Renovation of the Stockholm sewerage system

Within a few years Stockholm will let all its treated wastewater into the Saltsjoen. The Saltsjoen is the inner part of the Stockholm Archipelago, which is one of the finest recreation areas in Sweden and where many Stockholmians spend a great deal of their leisure time. That is one reason why it is so important that all wastewater is treated before it reaches the recipient. A sewerage system in bad condition means that the risk of not allowed/unplanned overflows is increased. In order to prevent overflows we drafted 'Plan 83' which tells us how our sewerage system shall be constructed in the future and what measures has to be made in the system. Renovations of the existing pipes is one method to get a well functioning sewer system. Today we have made about 50 different objects at a total length of 6000 m. Why do we renovate sewers and why are we attempting to develop methods for pipe renovations?