In the plumbing industry, improving building performance means finding opportunities that enable complex systems to be more efficient and sustainable.
Learning objectives
- Identify where more efficient, sustainable and complex plumbing building systems can be leveraged to reduce energy and water use.
- Evaluate the use of specific plumbing strategies and technologies.
- Understand how sustainable plumbing designs enable a more sustainable building system.
Plumbing insights
- Systems such as sewage wastewater energy exchange, graywater reuse, condensate recovery and rainwater harvesting can significantly reduce potable water use in buildings.
- Successful sustainable plumbing systems depend on early coordination, life cycle cost analysis and compliance with evolving codes.
When designing plumbing systems with the goal of improving sustainability, certain factors must be considered. Variables such as building scale, initial costs, operating costs and system complexities can create impacts on their own — and add up to larger savings when combined. Each factor plays a crucial role in providing a plumbing system designed for a client’s sustainability goals. Another concern is the ability to maintain such systems throughout the life of the building.
Traditional plumbing systems are the most familiar to engineers and clients and often the most first-cost friendly. Sanitary waste piping discharges to city sewer, domestic water is brought in to serve plumbing fixtures and rainwater discharges to the city storm drain system. In their base forms, these systems are often not the most efficient or focused on water conservation.
Recent credential and code changes including mandated sustainability requirements such as U.S. Green Building Council LEED, Green Globes or the California Plumbing Code (CPC) provide guidelines on strategies such as low-flow fixtures. There are also initiatives such as the MEP 2040 Commitment and AIA 2030 Commitment that design firms across the country are committing to, which set goals for progress toward net zero energy and challenge the design community to reach these goals.
Each of the baseline plumbing systems can be improved by using specialized plumbing equipment, newer technologies and system optimization. These tools can be used to create a more efficient and sustainable plumbing system and impact the entire building operation. Some of these strategies include sewage wastewater energy systems, graywater systems and reuse of rainwater.
Sewage wastewater as a sustainable energy source
During the early design stage, most project teams do not consider using a building’s sanitary system as a source of energy. Instead, many tend to discount the use of a sanitary waste energy exchange (SWEE) system due to lack of familiarity. Additionally, due to complexity, the probability of low fixture usage, high first costs and limited budgets, this type of system is often seen as unpopular.
However, it can also greatly reduce energy consumption if properly implemented. Depending on the building or site size, a SWEE system can be used as an energy source for heating, ventilation and air conditioning and plumbing water-source heat pumps. In North America, billions of kilowatt hours are wasted generating heat for domestic water use that serves faucets and showers.
The SWEE system uses residual heat from the sanitary waste piping system, which is consistently around 70°F. Raw wastewater is passed through a solid-liquid separator to remove solid waste and send it back to the sewer system. Liquid waste then passes through a specialized water-to-water heat exchanger, transferring thermal energy to or from a separate clean fluid stream (see Figure 1). The now hotter (or colder) sanitary waste continues to the sewer and is replaced by incoming 70°F sewer flows to repeat the process.
Graywater applications in sustainable plumbing design
The significance of water use reduction is growing due to environmental concerns and increasing water scarcity. Graywater, which is defined as lightly used wastewater from sources such as lavatory sinks and showers, presents an opportunity for water conservation. Water from these fixtures can be captured and repurposed for nonpotable use, such as flushing fixtures and irrigation supply. Graywater systems play a crucial role in minimizing reliance on the domestic cold-water supply for building areas that do not require potable water (see Figure 2).
Implementation of graywater systems involves careful planning and integration into building designs and early collaboration to ensure project compatibility, especially regarding cost. Codes such as the International Plumbing Code and CPC contain specific requirements to ensure that cross-contamination does not occur within domestic potable lines as it is routed throughout the building.
The 2025 CPC requires that any connection between a potable water system and a graywater system is protected by an air gap, a reduced-pressure principle backflow preventer or another approved backflow prevention device. Graywater system equipment is typically designed so that domestic potable makeup water is automatically supplied when graywater alone cannot meet flushing demand. Although the CPC permits certain direct connections when appropriate backflow protection is provided, designing the system without a direct connection is considered best practice because it offers the highest level of protection against graywater contaminating the domestic potable water system.
Local codes and regulations are another factor that must be considered in graywater system design. For example, Colorado’s Department of Health and Environment contains Regulation 86, which includes further requirements to be considered when designing a graywater system. Regarding graywater equipment, Regulation 86 says it must be NSF 350-certified and that ultrafiltration and chlorination must be used to disinfect the piping system.
To ensure there is no confusion between the domestic and reclaimed graywater piping, the regulation requires that the graywater piping is installed as purple pipe or that it features purple identification tape or wrap. Another safety requirement calls for proper signage within restrooms and the graywater equipment room to ensure all building occupants understand the type of water being supplied to the flushing fixtures.
Condensate recovery in sustainable plumbing design
In certain climates or with some types of building programming, mechanical air handlers and/or fan coil units may see significant amounts of condensate production. In these cases, a condensate recovery system can be used to minimize potable water use, primarily within the irrigation water system, by redirecting condensate drainage. The tonnages, gallons per minute and monthly use must be calculated to ensure proper sizing of the storage tank and distribution system.
Condensate water is typically clean because it is not regularly exposed to dust, debris and chemicals and therefore is not typically filtered. However, some authorities having jurisdiction (AHJs) such as the Los Angeles Department of Building and Safety also require an NSF-350 certified filtration system. Checking AHJ requirements for this type of system is best conducted early on because the addition of a filtration system can quickly turn a low-cost approach to water reuse into a target of value engineering.
Reusing rainwater to achieve sustainable plumbing design
Another strategy to reduce potable water use is to reuse rainwater and repurpose it. This water is already being collected by the stormwater and overflow piping systems within the building. By redirecting stormwater piping to the landscaping using bioswales and other infrastructure methods, potable water use within the site’s irrigation system can be reduced. Integrating rainwater harvesting systems for landscapes requires early coordination with both the design team and client to evaluate implementation, cost and ongoing maintenance needs. These conversations can help develop solutions that best serve the client, architectural aesthetic and region (see Figure 3).
