Notes from the Field Station: Greywater Irrigation

4 Nov

As mentioned in my post detailing many aspects of the Bechtel Environmental Classroom (BEC) design, the use, and reuse, of water is of extreme importance to the project. I have already discussed the proposed water conservation efforts in the context of composting toilets, and will now focus on the role a greywater system will play in limiting fresh water consumption and wastewater discharge.

is defined as any water leaving a building that was not used in toilets (water that includes toilet discharge is know as blackwater). This includes any used water draining from sinks, bathtubs, and washing machines. About 65% of all domestic wastewater is greywater, and if composting toilets are being used, 100% of wastewater is greywater. Although greywater is not without impurities (such as grease, hair, and food particles) it may still be suitable for reuse on site. Depending upon the amount and quality of the greywater available it can serve a variety of applications. Most commonly greywater is used for irrigation and toilet flushing, but nearly any use that avoids contact with humans is a possibility.

Greywater irrigation is suitable for trees, ornamentals, and food crops, although it is advised that greywater be applied directly to the soil through a subsurface irrigation system to avoid contact with the aboveground portions of the plant.  In the case of the BEC, all greywater generated will be used to irrigate surrounding permaculture gardens, containing teaching gardens and a variety of ornamentals and edibles, as well as an orchard.

When harvesting greywater for reuse, it is important to consider all additives, such as soaps and detergents, which may act to contaminate the water, rendering it impractical for its proposed application. In the case of irrigation, it is crucial that cleaning products be biocompatible, or suitable for the plants and soils of the greywater-irrigated landscape. Specific ingredients to avoid include sodium compounds, boron, and chlorine or bleach. Instead, seek out traditional liquid potassium-based soaps, such as Dr. Bronner’s, or handcrafted soaps. Furthermore, according to the Greywater Guide most soaps and shampoos will not damage plants if concentrations are kept low.

Greywater irrigation systems vary tremendously, from the basic mulch basin, to branched drains, constructed wetlands (like the one seen at the Omega Institute) and solar greywater greenhouses. When choosing a system it is important to follow ecological design principles, which state that the greatest efficiency is reached when the power of the tool is well matched to the task at hand. In essence, select the simplest design that meets your needs and build it as well as you can.

There are many benefits to recycling greywater for use in irrigation, including but not limited to recharging groundwater, maintaining soil fertility, facilitating plant growth, reducing fresh water use, and reducing wastewater discharged into sewer/septic systems. Furthermore, biological water purification that utilizes the bacteria located within the topsoil layer rapidly breaks down greywater, rendering nutrients available to plants and producing purified water of a higher quality than produced by any engineered treatment.

Although the recycling of greywater makes huge strides to improve water use efficiency and conservation, it is essential that all efforts be made to conserve water before it hits the drain prior to implementing a greywater system. For example, utilizing low-flow showerheads, efficient front-load washing machines, low flush/composting toilets, and rainwater harvesting. With these overlapping efforts in place, it is hard not to be satisfied by this direct participation in the responsible management of global nutrient and water cycles.

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Brad Lancaster, author of Rainwater Harvesting for Drylands and Beyond, has a wonderful website that serves as a great resource on greywater use, as well as rainwater harvesting.

Jessa Finch (’12)

CEEDS MacLeish Intern

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