Historically, sewage treatment in Sweden shows a development towards the construction and use of package plants. These package plants, however, present several problems associated with flow and temperature. Year-long studies of chemical precipitation using slaked lime in stabilization ponds have demonstrated an overall reduction of organic matter (as COD (chemical oxygen demand)) of about 75% to a level somewhat below 100 mg/L, and a reduction of total phosphorus of 90% to approximately 0.7 mg/L as P. These values were reached under ordinary operating plant condition. Two tracer studies using Rhodamine B dye have demonstrated the appearance of strong short-circuiting flows in ponds systems and the inserting of simple baffle walls in the pond has been recommended. Dewatering the sludge in the ponds by natural freezing has proven to be an excellent process, as the time for collecting sludge can be chosen almost arbitrarily during the year.