The Primary Settlement or sedimentation tanks are designed to reduce the velocity of the wastewater flow, allowing heavier organic solids (called raw sludge) to settle. They are the first stage of treatment after the removal of rags and grit in the inlet works. Scrapers present in the tank move continuously along the floor of the tank to deposit the raw sludge in hoppers for removal. The scum which floats to the surface is directed by water jets or scum boards to the sludge sump. The raw, settled sludge is removed by pump or gravity feed to a sludge treatment process, either on site or via tanker to a larger processing centre. Approximately 60% of suspended solids and 35% of BOD removal efficiency can be achieved at this stage.

The supernatant will still contain relatively high levels of suspend matter which will in most cases be passed on to some form of secondary treatment process. On sites where there is no secondary treatment discharge will be straight to the local watercourse. Monitoring of the suspended solids or turbidity level in the overflowing water is an important element of process control, both as a pollution prevention tool or as a dosing control parameter.

Monitoring the sludge blanket interface level allows automatic control of the de-sludging process ensuring that pumps are only used when required. This has the added benefit of producing better quality sludge for processing in digesters, thickeners and incinerators.