How to Design and Install a Graywater System for Irrigation

Graywater is water that is not safe for drinking but is not completely polluted or contaminated with sewage. Generally coming from sinks and showers and only containing soaps or other cleaning products, graywater is essentially harmless depending on the cleaning materials you use, making it suitable for irrigation purposes. First you need to contact a plumber to reroute plumbing pipes to direct graywater outside of the home and not into the sewage system.

Items you will need

  • Sealable, durable plastic box
  • Drill
  • Measuring tape
  • Shovel
  • Gravel
  • Sand
  • Backflow valve
  • PVC pipe (various sizes may be needed)
  • Waterproof adhesive
  • Epoxy glue
  • Hay
  • 3/4-inch garden hoses
  • Hose splitter
  • Knife

Obtain a durable plastic box with a lid. For a typical household of four or five people, a box that can hold about 11 gallons of water should be sufficient. If your household is larger or produces a large amount of graywater, consider using a box that holds 13 to 16 gallons.

Drill three holes in the plastic box, making each hole 3/4 inches in diameter. Drill the first hole in one of the shorter side walls of the box, centered about 2 inches below the top edge. Drill the second hole in the side opposite the first hole, also centered about 2 inches below the top edge. Drill the third hole on one side of the box, directly below one of the existing holes. Place this new hole about 1 inch from the bottom edge of the box. The water will ultimately exit the container through this bottom hole; the top hole on the same side is an overflow hole, and the lone hole on the opposite side is where the graywater enters the box.

Locate a spot for the box on the side of the house from which the plumber routed the graywater. The box needs to be in a landscaped area so that any overflow can be routed easily to plants. The box also needs to be at least 3 to 4 feet from the wall of the house in the direction of the yard to ensure that overflow doesn't pool near the walls of the house, and that the box is close to the grass you are going to irrigate.

Measure the box to get its dimensions. Dig a hole for the box with a shovel. Consult the measurements you took and make the hole you are digging at least 1 inch wider and longer than the box, so that it easily slides in and out of the hole. Also, dig the hole 2 inches deeper than the box is tall, so that you can cover the box with mulch or other landscape material. Note that the side of the box with one hole should face the house, and the hole should roughly align with the graywater spout.

Dig a shallow trench — 4 inches deep and 3 inches wide — stretching from the graywater spout to the hole in the ground. This trench will contain the drainpipe that carries the graywater from the house into the filter box. The trench should lead right up to the hole in the box.

Dig a second trench on the opposite side of the hole in the ground. This trench should line up with the pair of holes on the opposite side of the box. To start building the trench, dig down as deep as the hole in the ground itself. Continue digging the trench toward the yard at a slight upward angle. You want a slope that progresses upward 1/2 inch for every 4 feet, which will help the filtered water work its way slowly up to the surface. Stop sloping up once the trench is only 8 inches deep, but continue digging the trench until you reach the edge of the yard you wish to irrigate. Dig additional trenches that branch out in various directions to cover the entire yard. You'll use garden hoses and hose splitters to send the graywater through the trench's branches.

Fill all of the trenches attached to the box and running through the yard with 2 inches of gravel. This keeps pipes and hoses off the dirt so that soil doesn't clog openings.

Screw a backflow valve onto the rerouted drainpipe on the side of the house, and then connect 3/4-inch PVC pipe to the drainpipe, running PVC pipe all the way to the filter box. Use pipe fittings to attach lengths of pipe together. Use PVC glue to secure the fittings so that they do not slip off.

Insert the pipe from the house into the hole at the top of the filter box; the box should not be inside the hole yet. Seal around the pipe with epoxy glue to keep the seam between the pipe and the box watertight.

Insert a second pipe into the bottom hole on the opposite side of the box.

Slide the box in the hole and check that the pipes rest in the trenches on either side of the hole. Continue to connect pipes to the exiting end of the box, working your way up the trench until you reach the edge of the yard. At the yard edge, attach a garden hose to the pipeline with a hose connector and run the hose into the yard. Use a knife to punch holes in the hose face down every 6 inches. Use a hose splitter to route lengths of hose along all branches of the trench. Punch holes in all hoses. Put caps on all hose ends to force the water out of the holes.

Cover over the hoses in the yard — put down 2 inches of gravel and 1 inch of sand, and top with soil. The gravel and sand allow for the water to drain away from the hose, and the gravel helps keep soil from clogging the holes.

Cover over the pipeline from the box to the yard edge with the dirt that you removed from the area. Fill up the entire trench to ground level, except for the section of trench extending about 2 feet from the box. Leave this section uncovered to a depth of 6 inches. Dig more trench from the end of this section out into a nearby landscaped area and layer the bottom of the trench with 2 inches of gravel. This new trench will serve as overflow management.

Connect a 3/4-inch garden hose to the top hole on the exit side of the filter box, and run it along the trench and around the landscaped area to allow any possible overflow to escape. Punch holes in the hose face down every 6 inches to let the water flow out. Screw a hose cap on the end to make the water flow out of the holes.

Cover over the overflow hose with 2 inches of gravel and 1 inch of sand, and top it with soil and mulch.

Put hay in the bottom of the box as a filter for the graywater and close the lid. Cover up the top of the box with mulch. As the water leaves the house it will filter through the hay and make its way out the bottom hole. The force of the full box of water will push the water out the bottom hole. If the box fills up too high, then water will filter straight out of the box through the top drainage hole.


About the Author

Tara Dooley has written for various websites since 2008. She has worked as an accountant, after-school director and retail manager in various locations. Dooley holds a Bachelor of Science in business management and finance.