In seeking to create a built environment based on regenerative principles, one goal often cited is to create closed loop systems of water and waste. Net-positive Water is the sole Imperative for the Living Building Challenge’s Water Petal, for instance. The idea is that we ought to be able to supply all our potable water needs using filtered and sterilized rainwater that falls on the site; wastewater recycling is another option but using it for drinking water complicates matters even more.
In our pursuit of lower construction costs and greater efficiency in energy, materials and land use, we often work very hard to reduce the geographic footprint of the project. This is especially true in the residential sector, and doubly so in affordable housing projects. Greater density is often heralded as a basic groundrule of sustainable design (at least in urban settings).
But as we will discuss, density also presents serious challenges to developing projects that are net-zero in energy and water.
Parallels Between Energy & Water
Even though it is not the focus of this article, let’s start with energy vs. density. The basic quandary is that as density goes up, the ratio of roof area to occupants goes down, pretty much by definition. Of course, rooftops are generally where we put solar panels and other forms of renewable energy. Ultimately, we reach a point where the occupant energy load exceeds the output of the solar panels (averaged on a yearly basis), no matter how hard we work to reduce each person’s energy footprint.
Water constraints work the same way, but are even more stringent. We can increase the solar collection area by covering the vertical surfaces in solar cells too (though this DOES drive costs up). Electricity is much easier to transport and share on a regional scale (especially in an urban grid environment). Water generally falls more vertically (at least on a consistent basis), limiting the opportunities to harvest significant amounts off vertical surfaces. Whereas many estimates indicate that buildings must be limited to 4-6 stories to achieve net-zero energy, our own experiences have suggested that even 2-stories may be too much when it comes to net-zero water. Residential applications, which generally have higher per capita water use than an office building, for example, are especially challenging.
The Example of a 3-Bedroom House
To illustrate this, consider a 3-bedroom single family dwelling. Occupied by a family (instead of a bunch of college students, for instance), this dwelling will generally accommodate 4 occupants (it COULD easily be as few as 2 or as many as 6, but let’s just say 4). Now, Americans typically use as much as 50 gallons per capita per day (GCD) of potable (drinkable) water. That including drinking water, showers, dish and clothes washing, toilet flushing. Even if we eliminated the use of potable water for non-potable uses like toilet flushing, how much could we reduce. Estimates vary but 15-20 GCD would be be considered a challenging target.
In Washington DC where I live, we get about 40 inches of rain per year. DC isn’t the wettest or driest place in the world; it’s pretty typical. Depending on the efficiency of your rainwater harvesting system, 40 inches of rain translates into about 24 gallons of water per square foot (s.f.) of catchment area. At 20 GCD of potable water use, each person needs about 7300 gallons per year (sounds like a lot, doesn’t it – hard to believe we typically use almost 3 times that now). That means that to meet our potable water needs using filtered rainwater, each person needs about 304 s.f. of roof area. So a family of 4 needs about 1200 s.f.
It is possible to design a snug but comfortable single-story 3-bedroom house at around 1400 s.f. If you go to 2 stories, that might be more like 1800 s.f. total (adding space for the stairs and so on), but that means the footprint of the house is only 900 s.f. Even with some MAJOR roof overhangs (eaves), it is tough to get the 1200 s.f. of roof area you need to make your potable water budget. It is particularly hard to do using a row-house type of dwelling (very popular in places like DC).
Projects that have achieved net-zero water use are often commercial buildings like offices, where potable water use is relatively low and where professionally maintained, larger water recycling plants are potentially more viable.
So in conclusion, there seems to be a conflict here between greater density (even at modest levels like a two-story dwelling) and net-zero water. It may suggest that the only way to make net-zero water work is to include more speculative and expensive propositions like on-site graywater and blackwater recycling for potable uses. Other options (also more expensive and site-specific) include using groundwater (wells) recharged with filtered wastewater, subsurface dams (which trap stormwater so it can be collected and filtered to potable standards), and atmospheric moisture condensers. Of course, there is also the possibility of harvesting water as well as power in adjacent areas of the city that are lower-density, but this raises a lot of questions also (land-rights, cost, liability, etc.).
I take it as an article of faith that we CAN make net-positive energy, water and waste management work, but the calculations cited above suggest how difficult this may be while increasing development density. The cost barriers are currently prohibitive, even in places where water shortages have become critical.