What Is Gray Water? Definition, Uses, Laws, Systems and Treatment Methods

Reusing graywater is one of the best things you can do to reduce water demands and become more sustainable and self-sufficient, especially if you want to live off-grid.

To get you started, here’s what you need to know:

What Is Gray Water?

Gray water (also graywater, gray water, or graywater) is a type of wastewater that contains fewer contaminants than black water, so it is easier to treat and reuse.

The legal definition of gray water varies from place to place but is usually defined as water coming from sinks, showers, bathtubs, and washing machines.

For more on the legal definition of gray water, read: Gray Water vs. Black Water

How Much Gray Water Do We Produce?

Depending on which source you go by, the average American uses anywhere from 25 to 156 gallons of water per day. Of this water, about 25% goes towards flushing toilets. 

The rest could be classified as gray water. This is a considerable amount of water that could be reused.

Water Use Breakdown:

  • One toilet flush: 3 gallons
  • Full bathtub: 36 gallons
  • 10-minute shower: 20 gallons
  • Washing machine load: 15 gallons
  • Dishwasher load: 4 to 10 gallons
  • Hygiene: 2.5 gallons
  • Drinking water: 1 gallon

Is Gray Water Dangerous?

Gray water can carry pathogens, including bacteria and viruses, and chemicals from detergents, soap, and cleaning products.

However, the contamination levels are very low and do not usually pose a health threat. Thus, even untreated graywater can safely be used for things where potable water isn’t needed, such as irrigating lawns or flushing toilets.

Can You Drink Gray Water?

Some advanced treatment systems can make gray water safe to drink. The drinking water in many municipalities actually comes from treated sewage water.

This treatment technology is usually out of reach for most households, so gray water is only used for nonpotable use.

This treatment plant recycles grey water

Why Reuse Gray Water?

Drought is becoming an increasingly big problem in many parts of the world. gray water recycling and reuse can help reduce the strain on public water supplies. 

It also reduces the amount of sewage sent to local treatment plants. Further, gray water can contain nutrients beneficial to plants for irrigation.

Gray Water Laws

There is no countrywide law or standard for gray water reuse in the USA.

Each state has laws about how gray water can be reused. The definition of what counts as gray water also varies by state. Most states require gray water to be plumbed separately from potable water, which must be labeled as nonpotable.

Many states define laundry and sink waste as blackwater and do not allow it to be reused in gray water systems.

For example:

  • Maine: Gray water can be disposed of in fields but needs to be treated with an effluent filter. You will also need a permit.   Laundry waste and hot tub water is not considered gray water and must be treated with a conventional sewage system. Read more about Maine off grid laws.
  • Alabama: The laws are strict and require you to filter and disinfect gray water before reusing it, even for flushing toilets. Read more about Alabama off grid laws.
  • California: Recycling washing machine gray water is legal, but only if the water stays on the property, is used outdoors, is discharged under a 2″ cover, and has a way to direct flow back to the sewer system (such as a 3-way valve). gray water cannot be used to irrigate crops where the edible portion touches the soil. Read more about California off grid laws.
  • Ohio: The state defines graywater as “wastewater discharged from lavatories, bathtubs, showers, clothes washers, and laundry sinks that does not contain food wastes or urine or fecal matter.” Using graywater for irrigation is legal, but you must first have your soil inspected and get a permit. Read more about Ohio off grid laws.
gray water labeling requirements
Many state laws require you to label gray water as nonpotable (even though I doubt anyone will drink water from a urinal!)

Uses for Gray Water

There are many other potential uses for gray water, the most common of which are irrigation and flushing toilets. How you can use gray water ultimately depends on the condition of the gray water. 

Some types of gray water are “cleaner” and thus better suited for specific purposes. For example, shower and bath gray water typically have lower contaminant loads than gray water from kitchen sinks or laundry machines.

You can improve the quality of your gray water by avoiding harsh chemical cleaners, using low- or no-sodium laundry detergents, and installing strainers and grease filters in kitchen sink gray water systems. 

gray water also shouldn’t be allowed to sit for more than 24 hours as it can start to smell.

Irrigation

gray water can generally be used for irrigating crops, gardens, and lawns with little or no treatment. Even gray water from sinks can often be used as the food particles can nourish plants. These guidelines should be used when irrigating with gray water.

  • Use gray water slightly below the soil surface.
  • Do not spray gray water on leaves, twigs, or stems of plants.
  • Do not use sprinklers to irrigate with gray water. It can cause pathogens in the water to become airborne.
  • Control the gray water discharge to prevent runoff and pooling.
  • Only use biodegradable soaps. Don’t use boron products for cleaning as it is toxic to plants.
  • Periodically do a soil test for salinity and boron.
  • Unless it is treated, do not use gray water to irrigate edible crops which will come in contact with the gray water.

Flushing toilets and other indoor uses

The average toilet uses 2.2 gallons of potable water per flush. Most people flush 5x per day. Using gray water to flush toilets instead of potable water can save 11 gallons of water per person daily.

Unfortunately, many states still don’t allow gray water to be used indoors for things like flushing toilets legally. You may be required to install a complicated treatment system even when permitted.

There are simple systems for using gray water to flush toilets where it is allowed. For example, the “Sink Positive” design has a sink that drains directly into the toilet top tank.

Sink positive gray water recycling for flushing toilet
The “Sink Positive” toilet uses water from washing hands to directly fill the toilet top tank.

Gray Water Reuse Systems

There are many different ways to collect and reuse gray water. The systems can be straightforward (such as a bucket system) or incredibly advanced with multiple stages of treatment and filtration. Below are some of the most straightforward gray water reuse systems.

You can find a great chart on the types of gray water systems here (PDF).

Buckets

The easiest way to collect gray water is to put a bucket under a shower or sink drain. The water can then be dumped into toilet top tanks for flushing or carried outside and poured onto the ground for irrigation.

Laundry-to-Landscape

Reusing laundry machine gray water is typically very easy because you can divert it without having to alter your home plumbing system. You redirect the laundry drain hose so it goes outside.

It’s also easy to install 3-way valves, which let you easily switch from discharging outside to the sewer system.  

Get a free book on laundry-to-landscape design here.

laundry gray water recycling
A 3-way diverter on a laundry machine gray water system
laundry to landscape grey water
Laundry-to-landscape gray water system design

Gravity Gray Water System

These systems are particularly good for recycling shower and bath gray water.

The drains divert from the sewer system and go directly to the yard. Or, depending on local regulations and needs, the water can go through a filtration system and into a storage tank before being released into the yard. If your yard is located uphill, you will need a pumped system.

Pumped Gray Water Systems

Pumped gray water systems typically divert gray water into a storage container (such as a large barrel). A pump in the container then moves the water through irrigation lines.

Branched Drain Gray Water Systems

These systems also rely on gravity to move gray water. However, the drainage pipes are divided, so the water flows to a different mulched basin.

For example, one outlet can take gray water to tree roots, whereas another can go to the garden. The pipes must slope downwards at 2% (or ¼”) for every 12 inches traveled horizontally.

You’ll need to dig up a large portion of your yard to install a branched drain system but once installed, these systems are very easy to operate and maintain.

grey water outlet
A gray water outlet with shield in mulch basin made from a 5-gallon pot

Laundry Drum/Surge Tank Gray Water System

Instead of diverting laundry gray water directly outside, it can be diverted into a storage drum (called a surge tank). The drum needs to have an outlet at the bottom where you can attach a hose. The system is easy and cheap to install, but you’ll have to empty the drum manually.

gray water laundry drum
A very simple laundry drum setup

Kitchen Sink Gray Water Systems

Kitchen sink wastewater often contains high grease, food particles, and other contaminants. To use it, you may need to install a filter and/or a grease trap.

gray water grease trap
A gray water grease trap: The bucket is removable. The holes are closed to a height of 1/3 to allow particles to settle. It must be emptied regularly to keep it functioning.

Gray Water Treatment Options

Settling Tanks

With these treatment systems, gray water first goes to a tank. The solid particles will sink to the bottom, and grease floats on the top, allowing them to be removed. Another benefit is that hot water can cool in the settling tank before it is reused.

There are even special gray water septic tanks that can be used for settling. These are separate from septic tanks used for toilet waste and other black water.

One potential problem with settling tanks is that they can clog easily. They need to be regularly pumped out. The gray water inside the settling tank does not contain much oxygen, so it isn’t ideal for irrigation unless you install an aerator.

gray water settling tank
A gray water settling tank

Disinfection

Many states require you to disinfect gray water before reusing it indoors (even for flushing toilets!). The most common ways to disinfect gray water are chlorine, iodine, and UV treatment.

Filters

A gray water filter can be as simple as a strainer put underneath a laundry drainage hose or a cloth bag at the end of a drain pipe.

There are also much more advanced gray water filters available, such as ones that use activated charcoal, ceramic or multi-media filters. It’s also possible to create your own filtration system using layers of sand and gravel.

filter for gray water system
Sieve-type filter for sink gray water

Constructed Wetlands

Constructed wetlands are used in situations where you have more graywater than you can use and need a safe way to dispose of it (but don’t want to put it in your septic tank or the municipal sewer system). 

They are especially useful if you live near a lake, stream, or other waterways. You generally cannot discharge gray water into these waterways because it can cause pollution. The large amounts of nitrates and phosphates in gray water can cause algae blooms in waterways.

The wetland acts as a natural purifier, absorbing contaminants from the gray water so they don’t get into nearby waters. It’s also possible to put particles from gray water filters into constructed wetlands.

constructed wetlands picture
A constructed wetlands in progress
constructed wetlands gray water
A constructed wetlands for gray water treatment

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