This credit focuses on achieving an appropriate number of condenser water cycles in a cooling tower based on the concentrations of various water quality criteria, such as dissolved solids.

The credit has been a part of LEED-EBOM for some time, but is a new addition to the BD&C rating systems.

What do cycles have to do with saving water?

The key parameter used to evaluate cooling tower operation is cycles (sometimes referred to as cycles of concentration or concentration ratio).

For a well-managed condenser water system, a cycle represents the extent to which water is used efficiently before being discharged via blowdown (where water is drained from cooling equipment in order to remove mineral build-up). A cycle is calculated as the ratio of the concentration of dissolved solids (or conductivity) in the blowdown water compared to the make-up water.

From a water efficiency standpoint, you want to maximize cycles as this will reduce the amount of waste blowdown water and reduce make-up water consumption. The trick, however, is that dissolved solids increase as cycles of concentration increase, which can cause scale and corrosion problems unless carefully controlled. So this LEED credit sets a cap for five control parameters that typically play the biggest role in scale and corrosion.

Calculating the maximum cycles

Your team’s job is to measure the level of each control parameter in the cooling tower make-up water and then achieve the maximum number of cycles without exceeding the maximum concentration level for any of the control parameters.

The example in the Documentation Toolkit from the LEED v4 Reference Guide shows how to calculate the maximum cycles for your system.

The maximum cycles of concentration will vary depending on your system and the concentration of solids in the makeup water serving the cooling tower. That said, it is common for cooling towers to operate in the range of 5–7 cycles.

If you can, leverage your building’s future cooling tower or water treatment vendor to assess the control parameters and implement a plan to meet the maximum cycles of concentration. These vendors can also help determine if additional chemical treatments can be employed in order to reach the 10 cycles required to qualify for an additional LEED point. If you don’t have a vendor on board yet you can engage another water treatment professional to perform the analysis. Alternatively, the cooling tower or evaporative condenser manufacturer may offer a water test kit.

What does each of the control parameters mean?

Here’s how each of the parameters can affect the performance of your cooling tower. 

Ca (as CaCO3): Calcium (as Calcium carbonate)

Calcium carbonate is a very common form of scale found in cooling towers. Scale reduces a cooling tower’s heat exchange efficiency by insulating equipment. Because of this, a major goal of most cooling tower chemical treatment programs is to prevent scale build-up.

Total alkalinity

Alkalinity is an indicator of acid neutralizing or acid buffering minerals in the water. High concentrations can lead to scale build up.

SiO2: Silica

Silica is one of the impurities that frequently play a big role in limiting the maximum cycles of concentration for a cooling tower. As concentrations increase past maximum levels, silica is likely to form scale deposits and insoluble sludge in the cooling tower.

Cl: Chloride

Chloride can be corrosive to most metals, which decreases the performance or longevity of the cooling tower.

Conductivity

Conductivity is a measurement of the water’s ability to conduct electricity. It’s also an indicator of the total dissolved mineral content of the water, since higher conductivity levels correlate to more dissolved salts in the water. By this logic, water with very little minerals present (think purified water) has very low conductivity.

High conductivity levels indicate increased risk of scale build up and lower performance for the cooling tower.

Preventing Legionnaires Disease

Legionnaires Disease is a serious disease, and it's worth taking a minute to learn about it and how to prevent it. The disease is caused by bacterium found in potable and nonpotable water systems, such as cooling towers. Outbreaks in 2015 occurred in New York and California, serving as a reminder that proper management of biological growth should be an ongoing priority.

When Legionnaires develops in a cooling tower, it is transmitted to people as small droplets of water that contain the bacteria are released into the air. The bacterium is spread by water vapor only; it cannot be transmitted by infected individuals. When contracted, Legionnaires is a very a serious illness that can be lethal if left untreated.

In order to prevent contamination of your cooling tower, the Occupational Safety and Health Administration (OSHA) recommends the actions below. Note that these activities represent best practice strategies for maintaining your cooling towers; the LEED v4 credit doesn’t require that teams implement these actions.

• Add chemical biocides to control Legionnaires growth. Obtain information on appropriate biocide selection and use from equipment manufacturers or from companies experienced with the particular system used.
• Treat circulating water for control of microorganisms, scale, and corrosion. This should include systematic use of biocides and rust inhibitors, preferably supplied by continuous feed.
• Clean and disinfect cooling towers quarterly or at least twice a year if the unit is not used year round. Do this before initial start-up at the beginning of the cooling season and after shutdown in the fall.
• Any system that has been out of service for an extended period should be cleaned and disinfected.
• New systems require cleaning and disinfecting because construction material residue can contribute to Legionnaires bacteria growth.
• Design features that minimize the spray generated by these systems are desirable.

In addition, we recommend locating your cooling towers away from air intakes and operable windows to minimize unnecessary exposure risks.

How cooling tower materials affect cycles of concentration

Owners and design teams should know that the material used to construct a cooling tower can determine the maximum concentration levels a given manufacturer allows without voiding the warranty. Stainless steel typically costs more than galvanized steel but allows for higher concentration levels of certain parameters.

Reviewing the options from this perspective is prudent when considering a new tower. Manufacturers can provide recommended concentration levels for galvanized and stainless steel. Keep in mind that recommended concentrations would be specific to the particular manufacturer and piece of equipment that is installed in your building.

What’s New in the LEED v4.1 beta

This credit has two new options that broaden the credit’s scope, and the new credit title—Cooling Tower and Process Water Use—reflects that.

• Option 1 is mostly the same as the v4 version of the Cooling Tower Water Use credit.
• Option 2 now gives credit for not having a cooling tower (if the baseline system would have had one).
• Option 3: Process Water Use allows projects to achieve 1 or 2 points if they use recycled water for process water uses.