Organic pollution and aerobic/anaerobic decomposition

Organic pollution of drinking water is contamination by biodegradable organic matter – human and animal excreta, sewage, food waste, dead plants and animals, and effluent from food, dairy, tannery and distillery industries. These substances are decomposed by micro-organisms, which use up the dissolved oxygen (DO) in water. The strength of organic pollution is measured by BOD (Biochemical Oxygen Demand) and COD. Organic pollution also usually brings pathogens, so it is closely linked with water-borne disease.

Effects: fall in DO and death of fish; bad smell, colour and taste; growth of pathogens; formation of harmful by-products (e.g. trihalomethanes when organic-rich water is chlorinated); eutrophication when nutrients are released. Rivers like the Bagmati in Kathmandu show these effects.

Aerobic decomposition

Breakdown of organic matter by aerobic bacteria in the presence of dissolved oxygen.

• Carbon → CO₂
• Nitrogen → NH₃ → NO₂⁻ → NO₃⁻ (nitrification)
• Sulphur → SO₄²⁻
• Phosphorus → PO₄³⁻

General reaction:

Organic matter + O₂ → (aerobic bacteria) → CO₂ + H₂O + NO₃⁻ + SO₄²⁻ + PO₄³⁻ + new cells + energy

Features

• Fast and complete; final products are stable and harmless.
• No foul smell.
• Consumes DO – if loading is high, DO falls and conditions turn anaerobic.
• Used in engineered systems: activated sludge, trickling filter, oxidation pond, aerobic composting.
• Nitrate and phosphate released can cause algal growth (eutrophication).

Anaerobic decomposition

Breakdown of organic matter by anaerobic bacteria in the absence of oxygen (after DO is exhausted, e.g. in the bottom of polluted lakes, septic tanks, biogas digesters).

• Carbon → CH₄ (methane) + CO₂
• Nitrogen → NH₃ (ammonia), amines
• Sulphur → H₂S (rotten egg smell)
• Phosphorus → PH₃ (phosphine)

General reaction:

Organic matter → (anaerobic bacteria) → CH₄ + CO₂ + NH₃ + H₂S + new cells + small energy

Stages: hydrolysis → acidogenesis (fatty acids) → acetogenesis → methanogenesis (methane).

Features

• Slow, and products are often unstable.
• Foul smell (H₂S, mercaptans), black colour of water and sludge.
• Methane is a greenhouse gas but can be used as fuel (biogas).
• Used in septic tanks, UASB reactors, sludge digesters and biogas plants.

Comparison

 Organic waste in water
        |
   DO present? --yes--> Aerobic: CO2, NO3, SO4
        |
        no
        +-----------> Anaerobic: CH4, NH3, H2S