The Ministry of the Environment has required that wastewater treatment plants with a population equivalent of more than 10 000 have to make a report of harmful and hazardous substances in wastewater. In the area of the ELY Centre in Northern Ostrobothnia it has been required in 11 treatment plants. The project called Harmful substances in wastewater was carried out in 2013-2014.

The aim of this thesis was to study the transport of harmful and hazardous substances of wastewater to treatment plants and from there to the recipients in the Northern Ostrobothnia region. The aim of this thesis was also to complement the wastewater purification process with a method, which removes also the detrimental elements from the wastewater. It was studied what kind of contaminants end up in wastewater treatment plants and which of these substances do not exit or degrade in the process. The data was collected from the materials of the Centre for Economic Development, Transport and the Environment in Northern Ostrobothnia.

As a result of this study it was found that the concentrations of many harmful substances were below the limit of determination in wastewater, and the concentrations decreased during the cleaning process. Furthermore, it was found out that some of the sewage sludge contained harmful substances and some substances were assumed to be biodegradable. Sludge samples were not, however, analysed in the study, which weakens the final conclusions. The harmful substances in the wastewaters were mainly anthropogenic chemicals. This thesis deals also with harmful substances in slurry discharge at general level.

Good results are achieved by preliminary treatment of waste water before it goes to the sewer system. Its effects can be seen in wastewater treatment plants both in the incoming and in the outgoing wastewaters. The actual costs of the treatment can be targeted where they arise.