Aerobic and anaerobic decomposition are two essential biological processes used in sewage wastewater treatment, differing in their oxygen requirements and end products.

Aerobic decomposition occurs in the presence of oxygen and is part of secondary treatment. It includes the activated sludge process, where a large volume of compressed air is passed through sewage to maintain aerobic conditions. Aerobic microbes break down organic carbon, nitrogen, and phosphorus into minerals. Complex, decomposable organic matter is eventually changed into oxidised inorganic materials such as sulphates, phosphates, nitrates, CO₂, and H₂O. This process is also called mineralisation. Other aerobic methods include trickling filters and oxidation ponds.

Anaerobic decomposition occurs in the absence of oxygen. The sludge collected after primary treatment is subjected to anaerobic digestion in a separate tank. Anaerobic microbes digest organic matter by degrading it into soluble substances and gaseous products. The gases produced include methane (60–70%), CO₂ (20–30%), H₂S, and small amounts of H₂ and N₂. This gas mixture can be used as fuel or for operating power for the sewage plant. Anaerobic sludge digestion requires 30 to 40 days for complete digestion.

Both processes depend on microbiological degradation activities such as hydrolysis, oxidation, and reduction on decomposable nutrients in sewage, ensuring waste stabilisation.