Saulekilen is a mechanical purification plant with three successive steps. At first the sewage is roughly treated in a metal mesh. Thereafter the water enters the basin of step two, where sand and fatty materials are removed. Finally the wastewater enters very fine rotating filters, which take out particles larger than 0,2 mm. The water is then pumped out at a depth of 70 metres in the sea at Aeroya.

Saulekilen has a capacity of 275 l/s, or 1 000 cubic metres per hour. Every week 130 tons of waste is transported from the plant for further treatment.

The average effect of the purification process till now have been:

• About 60-70% reduction in suspended material.
• About 30-40% reduction in easily degradable organic material.
• About 25-30% reduction of phosphates.

As a consequence of international agreements and national goals, governmental authorities have imposed Arendal to undertake a better purification of the wastewater. The municipality is now working on a project to expand the process in Saulekilen by adding a chemical step of treatment. In addition, the waste removed during the purification process will be transported to a new sludge treatment plant. Bad-smelling air from the ventilation system of the plant will be treated separately in a special system. At the moment, the municipality is not imposed to build a special purification step for nitrates.