Upgrading and expansion of the Käppala wastewater treatment plant. Operational experiences and results.

Käppala Association has the responsibility to receive and treat wastewater from 11 municipalities situated just north of Stockholm in Sweden. Running a tunnel system, 60 km long, and a treatment plant meets this responsibility. The plant is situated in Lidingö, northeast of Stockholm. The load 2002 corresponds to about 520,000 p.e. During the 1990s the plant was upgraded and expanded to meet an increasing population and more stringent discharge limits. Nitrogen removal was introduced. The expansion was done in two steps; the first step comprised a new plant beside the old one both situated in rock. This part, which included the filtration step, was taken into operation in 1998. Then the old plant was upgraded to the same technical standard as the new part. His Majesty the King of Sweden inaugurated the whole new and upgraded plant in April 2000. A total of 1.3 billion Swedish crowns were invested corresponding to about 140 million euro. The plant's performance is good. There is no trouble keeping the discharge limits, 10 mg/l for BOD7 and total nitrogen and 0.3 mg/l for phosphorus. However the bio-P process has not been altogether successful. We haven't been able to fulfill our goal to reduce the use of iron sulphate. We will during 2003 go on in our investigations concerning running the plant with a combination of bio-P and chemical precipitation.