Combined sewer overflow in a historical perspective

The municipal sewage networks were originally built to divert the untreated wastewater and stormwater directly to the nearest recipient. When, at the beginning of the 1950s, wastewater treatment in Sweden began in a rational way, cut-off lines were often built parallel to the receiver to collect the wastewater. For economic and practical reasons, these cut-off lines were only dimensioned for a certain flow intensity that was allowed to pass through the treatment plant. At intersections between the collection line and the cut-off line, where the flow load occasionally became too great, overflow drains were installed, combined sewer overflows (CSO) as they are called. CSOs have therefore become the most common way to regulate flow in combined sewer systems. In case of overload in connection with rain or snowmelt, wastewater overflows from the system without prior treatment to the nearest recipient. CSO flooding is usually the least known part of the urban water balance. This was considered particularly serious in connection with the establishment of remediation plans for the municipalities' wiring networks. Good knowledge of the overflow conditions in the pipeline network means that the chapter set aside for rectification can be used optimally both with regard to the function of the pipeline system and the conditions in the recipient. The most common shortcomings in connection with overflows have poor flow control ability (unnecessary overflow), poor compoundreducing effect, poor accessibility (maintenance problems), and poor working environment conditions. The following considerations should be made for the recipient; what is the biological status in the recipient today, how large are the CSO discharge today and what amounts of pollution burden does it give to the recipient, what will the recipient be used for (bathing, fishing, bird ponds, water pollution, etc.), which quality requirements must then be applied, is any overflow situation at all acceptable today in relation to recipient use and established requirements. Many municipalities in Sweden have today removed the CSOs and recipient water quality has in most cases locally been improved. Note. Part of this presentation material and text is taken from the report “Bräddavlopp” by Hogland, Berndtsson and Larsson, Byggforskningsrådet, 1986. ISBN 91-540-4580-0 (In Swedish).