Facilities are under continuous pressure to reduce costs and operate in a more environmentally responsible manner. Historically, energy conservation has overshadowed water conservation in these efforts. In many locations, however, the reduced availability and increased cost of water make water conservation a much more attractive target than in the past. More facilities are being limited on how much water can be used and how many gallons of wastewater can be discharged. While there are alternatives to oil and gas, there is no substitute to water. The economic and environmental payback on reducing water usage is greater than ever.
As a large consumer of potable water, evaporative cooling water systems, including cooling towers, are an obvious target for water conservation efforts. Because of supply and cost pressures, more facilities are considering alternate makeup water sources such as air-handler condensate, rain water and reclaim water. In some cases, alternate water sources are being blended with potable water to improve water use efficiency.
It is important to understand the impurities in an alternate makeup water source and how they will impact cooling tower operation before substituting for potable water. Some water sources can be used successfully without further treatment while others require additional measures to control problems related to corrosion, scale and microbiological growth. An engineering and economic analysis is necessary to determine both the feasibility and potential cost savings. In general, the higher the cost and poorer the quality of the potable water source, the better the payback associated with using an alternate source. As with any water conservation product, water meters are essential to monitor water usage and benchmark improvements.
Air-Handler CondensateIn warm, humid climates, the normal operation of air-conditioning equipment produces large quantities of condensate from the air-handler cooling coils. This cold, near distilled quality water typically is sent to the drain. In some cases, this water can be readily recovered for other uses, including cooling tower makeup. Because air-handler condensate does not contain the dissolved mineral impurities present in potable water, using this cold, nearly pure water for cooling tower makeup water also allows operation at lower bleed rates.
Because the quantity of condensate generated by a cooling coil is a direct function of the airflow rate and change in the relative humidity, it will vary considerably throughout the year. In some locations, the amount can be staggering. For example, a 1,000 ton cooling load in a commercial facility with 25 percent outside air produces 4.7 gal/min of condensate when the outside air temperature is 85°F (29°C) with 80 percent relative humidity and 2.7 gal/min with 60 percent relative humidity. Case studies have shown that 10 to 40 percent of cooling tower makeup requirements often can be met with air-handler condensate in facilities that operate in warm, humid climates such as Florida, Georgia, Alabama, Mississippi, Louisiana, East Texas and along the east coast of the United States.
Depending on the location of the air-handling units and the cooling towers, the collection system can be as simple as a gravity fed tank with a level controller and pump. Corrosion-resistant tanks and piping are recommended due to the corrosivity of such pure water. Typically, an oxidizing biocide is added to the condensate storage tank to prevent microbiological problems. In facilities with a central chiller plant, a system can be engineered to use the air-handler condensate for makeup water to the chilled water loop. A competent water treatment professional should be consulted to design a program to accommodate air-handler condensate quality makeup water. Each facility is different, but the water savings and payback on using air-handler condensate for cooling tower or closed system makeup water can be significant.
Rain WaterAlthough unpredictable, rain water is a good alternate source of water in some areas. Approximately 0.62 gal of water can be recovered per inch of rain per square foot of collection surface. For a 2,000 ft2 roof, this translates to more than 1,200 gal of water per inch of rain.
A typical rain water recovery system directs rain water from the roof to a storage tank rather than a storm sewer. This cache of water then can be used for various purposes throughout a facility, including irrigation and cooling tower makeup water. Parking decks also can be used to harvest rain water, but special media filters are required to remove oil and other contaminants prior to use.
Like air-handler condensate, rain water is relatively pure. When collected from a roof, it can be used as cooling tower makeup water with little treatment other than the addition of an oxidizing biocide to the storage tank to minimize microbiological growth.
Reclaimed WaterReclaimed water, or recycled water, is highly treated wastewater from a municipal wastewater treatment plant. This non-potable water source is delivered to a facility in purple or lavender colored piping to distinguish it from potable water supplies. It is suitable for irrigation, deep well injection and possibly for open recirculating cooling systems.
As compared to potable water, reclaim water contains a higher concentration of hardness and alkalinity, which increases the tendency to form calcium carbonate scale deposits. It also can contain appreciable levels of phosphate and ammonia - components not normally present in significant amounts in potable water.
The increased phosphate content in reclaimed water also makes calcium phosphate scale formation in heat exchange equipment a concern unless the proper treatment measures are taken. These measures may include operating the cooling tower at a higher bleed rate, using sulfuric acid for pH control, and adding a phosphate-specific polymeric dispersant.
Although reclaimed water is chlorinated to control pathogenic microorganisms, there is an increased potential for microbiological problems when used in cooling tower systems. In part, this is related to the presence of ammonia and other nitrogen compounds in reclaimed water, which are nutrients that support bacteria growth. Reclaimed water also has higher background bacteria levels than potable water.
Because of the increased potential for corrosion and deposit problems related to bacteria growth, additional biocides usually are required. A combination of oxidizing and non-oxidizing biocides usually works best. Due to its unique properties, chlorine dioxide is often an excellent biocide for this application. It is a selective oxidizing biocide that can provide effective microbiological control at low levels. New technologies greatly simplify chlorine dioxide generation and application, making this an attractive alternative biocide.
Reclaimed water can provide a large cost savings. For example, a facility in central Florida with four 1,250 ton chillers was offered free municipal reclaimed water for use in the cooling towers. By modifying the water treatment program, the facility was able to use reclaimed water successfully to reduce potable water use by more than 35 million gallons a year. This provided a net savings of more than $65,000 annually and a five-month payback on the required equipment modifications.
Blended WaterBlending one or more alternate water sources also can reduce the overall water usage and costs associated with operating a cooling tower system. In some applications, it may be necessary to blend potable water with reclaim water to provide a makeup water quality that is suitable for use in cooling towers. Where there is excess water softener or reverse osmosis (RO) capacity, blending with high alkalinity, high hardness potable water can allow a cooling tower to operate at greatly reduced bleed rates. Alternate water streams such as RO reject water also may be suitable for use as tower makeup with blending.
Consistent blending is important when considering a blended water supply for cooling tower makeup. It is difficult to control the bleed rate and treatment levels when the makeup quality varies widely. Equipment systems can be engineered to accurately blend two or more water supplies.