Engineered earth with greywater systems

Propex incorporates a system of engineered earth with greywater irrigation on slope at residential site in Malibu, Calif.

By Jeff Hoilman, P.E., market development manager, GeoSolutions, Propex Operating Co. LLC

The increasing need for sustainable design coupled with droughts and water shortages have resulted in increased implementation of on-site wastewater treatment systems among residential and commercial developments. Even non-arid regions, like the Southeast, are not exempt from water shortages. Population growth is also driving water demand in many of the nation’s most water-scarce regions, including Arizona, California, Florida, Nevada and Texas. As an innovative way to save water, some state and local governments are incentivizing the use of on-site wastewater treatment systems to recycle greywater.

Greywater is lightly used water from bathroom sinks, showers, bathtubs and clothes washers. It does not include wastewater from toilets and is generally safe to handle and reuse for landscape or crop irrigation.

Reusing greywater for irrigation can be done using drip systems or spray distribution. These types of systems are designed to work in areas that are difficult to access or where steep slopes exist. Reusing greywater to establish vegetation on steep slopes can reduce erosion and sediment impacts, promote groundwater recharge, increase water quality, improve aesthetics and increase property value. One study commissioned by the city of Santa Rosa, Calif., found that a “laundry-to-landscape” greywater system could save 15 gallons (57 L) of water per person, per day.

Engineered earth with greywater systems

Propex Operating Co. LLC’s engineered earth armoring solutions can be used in conjunction with greywater irrigation systems to provide optimal slope stabilization and/or prevent erosion. Combining these two systems has been successfully used on residential and commercial properties.

In 2019 a developer purchased two homes located adjacent to one another near Coral Beach in Malibu, Calif., with the intent to renovate and resell both structures.

A 1:1 unvegetated slope was located behind the homes. Pacific Coast Highway 1 runs along the top of the slope. Due to runoff from the highway, the slope was experiencing severe erosion. The erosion was depositing sediments onto the driveway utilized by both residences. Ultimately, the sediment would enter the closed stormwater system during precipitation events. If this continued, these sediments would impact the hydraulic capacity of the sewer system, reduce water quality and increased maintenance costs for the City of Malibu.

To incorporate sustainability into the renovation, improve aesthetics, prevent further erosion and ultimately increase property values, the developer installed a drip and spray irrigation system using greywater to provide irrigation on the steep slope. To further protect the slope and the investment of the irrigation system, the developer used the ARMORMAX 75 Slope Stability System to protect the hill from further erosion.

The slope stability system is designed for areas of severe erosion and surficial slope stabilization that provides vegetated reinforcement, improves the factor of safety and significantly reduces the probability of failure. The system is composed of Engineered Earth Anchors that are designed and tested for compatibility and performance with our PYRAMAT 75 High-Performance Turf Reinforcement Mat (HPTRM) to increase slope stability for up to 75 years. The system also uses metal securing pins to temporarily hold the HPTRM in place.

The slope stability system is designed for areas of severe erosion and surficial slope stabilization that provides vegetated reinforcement, improves the factor of safety and significantly reduces the probability of failure.

The PYRAMAT 75 component is a homogenous woven HPTRM that provides erosion protection from overland slows while promoting the establishment of vegetation. The homogeneous construction of the HPTRM allows for the material to be easily cut to accommodate landscape plantings, drip lines, spray irrigation heads and other protrusions. While the HPTRM component can be cut, it is suggested that the irrigation piping, drip lines and spray irrigation heads be installed prior to placement. Landscape plantings can be installed post-installation of the slope stability system. See Figure 2.

Figure 3 shows vegetation establishment on the slope.

Since the installation of the engineered earth with greywater systems, the City of Malibu and other local municipalities are encouraging developers and builders to install on-site wastewater treatment systems along with the system. The combination of these technologies offers a sustainable way to save water while simultaneously reducing operations and maintenance costs to the city by eliminating sediment deposition into the stormwater system. Additionally, the slope stability system is more cost-effective and has a significantly lower carbon output than traditional hard-armoring solutions like rock riprap. Vegetation quickly established and continues to flourish two years later. The slope is stabilized and there is no longer erosion or sediment deposition.

Combining a slope stability system with an on-site wastewater treatment system that recycles greywater into irrigation has resulted in the establishment of vegetation, groundwater recharge, improved water quality and lower operations and maintenance costs through the reduction of sediment loss. Ultimately this has also improved aesthetics and increased home property values for the developer and homeowners.