Reclaimed Water for Data Center Cooling

Reclaimed water, also called recycled water or treated effluent, is wastewater that a municipal or regional treatment plant has cleaned to a standard suitable for a specific reuse. Instead of being discharged to a river or ocean, it is piped to customers such as golf courses, irrigation districts, power plants and industrial users. In many places these pipes are color-coded purple to separate them from drinking water lines.

Treatment levels vary. Secondary treatment removes most organic matter and solids. Tertiary treatment adds filtration and disinfection, and sometimes nutrient removal. States set the quality classes and the uses each class may serve, and the rules differ from state to state. Cooling tower make-up is a common permitted use in states with reuse programs, often with conditions on treatment and on managing aerosols from the towers.

For basics on how cooling design drives total water demand, start with our guide to data center water requirements. This guide goes deeper on reclaimed water specifically.

Reclaimed water is most valuable when the data center uses evaporative cooling, such as cooling towers or evaporative air systems, because those designs consume large volumes. A mostly dry-cooled or closed-loop campus uses little water and may not justify a pipeline. The choice of cooling design, covered in our guide to air vs. liquid cooling site implications, should come before the water source decision.

• Water-stressed regions, where drinking water supplies are limited or under scrutiny.
• Communities where water use is a central concern in the approval process.
• Sites within a reasonable distance of a treatment plant with spare effluent.
• Utilities that want a large, steady reuse customer, sometimes to reduce their own discharge.

It fits less well when the nearest plant is small or far away, when its effluent is already committed to other users, or when state rules make reuse permitting slow.

Many campuses use more than one source: reclaimed water for cooling, potable water for domestic use and fire protection, and potable or groundwater as a backup for the cooling supply.

Reclaimed water usually carries more dissolved solids, nutrients and organic material than drinking water. In a cooling tower, evaporation concentrates those constituents. That raises the risk of scaling, corrosion, fouling and biological growth, and it limits how many times water can be cycled before it must be bled off as blowdown.

• Phosphate, ammonia and other nutrients can encourage biological growth and affect corrosion.
• Hardness, silica and total dissolved solids limit cycles of concentration, which increases blowdown volume.
• Chloride and sulfate affect corrosion of piping and heat exchangers and may influence material choices.
• Biological control matters for both equipment and health. Legionella risk management for cooling towers, guided by ASHRAE Standard 188 and state or local rules, applies regardless of source.

Most projects handle this with on-site treatment ahead of the towers, such as filtration, softening or other side-stream systems, plus a chemical program tuned to the supply. Ask the treatment plant for several years of effluent quality data, not a single sample, so the design team can plan for seasonal variation.

The pipeline is often the largest single piece of a reclaimed water plan. Someone has to design, permit, fund and build it, and secure easements along the route. It may cross roads, streams and private land, which brings road permits, possible wetlands review and easement negotiations. Shorter and simpler routes make a big difference.

The supply agreement should answer:

• How much water is committed, at what peak daily rate, and for how long.
• What quality the plant will deliver, and what happens if it falls short.
• Who owns, operates and maintains the pipeline and pumping.
• How the price is set and how it can change.
• What priority the data center has if effluent flows drop or other users compete.
• Whether blowdown can return to the plant or sewer, and under what limits.

Effluent flow follows the community’s wastewater pattern, which varies through the day and by season, and can drop during plant upsets. On-site storage and a backup source, usually potable water, are standard parts of the design. In some western states, treated effluent that would otherwise return to a river may be tied to downstream water rights, which can limit how much is available for consumptive reuse. That is a state-specific question for water counsel.

1. 01Confirm the cooling design and its annual and peak-day water need.
2. 02Identify wastewater treatment plants within a reasonable distance and their approximate effluent flows.
3. 03Ask whether effluent is already committed to other reuse customers or required for discharge.
4. 04Map a likely pipeline route and flag crossings, easements and wetlands.
5. 05Review state reuse rules for cooling tower use and any water rights limits on effluent.
6. 06Request effluent quality data and plan on-site treatment and blowdown disposal.
7. 07Plan a backup source and on-site storage.

When we screen a site, proximity to a treatment plant is a flag we record early, even if the buyer has not decided on its cooling approach. It is much easier to keep the option open than to find it later.