Study On The Residual Chlorine Decay In Drinking Water Distribution Systems

The paper studies the regularities in the performance of chlorine disinfection in the drinking water distribution system and developeds a predictive model for the decay of residual chlorine, which have important implications to the design and operation of the disinfection system in China. The jar experiment is conducted on the granular activated carbon (GAC) filtered water (the concentration of TOC is around 1.00mg/L) of a waterworks in Guangzhou with disinfection being chloramine. The effects of organic matters, initial concentration of Chlorine, pH, temperature, NO2- on the bulk decay of residual chlorine are studied in the experiment. The results are as follows: with the increase of the concentration of organic matters, the decay rate of residual chlorine will increase; the higher the initial chlorine concentration is, the larger the amount of the consumption is and the lower the decay coefficient becomes; the lower pH is, the higher the decay coefficient is; within the range from 4℃to 30℃, the higher the temperature is, the faster residual chlorine decays; NO2- can accelerate the decay of residual chlorine and NO2- can't react with residual chlorine quickly. On the basis of the analysis of the effects by various factors, a decay model of residual chlorine in bulk water is developed and validated and the model shows good predicative results.The simulation system of drinking water distribution system designed by the author is experimented with the finished water of a waterworks in Guangzhou. The effects of initial chlorine concentration, pH, flow rate, pipe material and pipe diameter on the decay of residual chlorine are studied. The results are as follows: within the range from 0.63mg/L to 2.58mg/L, the higher the initial concentration of chlorine is, the higher the decay coefficient of residual chlorine is. But the increase rate of the decay coefficient decreases with the increase of the inital concentration of chlorine. pH poses an obvious impact on the decay of residual chlorine– the lower pH is, the faster residual chlorine decays and the higher the decay coefficient is; the lower pH is, the faster turbidity increases. The decay coefficient increases with the rising of flow rate. The increase of the flow rate will accelerate the striping of the attachment of the interior wall of the pipe, which will in turn increase the turbidity. With regard to new pipes, pipe material (cement-lined ductile pipes and PVC pipes) makes no differences on the performance of residual chlorine. Within 24 hours, the extent of the increase of pH for iron pipes is much larger than that in PVC pipes. The smaller the pipe is, the more influence the interior wall of the pipe puts on the quality of water, for the contact ratio between water and pipe wall will increase in smaller pipes. As a result, the faster residual chlorine decays, the larger the increase extent of pH is.From the investigation to drinking water distribution system in field of Guangzhou city, it is found that pipe wall including biofilm and corrosion deposit is most important factor causing residual chlorine decay in drinking water distribution systems. The conclusion from the experiment in laboratory is the same to the one got from the investigation in field. The key point to control residual chlorine decay in drinking water distribution systems is to control the corrosion of interior pipe wall.