By Cliff Lebowitz
Replacement of a low-frequency, shell-and-tube ozone generating system with a much higher efficiency, medium-frequency system at Clean Water Services' 64-million-gallons-per-day (MGD) Southwest Wastewater Treatment Plant (WWTP) in Springfield, Mo., has increased ozone concentration from a range of 3 to 3.5 percent to an average of 6 to 7 percent, and as high as 12 percent.
The new system has eliminated previous annual maintenance costs of $100,000/generator and ended increasingly frequent unavailability of spare parts, thereby improving reliability. It has also decreased power consumption for ozone production from 4,800 kWh/day to 2,500 kWh/day, providing for additional annual energy savings in excess of $100,000, even though ozone production capacity has increased by 56 percent.
The replacement ozone system was designed, manufactured and commissioned by Mitsubishi Electric Power Products Inc. (MEPPI) of Warrendale, Pa. The system qualified for funding through the American Recovery and Reinvestment Act (ARRA), including the "Buy America" requirement. It has increased ozone production capacity from 800 lbs/day/generator to 2,800 lbs/day/generator, allowing the plant to assure meeting peak demand. It also has increased disinfection capacity under those conditions and is expected to help with future compliance needs for endocrine disrupting compounds (EDCs) and other pharmaceutical concerns.
Part of the discharge of the plant, recently renamed as the city's Southwest Clean Water Plant, is reused as cooling water for a local electric power generating station, allowing the plant to realize value from its treated wastewater. In dry weather conditions, the plant's effluent makes up the entire flow of the stream it is discharged into.
"Our original ozone system was packaged with a good cryo plant that we were happy with, and we were pleased with the quality of effluent the ozone treatment was giving us," recalled Kelly Green, plant superintendent. "So when we decided we had to both operate more efficiently and assure continued compliance with our permit under some regulation changes, we didn't want to go backwards and wanted to stick with ozone, including continuing use of the cryo plant."
The plant's National Pollutant Discharge Elimination System (NPDES) permit, administered by the Missouri Dept. of Natural Resources (DNR), mandates needs for fecal coliform and E. coli disinfection that had to be met at planned higher flows. In addition, regulatory enforcement had been tightening up in general in recent years on WWTP discharges to lakes and streams, which also included limits on ammonia, phosphorus, biological oxygen demand (BOD), and total suspended solids (TSS).
"The technology available for ozone treatment had advanced so far since our first installation of a plate and frame system in the 1970s and our upgrade to a shell-and-tube, low-frequency system in the 1980s that it was comparable to the advances in computers during that time," Green said. "Working with our consulting engineering firm, we saw that even a less problematic shell-and-tube type could no longer serve our needs. So we decided to go with a medium-frequency system and put that out for competitive bidding."
He continued, "The medium-frequency system vendor we chose had previously done large drinking water plants. However, their system for our large WWTP has run very well as a good demonstration for their equipment on the wastewater side. We've handled several high-flow events that took us to our peak capacity of 100 to105 MGD, and that spoke volumes for the new system."
"And in addition to doubling our ozone concentration with the new system and ending a heavy maintenance burden with the previous system, we are now getting ozone into water by side-stream injection instead of through the previous surface aeration method," he added. "The better control that gives us means the side-stream pump in the process requires only 100 HP to run sufficiently compared to the nine surface mixers that required 270 HP previously."
He said, "Our ozone production power cost has been reduced from 4,800 kWh/day to 2,500 kWh/day. The vendor also warranteed we wouldn't have to open up an ozone generator for maintenance for at least five years, and we've never had it happen since installation in July 2012."
Ronnie Box, the plant's operations supervisor, doesn't miss the maintenance burden from the previous ozone system. "The units had outlived their usable life and had developed into an extreme maintenance investment," he recalled. "It cost us $100,000 for labor, materials and downtime every time we had to open a generator for rehab of the dielectrics, fuses and wiring. And there were also safety concerns, with the dielectrics being hard to handle."
He continued, "We had to do it annually for each generator, with the typical episode lasting three to four days but sometimes lasting several weeks when getting parts was a problem, while at the same time parts prices were going up. There were four generators, and sometimes it was back-to-back. Your redundancy gets reduced rather quickly when that happens."
"Our ozone residuals were safe," he said, "but we want to be more than safe." Ultimately, they wanted effluent looking like a mountain stream, with an effervescent quality coming out -- something they would like to show visitors. "With the new system, we are accomplishing all that."
The plant has two separate treatment trains that receive flow from common headworks and primary clarifiers and that also receive common ozone treatment. One train functions as a pure oxygen, 42-MGD activated sludge plant and the other as a conventional biological nutrient removal (BNR), 22-MGD avg./58-MGD peak flow activated sludge plant. The two trains come together at the ozone contactor.
Each of three ozone trains is limited to 33 MGD. There are three cells per train. Average ozone demand is about 1,000 ppd, based on a 3.5-mg/L ozone dose.
Off-gas oxygen is recycled to biological oxidation basins in the BNR process. Two 100-percent ozone-destruct units, each with a liquid ring compressor, are used to destroy any leftover ozone gas before oxygen is fed to high-purity oxygen tanks. Enhancing operator safety was another reason for deployment of the ozone-destruct units.
The previous ozone generators received all of the oxygen gas produced by the cryo plant. The replacement ozone generators only receive a portion of the total gas, with the rest fed directly to the biological oxidation basins.
Ozone gas dosing is based on plant flow, and to maintain minimum detectable ozone residual in the first cell of ozone contact tanks, one or two generators can be operated as needed to disinfect peak flows. Further, two eductors were provided for each tank. Space was left for a third eductor, which would increase disinfection capacity from 100 to 150 MGD.
Among design considerations, ozone dosing had to account for reliability of ozonation downstream from denitrification filters. Dosing also had to effectively meet disinfection requirements of the fecal coliform average monthly limit of 200 MPN/100 mL and the E. Coli monthly average/daily maximum limit of 126 MPN/100 mL; avoid production of disinfection byproducts that could impact effluent toxicity; and account for ozone demand by nitrite and methanol.
In addition, the increased disinfection capacity of the replacement ozone system is expected to help considerably with future regulatory compliance that is expected to call for peak flow disinfection, oxidation and/or mineralization of EDCs and pharmaceutical and personal care products (PPCPs).
The system has met the need to provide guaranteed power consumption at various production rates. Lessons learned during installation of the replacement system were used to make ozone system programming modifications as well as related changes.
For example, the system supplier modified the programming of ozone generator power supply units to keep cooling fans and cooling water flows running through the PSU in the event the ozone system is automatically shut down for safety reasons. In another matter, when sunlight caused false alarms of high oxygen gas temperature on temperature transmitters, the sensor was relocated inside the building to eliminate the problem.
Overall, successful emphasis was placed on developing a good working relationship with the ozone system supplier, maintaining accountability and achieving successful deployment. The city's consulting engineer for the project was Black & Veatch, from Kansas City, Mo., and their general contractor was Garney Construction, also of Kansas City.
About the Author: Cliff Lebowitz is an independent reporter and editor. His third-party case histories, mainly industrial equipment applications, are based on interviews with end users and are approved by them for accuracy and completeness.