City Tests Use of UV Disinfection for Drinking Water

The town of Ontario, NY, installed a pilot ultraviolet disinfection system at its drinking water treatment plant in March 1999 as part of a program to determine the feasibility of using UV disinfection as an alternative to chlorine.

The town of Ontario Water Treatment Plant, in Wayne County, is a 3.5 MGD facility. The plant is divided into two parallel treatment sections (north and south), both of which use upflow clarifiers in combination with sand filters for treatment. Raw water is pumped from Lake Ontario, chlorinated, and then split to the north and south side treatment units. Once through the treatment units, flow from the two sections is combined at the clearwell where post chlorination occurs.

The equipment chosen for the pilot was a Trojan UV8000 ultraviolet disinfection system manufactured by Trojan Technologies. The system uses two high intensity, medium pressure lamps within a quartz sleeve and enclosed by a stainless steel reactor chamber. The unit was installed in the south section of the plant and connected into a 14-inch main which conveys filtered water from the Microfloc units to the plant clearwell.

The UV dose delivered by the unit was designed to meet the requirements for a 3-log virus inactivation as set forth by the EPA in the Surface Water Treatment Rule. All water leaving the plant is dosed with chlorine to maintain a residual of approximately 1.0 mg/l.

In order to evaluate the effectiveness of the UV system for disinfection, pathogenic organisms entering the system must be measurable. However, negligible concentrations of organisms before and after the UV disinfection system were noted during the weekly testing runs.

To verify the dose delivery of the reactor, a bioassay was conducted at the plant. As a part of this process, a test organism which is non-pathogenic to humans (MS-2 bacteriophage) was injected at a known concentration and rate into the filtered water upstream of the UV disinfection unit. With a measurable concentration of phage entering the system, an accurate measure of phage kill versus ultraviolet dosage could be obtained by testing for phage downstream of the UV system.

To evaluate the dose delivery in the reactor, the dose-response characteristics of the MS-2 bacteriophage was predetermined under controlled conditions in the laboratory. The test organisms were given a known dose (defined as a product of intensity and exposure time) by irradiating at precise time periods at a known intensity to generate dose response curves. These same organisms were then injected upstream of the UV reactor and the survival was measured at a point downstream of the reactor. UV dose was then calculated from the laboratory dose response curves.

The bioassay performed at the plant confirmed the Trojan UV 8000 was delivering a dose exceeding the original design dose.

?The results of the pilot program have exceeded our expectations? said Robert Wykle, Superintendent of Utilities, town of Ontario.

The town of Ontario currently is planning to purchase the existing pilot unit and plans to add a second Trojan UV 8000 in the north section of the plant in the near future. In recognition of his pioneering efforts relating to the UV pilot study, Robert Wykle was recently awarded the local chapter?s American Public Works Association (APWA) 1999 Technical Innovation Award.

Recent research highlighting the effectiveness of UV for inactivating waterborne protozoa such as Cryptosporidium and Giardia has fueled the town?s interest in UV technology. Looking ahead, the long range goal of the town?s UV pilot study program is to position UV as the Ontario Water Plant?s primary disinfection, with chlorine providing residual disinfection.