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.).
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