- An Integrated Water Plan
- Why Onsite Wastewater Treatment?
Waste Water, Want Water
Options for small-scale, onsite wastewater treatment and reuse are improving—but can we solve a global crisis one building at a time?
By Paula Melton
Walk into almost any highway rest area bathroom in the U.S., and what do you hear? The constant whine of hand dryers and an equally constant drone of high-pressure flush after flush after flush. Before you grab that cuppa and get back in your car, stop and think: from this building in the middle of nowhere, devoted almost exclusively to the disposal of human pee and poop, where do those toilets flush to?
Our wastewater infrastructure is largely invisible and little understood. Most of us never question the wisdom of polluting pristine drinking water, losing valuable nitrogen and irreplaceable phosphorus in the process, just so we can make our poop disappear. Our own waste is a classic SEP—Somebody Else’s Problem.
The result? “Aging pipes and inadequate capacity lead .... to the discharge of an estimated 900 billion gallons of untreated sewage each year,” warns the American Society of Civil Engineers in its report Failure to Act. “Water infrastructure in the United States is clearly aging, and investment is not able to keep up with the need.” (For more, see “,” EBN Feb. 2012.) Though most rest areas still use one-way water cycling (onsite septic fields or long-distance connections with centralized systems), a few throughout the U.S. are incorporating constructed wetlands and other natural filtration systems that close the loop, permitting safe and sustainable wastewater treatment and reuse right on the site.
We’re seeing more wastewater reclamation in building types that aren’t devoted primarily to human waste disposal too, even in cities: commercial offices, multifamily buildings, and even hospitals have been experimenting with these systems for more than a decade, producing not only clean water but also important lessons that can be applied in the buildings we’re designing today.
Whether onsite wastewater treatment is a good choice for your next project will depend on a lot of different factors, from local codes to annual rainfall to the age of municipal infrastructure. If your project team does choose to treat and reuse wastewater on the site, different types of systems can have broad implications for energy consumption and other aspects of the project—but decentralized treatment and reuse can also contribute to improvements in centralized infrastructure, sometimes in more ways than one.
An Integrated Water Plan
Fifteen years ago, “it seemed like a great idea, every chance we had, to treat wastewater onsite,” says Russell Perry, FAIA, corporate sustainability director at SmithGroupJJR. “It’s not that clear anymore.”
During his time with William McDonough + Partners, Perry helped design the intensive natural filtration system in the Adam Lewis Center at Oberlin College, one of the first so-called living machines to be built. (Living Machine is a registered trademark owned by Worrell Water Systems; the more general term is simply “natural treatment system”.) Perry isn’t convinced that every building needs to do its own wastewater treatment, though: “The deeper you get into this, the more you realize certain decisions that solve one set of problems cause other problems.”
Even by usual standards of green building, the considerations around projects considering onsite treatment can be remarkably specific to the climate, the site, and the local infrastructure.
“Salt Lake City has a sophisticated wastewater system, including wetlands,” says Perry, describing discussions his firm has had with the University of Utah about a new building that will eventually house the Wallace Stegner Center for Land, Resources and the Environment. The project team is open to onsite wastewater treatment because of the client’s environmental mission, but it’s not necessarily the most sensible choice due to the exemplary municipal infrastructure, Perry said. Despite the region’s minimal rainfall, by using ultra-low-flow plumbing fixtures and collecting rainwater and graywater to flush toilets, the building will already have more water than it needs and can use the excess for irrigation. “In a certain way of looking at it, you’re solving a problem that doesn’t exist,” Perry notes.
Project teams looking to address water consumption and sewer loads—whether for sustainability reasons or financial ones—will want to default to low-flow plumbing fixtures, look holistically at conservation (see “,”), and then consider other options, usually in the order discussed below.
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, thus avoiding pollution of potable water just to move human waste around. Although the humus and liquids generated by composting toilets cannot legally be used to amend soil in some places, composting toilets could eventually help us recover valuable nitrogen and phosphorus in the future if these regulations change (see sidebar, “”). Energy use from pumping wastewater over long distances also “represents a significant portion of the overall impacts” of centralized wastewater treatment, according to a Cascadia Green Building Council life-cycle analysis, detailed in its report Clean Water, Healthy Sound. Composting toilets use relatively little energy and do not pollute water, making them the most sustainable choice, according to the analysis.
Most sites have plenty of rainwater that can be harvested and used not only in toilets but also as make-up water for cooling towers and to irrigate landscaping (see “,” EBN May 2008). Again, though, what might seem like a logical solution for a given project may not make sense on paper. Common sense dictates that the Pacific Northwest is rainy, but “Portland [Oregon] is a little difficult in that we have eight months of a lot of rain—39 inches total is normal—and then four months when it hardly rains at all,” explains Doug Sams, AIA, of ZGF Architects. While working on new headquarters for the Port of Portland, ZGF realized that, to span the dry period, the size of a proposed rainwater collection tank would be so large that the approach simply “didn’t pencil out.” Wastewater treatment was a better option in this case, and Sams oversaw the design and installation of a Living Machine in the building.
Additionally, many sites do not even have rights to the water that comes off their own roofs, particularly in the West and Southwest.
Graywater is wastewater from lavatory sinks, showers, and laundry facilities that does not include food waste or human waste; it may also include condensate from boilers. Indoor graywater reuse requires dual plumbing with purple pipes for the reclaimed water; but minimal water treatment and no purple pipes are needed if graywater is used only outdoors for irrigation. Graywater reuse may be a good supplement to composting toilets, but because of the expense of dual piping, for some projects it may make better financial sense to treat graywater and blackwater together.
Onsite wastewater reclamation systems represent significant first costs and a long-term maintenance commitment that not every client will be able to take on. So why consider them?
The most obvious time to consider onsite wastewater treatment and reuse is when a site is not connected to centralized infrastructure. 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 nutrient cycling, typically overloading the groundwater with concentrated nutrients instead.
Even if connecting with a municipal system is the least expensive option, a client may have other reasons for wanting to treat onsite. When the Boy Scouts first chose its Jamboree site in West Virginia’s New River Gorge, “we looked at upgrading the nearby municipal system, which would have been really easy to do there—and from a cost point of view would have been by far the cheapest solution,” says Allison Schapker, director of design and sustainability at Trinity Works, which has conducted site selection and design and construction management for the project. But the area’s rivers and streams attract tourists, who fuel the local economy. “Once we understood where we were, we had a firm commitment that we needed to manage our wastewater onsite.”
The Boy Scouts’ choices have had a ripple effect. “What’s really exciting about this for us,” Schapker told EBN, “is that our site was heavily mined and has extremely degraded soils, and returning nutrients back to the soil actually helps us rebuild the soil.” Treatment combines decentralized graywater systems—which will reuse shower and sink water for toilet flushing in more than 300 bathhouses—with a blackwater system comprising an equalization lagoon and trickling filters. Stormwater will be carefully managed with rain gardens and swales, specifically to keep it separate from both of the other systems, preventing concentrated nutrients from polluting the watershed before the wastewater has been treated.
No matter where your site is, she urges, “Once the water is treated, if you could identify a use for the wastewater, you could have nutrient cycling within your own site. You really start to find the value of your wastewater rather than just sending it downstream.”
Even in urban settings, there may be environmental advantages to treating and reusing wastewater on your site.
A full 3% of total energy in the U.S. is consumed by piping water and waste from place to place, according to the U.S. Environmental Protection Agency—just one reason the portmanteau word watergy is increasing in popularity (see “,” EBN Oct. 2010). Although small-scale wastewater treatment typically consumes more energy per gallon than centralized treatment, it does greatly reduce the use of potable water, which typically has a massive energy footprint even before we contaminate it with human waste.
And 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 your own wastewater makes your waste your own problem instead of someone else’s. Keeping it onsite may also encourage occupants to think twice before putting hazardous or nonbiodegradable waste down the drain, a common problem in municipalities.
“You’d be amazed how much we let flow off our sites that then goes on to cause another problem,” says Erin English, P.E., associate engineer at ecological restoration and regenerative design firm Biohabitats. “If you’re harvesting your water onsite, you’re disconnecting yourself from that whole cycle. That’s a pretty profound thing to do.” English characterizes potable water as “almost criminally cheap” and argues that once we begin to pay the true cost of water, “the whole [decentralized] approach is going to make a lot more sense.”
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 wastewater treatment for certain projects. 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.
In New York City, sewer connections aren’t optional, but there is a financial incentive for water reuse: buildings that replace at least 25% of their potable water with reclaimed water receive a 25% reduction in remaining water and sewer fees. Because of this incentive, onsite wastewater treatment has become “relatively common” in the city, particularly for project teams that are also seeking deep potable-water-use reductions for green building certification, according to Edward Clerico, P.E., president of Natural Systems Utilities and a co-designer of the pioneering wastewater treatment system at The Solaire high-rise in Battery Park.
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. “What small-scale projects do is break down the number one impediment to reuse of wastewater: the fear factor,” explains Pete Muñoz, P.E., senior engineer at Biohabitats. “There’s a perception that reused water is more dangerous or that we don’t have the safeguards in place.” Pointing to high-profile projects like the one at Sidwell Friends School—where President Obama’s daughters flush toilets with wastewater treated in an onsite constructed wetland—Muñoz argues that “breaking down that psychological barrier is reason enough to have single-building or small-scale wastewater treatment. But the economic case isn’t there most of the time.”
Onsite systems require frequent testing and provide research opportunities for students and scholars alike. “One of the big things for Oberlin College is the educational value,” notes Sean Hayes, facilities manager and community outreach coordinator at the school’s Lewis Center. “To us that is the biggest thing. We have a dozen students or so who work with it every semester. The value really can’t be quantified for us.”
The San Francisco Public Utilities Commission will be using its new Living Machine as a research tool as well. “There are some exciting future possibilities there at the district scale,” says Megan Koehler, associate at KMD Architects, which designed the building. As a public utility, “they understand that and are trying to study this.” An in-house team of wastewater experts is conducting ongoing research on water chemistry and quality during all stages of treatment and gathering data the utility hopes to apply on a much larger scale in the future.
How Wastewater Treatment Works
Every wastewater treatment system, from the smallest basement bioreactor to the largest centralized plant, needs to do certain things to wastewater. Typically, wastewater is treated just enough that it can be legally released into soil or waterways; treatment for reuse, at any scale, requires extra filtration, or polishing, during tertiary treatment. (Even more steps can make treated wastewater potable, but this is extremely rare—only NASA, Singapore, and a smattering of municipalities do it—and won’t be discussed here. More common is for treated wastewater to be released into rivers and then reused by cities downstream—without the psychological barrier of a closed loop.)
The goal of wastewater treatment is to remove contaminants—which make up just 0.06% of typical wastewater, according to the Water Environment Federation (WEF)—and reduce biochemical oxygen demand (BOD) so that released wastewater will not damage aquatic ecosystems by starving them of oxygen (for example, by causing algae blooms). This is accomplished primarily by encouraging bacteria to break down the organic materials in the water. Wastewater treatment typically follows these basic steps:
• Screening that removes sanitary products and large items like sticks and litter that wash downstream in stormwater (preliminary treatment)
•Settling and skimming of solids (primary treatment)
•Microbial digestion of suspended and dissolved organics, typically an aerobic process using aerated wastewater, or activated sludge (secondary treatment)
•Nutrient removal to reduce BOD further and, sometimes, polishing to remove small microbes and chemical contaminants (tertiary or advanced treatment)
•Disinfection, using chemicals or UV light
The solids left behind by the process can also be composted, either aerobically or anaerobically, and in some places can be used to amend soil. In very advanced wastewater treatment plants, methane produced by anaerobic digestion of solids can be harvested for power generation or use as a transportation fuel or heating fuel. More commonly, untreated solids are incinerated or trucked to landfills. A full understanding of local wastewater infrastructure should guide project teams in their decision-making about onsite wastewater management.
Onsite Treatment Options
Once a project team chooses to treat wastewater on the site, there are two basic types of systems. Most onsite “natural” systems rely on plant roots to create ideal conditions for bacterial growth and cycle water relatively slowly through a variety of filtration media. Parts of these systems may include lagoons, indoor or outdoor constructed wetlands, and a variety of gravel or sand filtration beds (along with less attractive pumps and polishing and disinfection tanks). Other onsite systems enclose the bacteria in a tank and cycle water relatively quickly; the bacteria are doing all the heavy lifting here, too, so in a sense these systems are just as “natural” as the kind that involve showy plantings and water features. In general, the more compact the system, the more energy it will require.
There’s not a sharp difference between the simplest natural systems and the more intensive ones: it’s more of a spectrum from the most basic constructed wetland to an energy- and space-intensive indoor natural filtration system. This is partly because, in general, the industry has moved away from tropical plantings (at least in temperate climates) that require greenhouse conditions and lots of water aeration. “Our Living Machine was built in 2000,” says Sean Hayes at Oberlin. “There’s been a lot of improvement to the design.”
Erin English of Biohabitats confirms that natural filtration technology has advanced considerably since the earliest Living Machines were piloted; her firm uses “a range of different approaches,” employing natural treatment that’s characterized by longer retention times and a variety of “different ecologies” that may include septic tanks, wetlands, and biofilm systems. “That way you have a variety of opportunities for different microorganisms to access waste in the water,” English explains. And for systems that go through soil, “that additional step tends to help things get broken down.” Although she cautions that all systems are different and that there are fairly few to study so far, preliminary evidence suggests that constructed wetlands can “substantially reduce” compounds like pharmaceuticals and endocrine disruptors in addition to reducing BOD and removing contaminants that are more conventionally removed from municipal wastewater.
Recent Biohabitats projects include a very simple system for the Dixon Water Foundation in partnership with designer Lake|Flato Architects. The remote, rural building will be used only a few times a month, and the system involves a septic tank that meters out about a hundred gallons of wastewater per day into a constructed wetland, which slowly treats and polishes the water; it will be used only for irrigation. “These are ranchers who are committed to rotational grazing and have a profoundly pragmatic approach to watershed management,” explains English. “The system is almost entirely hands-off, as it should be.”
The system at KieranTimberlake Associates-designed Sidwell Friends School, by contrast, is urban and intensively occupied during most of the year. It’s also one small portion of an onsite water management system that includes wastewater recycling and rainwater harvesting, and it produces non-potable water that’s reused to flush toilets; this system is commensurately more complex and includes a trickling filter in addition to the wetland, making it a hybrid of natural and enclosed. English provides a cautionary tale about the project, though: “There were some issues with quality of construction. If you’re going to do this as an architect, make sure contractors are getting things like liners and planting media down correctly; it’s really essential. Make sure it’s not just overlooked as ‘some landscape component.’”
One of the advancements since the days of tropical greenhouse Living Machines is that not every “natural” treatment method needs plants—although most use them anyway. Some recent projects, like the San Francisco Public Utilities Commission building, involve very deep planters, explains Pete Muñoz. Although the systems are being called tidal wetlands, he argues, “plants don’t grow ten feet deep,” so their root systems don’t have a major role to play, making the systems more like “tidal gravel filters.” But the plants have aesthetic and educational value, and such systems “are still an effective method of treatment for small footprints,” he told EBN—and they are far less energy-intensive than enclosed mechanical treatment systems.
“We’re always working on the spectrum of space and energy,” says Muñoz. “Our practice is definitely more of a land bias, larger systems with smaller energy footprints.” In cities, though, that approach doesn’t always make sense. “A small footprint and larger energy load does have a place in certain communities.” The most common enclosed systems, membrane bioreactors, are very compact and work relatively quickly compared with natural filtration systems, but they require very high energy inputs. Biofilm-based systems like trickling filters also push water through fine membranes, though they tend to use less energy per gallon (and also work at a slower rate). When might these higher-energy approaches make sense for a building?
Probably when a few buildings are able to process wastewater through one system, argues Edward Clerico of Natural Systems Utilities. “With multiple buildings, you quickly improve your energy profile and achieve efficiency comparable to municipal-type energy consumption,” he claims. “In the 200,000-gallon-per-day range, you’re comparable to municipal on energy.” That’s roughly 1,000 multifamily residential units, Clerico says.
He also sees new promise for the idea of recovering energy from membrane bioreactor systems. “There’s great technology coupled with thermal energy recovery of the water in the building,” meaning that heat from the bioreactor can be used to heat domestic hot water. “That you can do in a building on a small scale much more readily than on a large scale,” Clerico adds. “When you start bringing that type of solution to reality, then the whole distributed model takes on more immediate value.” His firm is proposing energy recovery from a membrane bioreactor for a project under way in Fort McMurray, Alberta.
For more about how membrane bioreactors work and the quality of the water they produce, see “”.
As Clerico notes, when it comes to wastewater treatment and energy, the question of scale can be crucial. 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.
“It’s great to have onsite wastewater treatment at the building scale,” Muñoz maintains, “but we find the most bang for our buck is with eco-district wastewater systems.” Rather than a LEED Platinum or Living Building Challenge “island of sustainability,” he argues, “it makes much more sense to do it at a block scale or neighborhood scale.”
Unfortunately, some of the best opportunities for onsite wastewater treatment involve remote greenfield development: if you’re building a suburb, a medium-scale wastewater treatment system that cycles water more than once—rather than mimicking large-scale, once-through systems—should be a no-brainer.
In already developed areas, it’s much harder to share wastewater treatment with your neighbors. “We can’t necessarily collect water in one building and use it in other buildings: we end up with other jurisdictional issues,” says Lisa Petterson, AIA, associate principal at SERA Architects. “You can’t become a utility; that’s why you might consider it on a single-building scale.”
If you are a utility, it might work out. According to Megan Koehler, the San Francisco Public Utilities Commission is currently dumping almost half its treated wastewater back into the sewer system, but they are looking into irrigating the landscapes of several surrounding buildings with the excess. The system at The Solaire was intentionally oversized when installed in 2002 and now serves two buildings owned by the same developer. District-scale systems can also work on college or business campuses, Petterson suggests.
Working with the city
And then there are opportunities for a college or other major developer to work directly with a municipality to improve local infrastructure.
Oberlin College is creating a 13-acre, net-zero-energy and potentially net-zero-water “Green Arts Block” as part of a public/private effort to make Oberlin, Ohio, a “climate-positive” city that also supplies 70% of its own food. Right now the project team is wrestling with “tradeoffs and issues around scale and effective cost,” says David Orr, Ph.D., professor of environmental studies and politics at the school. On the one hand, they could choose to do onsite water treatment for the Green Arts Block and “integrate the system in a compelling way,” with a gorgeous two-story waterfall and various other features that “make an iconic statement.” On the other, they could “do something cheaper but larger-scale” by building an industrial-scale constructed wetland outside of town that would partially treat the water for the entire community but would not attract as much interest.
Although a hybrid of the two approaches is likely, Orr sees the decision as an object lesson for designers: “How do we build a larger fabric and a larger pattern of resilience in the community? Some of that has to be visual; it can get people excited. But clock speed and visual impact are entirely different.”
The ultimate goal, many designers feel, should be better municipal infrastructure, and architects may be able to participate in that in ways that don’t involve multi-million-dollar water features.
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.
Looking at existing systems and areas with lots of infill projects going on, he and district systems lead Scott Shumaker, P.E., have proposed four purple-pipe districts that would make sense for Portland. (Purple pipes convey reclaimed water, and a few cities have purple-pipe infrastructure that carries reused wastewater both to and from buildings.) “We have abundant water in Portland but combined sewer overflow,” he told EBN. Although Portland recently built a big-pipe system to deal with most of the overflow and to come back into compliance with federal standards, the system still dumps raw sewage into waterways about four times a year, Brockman says. Their decentralized infrastructure would help solve this problem by “scalping” wastewater from sewer pipes (taking the clearer water from the top of the pipe), treating it at small neighborhood plants, and delivering it to nearby buildings. “You could create an overlay zone and say that buildings in these zones have to build with purple pipes.” San Francisco, he notes, now requires dual piping for all new buildings.
Brockman recognizes that his scheme is “very specific to Portland,” but he encourages all architects to think bigger. Although small-scale wastewater treatment certainly has a place, he says, “I don’t want architects to start imagining they all need to put membrane bioreactors and constructed wetlands in their projects with cool videos on their websites. They should be looking at their community and their place and their watershed. What can you do?” Brockman challenges.
Although the answer will be different for everyone, it’s clear that we all need to be asking ourselves that question.
For more information:
International Living Future Institute
U.S. Environmental Protection Agency