Water Reuse 101: Back to Basics

Humans have employed water reuse and recycling strategies long before the advent of modern municipal water treatment plants and the widespread supply of potable water to homes and businesses. As new technologies were developed, providing clean drinking water throughout the world became more common. However, there are areas that still rely on tried-and-true strategies to remove pathogens and ensure an adequate year-round supply of water. 

Some of these strategies cannot be exactly replicated in commercial and residential buildings; however, much can be applied to modern-day buildings. 

Before diving into the subject, I want to bring up an area of improvement for plumbing engineers who have been in the industry for a long time. Engineers can develop what I like to call “blind spots.” Some of these are borne of project types or geographical location, but one that almost all will develop universally is forgetting what it was like starting in the engineering industry.  

Water reuse can be a very complicated subject, with varying codes, authorities having jurisdiction (AHJ) requirements, and complicated system designs. The goal of this column is to provide beginner-level information and be accessible to all designers and engineers. 

Alternate Water Sources 

Many sources of untreated water can be reused on-site to reduce a building’s reliance on potable water. Deciding on the source(s) of water to be reused is the first step in the design of a water reuse system. The decision will inform the types of coordination, filtration, disinfection and end-use application of the recycled water. 

Before selecting a source, the plumbing engineer shall evaluate the suitability of the building for each option and weigh the costs and benefits to each system. Sometimes this evaluation is simple, e.g., a high-rise building has a very small roof area compared to building size, thus eliminating a rainwater collection system.  

1. Alternate water source: Nonpotable source of water that includes but is not limited to greywater, on‐site treated nonpotable water, rainwater, atmospheric generated water, air-conditioning condensate and reclaimed (recycled) water.* 

2. Blackwater: Wastewater containing bodily or other biological wastes, as from toilets, dishwashers, kitchen sinks and utility sinks.* 

3. Greywater: 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.** 

4. Groundwater: Natural flowing water below grade, typically collected via the building's subsoil drainage system. Flows can vary seasonally, and the reuse of this water can sometimes be federally or state-regulated. 

5. Rainwater: Natural precipitation landing on a manmade impervious aboveground surface and can be collected on‐site for beneficial uses. It is important to ensure rainfall is not too acidic in reuse applications.* 

6. Condensate: Water collected from the drains of air conditioning and other HVAC equipment. It can also be referred to as clear water waste. 

7. Seawater: Water collected from an ocean, sea or other saltwater body. This is more commonly seen on islands or other areas that cannot be served through traditional municipal water treatment systems. 

* IAPMO IGC 324, Alternate Water Source Systems for Multi-Family, Residential, & Commercial Use

** NSF350, Onsite Residential and Commercial Water Reuse Treatment Systems 

Recycled Water End Uses 

Just as there are many sources for recycled water, there are many end uses for this water. It is the responsibility of the plumbing engineer to verify the applicability and feasibility of these options before making a design choice. Typically, it is best to try and match source and end usage to ensure a water balance in the system and avoid needing too much makeup water or having an oversupply of recycled water.  

1. Laundry. Recycled water can be used for laundry in both residential and commercial settings. Typically, harvested rainwater is used for residential applications; commercial laundry systems employ mostly closed greywater systems using potable water only for final rinses. 

2. Irrigation. There are two main types of irrigation systems: subsoil drip and spray. Most AHJs do not allow recycled water to be sprayed/aerosolized due to a heightened risk of infection such as Legionella or other respiratory diseases that may be present in recycled water. 

3. Cooling tower. Cooling towers work by using water to absorb and reject heat from a building through evaporation. These are still commonly used HVAC systems for certain building types. However, cooling towers are being phased out of designs, with more energy- and water-efficient equipment taking their place.  

4. Flushing. Water is reused to flush water closets and urinals. This is the most common application of water reuse in commercial buildings as it represents the main source of nonpotable water needs for many building types. Smaller versions of these systems can be used in residential applications. 

5. Potable. In limited cases, it is possible to reuse water for potable consumption. This is mainly done for off-grid-style homes in remote locations such as islands or mountains where traditional systems are not possible. Rainwater typically provides the cleanest source water for this application.  

Recycled Water System Types 

Once the plumbing engineer evaluates potential alternate water sources and reviews end use(s) for the recycled water, the next step is the design of the actual water recycling system. When evaluating system designs, the engineer should focus on a few key factors to ensure the final design meets the project’s needs, including: 

• Compliance with local codes. Knowing the requirements of the AHJ is the first step in designing a system that can be permitted and constructed. As water reuse regulations are in varying stages of development throughout the United States, local health departments and code officials should be contacted to ascertain specific design requirements in the project’s location. 

• System complexity. As engineers, sometimes we can focus on the most technically advanced solution when a simpler system can meet the needs of the project at a lower cost; for example, using too fine of a filter or adding an unnecessary UV lamp. Ensure filters are easily located for service and consider the long-term maintenance needs and equipment replacement strategies to provide the building owner with a quality system. 

• System costs. From personal experience, if the system doesn’t fit in the project budget, it doesn’t matter how much water will be saved. Consider the owner’s objectives for the building and tailor recycled water systems to meet the project’s needs. While saving more water is preferred, a system saving 50 percent water usage is better than a system that could save 90 percent water usage but is too expensive for the project, ending up saving no water. 

In addition to these aspects, the following considerations are specific to each type of recycled water system: 

• Greywater. Ensure waste streams are kept separate from blackwater and consider the end use when selecting filtration equipment. If the recycled water is used for subsoil irrigation, it may not be required to select the same level of filtration and disinfection as would be needed for flushing needs within a building.  

It is also good practice to compare the daily flow needs of the irrigation system vs. the daily output of the greywater plant to ensure an adequate supply of water is needed and the system is not filtering excess greywater. 

 • Rainwater. When selecting roof areas for capture, the engineer should consider the roof surface type and avoid areas that are open to pedestrians to avoid contamination of the rainwater. The levels of filtration shall also take into account the potential for debris on the roof, from fine dust to larger dirt and rocks. 

In areas with heavy rainfall rates, it is necessary to safeguard the building against stormwater overflows from the system. Locations subject to snow should consider adding heat tape to prevent roof drains from freezing and maintaining a constant flow of water into the rainwater system. 

Pathogen Reduction 

The primary goal of any reclaimed water system is to maintain public safety. When water reuse systems became more common, the first codes and standards were performance-based, meaning that if you used certain types of filters or equipment, the system was acceptable.  

Newer standards such as those developed by San Francisco’s Department of Public Health Environmental Health Branch focus on measured performance by taking water samples to guarantee water safety (https://bit.ly/31m8HxQ, https://bit.ly/31p91fe). The samples must demonstrate compliance with log-reduction targets to reduce parasites, microorganisms and bacteria.  

In large downtown city cores where different building types such as residential buildings and commercial offices are located adjacent or in close proximity, district-sized systems can be used. They allow for water to be shared across property lines to mix and match alternate water sources and end uses. This enables water balances to be taken over longer periods and to take peak loads in one building and move water to adjacent buildings for future usage.  

An example of this would be collecting greywater from a hotel over the morning's showers and using that water for flushing needs in an adjacent office building. 

District systems have gained traction as a way to share and conserve energy usage between buildings, such as using waste heat from a commercial office building to preheat domestic water in a residential building. As more cities struggle to keep up with skyrocketing demand for energy and clean water, district water systems can alleviate the loads on utilities and allow developers and building owners to work together to create a mutually beneficial relationship.