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How to combine engineered earth armoring solutions with onsite greywater treatment systems


The increasing need for sustainable design coupled with droughts and water shortages have resulted in increased implementation of onsite wastewater treatment systems in residential and commercial developments. 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 onsite 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 and/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, CA, found that a “laundry-to-landscape” greywater system could save 56.8 L (15 gal) of water per person, per day.

Engineered earth armoring solutions can be used in conjunction with greywater irrigation systems to provide optimal slope stabilization and prevent erosion. Combing 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, CA 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 eventually 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 increase maintenance costs to the City of Malibu.

To incorporate sustainability into the renovation, improve aesthetics, prevent further erosion, and 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 an engineered earth armoring system designed to lock soil in place to stabilize the slope and protect against further erosion.

Since the installation of this project, the City of Malibu and other local municipalities are encouraging developers and builders to install onsite wastewater treatment systems along with the engineered earth armoring 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, engineered earth armoring solutions are more cost-effective and have a significantly lower carbon output than traditional hard armoring solutions like rock riprap.

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