Formation water that is co-produced with the production of petroleum needs to be collected and treated prior to re-injection into the reservoirs. The treatment of the produced water is important in order to keep the injection pressure as low as possible. A new water treatment plant is under construction at the moment and will start operating by 2014. Since 2012 OMV Austria Exploration & Production GmbH operates a polymer flooding pilot in order to increase the recovery factor of their mature oil fields. As a result polymer will also be backproduced and could therefore enter the new water treatment plant within the feed. This would lead to a decreased separation efficiency in single processing steps and to a reduced injection water quality. The water treatment process under the influence of the polymer was imitated with a water treatment pilot plant, which works with the same process steps than the future new water treatment plant. Chemical sets of flocculants were tested regarding the achievable hydrocarbon content and filterability of the water after dissolved gas flotation. In a second set of tests, the separation efficiency and the operating behavior of the final filtration process with nutshell granules was tested as well. The application of polyaluminum chloride for flocculation and subsequent flotation has proofed to be promising at low polymer concentrations. In the tests with the nutshell filter the required water quality for re-injection could be achieved. However the operating behavior of the nutshell filter was influenced by residual polymer. The long-term impact of the polymer on the nutshell granules’ capacity for oil separation and on the operating behavior of the nutshell filter needs to be investigated in further tests.