed (low flow). Even if 2,300 MLD were available, the reach would sit at the compliance limit with zero margin. The correct hierarchy is:
- Sewerage and interception of the storm-drain spill so that the plant receives design flow;
- Capacity augmentation to at least 45 MLD with flow measurement and a weir;
- Process rehabilitation plus disinfection (chlorination or UV) with contact time, to meet FC and to protect the ghat;
- Real-time online monitoring on the outlet, because the failure is operational, not design-only.
Use of dam release: only as a short-term emergency flush during the festival, and only in parallel with the above - never as the plan.
3. Applying the polluter-pays principle, how would you structure environmental compensation and the enforcement sequence?
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Step 1 - establish the breach on paper. Two consent conditions have failed: exceeding BOD 30 mg/L and exceeding design capacity, plus the unauthorised storm-drain discharge (a discharge without consent). Each is separately actionable under the Water Act 1974 (Sections 25/26) and Section 5 of the Environment (Protection) Act 1986.
Step 2 - quantify compensation on the CPCB environmental compensation approach. Compensation is built from three factors, all of which are documented here:
- Scale: 38 MLD of untreated or partly treated sewage, i.e. a known volume;
- Severity: exceedance factors of roughly 1.7 on BOD, and 400 times the coliform criterion in the river, so the severity band is high, not marginal;
- Duration: the number of days of non-compliance, evidenced by plant logs and the fortnightly samples, not just the inspection day. EC is then computed at the Board's notified rate per MLD per day for the severity band and multiplied over the period; it must be defended with the calculation sheet.
Step 3 - distinguish the three money flows. EC is a deterrent, not a cleanup budget; the cost of restoring the ghat and desilting the drain is recovered separately as remediation cost; and a bank guarantee secures the time-bound upgrade. Merging them lets the polluter argue double recovery.
Step 4 - sequence. Show-cause and personal hearing, consent direction with a dated milestone plan, EC demand recoverable as arrears of land revenue, bank guarantee for the upgrade, and prosecution only if the milestones lapse. Section 33A gives the power to restrain the continuing discharge; a closure direction is disproportionate while the plant remains functional.