Greywater Irrigation: Branched Drain

The basic concept is to allow the graywater to flow from the house by gravity alone, typically through 2″ pipes. To distribute the water into more than one location, a double Ell (a special type of T fitting) is used to split the flow into two. More double Ells can be used to split the flow further.

Theoretically the flow could be broken into 16 or more directions, although 8 is about the practical limit for a typical household (beyond this the pipes and Ells are too sensitive to water flow thus uneven distribution would occur.)

This tree has been planted in a large mulch basin, serviced by a branched drain outlet slightly uphill of the mulch basin. The water will flow to the mulch basin via a depression in the ground between the outlet and the basin.


Low material cost

Many small networks can be made with just $100 worth of materials. Larger systems may cost $400+ worth of materials.

Relatively low maintenance:

Apart from keeping a good layer of mulch in the mulch basins, the only regular maintenance required is a monthly check of the double Ell’s (removing a plug to inspect and clean if necessary).

We suggest an annual check to ensure soil settling has not altered the Ell position, resulting in uneven splitting / flow.


Irrigation inefficiency:

Very low water efficiency, typically the lowest efficiency of any water re-use method. Almost all of the graywater passes through the soil into the subsoil beneath the plant / tree root layer. If this method must be used, trees would be the best candidate. Keep in mind the tree roots will concentrate in the mulch basin, and will not spread far from the base of the tree. Additional feeding (fertilizer) will also be required due to the lack of root growth.

Consider a branched drain network with eight 4‟ round mulch basins. The total basin coverage is 100 square feet.

While some capillary irrigation will occur outside the basin zone, it will not benefit surface plants because the bottom of the mulch basin is well below the height of the surrounding soil.

However, for discussion, lets assume capillary effects double the effective radius of the basin, bring the coverage up to 400 square feet.

A typical 3 person household will generate about 120 gallons of graywater per day.

A dripperline system (laid under mulch) would irrigate about 2,000 square feet, based on typical planting to suit the locality.

Therefore, the branched drain network typically saves 25% or less water than a below mulch dripperline system (either gravity or pumped).

This method is exacting. The 2″ pipes must run to grade (approximately 2%) so that water splits properly at the Ell’s. Each Ell must be positioned exactly level to ensure even splitting of the water flow. The Ell should also be protected by a valve box (as seen top right) for easy access, and adjustment as required.

Before considering a mulched drain system, we strongly recommend viewing the mulch basin video we have created.