In order to solve the problems occurring in the North Sea and the Baltic Sea, programmes for the reduction of nutrients in wastewater treatment plants were initiated in the Schleswig-Holstein region. In 1988, all wastewater treatment plants with capacities greater than 17 000 population equivalents were upgraded for phosphate reduction, to give effluent concentrations of less than 2 mg P/l. By 1995, standard values of Ntot < 10 mg/l and P < 0.5 mg/l are expected.
Designs for the expansion of several wastewater treatment plants are presented in this paper. In particular, the problems which result from seasonal peaks, high phosphorus concentrations, and the combination of trickling filter and activated sludge processes are discussed.