An upgrade to the Mandeville (La.) Wastewater Treatment Plant from 2 to 4 mgd included replacement of an aging UV disinfection system that used electrode lamps with receptacle plugs and ground fault circuit interrupters.
The facility had two banks of UV lights in one channel and one bank in the second channel, for a total of 264 bulbs. Burning continuously, the horizontal bulbs heated to 160 to 180 degrees F, hot enough to bake debris onto the sleeves and shorten lamp life. Operators watched for rising bacteria numbers to tell them when bulbs had burned out, but they still didn’t know which ones had failed.
After researching replacement options, director of Public Works David deGeneres chose the MicroDynamics microwave UV disinfection system from Severn Trent Services. “We gambled a little because the technology, although successful overseas, isn’t well known in this country,” he says. “However, the bulbs have self-cleaning wipers and require no maintenance. We’ll save $300,000 in the first three years of operation and 416 labor hours annually.”
The eight OCS 660 UV units with 16 bulbs each were installed in two trains of four. With 136 fewer bulbs, the facility also will save electricity.
Before the replacement, the city was paying a contractor $100,000 a year to replace sleeves and bulbs in the UV system rather than divert operators from their duties. “Properly seating and sealing the sleeves and O-rings is a delicate operation,” says deGeneres. “If done incorrectly, a leaking seal might short out the entire rack.” About 85 percent of the contractor’s expense was parts.
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Cleaning the bulbs was the biggest issue, requiring eight hours per week. “The racks weren’t easy to extract,” says deGeneres. “After hanging them up, the operators would scrub the bulbs with a light acid. Frequent removal and replacement damaged the lamps and broke sleeves.” The operators also pressure washed the channels weekly.
The 1.5 mgd (average) treatment plant uses three gravity-fed aeration ponds in series to treat the wastewater before disinfection and discharge to a wetland. The upgrade by Gilmore & Son Construction Corp. of Baton Rouge included three new disinfection channels 49 inches deep.
“One UV module treats 1 mgd, but the engineers designed for redundancy,” says deGeneres. “By putting four modules in each channel, we can divert our entire flow to one channel if the other goes offline.”
Based on flow, automatic gates on the channels open or close to maintain water levels. The third channel, for future expansion, will probably receive UV units in 2013.
The technology uses microwaves to energize gases in the bulbs and generate consistent-strength UV disinfection. The modular, open-channel design allows the tops of the vertical bulbs to remain above the water.
Each assembly, driven by a magnetron powered by a dedicated power unit, has four UV bulbs with waveguides contained in a quartz sleeve. If a bulb needs replacing, the operator removes the cover and three bolts, lifts out the assembly without touching the sleeve, and snaps the bulb out. “Every component is modular with quick-connect plugs,” says deGeneres. “Repairs are simple and take minimal time.”
Switching to the new UV system involved closing one valve and opening another. “We were probably offline for eight hours for piping and installs,” says deGeneres. “The modules arrived on a pallet, and all the contractor did was lower them into the channels.”
A Severn Trent technician programmed the software, set the parameters, and trained the operators for two days. The modules use MicroPace flow-pacing technology, which measures the transmittance of the water and the flow. An external flowmeter signals the computer to reduce power to the magnetrons or turns them off as flows decline, then reactivates them as flows increase.
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At full power, the quartz sleeves reach 115 degrees F, making them less susceptible to fouling. The team programmed the wipers to activate every eight hours, sending stainless steel brushes up and down the length of the sleeves to clean them.
Although the UV software isn’t tied to the plant SCADA system yet, it is connected to a central computer in the rear of the plant. Operators receive alerts to problems and their locations. For example, if a probe on the outside of the sleeve doesn’t sense enough light, it sends a low-intensity output warning for a specific bulb.
“The first thing we do is hit the manual override button on the wiper to see if it’s working,” says deGeneres. “If it is, we have a different issue, but troubleshooting is as simple as following a recipe in a cookbook.”
Although the new system costs 15 to 20 percent more than the original UV system, the savings from reduced maintenance make up the difference: “The system has many benefits and my guys love it,” deGeneres says.