Ultraviolet disinfection for small water supply systems

Abstract

In small water supply systems (e.g. Tube well, rainwater harvesting), disinfection is a challenge since a family or small communities often do not have necessary facilities to disinfect water properly. Also many commercially available household-level treatment systems are not very effective in removing pathogens. UV disinfection is effective against Cryptosporidium, which is resistant to commonly used disinfectants like chlorine and has the ability to inactivate pathogenic microorganisms without forming regulated disinfection byproducts (DBPs). Some recent research works suggest that UV disinfection could be effective for disinfection of water in RWH systems; however, there is no systematic data for assessing effectiveness of UV disinfection. The main objective of the present study was to evaluate the effectiveness of UV disinfection for small water supply systems. A range of laboratory experiments have been carried out in cylindrical water tanks (15L and 60L capacity), commonly used for storage of water at household level, particularly focusing on the effects of important operational (e.g., intensity of lamp, exposure time, distance/location) and water quality parameters (e.g., initial FC concentration, turbidity, color, pH) on UV disinfection. The effectiveness of disinfection was evaluated in terms of reduction in fecal coliform (FC) concentration in treated water. The effectiveness of UV disinfection carried out in a tank/container has been found to depend on a number of operational factors including intensity of lamp, exposure time, and position/distance of a particular point (within the tank) with respect to the lamp. UV disinfection is least effective for areas (within the tank) located below the tip of the UV lamp. Initial FC concentration, Turbidity and Color have been found to affect disinfection efficiency. Turbidity of water to be disinfected should be below 5 NTU, and Color intensity should be below 50 Pt Co. While lower initial FC concentrations are preferable, if other conditions/criteria (discussed below) could be fulfilled, initial FC concentration is probably not a major concern. For effective design and operation of UV disinfection for small (tank based) water supply systems: (1) Water tank should preferably be cylindrical and position of UV lamp in the water tank should be at the center. Height of water tank should approximately be the same as effective length of the UV lamp; (2) The UV lamp(s) should be selected such that the entire height of water column within the tank is covered by the length of the lamp; (3) Treated water collection port should be located at a position perpendicular to the UV lamp, few centimeters above the bottom of the tank; (4) If the above criteria could be fulfilled, an exposure time of about 30 minutes should be enough for effective disinfection of a particular volume of water (i.e. reducing FC to zero level in the treated water); and (5) Disinfected water should be consumed within 6 to 8 hr to avoid the photo reactivation or dark repair. More study is needed to better understand the effects of water initial FC concentration, Turbidity and Color on the effectiveness of UV disinfection in small water supply systems. Results from this study should also be utilized in the design of effective disinfection system for rainwater harvesting systems.