Use alternatives to municipal drinking water for irrigation

Rain Barrel. Image credit: wikimedia commons

With a little ingenuity, gardeners can use non-potable water from a variety of indoor and outdoor sources to irrigate their garden.  Water that is non-potable is not safe for humans to drink but can be a great resource for vegetation.  Such alternative water resources as rainwater runoff, air-conditioner condensate and greywater are typically considered waste products in conventional landscapes and quickly removed from the site.  Sustainable landscapes capture this water and use it to benefit the garden.  In doing so, downstream aquatic ecosystems are protected from pollutants commonly carried in stormwater.   Further, the volume of water going to storm sewer systems is reduced,  lessening the likelihood of flooding and combined sewer overflows.

Rainwater collection

Rainwater collection has been used for centuries to support households and grow beautiful, productive gardens. The practice typically falls into two categories, passive and active.

Passive rainwater harvesting

Passive rainwater harvesting diverts water overland to vegetated areas for immediate use.  Roads, driveways and sidewalks are designed to direct water to vegetated areas instead of storm drains.  Design features such as shallow bioswales or terraces can be integrated into the landscape and used to slowly convey and disperse the water throughout the garden.

Active rainwater harvesting

Rainwater cistern collecting water from a residential roof top. Image credit: US EPA

Active rainwater harvesting captures water in a barrel, cistern or tank where it is held for reuse. The containers are typically manufactured in a variety of shapes and sizes and connected to the downspouts of the roof gutter system.  Rain barrels generally hold around 50 gallons and come with a screened cover and an overflow spout.  Cisterns are used to store larger volumes of water and may be buried below ground or integrated into the landscape or building design as a freestanding structure. Active rainwater harvesting is prohibited in some areas.   Local water laws should be understood prior to designing a rainwater collection system.

The amount of rainwater available for reuse varies with precipitation patterns and should be estimated for each month.  Approximately 600 gallons (2271 L) of water can be collected for each inch (2.54 cm) of rain falling on a 1,000 ft2  (93 m2) of impervious catchment surface.  It is important to note that all of the rain that falls on a surface will not be collected.  Different surface materials and collection systems have varying rates of loss.  For example metal roofs typically generate more runoff than asphalt shingles and some rain will overshoot the gutters during heavy rain events. When designing for rain water collection, materials and system components should be carefully selected to minimize waste.

Air conditioner condensate

Condensate is produced when warm, moisture-laden air passes over the coils of an air-conditioning system. Collecting condensate is an attractive irrigation option because unlike rain, which is sporadic and unpredictable, condensate is produced regularly during the hottest months when the need for irrigation is greatest.

To collect and reuse condensate, the water is simply piped and gravity fed or pumped to a storage cistern where it can be used for irrigation or other landscape purposes. A/C condensate is essentially distilled water that does not contain chlorine, minerals or other additives making it a high quality water source for irrigation.  Rainwater harvesting and condensate recovery systems use similar tanks for storage and can be combined to create a more efficient system and to reduce costs. The average single-family home produces 5 to 10 gallons of condensate per day. Condensate recovery systems are best suited to hot and humid climates.

Greywater collection

Image credit: Microsoft images

A variety of appliances and fixtures produce used water called greywater. An estimated 50 to 80 percent of residential “wastewater” is dish, shower, sink and laundry water.  Many people find greywater an attractive option because it provides a steady water source year round and saves potable water resources.  Greywater can be safely used to grow beautiful gardens, however due to the soaps, detergents, skins cells and other organic substances commonly found in the greywater, direct human contact should be avoided.

Landscape characteristics that may prevent the use of greywater include:

  • Areas prone to water ponding or flooding
  • High water tables
  • Shallow soils
  • Steep slopes that may lead to runoff or seepage

 

A typical U.S. household generates an average of 35 gallons of greywater per person per day (Roesner, Qian et al. 2006). Greywater is most often recommended for subsurface irrigation of non-food plants. Greywater systems vary from simple and low-cost to complex and costly. Systems can be designed to convey greywater directly from the source to the landscape, or plumbed to capture, treat and temporarily store greywater for later reuse. The simplest way to collect greywater is to plug the drain and employ a bucket to transport bath or shower water for use outdoors. Another common practice (but illegal in some locations) is to drain the washing machine directly onto outside vegetation. Sophisticated systems involve separate plumbing for greywater as well as settling tanks and sand filters to remove solids and pathogens.

The quality of greywater depends on the source of the water and how it has been used.  The alkalinity of the soil can be increased by sodium, potassium and calcium salts commonly found in greywater.  High salinity can damage soil structure and limit the ability of roots to absorb water.  Applying greywater over a broad area will help avoid the buildup of harmful substances.

Cenzio or Sage (Leucophylum frutescens) is an example of a colorful shrub that grows well when irrigated with greywater. Image credit: Sally and Andy Wasowski

Rain and fresh water irrigation can help flush the soil and reduce the concentration of salt and other chemicals. Landscape friendly detergents and other cleaning agents are commonly used in households with greywater systems to minimize any negative impacts.

Shade loving and plants that grow in acidic soils such as rhododendrons, hydrangeas or camellias typically do not do well when irrigated with greywater due to its high pH. Plants that grow well in alkaline soils are commonly recommended and vegetation irrigated with greywater should not be overly sensitive to elevated concentrations of salts. In general, seedlings and young plants tend to be more sensitive to elevated salinity than well-established vegetation.  Plants that are especially sensitive to high levels of salts include crepe myrtle, redwoods, star jasmine and hollies.  Plants that commonly do well when irrigated with greywater include oleander, bougainvillea, rosemary, oaks, agapanthus and juniper.

Note on State And Local Regulations 

States and local governments regulate greywater differently. Some prohibit the collection of greywater entirely.  You will need to investigate what applies in your area. A list of some states and municipalities with greywater policies can be found here:  (www.oasisdesign.net/greywater/law/index.htm)

In regions where greywater use or rainwater harvesting is not a legal option, homeowners can educate local officials on the benefits of water reuse and encourage policy changes.

Use water-thrifty irrigation practices

Conventional irrigation practices waste a lot of water. Watering with traditional sprinklers or when it is hot or windy leads to water loss through evaporation. Watering too quickly or too much leads to runoff and excessive water waste.