PNNL

Abstract

This report explores the concept of water reuse systems for cooling tower water makeup, specifically focusing on the use of alternative water sources, and the viability of these systems for Air Force installations and operations. Cooling is a critical requirement for DAF operations, and alternative water supplies that enable cooling tower use during a disruption of utility-supplied water increase Air Force resilience. The components of a water reuse system for cooling towers include alternative water supplies, storage, treatment, and distribution in addition to the cooling tower itself. Alternative water sources are not derived from fresh surface water or groundwater and can provide a redundant water supply to utility supply or to fresh water produced on site. Alternative water sources include the following, as shown in Figure ES.1: • Heating, ventilation, and air conditioning (HVAC) condensate: moisture accumulated from mechanical equipment • Atmospheric harvesting: moisture harvested from the ambient air • Rainwater harvesting: water collected from above ground surfaces • Stormwater harvesting: water harvested from ground surfaces • Greywater: lightly contaminated water from sinks, showers, or laundry • Reclaimed wastewater: (aka blackwater) collected from toilets, urinals, and other high contaminant building processes • Cooling tower blowdown: discharge from existing process • Desalinated water: from seawater or other brackish sources Water reuse components are generally widely available on the market, with scales ranging from small units serving a single cooling tower to large systems serving multiple buildings on an installation. Although onsite storage and distribution infrastructure may be available for these systems, water treatment will be needed on site for many forms of water reuse systems. The level of treatment will depend on the requirements of the cooling tower and the type of alternative water source. Treatment can range from a simple strainer for removal of large objects, to filters that remove small to microscopic particles, to a complex series of biological, chemical and/or mechanical processes (referred to as a treatment train) to achieve a specific level of non-potable water quality appropriate for cooling towers. Infrastructure may be less complex in the case of alternative water sources that require minimal treatment or those that are co-located with the cooling tower. For example, an HVAC condensate or rainwater harvesting system located near the cooling tower served would require relatively little infrastructure compared to a wastewater treatment plant on the far side of the base. Appendix A provides a list of DAF installations ranked by availability of high-quality alternative water sources applicable for cooling tower installations. It is recommended that the DAF develop a strategic approach to deploying water reuse systems across its portfolio. Key parameters to strategically target sites include installations with large cooling loads served by cooling towers, existing water infrastructure, mission critical water source deficiencies, high mission priority, and location in a state that has a supportive regulatory framework. A focus on sites with a sufficient source of high-quality alternative water (e.g., condensate capture or harvested rainwater) to meet the demand will reduce costs for additional components such as storage, treatment, and distribution. A demonstration project of a water reuse system could illustrate technology feasibility at a relevant scale for a cooling tower application. The pilot system should be designed with specific requirements for the site using modular processes that would allow various technologies to be tested to determine the most effective and cost-efficient treatment approach. Pilot testing also would allow operators to engage with the process and develop familiarity with the alternative water source storage and treatment technologies and O&M requirements.