Water UK

The treated water (effluent) is returned to rivers and seas while the solid matter (sewage sludge or ‘biosolids’) is disposed of in a number of different ways, including incineration, landfill, and application to certain types of agricultural land.

Sewage treatment is strictly controlled, to comply with UK codes of practice and European directives. In accordance with European standards, the dumping of sewage sludge at sea ceased on 31 December 1998.

Sewage treatment processes

Preliminary treatment
Larger solids are removed or disintegrated from the wastewater by the use of screens. Grit from road drainage is also removed. In some works, oil and grease are also removed at this stage.

Primary treatment
In this first major stage of treatment, the sewage flows into settling tanks where the heavier solid material settles to the bottom of the tank. This reduces the biochemical oxygen demand (BOD) by 20 per cent, while total suspended solids are reduced to at least 50 per cent before the sewage can be discharged.

Secondary treatment
There are two main types of secondary treatment. Both use the natural action of bacteria to break down any organic matter left after primary treatment.

Biological filtration - the sewage is spread over a granular filter bed, where bacteria, fungi and other organisms feed on the organic matter to leave a virtually clear liquid that then flows from the base of the filter bed.

Activated sludge – micro-organisms break down the organic matter in a concrete tank through a natural process which is accelerated by rotating paddles and jets of air.

Tertiary treatment
Before discharge back into water, suspended matter and nutrients such as nitrogen and phosphorous are removed. UV treatment (‘polishing’ or advanced treatment) may also be used for disinfection.

Sludge (biosolids) treatment, recycling and disposal
The byproduct of sewage treatment (sludge) may be treated in a variety of ways and is viewed as a valuable agricultural resource.

Treatment of sludge

Thickening or dewatering – the water content of the wet sludge is reduced by gravitational or mechanical processes (centrifuges, presses and vacuum filtration).

Digestion – bacteria and micro-organisms are used to break down the organic matter in the sludge to reduce the volume of solids.

Composting – aerobic fermentation.

Thermal drying – using high temperatures to reduce the sludge to a granular form (80 per cent solids).

Incineration or combustion – high temperature oxidation to produce an inert ash; the heat is usually recovered for other uses.