Water Resources in Buildings

Water Resources in Buildings

Water use generally refers to municipal potable water use on the site.  It includes the use from fixtures (faucets, toilets, sinks, etc.), the use from equipment (dishwashers, etc.), and the exterior use for landscaping.

Good system design and good specification of products can easily reduce water use by 50% or more.  At least one green building certification system requires buildings to be net zero water use.

There are several ways to get the most out of every drop: water-efficient fixtures and equipment, water-efficient irrigation and landscaping, recycling water so it can be used more than once, and capturing rainwater. You can also purify the water on-site with living machines or advanced septic systems.

Water-Efficient Fixtures and Equipment

Fixtures that save water include low-flow shower heads, sinks with auto-shutoff mechanisms, and water-saving toilets and urinals. Equipment that saves water includes dishwashers, clothes washers, other commercial kitchen equipment such as sprayers and steam cookers, as well as industrial process equipment.

Reducing water use from fixtures and equipment is perhaps the easiest method to reduce total potable water use. It does not require extensive design solutions, just specifying certain products. Avoiding large fountains, pools, and other water features will also save water use.

Predicting Water Savings

How much do these products save? You need to calculate it by calculating the total building water use based on status-quo “baseline” products, and the total water use with the water-saving products. In each case, this is simply the sum of water use for all products in the building. The water use for each product is the number of liters per use multiplied by the number of uses per year.

Total Water Use = Σ (liters per use • number of uses / year)

Standard estimates for the amount of water per use may vary by region, but some government organizations and non-government certification systems have standards for baseline water use.¹

The number of uses is determined by the number of occupants, their gender, the amount of time they spend in the building, and the activities they engage in there.² These details will be defined by the building’s program.³ The ratio of women to men should be assumed to be 50/50 unless it is known to be otherwise for the building’s program.

For example, a retail store’s bathroom may have a toilet, a urinal, and a sink. Visitors will use the restroom much less per person than a full-time employee (“FTE”) would, but there will be many more visitors than employees. Men will use the urinal, but as a result will use the toilet less often than women. Both genders are assumed to use the sink equally.

For each fixture, its use can be calculated as follows:

Once the number of uses per day is calculated, it can be multiplied by the number of days per year that the building is occupied (250 or 260 days/year for many offices). That will be the total uses per year. That can then be plugged into the first equation above, calculating total water use by multiplying liters per use by number of uses per year.

Wastewater Recycling

Most buildings use municipal drinking water for all uses, but many applications (such as irrigation, toilet flushing, decorative fountains) do not require it.  Wastewater recycling is the reuse of water after it is no longer potable.

Wastewater recycling can significantly reduce total potable water use without requiring austerity, as the same water can be used more than once.

Water recycling reuses “greywater“.  This is water that has been used for washing, and is still relatively clean, unlike sewage water which is called “blackwater”.

Some greywater systems deliver the water as it is.  Others filter and purify the water before delivery, removing solids, chemicals, and pathogens.  These purification systems may be physical and chemical, or can even be artificial wetlands.  Almost none purify it to the point of being drinkable again, though it is possible.

Greywater for Irrigation

Greywater is often used for irrigation, because toilets do not use all of the greywater supply from most buildings.  However, there are some considerations that must be addressed when using it.

Water used for irrigation should not harm plants, so wastewater coming from sinks, showers, and process equipment should not contain harsh chemicals.  This can be accomplished by instructing occupants to only use non-toxic biodegradable soaps, and/or by purifying the greywater before it is used.

Water used for irrigation should clearly not harm people, either.  This can happen if water is not purified adequately for consumption, but is inadvertently used for consumption, by people eating food grown with the water, or breathing in mist from sprinklers.

To avoid people breathing the water from sprinklers, drip irrigation can be used.  To avoid people eating food grown with unclean greywater, landscaping can be entirely inedible plants, or garden areas can be irrigated separately with potable water.

Municipal Non-Potable Water

Some cities recycle greywater on a municipal scale.  They have separate plumbing lines for non-potable water, which homes and businesses can use for irrigation, decorative water features, or process water.

This water is filtered to be free of solids and suspended particles, and will be purified to an extent.  However, since very few cities provide this infrastructure, there are not widely-accepted standards for the quality of the water.

Water-Efficient Irrigation and Landscaping

Water-Efficient irrigation and landscaping are ways to save water by choosing different irrigation equipment, different plants, and siting plants differently.  They can also be combined with water reuse.

Landscaping often uses more water than fixtures and equipment within the building, so water-efficient landscaping can be the biggest source of water savings in a project.

Water-Efficient Irrigation

Water-efficient irrigation reduces water use by avoiding evaporation, and avoiding over-watering.

Avoiding evaporation can be done by delivering water more directly to the soil, or by delivering larger water droplets so they will not evaporate so easily, or by timing irrigation to avoid hot sunny times of day that cause more evaporation.

Delivering water to the soil can be done by “microirrigation” or drip irrigation.  Microirrigation is where irrigation nozzles are very near to the ground, but more numerous to make up for the lack of range of each nozzle.  Drip irrigation does not spray water, but drips it from holes in a pipe that lies on the ground or underground, to avoid evaporation entirely.

Drip irrigation hose

Drip irrigation is 90% efficient in delivering water where it is needed, while sprinkler irrigation is generally only 63% efficient.¹

Avoiding over-watering can be done by not irrigating when it rains, having sensors in the ground shut off irrigation when the soil has enough moisture, or having evapotranspiration sensors shut off irrigation when plants are losing less moisture to the air.  Microirrigation and drip irrigation can also avoid over-watering by being more precise about delivering the right amount of water to different locations.

Water-Efficient Landscaping

Your choice of plants greatly affects your water needs; so does the density of planting and the climate conditions of each different part of the site (direct sun vs. shade, high winds, etc.)

Succulents need much less water than some other plants

For examples of how much these three variables can change, turf grass can use up to three times as much water as trees, shrubs, or groundcover in the same area.  Many plants all planted close together can double the water use compared to sparser planting.  Finally, a location in full sun and wind might use nearly three times as much water as a shaded and secluded location.

Choosing plants that require no more water than natural rain is an ideal way to eliminate the need for irrigation.  In dry climates, this is called xeriscaping.

Rainwater Harvesting

Rainwater harvesting means capturing and storing rain that falls on-site (usually on roofs). It is generally used for irrigation and toilet flushing or other greywater uses, though it can also be used for drinking water if it is adequately treated.

Capturing rainwater can be a valuable way to reduce or even eliminate a building’s use of municipal potable water, without requiring reductions in water use by occupants. However, it is of course more effective in rainy climates than dry ones.

Rainwater harvesting systems are measured by their area for collecting water (in m² or ft²) and the volume of water they store (in liters or gallons).

Simple rainwater collection systems have three main elements: the roof or other catchment area, the storage tank(s), and the gutter and other piping that directs the water from the catchment area to the tank.

The Aldo Leopold Center turns a rainwater-harvesting gutter into an aesthetic water feature

Advanced systems may also use a pump to pull water from the tank to where it is used, and may purify the water with additional devices such as filters and ultraviolet disinfection.

If the rainwater is meant for drinking or watering gardens, be sure to choose a tank material that does not leach toxins or foster pathogens. For example, galvanized steel tanks are lined with polyethylene or other food-grade liner.

If the rainwater is collected from a roof is meant for drinking or watering gardens, be sure to choose roofing materials that do not leach toxins. For instance, asphalt shingles leach toxins into water, while metal roofs or slate shingles do not.

Predicting Rainwater Harvest Rate

To size a system for a site, you must choose the water collecting area to supply enough volume of water for the site occupants, given the site’s rainfall patterns.  The simplest equation for system sizing is this:

(Volume) = (Area) • (Precipitation) • (% Efficiency)

Volume is the amount of rain harvested in that time period, measured in liters. Area is the rainwater capture area, measured in m2. Precipitation is the amount of rainfall in that time period (in mm). Efficiency is the percent of water actually captured, as opposed to splashing out of the system somewhere; it is usually 75% – 90%.¹

In English units, a coefficient must be added:

(Volume) = (Area) • (Precipitation) • (0.62 gal/ft²/inch) • (% Efficiency)

Here volume is in gallons, area is in ft², precipitation is in inches.

Occupant Needs

The volume of water needed by the occupants will vary based on the number of occupants, the amount of time they spend on site, the activities they engage in, and the equipment or processes used on site. See Water-Efficient Fixtures and Equipment and Water-Efficient Irrigation and Landscaping for calculations to determine water usage needs.

These needs are often constant throughout the year, but if they vary by season, be sure to incorporate that in your calculations.

Rainfall

Weather data from TMY files can be used to determine rainfall patterns. These will be in mm or inches of rain.

Be sure to calculate average monthly rainfall for the different months of the year, not simply an annual total. Most sites have much more rainfall in some seasons than others, and excess water can always be drained, but a lack of water requires municipal water use to compensate.

Efficiency

Different gutter systems, different roof pitches, and different materials can affect system efficiency. For example, lower-pitch roofs cause less loss than steeply-pitched roofs.

Sizing Rainwater Tanks

There is no one standard recommended size for rainwater storage tanks. The size depends on the site’s water needs, the weather, and whether the site is connected to a municipal water supply or not. While bigger tanks allow for more water independence, the tank is usually the most expensive part of the system.

Systems that do not have municipal water backup (called “off-grid”) must hold much more water, in case of shortage. The amount of oversizing depends on how crucial the water needs are–discretionary water use like lawns or water features can be done without for days or weeks at a time, while drinking water cannot.

Off-grid residential rainwater catchment tanks

The main consideration for sizing a storage tank is the worst-case length of time between rains. This can be seen by graphing the TMY precipitation data by day, rather than simply finding monthly precipitation averages. After you have calculated the occupants’ water needs and the average frequency and magnitude of rain in the dry season, you should multiply the resulting tank size by a “safety factor” to provide room for error or extreme weather.