Sewage and its strength
Sewage is used water from homes, shops and industry, carrying organic matter, solids, nutrients and pathogens. Released untreated, it uses up the oxygen in rivers and spreads disease.
The strength of sewage is measured mainly by how much oxygen it would consume. Biochemical oxygen demand, BOD, is the oxygen used by microbes breaking down the organic matter over five days at 20 °C. Chemical oxygen demand, COD, uses a strong chemical oxidant and is always higher. Suspended solids, nitrogen, phosphorus and faecal coliforms complete the picture.
A treatment plant is sized on both flow and load. Flow follows water supply, since most supplied water returns as sewage, with allowances for infiltration. Load is flow times concentration, expressed in kilograms of BOD per day.
Treatment removes these pollutants in stages: screening and grit removal, primary settling, biological treatment such as activated sludge, and often tertiary treatment and disinfection, so the effluent meets discharge standards or can be reused.
BOD load (kg/day) = flow (MLD) × BOD (mg/L)BOD₅ from dilution test = (D₁ − D₂) / P, with P the decimal fraction of sample
Problem. A town discharges 8 MLD of sewage with BOD 250 mg/L. Find the daily BOD load.
- 8 × 250 = 2 000 kg/day (since 1 MLD × 1 mg/L = 1 kg/day).
Answer. 2 000 kg BOD per day
Quick quiz
1What does BOD measure?
Oxygen used by microbes decomposing organic matter (5 days, 20 °C).
2Is COD higher or lower than BOD?
Higher.
31 MLD at 1 mg/L equals?
1 kg per day.
- CPHEEO Manual on Sewerage and Sewage Treatment Systems (2013) (India): Central government manual for sewer networks and sewage treatment plants.
Cited by number and clause only. We do not reproduce clause text, tables or figures (BIS Act 2016, s.10(5) and s.11). Check the current edition and amendments before use.
References and further reading
Standard texts, listed in no order of merit. We do not copy text, figures, tables or solved problems from any book; our explanations and examples are original.
- Metcalf and Eddy, Wastewater Engineering: Treatment and Resource Recovery, McGraw Hill, 5th edition.
- S. K. Garg, Sewage Disposal and Air Pollution Engineering (Environmental Engineering Vol. II), Khanna Publishers, latest edition.
Free and official sources
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