Surface Water Treatment In Northern Cold Region By Combined Process Of Sedimentation And Dissolved Air Flotation (DAF)

In northern cold areas of China, the large extent of seasonal water quality variation cause increasing drinking water quality problem. The raw water with low temperature and low turbidity is not easy to treat, and the sudden high turbidity water during rainy seasons cause treatment difficulties . The process of coagulation / dissolved air flotation (DAF) has its unique advantages in treating the water of low temperature and low turbidity, which attracts more and more attentions. This paper introduce the research related to the process of coagulation and dissolved air flotation for optimizing the treatment of the surface water of low temperature and low turbidity, and proposed the combined process of sedimentation and flotation, which may provide a suitable method for the treament of surface water in cold areas. It is found that the process of coagulation and dissolved air flotation,is effective in the treatment of surface waters with low temperature and low tubidity. With improving treatment efficiency, the process of coagulation and DAF can greatly reduce the coagulant dosage. It also exhibite a wider optimum coagulation range and decrease coagulation time.It is found that the formation of bubbles in water results in very complex gas, liquid, solid-phase interface. the bubbles may adsorb hydrophobic organic compounds, which take place in gas extraction for small molecular organic compounds, stripping, the bubble enhanced coagulation and so on.For these complex reasons, the process of coagulation and DAF have different ability for the removal of organic matter in comparison to traditional coagulation and sedimentation process.This paper proposed a technology which combine sedimentation with dissolved air flotation, and studied its performance for the treatment of surface waters of low temperature and with low turbidity, and the waters of normal temperature water and high turbidity. Results showed that the combined process of coagulation-sedimentation and filtration is satisfactory for treating both types of water. By using the proposed process, turbidity, organic matter and other indicators of water quality are improving. The sedimentation time can be greatly reduced, which means that the sedimentation and flotation process will not need more time than conventional coagulation and sedimentation process. So the existing sedimentation tank can be upgraded into the combined process of coagulation-sedimentation and flotation by inserting flotation stage at the rear part of sedimentation tank. Sedimentation tank can remove large floc which will reduce the load of flotation tank, and the flotation tank can remove small floc which can not be removed by sedimentation process. Because of the higher treatment efficiency and adapbility for the variation of raw water quality, it will be a reliable process for the security of the drinking water quality in northern regions.