Wastewater precipitation ponds (fellingsdams) are conventional stabilization ponds adapted to cold climate by the use of chemical precipitation to attain sufficient removal efficiency of impurities, primarily phosphorus. The objective with this investigation was to study the influence of an interruption of the dosage of coagulant during summer periods at two fellingsdam systems (Orrviken and Lockne) in the middle of Sweden. The investigation took place over two years characterized by unusual precipitation conditions; 2001 was intense in precipitation whereas summer 2002 represented a dry season. The results showed that there is a potential to utilize the summer biological activity in fellingsdams. At Orrviken the effluent quality measured as organic matter and phosphorus in the effluent was just slightly above the values that were reached by chemical precipitation. At Lockne the performance was lower. The organic matter reduction at Orrviken in the summers of 2001 and 2002 were 71 and 67%, respectively, compared to previous years using precipitant when the average was 78%. At Lockne, however, the values in the summers of 2001 and 2002 were 36 and 18%, respectively, compared to previous years using precipitant when the average was 55%. The phosphorous reduction at Orrviken in the summers of 2001 and 2002 were 85 and 89%, respectively, compared to previous years using precipitant when the average was 95%; at the Lockne plant, the phosphorous reduction during the summers of 2001 and 2002 were 60 and 66%, respectively, compared to the previous years' average of 86%. The nitrogen reduction varied considerably over the two summer periods. The reduction at Orrviken was 13% in 2001 and 58% in 2002; the reduction at Lockne was 13% in 2001 and 33% in 2002. Reference values of nitrogen reduction during normal operations were not available