Secondary treatment

SECONDARY TREATMENT

Secondary treatment involves further treatment of the effluent, coming from the primary sedimentation tank. This is made possible through decomposition of organic matter under either aerobic or anaerobic conditions. In these biological units, organic matter is decomposed by bacteria and clear water emerges as the effluent.

The treatment tanks where organic matter is decomposed by aerobic bacteria are known as aerobic biological units. They consist of

• Filters
• Aeration tanks
• Oxidation ponds and
• Aerated lagoons

1. They are called secondary treatment units since they use the settled sludge from primary treatment units like primary sedimentation tank.
2. The treatment tanks in which the organic matter is destroyed and stabilised by anaerobic bacteria are called anaerobic biological units. Examples of such tanks are:
1. Anaerobic lagoons
2. Septic tanks
3. Imhoff tanks
3. Among the above listed treatment units, only anaerobic lagoons make us of primary settled sewage and hence are classified as secondary biological units while septic tanks and imhoff tanks cannot be classified as secondary units as they use raw sewage.
4. Effluent from secondary biological treatment contains very little BOD (5 to 10% of the original)
5. The organic sludge separated in the primary and secondary settling tanks is disposed off by stabilizing it under anaerobic process in a sludge digestion tank