Module 4: Water Efficiency

Common Options for Reducing Domestic Water Use

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Full-service kitchens/cafeterias

Many institutional/commercial buildings have onsite full-service restaurants or cafeterias that use water in sinks, pre-rinse sprays, steam tables, ice machines, drip trays, cleaning, and dishwashers. Sinks are used for supplying water in preparing food and drink, and for dishwashing. The EPAct standard for non-lavatory faucets is 2.2 gpm. Pre-rinse spray valves are used for pre-washing dishes before they are put through a commercial dishwasher. The Energy Policy Act of 2005 sets a federal minimum efficiency standard for pre-rinse spray valves at 1.6 gallons per minute. No EPAct standards exist for steam tables, ice machines, drink machines, or commercial dishwashers. Typical water use can average 40.5 gal/h for steam tables, up to 90 gallons of water per 100 pounds of ice for ice machines, 3 gpm for soda machines and ice cream drip trays, and 7.0 gpm for commercial dishwashers.

Low-flow faucets and aerators are available for kitchen sinks as previously discussed. Steam tables can be replaced with units that are more water-efficient or that recirculate water, such as connectionless units that use less than 2 gallons per hour or 95% less than older continuous fill types. Air-cooled ice machines can use up to 10 times less water than older water-cooled units. You can replace commercial dishwashers with more-efficient units that use as little as 2.0 gpm or approximately 70% less than older commercial dishwashers.

Figure 6. Pre-rinse Spray Valve

Studies show that garbage disposers can waste a significant amount of water, and contribute to fats, oils, and grease blockages in sewer lines. You should minimize their use, or eliminate them from your kitchen operations. Many facilities use strainers or traps that employ a mesh screen to collect food waste for proper waste treatment. Another option is to install strainers in sinks, leaving the food matter in the sink for disposal in trash receptacles or composting units.

Cooling towers

Cooling towers can account for up to 20 percent of an institutional building’s water use. They represent the largest single opportunity for water reuse in institutional and commercial buildings. Cooling towers work by pumping water that is to be cooled to the top of the tower, where it is sprayed or dripped through wet decking. Air blown through the falling water over the wet decking causes evaporation. The heat lost or rejected through evaporation lowers the temperature of the remaining water. The water then circulates though a cooling system or equipment, where it removes heat from the process, and returns to the tower.

Cooling towers need to be expertly operated and maintained to ensure efficient operation. Depending on the quality of makeup water that is used, you can attain various cycles of concentrations. In general, you should strive for a minimum cycle of concentration of 6 to achieve significant water savings. You can rely on regular water-quality testing to help ensure suspended-solid concentrations remain at recommended levels and to reduce bio-fouling.

For more detailed information on cooling towers, see North Carolina’s guide on Water Management Options.

Some water-efficiency opportunities for cooling towers include:

Optimizing cycles of concentration
Using alternative sources of makeup water
Installing and properly maintaining drift eliminators
Using automatic controls to control blowdown
Sub-metering makeup water and blowdown
Implementing an effective maintenance program
Ensuring that towers are appropriately sized for the cooling load
Installing variable-frequency drives (VFDs) on motors
Installing automatic shutoff control devices
Using sidestream filtration
Working with cooling tower vendors on performance contracts or alternative-treatment programs

Some facilities have the opportunity to reuse water from other processes for cooling tower makeup water. For example, Emory University uses recovered condensate water from a laboratory building as makeup water for its cooling tower. This approach is saving an estimated 200,000 gallons of water per year. Other examples of water that can be reused as makeup water is reject reverse-osmosis water, wastewater from once-through cooling systems, and municipal wastewater effluent.

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Single-pass cooling

Frequently facilities use water for “once-through” cooling of small heat-generating equipment. Typical once-through water-using equipment includes CAT scanners, ice machines, degreasers, vacuum pumps, welding machines, air compressors, condensers, X-ray equipment, and even some air conditioners. This is an extremely inefficient use of water as it is used only once before being sent to the sewer. Cooling towers that have a cycle of concentration of 1 are also considered single-pass cooling.

Some cities—such as Phoenix and Seattle—have banned the use of certain types of once-through cooling equipment.

Some water-efficiency opportunities that can be considered for single-pass cooling include the following:

Reusing the cooling water for other processes, such as boiler makeup water, landscaping, rinsing, or washing.
Reducing the cooling water flow and monitoring the temperature differential between the influent and effluent water in the single-pass system to make sure it’s within the parameters established by the manufacturers.
Installing automatic shutoff controls to turn off the system when not in use.
Using a cooling tower or other heat-absorbing medium to allow the water to be recycled.

Boilers

Boilers are typically used in commercial/institutional buildings for heating, steam generation, and hot-water delivery. Boilers use varying amounts of water, depending on their size, design characteristics, amount of condensate returned, and the presence and severity of leaks. They require makeup water to compensate for uncollected condensate or to replace blowdown water. These units have a tendency to develop leaks as they age which wets the insulation and causing both heat and water losses.

“Scaling is one of the leading causes of boiler tube failures. Scale is equivalent to having a thin film of insulation between the furnace gases and boiler water. It can drop a boiler’s efficiency by as much as 10–12%.” (Office of Industrial Technologies, U.S. DOE)

For more detailed information about boiler efficiency, see the U.S. Department of Energy Industrial Technologies Program’s Steam Systems Energy Efficiency Handbook at: www1.eere.energy.gov/industry/bestpractices/pdfs/steamhandbook.pdf].

Installing a condensate recovery system that reuses condensate water as makeup water. These systems can save up to 70 percent of the water used in a boiler, and significantly reduce energy costs.

Some water-efficiency opportunities that can be considered for boilers include the following:

Adopting a leak-detection program to guarantee that all of the water in the condensate recovery system is being returned.
Controlling scale in boilers by adopting a preventive-maintenance program for mechanical cleaning and chemical treatment.
Installing automatic controls for chemical-treatment systems.
Re-insulating the boiler or hot water lines that have become worn or damaged by water.

Source: Dept. of Energy, Federal Energy Management Program, Water Efficiency, Best Management Practices

Another way to maximize water and energy efficiency in boilers is to control blowdown, which reduces the concentration of impurities in the boiler to acceptable levels by releasing water from the boiler and making it up with fresh water. The precise volume of blowdown is important because excessive blowdown leads to the loss of water and energy, but insufficient blowdown can lead to the buildup of impurities and scale. Blowdown can range from 1–25 percent, based on the condition and age of the boiler as well as the quality of the local water.

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