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- Sludge should be remove from the primary sedimentation tank before anaerobic.
- Quantity of sludge remove depend on:
1. Influent wastewater strength
2. Clarifier efficiency
3. Sludge condition
- Remove 50-60% of suspended solid

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- Sludge scapers run through surface as they return to effluent
- Floating materials is carried to a collection point
- Separated scum is usually disposed of with screening, unwashed grit or digested sludge.

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Use in smaller plants : due to simplicity of sludge removal

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Use in larger plants : to reduce construction costs and space requirement

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1. To settle down Organic materials
between 1.0 - 2.5 m/hr
2. Floatting materials suh as oils and
grease are skimmed out

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Known as Type-2 Settling
(without the addition of chemical coagulants and floccution operations)

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1. To reduce waste loads to receiving streams
2. To reduce the velocity of the wastewater flow, allowing heavier organic solids (called raw sludge) to settle.
3. They are the first stage of treatment after the removal of rags and grit in the inlet works.

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A unit operation designed to remove suspended organic solids