Sustainability: A Deep Dive on Greywater Irrigation - The Edge from the National Association of Landscape Professionals

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Sustainability: A Deep Dive on Greywater Irrigation

If you’ve looked into water conservation methods that can be utilized in the landscape, you’ve probably run across the term greywater or greywater irrigation.

Greywater is the used water from showers, tubs, bathroom sinks and washing machines. It is not used water from toilets, kitchen sinks or dishwashers. That is known as blackwater. In a typical household, about 50 percent of the wastewater generated is greywater and it could supply about 30 percent of the water needed to irrigate a typical residential landscape.

Greywater irrigation systems can be used to irrigate large plants like trees and shrubs. These systems are not built to irrigate turfgrass and they should not be used to irrigate root crops or other edible crops that touch the soil.

Before offering this as a possibility to your clients, below are some of the pros and cons that need to be considered as well as some of the different irrigation system options.

Pros and Cons

The primary benefit of using greywater is lowered water usage and, consequently, lower water bills as well. According to Water Wise Group, homeowners who install a greywater system can save up to 40,000 gallons in water per year. In drought-prone areas, greywater is particularly significant for its ability to offset the need for potable water to be used on the landscape.

For those who have septic tanks, having a greywater system can also prolong the lives of their tanks. Studies on the effects of greywater on plants have shown it can benefit plant growth as it can contain plant macronutrients.  

While the water savings are significant, there are also a number of challenges that come with adding a greywater irrigation system. One of the main inhibiting factors for the utilization of greywater is state laws. Fourteen states define greywater as wastewater and another 10 define it as septic, making the usage of greywater in these states illegal.

States like Arizona, California, New Mexico, Washington and Oregon permit greywater usage with a tiered approach based on the different-sized systems. The required permitting and legality of greywater has evolved over the years, so always check with your local ordinances to know what the latest guidance is.

The main reason the use of greywater is regulated is due to public health and safety concerns. While it’s not as contaminated as blackwater, greywater does contain traces of dirt, grease, soap and other organic matter. If stored, it will turn foul, so it must be used quickly. Coming into direct contact with untreated greywater also carries a risk of transmitting pathogens. The risk is small but does exist.

In the case of most houses, the greywater mixes with the blackwater, so elaborate plumbing changes are needed to separate these two water types. This can be expensive, and depending on what type of irrigation system the client wants, the complexity of the project and cost will vary greatly.

As for the long-term effects of greywater on plants and the soil, studies have found that most plants show positive impacts such as high shoot growth, better density and color. Sodium accumulation was the main problem that occurred in soils that used greywater. These soils also tend to have a higher pH.

Types of Greywater Irrigation Systems

If you’re in a state that allows the usage of greywater irrigation and your client is still wanting to proceed, remind them that plant-friendly products are key when reusing their greywater. There is no point in installing a greywater irrigation system if they consistently use products with lots of salt, boron or chlorine bleach, which are harmful to plants.

There are two main greywater irrigation system types: diversion and treatment. Diversion devices carry the water from the source to subsurface irrigation without treating the water. Meanwhile, a treatment system collects the greywater and disinfects it to varying degrees before continuing on to the landscape.

Diversion systems can use gravity or a pumped system to push water through the lengths of the dripline. Pumped systems are commonly needed if the landscaped areas are uphill or the yard is flat and the only irrigation needs are far from the house. Because the drainage line for most washing machines is separate, laundry-to-landscape systems are a popular choice that doesn’t require as much plumbing. These systems have no filters, storage tanks or external pumps.

Another option is the branched drain system. Greywater fixtures drain into a diverter valve and this water is spread through a network of flow splitters and underground emitters. Whichever system used, the greywater should typically be directed to mulch basins as they allow greywater to spread into the ground subsurface avoiding pooling, runoff and odors.

Jill Odom

Jill Odom is the senior content manager for NALP.