Mixed Oxidant Replaces “Cocktail” of Chemicals in Power Plant Cooling Tower Water System
In 2010 NIPSCO installed two MIOX RIO Mixed Oxidant Solution (MOS) chlorine generator systems on two of the four cooling towers at R.M. Schahfer Generating Station. After switching to MIOX for cooling tower water treatment, the two towers experienced minimized algae growth, better biocontrol and lower Legionella counts, cleaner condenser tube sheet and reduced differential pressure, and zero negative impact on corrosion rates.
A new onsite mixed oxidant system at the R.M. Schahfer Generating Station of Northern Indiana Public Service Co. (NIPSCO) in Wheatfield, IN, has yielded substantial treatment chemical cost savings and improved summer operation compared to the control unit cooling tower.
NIPSCO, an electric and gas utility and subsidiary of NiSource Inc., was approached about the system during a biannual technology meeting of one of its vendors, Equip Solutions Inc. Equip Solutions is currently a Midwest representative of MIOX Corp. and also handles the majority of NIPSCO’s chemical feed systems, chemical feed piping, chemical tanks, and the continuous emissions monitoring at the company’s Wheatfield generating station.
MIOX, of Albuquerque, NM, has manufactured onsite disinfection generators since 1994, with more than 1,500 installations in over 30 countries treating better than 6.5 billion gallons of water a day. The NIPSCO project holds the double honor of being MIOX’s first power plant cooling tower installation as well as its largest cooling tower installation.
Until Spring 2010, NIPSCO’s Chemical and Environmental Compliance Department had used a “tri-cocktail” regimen of sodium hypochlorite, sodium bromide, and a photosynthesis blocker (or algaecide) to treat water supplied to four cooling towers at the 1780 MW coal-fired power plant. The cooling water is allowed to reach 2-3 cycles of concentration, with the cooling tower blowdown being discharged into a 200-acre settling pond, which discharges into the Kankakee River.
Due to the NPDES permit restrictions, disinfection dosing was limited to only two hours a day for each tower. During the warm weather summer months, it was not uncommon to see spikes in Legionella above 200 colony forming units per milliliter (CFU/mL). An action level of 100 CFU/mL triggers a recommendation to workers servicing the towers to wear respiratory equipment for health safety. NIPSCO has adopted the policy of posting Legionella level warning signs at all of the cooling towers. Due to open-air hotdecks exposed to the sun on two of the towers, algae control was also an ongoing challenge.
At the time the utility first began considering a new disinfection regimen, sodium hypochlorite prices had nearly doubled to about $1.60-1.65 a gallon, so saving money was central to the idea of a new regimen. The savings potential also reducing the number of chemicals used in biocontrol from three to one only made the proposal more attractive to officials at NIPSCO, who have a forward-thinking reputation of working closely with regulators to stay ahead of the regulatory curve.
NIPSCO has invested significantly in both the boiler water and cooling water areas concerning automated chemical feed control, process chemistry data acquisition, and compliance with EPRI (Electric Power Research Institute) recommended cycle chemistry guidelines.
NIPSCO installed a MIOX RIO on-site mixed oxidant disinfection system in late 2009 – early 2010 on one of the four cooling towers at R.M. Schahfer Generating Station, Unit 15, which has a flow rate of 168,000 gallons per minute (gpm). Unit 14, the tower most like Unit 15 (but with brass versus stainless steel condenser tubes), was kept on the biocide/algaecide tri-cocktail treatment as a control comparison to help evaluate the new system’s effectiveness. The hypochlorite-to-bromide biocide ration on Unit 14 ranges from 4:1 to 8:1, depending on the tower’s biological condition. Unlike the two towers, towers 14 and 15 have open-air hotdecks, which make algae control more problematic.
The ongoing evaluation criteria for comparing Unit 15 to Unit 14 are:
- Mild steel corrosion rate data
- Dipslide total bacteria counts
- AquaFlour (chlorophyll A) counts
- Algae growth (and other microbially induced corrosion – MIC) on hotdecks and other cooling water surfaces, determined by visual inspection
- Chlorine (mixed oxidant) production test, confirming that the daily ppd FAC, daily total product volume, and the 0.40% mixed oxidant solution (MOS) concentration are consistent with O&M Manual specifications over time
- Program cost/financial efficiency
- Condenser visual cleanliness
- Legionella bacteria counts
After two full quarters of use, the outcome has been well worth the investment, noted Bert Valenkamph, NIPSCO’s Director of Chemical and Environmental Compliance.
“MIOX representatives claimed that we would see better biocontrol with this system. And I would say, based on this summers’ data, that is definitely true for Legionella bacteria counts,” Valenkamph said. “The Unit 15 cooling water system also experienced a longer summer run without a cleaning than we have historically experienced.”
Unit 15 is normally the dirtiest cooling tower and has to be cleaned regularly to clear condenser tubes. After a long run, though, the condenser was brought down for a scheduled maintenance check in June.
“The U15 condenser was cleaner than we’d ever seen it before,” Barnes said. “And there isn’t any other change in the system to explain these results, other than the conversion to this new biocide system.”
Financially, the system has delivered very real savings. The Unit 14 tower’s treatment employs sodium hypochlorite ($65,000/year), sodium bromide ($85,000/year), and an algaecide ($75,000/year) – for a total cost of $225,000/year. Meanwhile, the MIOX-treated Unit 15 tower (with yearly forecast estimated on the high side) has annual expenses of about $34,000 in salt, $30,000 in electricity, and $2,500 in pleated 5-micron iron filters – for an approximate cost of $66,500/year. The annual savings with the system amounts to about $158,500/year.
Additional savings have been realized by eliminating the man-hours for scheduling, delivery/unloading, and the separate tanks and containments required for the previous treatment regimen – not to mention the elimination of the risk and handling hazards associated with the treatment chemicals.