FEASIBILITY STUDY OF BROMINE DISINFECTION TO COMBINED SEWER OVERFLOWS BY FULL-SCALE EXPERIMENT

The sewers in the Tokyo metropolitan area have been developed for more than a century since 1885, and 1995 saw percent of sewered population nearly 100% within the 23 wards of Tokyo. While sewage was taken into the sewers, however, flood prevention measures for lowlands were considered at the same time, and as a result about 80 % of the existing sewerage facilities are combined sewers in which wastewater and storm water are discharged in the same sewers. In the combined sewers, there are problems with CSO (combined sewer overflows) in case of rain.Several years ago, a weekly magazine reported that a high concentration of coliform group was detected in Odaiba Seaside Park, an artificial waterfront facing Tokyo Bay, after rainfall. After this report, the actual situation of CSO was widely discussed in the mass media like TV, newspapers, etc. Since then, immediate promotions of improvement measures for the combined sewers have been strongly required in the public interest. Considering that there are more than 20 waterfront and quayside parks in Tokyo where the citizens have increasing opportunities to come into contact with water, it was decided to introduce disinfection facilities to remove coliform group contained in CSO.For the introduction of disinfection facilities, disinfectants having high disinfection effects that are not influenced by rapid changes of water volume and quality which are characteristic of CSO as well as faster disinfection capabilities were required. Sodium hypochlorite which has been used as disinfectant in sewage treatment plants, needs a contact time of 15 ∼ 20 minutes to reach sufficient disinfection effects. Therefore it is not possible to secure such a long contact time for CSO in pump stations. In addition, in sewage with high concentrations of ammonia, chloramine and other substances with inferior disinfection power are formed, whereas the consumption of disinfectants is high in sewage containing a large amount of turbid materials. There were also concerns that in both cases a high volume of chemical injection would negatively impact the environment where the effluents are discharged.Given this background, we have selected a bromine disinfectant, which has been used for disinfection in swimming pools and spas, slime control in cooling towers and white liquor in paper manufacturing, as a substance that enables effective removal of coliform group in a short time and leads to space saving as well. Since 1999, we have been verifying its feasibility in a pilot plant in the Narihirabashi pump station.The results of our feasibility study in the pilot plant have confirmed that an injection rate of 2 ∼ 7.5mg/l (chlorine equivalent) reduces 103∼106 counts of coliform group to less than the standard value of 3,000 CFU/ml. Based on these results, we have investigated a method of determining appropriate injection rates by estimating the number of coliform group density (CGD) from turbidity and precipitation data, and with a supplementary use of turbidity. Also to estimate the biological safety of the chemical injection rates that are expected in real disinfection facilities, evaluations have been made in terms of acute toxicity, mutagenesis, impacts on aquatic organisms, formation of trihalomethane and bromate ions, proving that BCDMH is comparable with sodium hypochlorite concerning biological safety.