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 meet this responsibility. The Association was formed in 1957 and the tunnel system and the Käppala plant were taken into operation in 1969. The plant is situated in Lidingö, northeast of Stockholm. The load will in 1999 correspond to nearly 500,000 p.e.
The plant built in the sixties was running until the 31 March 1998 when a new part was taken into operation. During 1998–1999 the old part will be upgraded and the expanded and upgraded Käppala plant will run under ordinary conditions in the year 2001. The expansion was decided on the basis of improvement on four points namely:
1. Capacity improvement. The design year is 2020 and the estimated load will then correspond to about 700,000 p.e. The design load of the old part is 500,000 p.e.
2. Nitrogen removal. The Swedish Parliament specified in 1990 that at least 50% nitrogen removal should be achieved in the major treatment plants in Sweden.
3. Working conditions. Since the plant is housed in the Käppala rock special demands must be put on the working environment. The demands have increased during the years and the environment had to be improved substantially.
4. Odour. Complaints about odour among neighbors have long been a hot issue in Lidingö and the need for improvement was highly desirable. Also along the tunnel-system there were complaints about odour.
The decision to expand the plant was taken by the owners, nine municipalities in 1993 and the necessary permit from the National Licensing Board for Environment Protection was received in December 1993. The basis for the application to the Licensing Board was a principal design proposal. In this proposal, which was approved by the authorities, the overall treatment process design was determined.