Treating wastewater onsite can save owners money, but there are other good reasons too.
The Living Machine at Port of Portland features both indoor and outdoor plant beds for filtration.Photo Credit: Eckert & Eckert, courtesy ZGF
Living Machines and other types of constructed wetlands are beautiful, but they’re not ideal for every client. Onsite wastewater treatment might make sense for your next project, though, depending on factors like the site, the local infrastructure, and the owner’s mission.
Here’s a quick guide to figuring out when and where onsite wastewater treatment makes sense. For a deeper look at the topic, read this month’s EBN feature article, “Waste Water, Want Water” (BuildingGreen member link).
Lower the flow first
Potable water has a massive energy footprint, even in water-rich areas. We don’t pay anything like the true cost of this nonrenewable resource, so most of us don’t think twice about polluting it just so we can make our own pee and poop go “away.”
Transporting and treating wastewater has energy and other environmental costs as well, but before you start doing the payback analysis on that membrane bioreactor, you first need to look at the water budget for the project holistically. What else can you do to reduce your use of potable water?
They’re not for every client, but they do warrant consideration for almost every project. Composting toilets use very little or no water, depending on the model: this means that we avoid polluting potable water just so we can move human waste around.
A recent high-profile project questions our assumptions about where composting toilets make sense. The six-story Bullitt Center, under construction in Seattle and pursuing Living Building Challenge certification, is currently in the process of installing composting toilets.
These composters in the basement of the six-story Bullitt Center in Seattle will capture waste from all the building's toilets. After high-heat aerobic processing breaks it down, the compost will be trucked to a nearby forest to amend the soil.Photo Credit: John Stamets
Now consider onsite treatment
Maybe you’ve gotten all the potable water savings you can by rethinking your mechanical system and harvesting rainwater. Here are five cases in which onsite treatment and reuse should definitely be considered:
- Remote sites and new developments—A remote site will have to be hooked into a municipal system, often at great expense. Many such sites simply opt for a septic field, but conventional septic systems are not always well managed, they don’t treat water for reuse, and they don’t do a good job of removing nutrients (typically overloading the groundwater with nitrogen and phosphorous instead). Increasingly, new suburban developments with low-flow fixtures also bump up the concentration of solids in the wastewater, creating conditions that centralized plants are not designed to handle. Decentralized treatment and reuse systems serving these new developments make a lot more sense.
- Overtaxed municipal system—Although centralized systems can boast economies of scale, many are aging, leaky, and overtaxed, and older ones combine stormwater and wastewater, which can lead to “combined sewer overflow”—the release of raw sewage into waterways. Treating and reusing onsite makes your waste your own problem instead of everyone else’s.
- Nutrient cycling— The aim of wastewater treatment is to protect us from exposure to disease pathogens. In the process, we remove nitrogen—and, more rarely, phosphorus—to reduce biochemical oxygen demand (BOD) before releasing effluent. These nutrients can damage ecosystems and are very much a part of the “problem” that wastewater treatment is “solving.” They are rarely viewed as a valuable resource. Natural onsite treatment systems can change that by returning nutrients slowly and safely to depleted soil, potentially repairing decades of damage.
- Expensive sewer fees—Potable water remains remarkably inexpensive even in regions where it’s scarce, but municipal wastewater treatment can represent a major cost for commercial buildings in some places, potentially creating a business case for onsite treatment. Some cities may waive considerable sanitation hookup charges if owners choose to treat their water on the site, and ongoing sewer fees are also avoided. On the other hand, energy use will offset cost savings, as will system maintenance.
- Education and research— One of the most compelling reasons to treat wastewater onsite is to educate building occupants, visitors, students, and professionals about freshwater scarcity and wastewater treatment. Natural onsite systems may include beautiful landscaping and water features. They also require frequent testing and provide research opportunities for students and scholars alike, and they can even serve as test beds, helping develop natural treatment methods that may someday work at the district or municipal scale.
The more expensive and energy-intensive your system is, the more it may make sense to rely on a centralized system’s economies of scale—particularly if the local infrastructure is reasonably sustainable. Since that’s not often the case in the U.S., though, many wastewater experts are advocating for larger decentralized systems.
Proposed purple-pipe networks in Portland, Oregon, could help solve issues with combined sewer overflow.Photo Credit: SERA Architects
Clark Brockman, AIA, principal at SERA Architects, has been working with his colleagues to get the City of Portland, Oregon, to rethink its systems and to get developers rethinking their neighborhood infrastructure—possibly even creating micro-utilities for sharing reclaimed water among multiple building owners.
Brockman recognizes that his scheme is “very specific to Portland,” but he encourages all architects to think bigger.
To learn more about Brockman’s ideas, find out about the latest breakthroughs in closed-loop nutrient cycling, and hear from designers of the Sidwell Friends School constructed wetland, the Port of Portland Living Machine, and lots of other projects, check out this month’s EBN feature.