The Study on UV Disinfection of Treated Wastewater for Sewage Treatment Plants in Japan

The Problem and the Purpose of this studyIn Japan, Treated wastewater has been conventionally treated mainly with chlorine. However, an alternative disinfection technology replacing chlorination is demanded for diverse reasons. For example, special circumstances of water areas (for preservation of rare species) to which treated wastewater are discharged or recycling of treated water for landscape use. There are about 1,900 places sewage treatment plants in Japan, and sewage treatment plants installed ultraviolet (UV) disinfection systems is 90 places of total about around 5%. The following matter is nominated for this reason.□ In Japan, there is little recognition about performance of UV disinfection.□ A technical matter about a plan of UV disinfection facilities, a design, and maintenance is not arranged in our country.This study aims to survey the background and observed effects of sewage treatment plants where UV disinfection systems have already been implemented, and clarify applicability of the UV disinfection technology.The ConclusionApplicabilityA survey was conducted of sewage treatment plants with UV disinfection systems installed, asking for the specific reasons why the UV disinfection technology was selected. About 90 % of the respondents mentioned preservation of aquatic resources and rare species, sources for tap water, and recycling such as for the landscaping use. As for the creature to protect, a sweet fish and laver hold total 30% in marine resources mainly. In addition, a freshwater fish holds a total half. After introduction of UV disinfection facilities, that the fish which were not watched till now came to be seen is evaluated by inhabitants of a basin person concerned. Our study has also defined these reasons as preferential applicable areas of the UV disinfection technology.Design specificationsUV transmittanceTo set the UV transmittance for a design, we actually measured UV transmittance of wastewater at each sewage treatment plant. For installation of a system to a newly constructed plant, we set a UV transmittance of 70% or more as the standard.UV disinfection performanceWhen a UV radiation dose of 300 to 500J/m2 was applied, the coliform count after disinfection was 10/cm3 or less in each sample case. Even if we take into consideration a 30-fold increase of coliform count from photoreactivation, the coliform count can constantly be limited within 3,000/cm3.According to the literature we consulted, the coliform count of wastewater is said to be around 30,000/cm3 at maximum. Considering the increase in the coliform count from photoreactivation, the average disinfection performance must constantly be maintained to within 3,000/cm3. Therefore, we concluded that the average disinfection performance immediately after disinfection should be set to around 99.9% and that a UV radiation dose of 300 to 500J/m2 (from 30 to 50 mWs/cm2) is required to achieve such performance.DesignWe analyzed design methods for open channel UV disinfection systems and closed chamber UV disinfection systems. In this study, we conducted model designing as shown in the reference document as well as sorting out design details of each system.