Sludge, produced in the waste water treatment process, can be used as fertilizer in agriculture. It is rich in nutrients but also contains other substances, such as heavy metals. Metals may enter crops which means that it also enters the food chain. Some heavy metals have been proven to cause severe damage to living organisms in high doses. It is therefore important to regulate the amount of heavy metals in the soil and in the sludge used as fertilizer. In this thesis the sources of heavy metals in the waste water system in the town of Västerås was examined. The sources of the heavy metals cadmium, chromium, mercury, copper and zinc were mainly analyzed by using the Excel tool "Source Finder". In this tool collected data of emissions or calculated emissions by the use of model values were entered. Households proved to be the single largest source of all the metals in this study, with the exeption of chromium. For cadmium and chromium water leakage from the ground water into the pipe system was a large contributing source. Business did not prove to be a large source of any metal with the exception of dental units which emitted large amounts of mercury. To be able to predict change of quality, in regards of metals, in the sludge and puried water an existing model of the water treatment plant was supplemented with processes regarding separation of copper. This was done by studying other models regarding metal partitioning and separation. The final model proved unable to describe variances in the measured data, but could describe the median concentration of copper in cleansed water and sludge.