Light Management: Choosing the Best Controls for a UV Disinfection System

(February 2006, as appeared in WE&T)

The challenge in operating an ultraviolet (UV) light disinfection system is ensuring that the water receives the correct UV dose: too much, and you’re wasting electricity; too little, and the water isn’t thoroughly disinfected. That’s why the proper control system is critical to the success of the process.

UV systems can be controlled manually or automatically, depending on system components. A successful control system will deliver the correct dose at the correct time to ensure compliance with discharge permit limits. If automated, it also will interface seamlessly with the treatment plant’s overall supervisory control and data acquisition (SCADA) system.

UV Disinfection Systems
UV radiation — electromagnetic energy emitted at wavelengths between 235 and 270 nm — is biocidal to waterborne bacteria and viruses. The pathogens’ deoxyribonucleic acids (DNA) readily absorb UV light, which alters their molecular structure so they can’t replicate. The result is disinfected wastewater.

UV disinfection systems have gained popularity during the past 20 years. They also have become more sophisticated, reliable, and cost-effective. Today, UV disinfection systems can be divided into three major classifications: low-pressure; medium-pressure; and low pressure, high-intensity.

Low-pressure lamps. Available for more than 20 years, low-pressure lamps are available in horizontal or vertical configurations submerged in relatively shallow flow channels. Enclosed and Teflon-tube systems are also available. Lamp control is limited to “on” and “off.” Low-pressure lamps typically are used at facilities where the design flow is less than 5 mgd (18,900 m3/d). Because more lamps are needed as flow increases, the related maintenance costs at large facilities may be higher than those for other UV systems.

Medium-pressure lamps. Medium-pressure lamps became available in open-channel and closed-pipe configurations during the last decade. Because they have higher UV output, medium-pressure systems use about one-tenth the number of lamps that a low-pressure system requires. However, they use more power and need an automatic cleaning system that periodically removes the solids that coat the quartz sleeves.

Low-pressure, high-intensity lamps. Introduced within the last two years, low-pressure, high-intensity lamp systems use about one-third the lamps of low-pressure systems but three times more than medium-pressure systems. Early installations were deliberately over-designed, involving multiple banks of lamps and cumbersome hydraulic diversion controls designed to turn lamp banks on and off as operating conditions dictated. When these systems were on, all lamps in the bank or channel operated at full intensity. Newer improvements allow the lamp’s wattage output to be varied to optimize dose delivery. These systems also include an automatic cleaning system.

Control Strategies
The control system is a key part of the UV disinfection system. Typically, one of three control strategies – manual, flow pacing, or dose pacing – is used, depending on the type of UV equipment involved.

Manual control. Many low-pressure UV systems are operated manually. Operators simply turn them on and allow them to operate continuously at full power, regardless of flow and water quality. This method can result in a lamp life of more than 20,000 hours or longer, but it wastes power when flow is not at capacity. Smaller plants (less than 5 mgd [18,900 m3/d] capacity) or those using low-pressure or low-pressure, high-intensity lamps may consider this option.

Flow pacing. Flow pacing can save energy. Typically used in low-pressure systems, this method involves measuring the flow and adjusting the lamp intensity accordingly. The controller receives data from a flowmeter, weir, or flume and a recording device, and then turns the UV lamps on and off and increases or reduces their intensity as needed. To ensure that the flow signal – and therefore the UV dose – is accurate, the U.S. Environmental Protection Agency recommends that flowmeters be calibrated quarterly (Ultraviolet Disinfection Guidance Manual, EPA/815/03/007).

Unfortunately, flow pacing does not take into account the effects of aging lamps or water quality changes, so the worst-case assumptions must be used in setting the UV dose.

Dose pacing. Typically used in medium-pressure and low-pressure, high-intensity UV systems, dose pacing involves setting the UV dose based on flow, lamp output, and water conditions. Measurements of flow, lamp intensity, and water transmittance can be taken manually or automatically, but they must be part of a long-term routine, so site-specific trends can be determined and a suitable algorithm developed to predict necessary system adjustments, maintenance, and component replacements. If the measurements are automatic and continuous, the controller also can be programmed to notify operators immediately when set points are exceeded.

This control strategy can enable the UV dose to match operating conditions in real time, saving power and maintenance costs by allowing the equipment to operate at less than full capacity, when appropriate. Larger facilities (capacity greater than 5 mgd [18,900 m3/d]) or medium-pressure systems should consider this control method.

PLC Considerations
As UV systems are developed that require more sophisticated controls, most manufacturers are turning to programmable logic controller (PLC) technology. Typically, the control systems will consist of a master PLC that interacts with the ballasts, sensors, and on-line monitoring technology on each UV unit. The PLC can match the UV dose to the wastewater characteristics by controlling the number of units on-line, the flow rate into each unit’s channel, the number of lamp banks energized, and the lamp intensity. It can be connected directly to the treatment plant’s overall control system.

The control system typically includes an operator interface terminal that enables operators to monitor and control the system locally by noting

• Whether the control system is in local manual, local auto, remote manual, or remote auto mode;
• Which banks of lights are on;
• Each bank’s power level;
• The water’s transmittance level;
• Whether the UV dose is low or below minimum; and
• Whether any unit’s critical or noncritical alarms have been triggered.

Originally, plant staff could control the UV disinfection system only through the operator interface terminal, but some suppliers have begun to offer control systems that allow system adjustments from the plant’s central control room or another remote location. However, some adjustments, such as equipment calibration, are still done at the operator interface terminal. Vendor-specific controls are password-protected and can only be changed by the manufacturer.

When powering up the PLC during startup operations or after a blackout, plant staff should reset the UV system controls either to previous settings or to the manufacturer’s recommendations. The setting selection may depend on the type or duration of electrical upset. Lamp life shortens each time a UV lamp is turned off and back on, and medium-pressure lamps may need up to 30 minutes to return to full output after a power loss.

There are several reasons why PLC-based control systems are preferred to proprietary ones. Distributors worldwide stock both PLCs and their components, making repairs and replacement easier. Also, PLC modules typically are plugged into a backplane, with panel wiring connected to the components by plug-and-socket connections, so failed components by can be replaced quickly. And because PLCs aren’t a proprietary technology, servicing is readily available (although the specific services and warranty terms should be carefully reviewed before choosing someone other than manufacturer-trained personnel).

To simplify operation and maintenance further, all PLCs connected to a plant-wide SCADA system – including the one for the UV disinfection system – should come from the same manufacturer. This will minimize communication problems, make training staff easier, simplify the spare parts inventory, and streamline obtaining replacement parts.

Gary Hunter is a senior wastewater specialist, Paul Wood is a senior instrument control specialist, and Ed Kobylinski is a senior wastewater specialist at Black & Veatch (Kansas City, Mo.).

Copyright 2006, WE&T