Gray Water: What is it and is reusing it practical?

gray waterThe world is rightfully worried about the sustainability of its fresh water supply. One of the methods sometimes proposed of conserving this resource is the recycling of what is known as gray water.

What is Gray Water?

Gray water is water that comes out of the sink, tub, dishwasher, and washing machine. It is not toilet water or water that came into contact with diapers that contained fecal matter. Toilet and diaper water are known as “black water.” Light gray water is water from the tub and bathroom sink alone, without any of the fats, salts and detergents that run off when washing dishes. [1]

Gray Water Access

Modern plumbing does not differentiate among the types of waste water, merging them all together and flushing them down the sewage system. Before you can even consider reusing gray water you have to either redo your entire plumbing system, or at least try to capture sink water by opening the indoor plumbing joints and using buckets to capture the water.

In many places, you are not permitted to isolate the gray water from the standard home plumbing system. But even if you are allowed to redo the plumbing for gray water access, you must be very careful because improper management of the gray water system can cause noxious odors, insect infestations, or even poison the water system. [1]

The Potential Uses of Gray Water?

There are proponents of using gray water for most water needs. While this might be a worthwhile goal, we are far from having the capability today of filtering the gray water in our homes. What is available and safe for use are toilets and toilet hookups that use gray water. With 30-40% of water used in the household being literally flushed down the toilet, unless you are using gray water you are using drinking quality water for your toilet needs. You can buy DIY systems for about $800 and the manufacturers claim that you can hook up the plumbing yourself in less than an hour. There is an additional cost of approximately $50 a year for a new filter. [2]

There are also home systems that reuse gray water for the garden. They claim to save 400 liters of drinking water a day and carry a cost of $6,000 not including installation, which is a minimum of $1,000. There is also a charge of approximately $200 a year for changing the filter. [3]

Some Basic Gray Water Safety Precautions

Due to the chemicals that are present in gray water these are just some of the basic precautions you should take if using recycled gray water for your garden:

  • Don’t store gray water more than 24 hours.
  • Minimize physical contact with gray water.
  • Ensure that the water will either drain into the sewer system or directly into soil. Gray water should never be accessible as drinking water for people or animals.
  • Ensure that the gray water permeates into the ground and doesn’t pool up or run off. This water can be a breeding ground for mosquitoes.
  • Keep the system as simple as possible. Avoid pumps and fancy filters. Gray water is harder than drinking water on these systems and repairs can be costly.
  • Do not overwater your plants.
  • Make sure that the water only reaches the base of the plants and not any part of the plant that is edible. [4]
Gray Water and the Law

Check your local laws concerning the legalities of gray water. Some examples:

  • In Israel, the usage of gray water in public buildings and industries needs approval from the Ministry of Health. The recycling of gray water is prohibited by law for domestic use. [5]
  • In the US there are 20 states that allow the reuse of gray water and that number is on the rise. Arizona, for example, changed their gray water policy after discovering that over 10% of the households in the southern part of the state were illegally reusing gray water. Additionally, the trend is to shift gray water regulation from the Plumbing Code to the various environmental departments. [6]
  • While the EU is working on guidelines for the reuse of water, it seems that the guidelines currently being drafted will not include the use of gray water. [7] While gray water usage is allowed in Germany, capturing rainwater is the preferred method of reusing water. Of all of the EU countries, Cyprus is leading the way with 90% of its waste water reused.
  • In Tokyo gray water recycling is mandatory by law for all facilities larger than 30,000 square meters, or with a potential reuse of 100 cubic meters a day. [8]
Final Note

While gray water might be part of future solutions to the water shortage, it is not the savior of today. It might be worth the money time and effort for industries, hospitals and hotels that use very large quantities of water, but it just isn’t practical for the average home. Though using gray water is potentially very good for the environment, it will not save you money. Even the DIY option for the gray water flush toilet takes 4 -5 years before you start saving money. If you were to install the home irrigation unit at a total cost of $7,000 and saved 400 liters a day, it would take a lifetime to get a return on the investment.

In any case, because recycling gray water is potentially dangerous to the environment—including people and animals—if not handled exactly as prescribed, gray water is illegal for home use in many jurisdictions.

With necessity being the mother of invention, and as our need for fresh water increases, perhaps better and more cost-effective solutions will be found for safely reusing gray water. We believe that water regulatory authorities and utilities should be more proactive in working with industry and entrepreneurs to help this option happen sooner rather than later.


[1] Derek Markam, How to reuse gray water in the home and garden, June 2, 2014
[2Antonio Pasolini, ReFlow reuses gray water, saves freshwater, April 28, 2015
[3] Mike Hanlon, Gray water treatment system for the home, February 9, 2007
[4] Laura Allen, About Gray water reuse, 2014
[5] State of Israel, Ministry of Health, Environmental Health>Gray water, 2018
[6] Laura Allen, Gray Water Use and Policy, 2014
[7] EU Water Directors, Guidelines on Integrating Water Reuse into Water Planning and Management, June 10, 2016
[8] Stephan Mcillwaine, CSBE Gray Water Reuse Project, 2003


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