Effects Of Water Source Switching By South-to-North Water Diversion Project On Distribution Systems In Beijing And Countermeasures

With the gradual switching of the water supply source from the Danjiangkou Reservoir in Beijing, the diversity of water quality tends to bring about the instability of water quality in drinking water distribution systems, even resulting in “colored water”. The factors contributing to this issue usually include the water quality, flow condition, water treatment process, and corrosion inhibitors. In order to investigate the effects of water source switching on the water quality in the distribution systems of Beijing and estimate the risk of “colored water”, the characteristics of water quality in Danjiangkou Reservoir were analyzed first. Then the following two experiments were conducted in turn:(a) the old pipe sections of Beijing were transported to the Danjiangkou Experimental Base and through the treated water, and(b) the pipe in distribution systems of Beijing were in situ and through the treated water brought from the Danjiangkou. The experimental results indicated that the relative low corrosiveness and good chemical stability of the water in the Danjiangkou Reservoir. Low risk of “colored water” correlated to the flow condition in the pipe, while excessive levels of iron content and turbidity might occur under prolonged stagnation state, but mitigate and settle out if flushed with water continuously. Traditional methods, such as Larson Index and Langelier Index, used to evaluate the stability of the distribution systems would present their limitations when dealing with the switching of water sources. Thus, a more appropriate method should be advanced based on the analysis of the effects of water quality parameters on the stability of pipes. Considering the water quality parameters closely related to the stability of pipes and the release of corrosion product, the diversity factor of water quality which involved dissolved oxygen, Larson Index, nitrate concentration, and hardness was proposed to study this issue. The lower the diversity factor is, the higher the similarity between the water qualities of initial and switched water sources, and the better the stability is. The applicability of this evaluation method was confirmed through verification between predicted and observed values, and its simplicity and validity were prominent when compared with the traditional methods. On the basis of diversity factor method and field investigation, the risk of “colored water” in the distribution systems of Beijing were identified and categorized. Furthermore, the oxidation reduction potential could be assistant to estimate the corrosivity, it would not accelerate the release of iron if the oxidation reduction potential value was not less than 400 mV. The countermeasures of “colored water” include flushing of the pipe network and the adoption of regulating techniques in water plants. Daily management, such as optimizing the operation of pipe network, enhancing the monitoring of water quality, and developing the emergency mechanism. It is desirable to replace the water source stage by stage to keep the water quality under control.