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Access to clean water is critical to the health and wellbeing of our society globally. In the United States, potable water comes from two main sources: municipal water supplies and wells. The U.S. Geological Survey notes that only 15 percent of the population relies on wells, with the rest tapped into their local water purveyor. While these municipal water supplies produce clean water, they are often underfunded and, as a result, poorly maintained, leading to water main breaks or mandatory water usage reductions. As climate change, extreme drought and growing populations continue to challenge our water systems, these occurrences will be more frequent and disruptive.
One way to reduce the collective demand on potable water systems is to use water reclamation systems. These systems have been common in the plumbing industry for decades now and can be applied in areas experiencing drought and those with plentiful natural water sources.
The goal of these systems is to reduce the amount of potable water used for nonpotable uses. Reclaimed water can be used for many purposes, including toilet flushing, irrigation and cooling tower makeup.
In the past, rainwater harvesting systems were the primary choice for reclaimed water systems within buildings. In many areas, rainwater is clean (nonacidic) and can be reused with minimal filtration or disinfection, especially if the roof surfaces are clean.
However, rainwater harvesting systems aren’t the right choice for every building, especially high-rises. They typically have a small roof area compared to the building occupancy and total square footage. For these projects, greywater harvesting can provide an excellent source of reclaimed water.
Greywater has different definitions according to local Authorities Having Jurisdictions (AHJs), but for this discussion, we will use the definition from the National Sanitation Foundation (NSF): “Wastewater from water-bearing fixtures, including laundry, such as clothes washers and laundry sinks, and bathing, such as bathtubs, showers or sinks, but excluding toilets, urinals, bidets, kitchen sinks and dishwashers.”
Before starting the design of a greywater system, you must determine the water reclamation requirements with the local AHJ. Also, state or county health departments may have additional regulations for recycled water quality that must be met.
Once the AHJ requirements are known, the design of a greywater system starts with determining which fixtures will be harvested and which fixtures will be connected to the sanitary sewer. In a typical high-rise commercial office building, greywater can easily be harvested from restroom lavatories as well as shower rooms.
Greywater collection piping can be an added project cost, so it is important to work with your architect to plan restroom layouts where the lavatories are separated from water closets. This will help simplify the collection piping and routing in plumbing chases. (See Figure 1.)
Collection and Primary Filtration
Once you have determined which fixtures are to be harvested, the design of greywater collection piping is identical to sanitary piping. Follow your local plumbing code for fixture unit loading and pipe-sizing tables.
Before the combined greywater waste can be filtered, many AHJs require a three-way valve. This valve aims to divert greywater to the sanitary sewer while the greywater system is being maintained. These valves are usually manually operated, but motorized valves can be used to allow for remote operation.
After the three-way valve, the greywater is routed through a “first flush” filter with a stainless-steel mesh basket to remove larger debris, such as jewelry, contact lenses, or other items that may inadvertently be dropped in a lavatory or shower. A high-pressure spray nozzle is provided to wash lint and hair from the mesh screen, and the basket is removable for cleaning. The filter has a bypass to the sanitary sewer to carry away dirty water and debris. (See Figure 2.)
After the filter, greywater passes into the biological treatment tank. This tank can be made of plastic, steel or cast-in-place concrete with a waterproof liner. The treatment tank allows solids and sediment to settle to the tank bottom and soaps and lotions to float to the top. The tank needs to be accessed annually for cleaning, so a manhole or other large opening must be provided along with a tank drain routed to the sanitary sewer.
Treatment options vary for greywater harvesting; however, the most common method is a moving-bed-bioreactor or MBBR. This method uses floating media within the treatment tank to attract biological growth. Aeration within the tank ensures the media is constantly circulated to ensure organic matter is collected.
It is important to match the size of the biological treatment tank to the building’s expected daily generation of greywater. A good place to start is using the LEED standard water calculations and the expected full-time equivalent (FTE) employee count. For a building with 5,000 daily FTEs, greywater generation can be estimated at nearly 6,000 gallons per day (see Figure 3).
Matching the tank size to daily use reduces the retention time for raw greywater and ensures that all usable water is captured. It is possible to size the tank smaller, but that will result in potential overflow to the sanitary sewer.
The next piece of the system is the primary filtration skid, which is typically factory-built and -tested by the manufacturer. The filtration system operates at low flow rates and maintains a constant draw from the biological treatment tank. The tank absorbs high periods of greywater inflow and holds it for future processing. The filtration skid is a multi-step process, with the following stages (see Figure 4):
1. Zeolite filter: removes ammonia, sediment, and metals;
2.Activated carbon filter: removes detergents/soaps, organics, odors, and sediment;
3. 5-micron filter: removes sediment;
4. Turbidity meter: ensures water clarity for optimal UV disinfection;
5. UV light: eliminates pathogens and other biological contaminants.
Once the greywater is processed by the filtration system, it is pumped into the purified water holding tank and is ready to be distributed back to flush fixtures such as toilets and urinals. A nonpotable water booster pump with a hydropneumatic tank should be sized according to the requirements of the system and local plumbing codes for water pipe sizing. (See Figure 5.)
To safeguard and maintain the water quality in the purified water holding tank as well as in the piping distribution system, the final stage of water treatment is a chlorine dose.
This ensures that the water remains safe for use in flushing applications and carries no residual biological contaminants. Municipal water systems provide chlorine dosing for the same reason.
A key component of any reclaimed water system is an adequate supply of potable water which can be used as a backup in case of maintenance or unexpected system downtime. The potable water makeup supply is connected to the purified water holding tank or into the suction side of the nonpotable water booster pump.
It must be guarded by means of a reduced pressure backflow assembly or an air gap, depending on the requirements of the local AHJ. The size of the potable water pipe must be adequate to provide the full flow of the reclaimed water system.
Along with maintenance, one of the important annual tests of any reclaimed water system is a cross-connection test with the potable water system. The purpose of this test is to determine if the potable and reclaimed water systems are interconnected at any point within the building. If a cross-connection is detected, it would indicate the potential for reclaimed water to flow into the potable water system and endanger the building occupants.
Cross-connection testing methods are similar in most jurisdictions and are based on the UPC and other standards, such as the EPA’s Cross-Connection Control Manual.
Piping and Materials
When selecting materials for greywater collection and distribution piping, the engineer needs to consider code requirements along with cost and constructability. Greywater collection piping typically matches the piping used for the building’s sanitary waste, typically PVC or cast iron. Both materials are suitable and offer pros and cons that must be evaluated.
Nonpotable and reclaimed water distribution piping is available in a multitude of options. Copper, PEX and CPVC are all suitable choices; typically, they are provided in purple to distinguish them from a buildings’ potable water system.
Pipe labeling requirements can be found in Chapter 6 of the Uniform Plumbing Code; it is important to verify if your local AHJ requires any additional modifications.
Maintenance needs vary greatly, depending on the size of the greywater system, but annual and preventative tasks are needed for all systems. The primary maintenance needs are inspection and cleaning of filters.
Additional requirements are listed in Figure 6 from the UPC, ranging from water quality testing to cleaning the greywater treatment tank.
Along with lower-flow and -flush fixtures, targeting LEED certification can use greywater reclamation to realize additional water savings. The calculation in Figure 7 shows the potential water savings that can be achieved by combining these two methods.
The extra water savings boost from greywater harvesting allows the maximum number of LEED credits to be reached, including bonus points for regional priority and exemplary credits. Compared to other building design choices to maximize LEED credits, such as glazing and high-efficiency HVAC systems, greywater reclamation can provide as much or more value for a building’s owner.
As water and sewer rates continue to increase, the system can provide long-term savings and payback of the initial investment.
Codes and Standards
As is the case for plumbing codes across the United States, there is not a singular nationally accepted greywater harvesting standard. In the absence of a national standard, many states enacted their own regulations through local health departments, while others adopted standards such as NSF/ANSI 350: Onsite Residential and Commercial Water Reuse Treatment Systems or IAPMO IGC 324: Alternate Water Source Systems for Multi-Family, Residential, and Commercial Use.
While these references are not adopted in all jurisdictions, their requirements should be used to create a baseline system design relating to water treatment and quality as well as public health concerns.
Jonathan Franzese is a Senior Engineer and the Plumbing Engineering Manager at McKinstry in Seattle. He currently serves as the ASPE Seattle Chapter President and is a member of ASPE’s Credentialing and Education Committees.