Chemical precipitation is a well-known water purification process and is used in both drinking water preparation and sewage treatment. However, it has been implemented and published relatively little scientifically about the process in stormwater contexts. In the thesis, process conditions for chemical precipitation of stormwater have been investigated, primarily with regard to the choice of precipitation chemical, dosage of precipitation chemical, pH, alkalinity and particle content (measured as suspended material and turbidity). The pollution separation was also studied with a focus on the metals that are common pollutants in stormwater, ie. cadmium, chromium, copper, nickel, lead and zinc in both loose and particulate phase. The hydrocarbon separation has also been investigated, measured as polycyclic aromatic hydrocarbons, and also aliphatic hydrocarbons.
In the studies, an urban snow-melt mix was initially used as a test sample, in which process conditions and separation of stormwater pollutants were studied. At a later stage, polluted road day water from a central place in Luleå was used as test water in the experiments. The experiments have been carried out on a laboratory scale in batch tests using a so-called flocculator.
The studies showed that the examined test water was characterized by a low alkalinity, which meant that pre-hydrolyzed precipitation chemicals were most suitable for use, as they consumed less alkalinity and in turn affected the pH at least. Over 95% particle separation was measured at low dose precipitation chemicals which resulted in charge neutralization as the major precipitation mechanism. As for the particulate pollutants, such as metals and hydrocarbons, the separation rate was high, above 90% on average. However, the separation rate for dissolved metals was somewhat lower. On precipitation of road water with metal salts, dissolved chromium, copper and lead were separated by an average of 40%, however, the dissolved phase of nickel and zinc increased. This was attributed to the pH decrease, which led to nickel and zinc being mobilized from particles. This did not happen when using chitosan, which was the precipitation chemical that had the highest degree of action for the separation of dissolved metal. In contrast, chitosan particles and particulate impurities secreted slightly less effectively than the metal salts, of which PIX-111, an iron chloride product, exhibited the highest degree of activity. PIX-111 was also the only precipitation chemical in which a separation of dissolved organic carbon could be measured. Overall, the results of the studies show that chemical precipitation as a purification process has a good ability to separate typical contaminants from stormwater.