Wastewaters generated from electroplating and metal surface treatment processes are hazardous for health and the environment. Therefore these industrial wastewaters must be purified before discharge into the sewage network or water systems. There are suggestive concentration limits for heavy metals which allow wastewater to be found to be sufficiently harmless to drain. Chemical precipitation is the prevalent technique for heavy metal removal from wastewaters. In principle heavy metals are precipitated as scarcely soluble salts which can be separated mechanically from the wastewater. The client of this thesis, Kierto Ympäristöpalvelut OY, executes chemical precipitation in practice in their wastewater purification plant exploiting the self-named HeavyReg technique, in which heavy metals are chemically precipitated as insoluble and harmless recyclable products. The focus of the theory part is on the principles of chemical precipitation, electroplating and metal surface treatment processes as well as the treatment of sewage from these processes.

Kierto Ympäristöpalvelut OY's HeavyReg technique was studied as a laboratory pilot. The aim of the study was to research the pH dependence of heavy metal precipitation and to determine pH values at which certain heavy metals were precipitated out of the wastewater below the concentration discharging limit. The Client provided metal plating wastewater samples, of which copper, nickel, chromium, and zinc were precipitated from different samples. Precipitation studies were carried out as jar tests and the filtrate separated from the precipitate was analyzed using the atomic absorption spectrophotometer flame technique. As a result pH values were discovered where heavy metals were removed below discharge concentration limits.

The Precipitation technique performed properly for all other waste water samples but failed with the sample containing nickel. Electroless nickel coating solution probably contains a complexing agent which forms complex compound with nickel and therefore nickel remains in the solution. The chemical complex must be disintegrated before precipitation can occur, or alternatively, nickel should be removed using some other removal technique, such as an ion exchange column.