The Light at the End of the Tunnel - How to Successfully Operate a UV Disinfection System

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Courtesy of Water Environment Federation (WEF)

ABSTRACT
Over the past ten years, ultraviolet (UV) technology has become a commonly applied method of disinfecting of wastewater effluent. UV disinfection involves passing wastewater effluent through a confined chamber with rows of underwater lamps emitting UV energy. Viruses and bacteria become inactivated upon exposure to dosages of UV energy, thereby disinfecting the wastewater.

The UV system is defined by the physical (electrical and mechanical) requirements of the equipment. These requirements must be matched with the water quality of the wastewater plant to provide effective disinfection. For the purpose of trouble-shooting UV equipment, the UV system can be divided into three components: process, electrical, and mechanical. The process system deals with the quality of the water to be treated by the UV system and the disinfection goals that must to be attained. The electrical system consists of the lamps, wiring, and control system. Mechanical system includes the quartz sleeves, frames, cleaning mechanism, and reactor configuration.

When the UV system functions as intended, it is easy for operations staff to maintain compliance with the treatment facilities NPDES permit requirements. This paper discusses the key operating requirements and presents helpful hits on how to improve the operations and maintenance and trouble shoot UV systems.

INTRODUCTION
Ultraviolet (UV) disinfection has several advantages when compared to other disinfection alternatives. Unlike chlorination and ozonation, UV disinfection is a physical process which does not involve the addition of chemicals. As a result, UV disinfection does not produce any known toxic residuals or form known byproducts which pose a risk to humans or aquatic organisms. In contrast, many municipalities are being required to remove residual chlorine before discharging treated wastewater due to concerns over chlorine's toxicity to aquatic life, requiring the addition of sulfur dioxide or other reducing agents (resulting in additional costs). Another benefit of UV disinfection is that it eliminates the need to transport, store and handle potentially dangerous chemicals. While this is primarily a safety issue, there are also costs associated with such transport and storage. The 1988 Uniform Fire Code (and subsequent revisions) includes requirements for the control of accidental discharges of gaseous chlorine as well as sulfur dioxide that is frequently used in the dechlorination process. The scrubbers required to meet these codes increase capital costs for chlorine  disinfection. Since UV disinfection facilities do not require scrubbers and occupy only minimal building space and basin volume, UV disinfection is often found to be a viable, cost-effective alternative to chlorination and ozonation.

In 2006, WEF published a manual on the operation and maintenance of disinfection facilities. As part of this manual, information on UV disinfection systems was presented. This paper highlights only the maintenance and trouble shooting activities. Readers are encouraged to obtain the WEF manual for further information on UV systems.

MAINTENANCE ACTIVITIES
A number of maintenance activities need to be completed to ensure the UV system will continue to function adequately. Operations staffs need to review these activities to ensure that the UV system will continue to function. EPA (2003) recommends the maintenance activities for UV systems in Table 1 be completed.

During startup of the UV system, operation staff need review activities presented in Table 1 with specific vendor representatives. Vendors have specific requirements for their respective system. This may range from determining lamp replacement frequency to cleaning to maintenance of the reactor.

Treatment plant staffs need to understand that activities like cleaning and calibration of on-line instrumentation is critical to the success operation of the UV system.

While not directly related to operation and maintenance, staffs need to understand the importance of following correct procedures for sampling and analytical procedures. Sampling of UV systems needs to be completed to ensure that no additional material is included into the samples. Clean sampling techniques may have to be implemented if the UV system has been designed to achieve high level disinfection requirements. In addition, analytical procedures need to be followed (including quality control and assurance practices). Microbial testing is the key to monitoring the performance of any disinfectant. If appropriate sampling procedures (40 CFR 136) are not followed either in the field or in the laboratory, results can be misleading. Samples should be collected in certified, clean containers using grab techniques and should be kept at a temperature of 4O C during transportation to the laboratory. Analysis should start within 6 hours of the time of collection. In some cases, rosolic acid may need to be added to the sample to ensure accurate counts. After collection, filled sample bottles must be kept in the dark at all times. This can be accomplished by wrapping the bottles in aluminum foil.

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