Water harvesting in commercial projects involves collecting rooftop rainwater, stormwater runoff, condensate from air handlers and greywater from showers, baths and lavatory sinks. This water is reused for non-potable applications such as toilet flushing, irrigation and cooling towers.
Because it offsets the use of drinking-quality water, this reuse has the potential to save millions of gallons per year for one single building.
To ensure public safety, public health agencies oversee the proper design of these systems.
TYPES OF WATER GATHERED
The National Plumbing Code and NSF International defined the types of harvested water as:
- Rainwater: Rain or snow falling naturally from the sky that is collected from rooftops.
- Stormwater: Rainwater that has fallen onto hardscapes or the ground and is more contaminated than rainwater, requiring additional treatment and monitoring.
- Greywater: Water from showers, baths, spas, lavatory sinks and laundries.
- Blackwater: Water from toilets, food and kitchen waste sinks and other heavily contaminated wastewater. Blackwater is not easily treated as harvested water for onsite reuse.
- Reclaimed Water: Blackwater that is dewatered by municipal water treatment facilities. This water is considered non-potable and its reuse options are limited by code.
All water from rainwater, stormwater and greywater harvesting treatment systems must meet certain performance standards for reuse. These standards vary by state, but most of those states are adopting the NSF-350 standards listed in Table 1.
To meet the demands of properly designed water harvesting systems, valves of many types are used. They control where the different types of water can and cannot go. Materials of construction vary by local code, but polyvinyl chloride (PVC), stainless steel and copper/bronze are the most common.
That having been said, there are exceptions. Projects designated to meet the “Living Building Challenge,” which requires rigorous green construction standards, preclude the use of PVC and some other materials that have been deemed harmful to the environment because of the manufacturing processes or disposal methods.
Beyond materials is design and valve type choice. The section of this article to follow looks at common rainwater and greywater harvesting system designs and how different types of valves are used within each.
In general, how harvested water will be reused and how local plumbing codes apply affect what types of valves are used. Also under consideration is the reality that, the volume of water available to be harvested may not be sufficient to meet 100% of reuse demand. In that case, domestic (potable) water may be included in the system to make up the shortfall.
The major concerns of public health and plumbing regulators are separating the domestic water source from the harvested water interconnection and potential contamination of the domestic potable water supply.
DOMESTIC WATER MAKEUP
In critical applications such as toilet flushing, codes require a backup water source when using harvested water as the primary source.
Two such backup sources are 1) direct connection and 2) air gap.
Direct connection (Figure 1) requires using a reduced pressure zone valve (RPZ) as well as an actuated valve for separating the domestic water source (potable) from the harvested water (non-potable). Most system designers use a normally open/held closed solenoid valve (NOHC-SV) for this. The solenoid valve needs to have a duty cycle of 100%, meaning the valve is rated as energized 100% of the time without excessive heat buildup or failure of the valve coil.
The advantage of using the NOHC-SV is that, if there is a power failure or a system fault, the valve will automatically open without any additional controls. This allows domestic water to flow to toilets automatically.
However, some states such as Illinois and California do not allow RPZ systems with direct connection to harvested water systems.
Air gap separation (Figure 2) for domestic makeup is the other option for backup and is universally accepted as safe. The air gap separation design requires the use of a day tank and a physical gap for the domestic water to freefall into the day tank. This method of protection against cross contamination is considered 100% reliable for keeping the integrity of the potable water system safe.
With these systems, booster pumps are used to generate water pressure for reuse. Level controls in the day tank allow the process control system to add domestic makeup through the air gap at controlled low-level start and high-level shutoff of a domestic makeup valve. This makeup valve is a normally closed valve that can be a solenoid or a motorized valve. Typically, spring-loaded valves (air to open/spring close) are not used because a compressed air source is not available. Many system designers normally use a solenoid valve with a water hammer-free design so that the valve has a built-in dampener to slow valve closing and prevent water hammer.
Rainwater harvesting involves collecting and storing large volumes of water in a cistern. Because it doesn’t rain every day, rainwater systems are designed to maximize the volume of water collected so that the supply can carry through to the next rain event.
Treatment of rainwater is fairly straightforward—a series of filters remove debris and suspended solids (turbidity) in the water. The filtered water needs to be sanitized before its reuse. Sanitation is typically accomplished by using ultraviolet sterilization or a chemical agent, such as chlorine.
Greywater harvesting involves collecting raw greywater typically from showers, baths, spas and lavatory sinks. By code, this raw greywater must be sent to a sanitary sewer every 24 hours to prevent it from deteriorating into blackwater. Greywater requires more rigorous treatment to become reusable and NSF-350 compliant.
An important note is that once the raw greywater is treated, it’s no longer classified as grey, which means it can be stored and reused for many applications. This is what is termed by the industry as onsite treated non-potable water. One advantage of using a greywater system in a residential building such as a dormitory, hotel, barracks or other institutional/recreational structure is that a new supply of water is created every day. Also, storage of processed water is greatly reduced as compared to a rainwater system.
A day tank may be used for the sanitizing vessel for toilet flushing and cooling tower makeup applications. For irrigation systems, pumping directly from the cistern to reuse is common. In this case, the water travels directly to the final filtration and sanitation step before it exits to the sprinkler heads of the irrigation system.
Water harvesting typically uses ball valves because they are quick opening and closing with a full-port flow profile and low pressure loss. Good design allows isolation of equipment for servicing without disrupting the entire system. For example, a common practice is to use ball valves on tank nozzles to allow servicing of downstream equipment without having to empty the storage tanks. Pumps have isolation valves to allow servicing of the pump without draining the entire pipeline. Also used in the isolation process is backflow prevention valves (check valves) (Figure 3).
Motorized, 3-way ball valves are used to allow one pump to direct flow for more than one purpose. This saves the cost of an additional pump as well as related controls. Figure 4 shows a motorized, 3-way ball valve used to send water to recirculation and sanitizing or to the process skid for treatment.
Prevention of backflow is an important part of any water harvesting system. Ball check valves are commonly used to guard against backflow in pipes when a pump is shut off and system pressure is lost. Check valves are also used to prevent domestic water or harvested water from backflow, which would cause contamination or intrusion of water where no one wants it to go.
Small check valves, which are called injection valves, are used when dosing pumps are adding chlorine or blue dye chemicals to a pressurized line.
Large wafer or disk check valves are used with overflow systems on storage tanks to prevent sewer backup and rodent intrusion into the harvested water system.
Butterfly valves with manual or motorized operation are used as shutoff valves for large-size pipes (Figure 5). For below-grade applications, a manual, gear-operated butterfly valve is used to shut off the flow from cisterns, which often hold hundreds of thousands of gallons that will allow safe and easy service to pumps in wet wells. A shaft extension allows control of the below-grade valve from the grade level.
Some designers also use lug-style butterfly valves, which offer the ability to remove downstream piping so the valve can be a blocking valve. These lug-style butterfly valves are bolted to mating flanges from both sides of the valve. (Wafer-style butterfly valves do not allow for this feature). Note that in Figure 5, the valve and extension are located inside the wet well, which allows service access to the valve without the need for a valve box.
When actuated valves are required for below-grade applications such as cistern draining, electric valves are not a practical choice because electric actuators often fail in the presence of water. Pneumatic valves, on the other hand, are usually precluded because of the lack of compressed air supply. Hydraulic (water-pressure) actuated valves are often the solution instead. An electric pilot solenoid located safely near the control panel can send pressurized water to a normally closed hydraulic actuator to open or close the valve even when the actuator is submerged. With a hydraulic actuator, no danger of water contact with the actuator exists, a situation that would be problematic with an electric actuator.
On-site water reuse systems aren’t that different from other systems where flow must be controlled. Most of the principles that apply to the use of valves and other mechanical water-handling systems are simply employed in a different manner to meet the unique requirements of this emerging new sector in the water industry. Still, with the call towards more sustainable buildings growing every day, it’s likely that this industry is one that could be important to the valve industry.