Opting for Ozone
The water superintendent for Winfield, Kan., Dan Defore, has reported a reduction in taste and odor complaints from his drinking water customers—from the level of hundreds during a two-month period to very few since the 2004 installation of an ozone system as the primary disinfection technique for the local 6 million-gal-per-day (mgd) surface water treatment plant (SWTP).
Defore has also noted dramatic reduction of total tri-halo methane compounds (TTHMs)—from compliance levels to nearly undetectable—and lowering of compliance levels of halo-acetic acids (HAA5s) by an additional 80%, while eliminating the need for carbon feeding.
Installed as part of a major plant renovation, the 100%-redundant ozone system was set up as a separate 2,000-sq-ft building and contact basin between the plant’s solid contact units and its filtering system.
Ozone from a 150-lb-per-day generator is injected into the contact basin underneath the building, where water is continuously monitored to comply with contact time regulations from the Kansas Department of Health and Environment. Residual ozone levels are controlled to meet disinfection credit requirements, and ranges vary with water temperature and production rates.
Defore noted that previous treatment techniques for taste and odor control had stopped working. “There’s not been anyone who could tell us why all of a sudden we had this taste and odor problem that we could not control,” he said. “It began in 2002 and hadn’t been this bad since I started in 1997. But we did know our head-of-plant chlorine treatment, as primary disinfection, was somewhat hampering the effect that the carbon normally took care of. Normal feed rates had moved up from 10 to 15 ppm to 25 to 30 ppm, and as high as 40 ppm of carbon, and this was not taking care of the problem. Neither did a temporary addition of potassium permanganate or hydrogen peroxide. So we started to look at something different.”
The water system serves a population of about 13,000, and the number of complaints during December 2002 and January 2003 reached the hundreds, according to Defore.
“Since we installed the ozone system in 2004, there have been very few taste and odor complaints about our product,” he said. “Meanwhile, we’ve also reduced our TTHMs, which are regulated at 80 ppb max, from around 60 ppb to nearly undetectable amounts, and our HAA5s, which are regulated at 60-ppb limit, from about 45 ppb to less than 8 ppb.”
Shopping for a Solution
The plant renovation that included the new ozone system was designed by Professional Engineering Consultants, P.A., Wichita, Kan. The project engineer was Bruce Allman, and the project manager was Kevin Rood.
Originally commissioned in 1969, the 6-mgd, 2.8-mgd average SWTP is fed by a 1,200-acre lake, nine miles from the plant, through a 24-in. line. It has solid contact basins for coagulation and sedimentation, plus a dual-media filtering system. In addition to establishing ozone as the primary disinfection system, the 2004 renovation included changeout of the filter media, addition of domes over the basins, new chemical feed equipment and air scour to aid in the filter backwash process.
The taste and odor problem emerged in late 2002 in spite of chlorine fed at the head of the plant and powder-activated carbon fed upstream. An attempt to solve the problem by adding batch-mixed potassium permanganate at 10 to 15 ppm added a color problem, while hydrogen peroxide was ineffective at the 1 to 2 ppm levels needed to avoid the removal of chlorine.
Unusual freezing of the lake had escalated the issue by not allowing the lake to off-gas as it should during normal decay of vegetation. It was the first time the plant had not been able to control taste and odor. Ultraviolet (UV) disinfection was then considered as a new primary or secondary technique, along with a different feed point for the chlorine.
“UV had worked in other states, but the state of Kansas had not yet allowed disinfection log credits for it,” Defore said. “Meanwhile, further reduction of TTHMs and HAA5s was also an issue. We had already stopped feeding chlorine at the lake to reduce chlorine contact time with the total organic compounds—the precursor for TTHMs and HAA5s—prior to enhanced coagulation and sedimentation. We preferred not to try chlorine dioxide because of our operators’ lack of confidence in the reliability of the generators, and we were also concerned about the byproducts associated with it. So we started to seriously consider ozone.”
“We were very encouraged by the city of Emporia’s [Kan.] 10 years of experience with it in a much larger plant than ours,” Defore said. “We decided to put it in not just to solve the taste and odor problem, which could have been a predisinfection treatment, but to serve as our primary disinfection system.”
In considering ozone system vendors, Defore noted a major distinction among them. “Ultimately, the plant renovation was a typical design-bid-build project, with three vendors considered separately for the ozone part,” he said. “The vendor we chose for the ozone had a very competitive price, but more importantly, we expected the best possible service from a company that was trying hard at the time to get established in the U.S. And they were also guaranteeing the dielectric and other key parts for 10 years, which was twice as long as the other vendors. That was very important to us because it would allow us five years just to learn how to take care of the routine maintenance and total operation for a technology that was new to us.”
Defore said the city is pleased with the way the system has turned out since its installation in July 2004. Few maintenance issues have happened in between regular six-month visits that could not be fixed by a phone call, he said.
The ozone generators were manufactured by Mitsubishi Electric Power Products, Inc., Warrendale, Pa. A subsidiary of Mitsubishi Electric Corp., Kobe, Japan, the company also provided system design and installation, and it continues service through a maintenance contract.
To help assure reliable delivery of ozone for disinfection, the ozone generators feature patented technology designed to maximize continuity of operation. The borosilicate glass dielectric is described as a durable design that resists thermal shock and evenly distributes the applied electric charge over the entire dielectric surface without arcing.
For more information, contact Mitsubishi Electric Power Products, Inc. at 724.778.5160 or by e-mail at [email protected].