Characteristics of Sewage — BOD, COD & DO — Environmental Engineering Notes

Last reviewed 16 Sept 2026 · 8 min read

Composition of sewage

Domestic sewage is about 99.9% water and only about 0.1% solids — but those solids (organic matter, nutrients, pathogens) make it polluting.

Organic matter — proteins, carbohydrates, fats and oils, urea, detergents.
Inorganic matter — grit, salts, chlorides, nitrates, phosphates, sulphates, heavy metals (from industries).
Microorganisms — bacteria (many billions per litre, including pathogens), viruses, protozoa, helminth eggs.
Gases — dissolved oxygen (in fresh sewage), H₂S, CO₂, CH₄, NH₃ (in stale sewage).

Fresh sewage contains some dissolved oxygen and is aerobic; stale sewage has used up its oxygen; septic sewage is anaerobic and putrefying.

Physical characteristics

Property	Remarks
Colour	Fresh sewage — grey (soapy, yellowish-grey); stale/septic sewage — dark grey to black
Odour	Fresh — musty, not offensive; septic — rotten egg smell of H₂S and other sulphur compounds
Temperature	Usually a little higher than the water supply; higher temperature speeds bacterial activity and reduces DO
Turbidity	Due to suspended and colloidal solids

Solids

DefinitionClasses of solids

Total solids (TS) — residue on evaporation at 103–105 °C.
Suspended solids (SS) — retained on a filter; dissolved solids — pass through the filter.
Settleable solids — settle in an Imhoff cone in 1 hour (mL/L) — indicate sludge production in primary tanks.
Volatile solids — burnt off at 550 °C (approximately organic matter); fixed solids — the inorganic residue.

$T S = S S + T D S$ ; each fraction can be divided into volatile and fixed parts.

Chemical characteristics

pH — fresh domestic sewage is slightly alkaline (around 7–8); septic sewage may turn acidic; biological treatment works best near neutral pH.
Chlorides — from urine, kitchens and industries; not removed by conventional treatment; a sudden rise indicates industrial discharge or sea-water infiltration.
Nitrogen — essential nutrient for bacteria; forms show the stage of decomposition:

Form	Indicates
Organic nitrogen and ammonia (free NH₃)	Fresh sewage — recent pollution
Nitrites	Partial oxidation — decomposition in progress
Nitrates	Well-oxidised, stabilised sewage/effluent

Total Kjeldahl nitrogen (TKN) = organic nitrogen + ammonia nitrogen.

Phosphorus — from human wastes and detergents; with nitrogen, causes eutrophication of receiving waters.
Fats, oils and grease — interfere with biological treatment, clog sewers and filters, form scum.
Sulphides and sulphates — sulphates reduced to H₂S under anaerobic conditions (odour, corrosion).
Toxic substances — heavy metals, phenols, cyanides from industries inhibit biological treatment.

Dissolved oxygen (DO)

Saturation DO in water decreases with temperature (about 9.1 mg/L at 20 °C) and with salinity.
DO in fresh sewage is low; its presence indicates aerobic conditions.
Measured by the Winkler (iodometric) method or DO meters.
Aquatic life generally needs at least about 4–5 mg/L DO.

Biochemical oxygen demand (BOD)

BOD is the amount of oxygen required by microorganisms to biologically oxidise (stabilise) the decomposable organic matter in a sample under aerobic conditions at a specified temperature and time.

Standard: 5-day BOD at 20 °C (BOD₅). In India, BIS (IS 3025) prescribes a 3-day BOD at 27 °C, which gives comparable values under Indian temperatures.
BOD is the most widely used measure of the strength of organic pollution and of treatment efficiency.

BOD test (dilution method)

Sewage is diluted with aerated dilution water (containing nutrients, buffer and seed bacteria if needed) in 300 mL BOD bottles; DO is measured initially and after incubation in the dark.

FormulaBOD by dilution

B O D = \frac{D _{1} - D _{2}}{P}

$D_{1}$ = initial DO of diluted sample; $D_{2}$ = final DO after incubation; $P$ = decimal fraction of sample in the mixture (volume of sample ÷ volume of bottle).

Seeded samples: $B O D = \frac{( D _{1} - D _{2} ) - ( B _{1} - B _{2} ) f}{P}$ , where $B_{1}$ , $B_{2}$ are seed control DO values and $f$ = ratio of seed in sample to seed in control.

For valid results, the DO depletion should be at least about 2 mg/L and the residual DO at least about 1 mg/L.

BOD kinetics

FormulaFirst-order BOD reaction

L_{t} = L_{0} e^{- k t} \Rightarrow B O D_{t} = L_{0} (1 - e^{- k t}) = L_{0} (1 - 1 0^{- K_{D} t})

$L_{0}$ = ultimate (first-stage) BOD; $L_{t}$ = BOD remaining after $t$ days; $k$ = rate constant (base $e$ , per day); $K_{D} = k /2.303$ (base 10).

Typical domestic sewage: $K_{D}$ ≈ 0.1 per day at 20 °C ( $k$ ≈ 0.23 per day) → $B O D_{5} \approx 0.68 L_{0}$ .

Temperature correction:

k_{T} = k_{20} θ^{T - 20}, θ \approx 1.047

Carbonaceous and nitrogenous BOD

First stage (carbonaceous) BOD — oxidation of carbon compounds; begins immediately.
Second stage (nitrogenous) BOD — nitrification of ammonia to nitrites and nitrates by nitrifying bacteria (Nitrosomonas, Nitrobacter); usually becomes significant after about 8–10 days in untreated sewage (earlier in treated effluents). This is one reason the standard test is limited to 5 days (or 3 days at 27 °C).

Limitations of BOD: long test period; needs acclimatised bacteria; toxic substances inhibit results; measures only biodegradable organics; nitrification interference.

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Characteristics of Sewage — BOD, COD & DO