Water Quality Management: DBU Classes, Consents and Effluent Limits
Water Quality Management tests the Water Act 1974, designated best use (DBU), monitoring and effluent standards. The exam favours applying criteria to a river reach or interpreting a laboratory result.
Legal and planning frame
The Water (Prevention and Control of Pollution) Act, 1974 prohibits disposal of polluting matter into streams (Section 24) and requires consent from the State Board (Sections 25–26). CPCB coordinates, sets criteria and runs the National Water Quality Monitoring Programme (NWMP). Water bodies are classified by DBU: A (drinking source after disinfection), B (outdoor bathing), C (drinking source after conventional treatment), D (wildlife/fisheries) and E (irrigation, industrial cooling, controlled disposal).
Criteria
Class A: DO ≥6 mg/L, BOD ≤2 mg/L, total coliform ≤50 MPN/100 mL. Class B: DO ≥5, BOD ≤3, total coliform ≤500. Class C: DO ≥4, BOD ≤5, total coliform ≤5000. Drinking water specification IS 10500:2012 gives pH 6.5–8.5 and TDS 500 mg/L acceptable, 2000 mg/L permissible. General effluent standards for inland surface water under EP Rules Schedule VI include pH 5.5–9.0, BOD 30 mg/L, COD 250 mg/L and TSS 100 mg/L.
Worked example
Upstream flow 2.0 m³/s at BOD 2 mg/L meets effluent 0.5 m³/s at BOD 30 mg/L. Downstream BOD by mass balance = (2.0×2 + 0.5×30)/2.5 = 15.0/2.5 = 7.6 mg/L. Against Class B (≤3 mg/L) this fails; against Class C (≤5 mg/L) it also fails. The calculation assumes complete mixing and no decay, so it is a screening check, not a full river model.
Traps
BOD is not COD; total coliform is not faecal coliform; MPN/100 mL and CFU/100 mL are different units. Do not compare an effluent limit with a DBU criterion. BOD is 5-day, 20°C, with seeding. A dilution calculation must use flow-weighted mass, not simple concentration averaging.