Aquatic Pollution and Coastal Management

Unit 2 : Sewage and domestic wastes, treatment and reuse

2.5.2. Primary Treatment
This is a settlement process to remove suspended solids by passing the sewage through large settlement tanks where most of the solid materials sink to the bottom by gravity. The retention period of wastes in this tank may vary from 90-150 minutes which results in the removal of about 70% of the solids and 50% of the whole biological oxygen demand. The settled solids are referred as “sludge”. The sludge is used on farms after a further treatment called ‘sludge treatment’.