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Q: What is greywater and how is it used?

A: Greywater refers to gently used household water from showers, bathtubs, laundry and sinks.

Although greywater carries small amounts of soap, hair, laundry lint and other particles, it can be used very successfully for irrigating the vegetation around your home.

Greywater cannot be used for freshwater irrigation, such as sprinkler systems. It must be distributed underground or on the ground covered by several inches of mulch. It is not practical for grass lawns because of the large amount of water that is required to keep a lawn green and healthy.

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Nonetheless, there are many benefits to reusing greywater — besides the obvious savings on your water bill.

Using greywater reduces your home’s carbon footprint, because moving and treating water consumes a tremendous amount of power.

It protects the aquatic ecosystems, reduces loads on sewage systems and puts water back into the local aquifer, which is far better than dumping it into rivers, lakes and oceans.

And if your home is connected to a septic tank, using greywater reduces loads on your septic system, which will prolong your service intervals.

The easiest way to start a greywater system is with your family’s washing machine. Since the water comes out of a hose on the back of the machine, you don’t have to worry about altering the plumbing under the house.

The most common washing machine greywater system is called a laundry-to-landscape system. It uses the washer’s internal discharge pump to help move water out to the yard, where it is distributed into mulch basins through a network of valves.

For bathtubs, showers and sinks, the most common method is a branched-drain system.

It is a gravity-flow system, so it requires no storage tanks, pumps or filters and relies on gravity and mulch to distribute water into the landscape. Drains from the various fixtures are combined into a single pipe and then diverted away from the sewer and outside the house.

Once outside, the flow can be divided and subdivided into multiple branches, so that water can be spread to various areas in the yard.

A gravity system needs to flow in at least a slight downhill direction, so it is not suitable for flat lots or for property that is located higher than the home.

There are many greywater systems available, and no matter how simple or complicated, the key to their success is having adequate mulch beds to filter the water. If the areas that you plan to irrigate are far from your home, it’s likely you will need a pump, typically a sump-pump system.

Greywater should never be stored in a tank. Trace amounts of organic matter will cause anaerobic bacteria growth and odors, which are not a problem when the greywater goes directly into the soil.

Greywater systems can be relatively simple or very complex, so it is a good idea to consult with a company that is experienced in installing them. They can also help you with local building codes and installation permits.

HomeWork is the weekly column by the Master Builders Association of King and Snohomish Counties’ Remodelers Council about home care, repair and improvements. If you have questions about home improvement, send them to

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