Targeting Process Water Reduction in LEED v4

One of the many changes with the new version of LEED is a spotlight on process water consumption in buildings.

What is “process water”?

Process water is water that is used by machines in everyday building operations and by industrial processes. In building operations large amounts of process water may be used by on-site equipment such as chillers, boilers and evaporative condensers or cooling towers which are part of the HVAC system.

Because process water use is mostly behind the scenes, its use is typically invisible to building occupants. However, according to USGBC, the volume of process water used in buildings can be more intense than water used for toilet flushing, fixtures or irrigation.

Where Does Process Water Come From?

Unfortunately, process water typically comes from the same potable (drinkable) municipal water we consume from the tap.

Green building design emphasizes the use rainwater or greywater, as a portion of building process water. Greywater is water that has previously been used for another process, such as handwashing or showers, but that has not come in contact with toilet water (aka black water).

Projects that incorporate rainwater harvesting, graywater reuse, or other reused water sources are eligible to earn additional points toward higher LEED certification levels.

LEED v4 and Process Water

Within the revised LEED v4 Water Efficiency credit category, there are five credits and/or prerequisites that aim to reduce water use inside green buildings:

• WE Prerequisite Indoor Water Use Reduction
• WE Prerequisite Building Level Water Metering - New Prerequisite
• WE Credit Indoor Water Use Reduction
• WE Credit Cooling Tower Water Use
• WE Credit Water Metering

The most significant opportunity to make an impact on process water in building efficiency, in both potable water and energy efficiency, is from within the Cooling Tower Water Use credit.

The new credit aims to help managers both better control cooling tower concentration cycles and introduce non-potable sources of makeup water.

Behind the Scenes

Building HVAC systems are constantly removing heat from the air to cool interior building spaces.

Evaporative coolers, such as cooling towers, rely on water to remove heat from the air through evaporation, which causes the water to absorb heat before changing to vapor. The vapor is then released through the top of the cooling tower.

If you’ve ever driven by a nuclear power plant and noticed plumes of white “smoke” rising from these hyperboloid structures, this is actually water vapor (fog or steam) rising as it’s released through the evaporation process. The evaporation process requires new water to be added to the cooling tower to continue the cycle.

This diagram from “The Alliance for Water Efficiency” shows how the cooling tower process causes water to constantly evaporate, requiring more fresh supply or “makeup water”.

LEED outlines several alternatives for non-potable makeup water sources for cooling towers including condensate from air conditioners, steam systems and ice machines condensate, rainwater, fire pump test water and food steamer discharge water.

Even with the continuous addition of fresh water, process water used in cooling towers becomes “hard” with mineral build up and must be flushed. The flush out, also called “blowdown” involves dumping the process water into a wastewater drain before replacing it with new fresh water.

However, blowdown water can act as an alternative water source to meet any outdoor irrigation needs

The Efficiency Tradeoff

Cooling tower blowdown has the potential for further reuse in landscape irrigation, thereby reducing outdoor water use. However, a tradeoff exists.

Cooling towers that save water by discharging less frequently may have a level of Total Dissolved Solids (TDS) that is too high for use in irrigation of most plants. In general levels between 501-1,000 ppm of TDS may kill the plants. Irrigation experts indicate that levels <500 ppm of TDS is best for plants.