By Timothy Keister
Due to purchase and operating economics, ‘wet’ cooling towers are the technology of choice for commercial and industrial cooling systems as water is the best material for both transfer of heat and evaporative cooling. One drawback is that such use presents a biological control problem as warm water loaded with nutrient and organics is an excellent medium for growth of microorganisms.
Cooling towers at a public school
Growth of microorganisms in cooling water is further encouraged by the use of reclaimed wastewaters in the makeup and increased cooling tower cycles of concentration. These are current trends driven by fresh water shortages, increased water and sewer charges and stricter environmental regulation. The uncontrolled growth of microorganisms in cooling water causes severe problems related to increased risk of Legionnaires disease, plugging due to physical blockage of cooling water passages, accelerated corrosion under biological masses and reduced heat exchanger efficiency due to biofouling of surfaces.
Present practice and problems
Current cooling water biological control technology depends upon various toxic and hazardous chemicals commonly termed ‘biocides.’ These include chlorine, ozone, chlorine dioxide, dithiocarbamate, isothiazolin, hydantoin and glutaraldehyde. While these biocides are often quite effective, their use represents substantial environmental, health, and safety concerns as there are over 300,000 cooling towers in the United States using an estimated 40 million pounds of such chemicals on an annual basis.
Use of biocides is everywhere, as cooling towers are found throughout our country in neighborhoods, towns and cities. In addition to typical industrial installations, cooling towers are commonly found at hospitals, hotels, grocery stores, office buildings, warehouses, apartment buildings, schools, colleges and retirement homes. It is anywhere air conditioning or process cooling is needed.
Gas form oxidizing biocides such as chlorine, chlorine dioxide and ozone present a serious occupational safety issue, as low water solubility, reagent spills and leakage can result in exposure of workers to toxic levels of the gas and explosion hazards.
Liquid oxidizers such as sodium hypochlorite and dibromosulfamate are corrosive and reactive. Exposing workers to these may generate chemical burns, toxic gas evolution and explosion hazards. Solid oxidizers such as hydantoin are quite reactive and can self ignite when mixed with many organic materials such as sawdust or flour.
Chlorine gas is commonly used in larger cooling water applications due to its low cost and because it is often present in large amounts on site. This chemical is extremely toxic if used improperly and any release in large amounts represents a major risk for fatalities and serious injury within both the using facility and the surrounding community.
Non-oxidizing biocides in common use represent a substantial worker hazard due to high toxicity values, with several of the products being readily absorbed through the skin. Table 1 summarizes some relevant toxicity data on five chemicals commonly used as cooling water biocides.
Smaller users such as the vast majority of cooling tower operators represent a special worker safety concern. This is because cooling water treatment and the application of biocides is often the responsibility of workers who may not be adequately trained in handling hazardous chemicals.
Environmental considerations
The widespread transport, storage and use of biocides presents many opportunities for accidents that would result in release of these products into the environment with generally severe results. Both oxidizers and non-oxidizers are extremely toxic to most aquatic life and even small product spills and leaks can produce catastrophic effects. Table 2 summarizes some aquatic toxicity data for several commercial cooling water biocides along with the typical cooling water dosage range.
Cooling towers, being basically evaporative coolers with about 80 percent of the input heat load being removed by evaporation, rapidly increase cooling water solids content. This requires that routine blowdown, concentrated cooling water wasted to sewer, is necessary to prevent scale formation. Typically operating at four cycles of concentration, a cooling tower will evaporate 2,655 gpd and blowdown 885 gpd per 100 tons of thermal load.
This blowdown has been recognized as a substantial source of highly toxic chemical input to the environment dependent upon the biocide(s) and discharge treatment in use. Several cases have been identified where environmental agencies have either banned the use of, or required treatment for, various biocides prior to direct stream discharge of blowdown. In the case of a smaller publicly owner treatment works (POTW) and a large blowdown discharge, the POTW mixed liquor bio mass could be easily wiped out by the biocide content of the blowdown.In addition to the intentional blowdown discharged from cooling towers, spillages of concentrated biocide products are a substantial threat to operation of POTW. For instance, a spill of 55 gallons of a biocide product like 30 percent carbamate could totally wipe out an activated sludge plant with a mixed liquor volume of up to 1.3 million gallons. Typical is a case a few years ago where a carbamate product was discharged into a POTW in Indiana resulting in a fish kill in the White River.
Since most non-oxidizing biocides are long lived, difficult to neutralize, and not easily degraded in the environment, oxidizing biocides which are easily neutralized by addition of a reducing reagent to the blowdown stream are preferred from the standpoint of minimizing the environmental impact of cooling tower blowdown, or product spills. Oxidizing biocides, however, still present significant hazards during transport, storage and use.
Bromine, an oxidizing biocide in its various delivery methods, has been recognized as a superior cooling water biocide for many years. Unfortunately, the delivery methods all suffer from the same environmental, health, and safety issues as other oxidizers. Plus they are somewhat costly. Use of on-site electrolysis to make aqueous electrolytic bromine is appealing as sodium bromide solutions are non-hazardous and relatively low in cost.
The electrolysis process is time proven, having been used for over 100 years with the industrial production of both chlorine and bromine. The problem with existing electrolysis technology for manufacturing aqueous electrolytic bromine is mainly economic, in that platinum-plated titanium is used in construction of electrolysis cells that operate with a typical bromide-to-bromine conversion efficiency of just 35 percent.
Green biocide delivery system
Given the advantages of bromine use for cooling water biological control, a project was started in 2001 to devise a cost effective electrolysis- based delivery technology to make aqueous electrolytic bromine on site. An initial patent application was filed in May, 2002.
The project resulted in development of a new delivery technology to produce it from a non-hazardous precursor bromide salt solution. The process is based on a unique containerless electrolytic cell constructed of impregnated electrolytic graphite. This is done at a much lower cost than existing design electrolysis cells.
Containerless graphite electrolysis cell
A second innovation is use of a mixed solution of sodium bromide and chloride salts to obtain a high conversion rate from bromide ion to electrolytic bromine. Both liquid and solid separate and mixed precursor salt products have been registered with the US EPA, as biocides and the electrolytic units are manufactured in a US EPA registered facility.
Electrolytic bromine produced by the new cell design has been determined to be an aqueous mixture of bromine, hypobromous acid and hypobromite produced by electrolysis (these are toxic- don’t know if I’d call this a “green” biocide despite the potential improvement). It is of a minimum 1:2 molar ratio of sodium bromide and sodium chloride according to the following equations:
- 2 Cl = Cl2 + 2 e
- Cl2 + 2Br – = 2Br + 2Cl- (bromine)
- 2 H2O + Cl2 = HClO- + HCl + H2O
- HClO- + Br- = HBrO- + Cl- (hypobromous acid)
- 2 Br = Br2 + 2 e (bromine)
- 2 OH- + Br2 = BrO- + Br- (hypobromite)
Note that both Cl- and Br- recycle within the reaction sequence, increasing the reaction
efficiency for bromide conversion to oxidizing species to about 85 percent at a 1:2 molar ratio of bromide to chloride, increasing to almost 100 percent at a 1:3 ratio.
Health and Safety
The oral toxicities of the two salts used in the electrolysis process, sodium bromide at 3,500 mg/kg and sodium chloride (table salt) at 3,000 mg/kg, are far higher than any other biocides, making them (for practical purposes) nontoxic as to worker and public exposure. Aqueous salt solutions are, of course, even less toxic due to dilution.
As the electrolytic bromine solution produced by the process is made ‘as needed’ and immediately fed into the cooling tower water, there is essentially no worker exposure to the oxidizing product, minimizing health and safety risks. At the design 0.8 percent oxidizer content, the output of the electrolysis cell is below Occupational Safety and Health Administration (OSHA) hazardous designation level of 1.0 percent for oxidizers.
Environmental considerations
When considering precursor spills, solid sodium bromide and chloride salts and their aqueous solutions and materials have very high aquatic toxicity values. These present very little danger of environmental damage from accidental discharges as compared to other biocides. For instance, a 40 percent sodium bromide solution has a 96 hr LC 50 of > 1,000 ppm for rainbow trout, as compared to 1.5 percent isothiazolin at 0.14 ppm.
The recommended dose of electrolytic bromine for typical cooling waters is 0.5 to 1.0 ppm measured as total bromine. Following a dose, the electrolytic bromine degrades within one or two hours to harmless bromide ion that is present in seawater at 65 ppm. Many cooling tower controllers can be programmed to ‘lock out’ blowdown during and for a set time after a biocide feed event. By properly programming the cooling tower controller, any discharge of electrolytic bromine in cooling water blowdown can oftentimes be avoided.
In some cooling systems, due to makeup water characteristics or specific thermal requirements, it may be impossible to lock out blowdown for the required time to degrade the electrolytic bromine. In this case an appropriate feed of a reducing agent (such as sodium sulfite) into the blowdown can be used to destroy the residual biocide; however, considering that typical sanitary wastewater is highly reducing, discharge of electrolytic bromine treated cooling water blowdown to sanitary sewers will not present a problem unless the blowdown flow is a very significant portion of the total flow to the receiving POTW.
What is “Green”
The word “green” is today being applied to more and more products in commerce and generally indicates that the product so designated has superior attributes from the environmental standpoint. In the cooling water management business, we are seeing “green” applied to a number of different products; non-chemical devices claim to be green as their use eliminates discharge of “hazardous” chemicals to the environment, solid feed products claim to be green due to reduced potential for hazardous chemical spills, while at least one chemical biocide is claimed to be “greener” than others due to reduced product toxicity.
Given the obvious commercial appeal of calling your product green in today’s marketplace, an independent definition of “green” is needed. Our friends at the USEPA have kindly provided their view point by using the definition in “The Twelve Principles of Green Chemistry”[1]. Review of this document shows that several of these principals can be applied to the cooling water management business as follows:
- Design safer chemicals and products
- Use renewable feedstocks
- Design chemicals and products to degrade after use
- Minimize the potential for accidents
The other eight principals, involved with chemical manufacture, do not apply to the majority of cooling water program suppliers as they do not make active product ingredients, such as polymers and phosphonates.
Is It Green?
Considering electrolytic bromine as a biocide against the four USEPA green principals which generally apply to the cooling water management field we find:
Design safer chemicals and products: The precursor chemicals used to produce electrolytic bromine, sodium bromide and chloride, are substantially safer than any biocide in current use. Electrolytic bromine solution, while a potent biocide, is much safer to handle than other products as it is a low strength, aqueous solution and is only made on demand and immediately dosed, so there is little product subject to accidental spillage.
Use renewable feedstocks: Ultimately, the sodium bromide and chloride used in the process return to the sea, which is a source of both compounds. For the ultimate “green” process, renewable power such as wind or solar, could be used to power the electrolytic reactions.
Design chemicals and products to degrade after use: Electrolytic bromine rapidly degrades back to harmless salts after use.
Minimize potential for accidents: As the precursor chemicals are non-hazardous and the electrolytic bromine is manufactured on demand and immediately dosed, the potential environmental and health and safety hazards associated with any type of accidental discharge is minimized.
Economic Considerations
Capital cost for new electrolytic bromine units is generally about 30 percent of the cost of equal capacity units based upon containerized, platinum-plated, titanium electrode technology. For cooling water biocide use, commercialized units are available in outputs ranging from one to 30 lbs/per day as bromine.
A one-pound per day unit would usually be suitable for cooling towers with a thermal capacity up to 500 tons and cost about $1,500. A 30-pound per day unit is currently in service at a 90 MW power station (approximately 37,700 tons of thermal load), with a selling price of $22,000.
Comparison of the cost to operate the new electrolysis process, as shown in Table 3 for a cooling tower in terms of $/1,000 gallons of cooling water treated, shows the process provides a substantial operating cost reduction over many commonly used biocides.
Power cost to operate the electrolytic process is minor, at $0.10/kwh. The power cost calculates as $0.17/lb bromine, or $0.04/1,000 gallons cooling water treated, increasing the total cost to $0.28. (Note: pricing current as of 2007)
Proven technology
Following six months of field trials, the first commercial electrolysis process units were installed in June, 2003 and have proven to be a cost-effective, reliable means of controlling the growth of microorganisms in cooling waters. The first three units installed, two in Pennsylvania and one in Indiana, are still operating. A paper presented at the 2004 International Water Conference in Pittsburgh reports upon a one-year demonstration, where chlorine gas use as a biocide was totally replaced by electrolytic bromine at an 1100 MW power station.
Of great interest was that the operating cost for this installation, using separate salt solution feeds, was determined to be the same as chlorine gas. This makes the electrolytic bromine process a very economical alternative technology.
Given the environmental, health and safety hazards presented by current biocide technology and the proven advantages of the new electrolysis process, it can be expected that electrolytic bromine will eventually become the biocide of choice.
A 30 lb/day bromine output unit; note the separate power supply, electrolysis cell and salt solution delivery pump.
The electrolytic bromine process discussed is patent pending and has been commercialized under the trademark names ElectroBrom and MiniBrom by ProChemTech International, Inc.
Case history reports and other referenced material can be downloaded from the ProChemTech web site – www.prochemtech.com
References
- Mantell, C. L.; Industrial Electrochemistry, 3rd edition, McGraw-Hill Book Company, Inc, 1950.
- Frayne, C.; Cooling Water Treatment Principles and Practice, Chemical Publishing Company, Inc., 1999.
- Gill and Keister; Development of an On-Site Hypobromite Generator, Cooling Technology Institute, paper TP04-15, 2004.
- Lawler, Keister and Teague; Demonstration of an On Site Hypobromite Generator at a Power Generation Station, International Water Conference, paper IWC-04-19, 2004.
- Keister; Electrolytic Bromine: A Green Biocide for Cooling Towers, Water Environment Federation Industrial Water Quality Conference, 2007
About the author
Timothy Keister, B.Sc. Ceramic Science, PSU 1973, 13 years experience as corporate water/wastewater manager for a Fortune 500 firm, Chief Chemist/President of ProChemTech International since in 1987 (a water management program supplier). Professional activities have included election as a Fellow of the American Institute of Chemists; Senior Member of the American Institute of Chemical Engineers and memberships in the American Chemical Society, Water Environment Federation, Cooling Technology Institute and Association of Water Technologies. Certificates and licenses include Certified Water Technologist (CWT) and operators licenses in New Jersey, New York, Pennsylvania and Indiana. Keister currently serves as Chairman of the Brockway Area Sewerage Authority and is Technical Director of the Toby Creek Watershed Association.