The most effective graywater “filter” is biologically-active soil covered in mulch! Mulch basins help you avoid removing large particles like hair or lint and help add organic matter back into the soil--organic matter in the soil results in healthier soil and makes it easier for your plants to absorb nutrients.
What’s a mulch basin?
Mulch basins catch large particles in mulch, help transform the mulch into compost over time, and help the graywater soak down into the soil to irrigate plant roots. A mulch basin is a trench dug around the outer edge of a plant's drip line and back-filled with mulch. A plant's "drip line" is an imaginary line drawn from the outside branches of a plant down to the soil (see inset image).
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Mulch basins can be any shape. They are typically as wide as a shovel. They should be as flat as possible to help graywater flow evenly.
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The graywater pipe is protected in a "mulch shield", separated from the surface above and more mulch below. This setup allows graywater to spread widely and deeply into your soil while preventing runoff or ponding of graywater.
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Maintenance typically includes checking the ends of the graywater pipe for obstructions to remove and replacing the mulch as it composts over time (once a year in general). Coarser mulch will last much longer than finer mulch and will be better at filtering out any large debris from the graywater.
How do you size a mulch basin?
The amount of area you need to contain mulch basins is determined by how much graywater you produce and your soil type to prevent pooling, ponding, or runoff of graywater. Healthier soils will reduce the size of mulch basins while making it easier for your landscape to benefit even more from graywater. When you apply for the rebate, Valley Water will provide you the total square footage you’ll need based on how much laundry you do and the type of soil in your landscape.
How do you install a mulch basin?
Avoid shallow utilities (water, gas, electric) or septic systems by calling the Underground Service Alert at (800) 227-2600 or 811 before you dig.
- To install a mulch basin, locate the outer edge of the canopy on each plant -- its drip line. If it’s a younger plant, imagine where its outer edge may be in a year or two. Too far out and the plant won’t benefit as much from the graywater; too close in and you may damage the plant’s roots when digging.
- Immediately outside of the drip line, begin creating a 6-18 inch wide and 6-12 inch-deep trench with a shovel. The mulch basin shall be deep enough so that the graywater pipe is 2-inches below the soil surface within a mulch shield, and the mulch shield rests on a 2-inch cushion of mulch within the mulch basin. The top of the mulch shield will be visible from the basin.
- A valve box can be used as mulch shields. These can be purchased at a hardware store, a home improvement store, or from a landscape supply store. Some garden centers and nurseries sell them as well. Larger, round valve boxes provide more space to place and adjust the piping as you finalize your system.
To avoid mistakes, dry-fit your piping and test how the graywater flows. It’ll be much easier to adjust your piping and mulch basins, and tighten loose connections when everything is still unglued and exposed.