Non-Potable Water as an Alternative Source
Where available, non-potable water sources can be a great way to conserve potable water, and a path to earning 2 LEED credits. Let’s look at four common categories of non-potable water: HVAC condensate, rainwater and stormwater, recycled municipal water, and gray water.
HVAC Condensate. Especially in the Southeastern U.S., high humidity and high cooling loads during most of the year affords a high potential for condensate capture. In Washington, D.C., for example, the condensate capture could be as much as 10 gal/cfm of OA each year and in Miami as much as 31gal/cfm of OA each year, according to a 2021 ASHRAE Journal article.
HVAC condensate is an ideal source of make-up water for cooling towers for two reasons. First, the timing of the generation of condensate from air conditioning systems aligns well with the timing of need for make-up water for the cooling towers. This alignment means that a storage tank may not be necessary. Second, condensate water is pure with a very low dissolved mineral content. However, a potential downside of HVAC condensate is that it sometimes contains heavy metals, such as copper or lead, which may require treatment prior to use as make-up water.
Rainwater. Rainwater and stormwater are commonly harvested from roofs and hard surfaces, such as roadbeds or parking lots. Regulations vary by state on the use of such water. This map
provides an initial assessment of the feasibility of implementing rainwater and stormwater capture. A 2012 study published by the University of Tennessee concluded that a high number of COC can be achieved with rainwater because dissolved solids are significantly lower than in tap water.
However, rainwater pH frequently is around or below 6 and therefore needs to be mitigated before use in a cooling tower to minimize the risk of corrosion and contamination. Also, control of microbiological growth must be included in any water treatment plan where harvested rainwater is being used. The level of treatment required for harvested rainwater depends on the source. Two common issues are bird droppings if the rain is harvested from a roof and oil if harvested from roadbeds and parking lots.
Recycled Municipal Water. Local municipalities are increasingly developing the capability to reclaim and sell treated wastewater (at a significantly lower price than potable water) rather than discharging it into a lake or river. “Purple pipe,” along with appropriate signage, is used to distinguish such distribution systems from potable water lines.
This water is often good quality, although the concentration of minerals is usually higher than potable water. An advantage is that the increased silica, alkalinity, hardness, and phosphate content in reclaimed water are often less corrosive than tap water. When using recycled municipal water, water quality management teams need to evaluate how corrosion inhibitors from the municipal process may impact water treatment strategies for cooling tower make-up water.
Gray Water. Unfortunately, typical commercial sources of gray water –- e.g., urinals and laundry –- are not appropriate for use as a direct non-potable water source without significant further treatment. Soaps found in laundry can be problematic because they act as a food source for microbiological growth.
Addressing water quality challenges
Depending on the quality of the water available, and based upon testing and recommendations of water treatment professionals, the strategy for conserving water may require the implementation of one or more mitigation methods. These methods divide into two basic categories: (a) improving the water with chemical treatment and filtration, and (b) protecting the system with materials of construction that offer high protection from corrosion.
Dissolved Minerals. Water with high mineral content can be particularly challenging because high levels of calcium, magnesium, alkalinity, and silica increase the risk of scale on heat transfer surfaces, which can rapidly degrade system performance. On the other hand, high levels of chlorides and sulfates increase the risk of corrosion on various metals used in cooling water systems, which could lead to increase maintenance costs and reduced asset life. Limiting dissolved solids is critical to achieving increases in the number of cycles of concentration. Three mitigation methods are commonly used:
Chemical treatment. Scale inhibitor chemicals cause a process called crystal modification to occur, which softens the hard edges of the crystalline precipitate into rounder material that stays in solution longer and is less likely to form scale. Chemical treatments also include dispersant polymers that coalesce and agglomerate these softer particles, and by hydrophilic and hydrophobic forces make these particles less likely to bond to the heat transfer surfaces. These chemical treatments work, but they have limits based on modern chemistry, and are often insufficient to address water with high mineral content.
Mechanical Pretreatment. Most facilities need to consider mechanical pretreatment, either water softener systems or partial reverse osmosis systems or a combination of the two. Water softeners use an ion exchange resin, which collects calcium and magnesium, along with a brine tank, which uses salt as a regenerant. These systems are relatively simple and have a low total cost of ownership, but they only remove calcium and magnesium and not other dissolved solids like chlorides; and they do not reduce alkalinity. Water softeners can increase cycles of concentration from approximately 2 to 4, but rarely allow for the large leaps necessary to reach 10 COC.
Partial reverse osmosis (RO) is more effective. Water is pushed through a membrane, removing 95-98% of all minerals. Normally RO is used to produce pure water, but that would be too corrosive for cooling tower applications. Therefore partial RO is used, which blends the RO water with municipal make-up water. For water with very high mineral content, the most cost-effective design is to use a water softener to remove the hardness minerals prior to the RO process.
Suspended Solids. The concentration of suspended solids from various water sources and from airborne particulates collecting on the cooling tower facilitates biological fouling and risks dangerous bacterial growth. For this application, cyclonic filtration is more effective and easier to maintain than sand filtration.
The most cost-effective solution is to design and install a side stream filtration system with basin sweeper piping. The system should be factory assembled and delivered with the new cooling tower. (Aftermarket systems require removing the fill pack which increases the risk of damage to the fill.)
Corrosion. Water conservation strategies often involve elevated corrosiveness of the system water, especially when using partial RO or high purity non-potable water such as HVAC condensate and rainwater.