Gray Water - Is It Safe For The Garden? - Garden Myths (2023)

Want to start an online battle? Ask about using greywater in the garden (also spelled greywater, greywater, and greywater). Opinions vary widely and both users and non-users feel very strongly about their position. You will see many opinions, and many statements that are incorrect.

There is a clear mistrust of gray water in highly developed countries, but as regions experience more drought, people are starting to see it as a viable option. Clean water, also known as potable water, is wasted on landscapes and as its cost rises, more and more gray water will be used. In this post, I will discuss its use in the garden.

What is Gray Water?

Such a simple question and yet we can't agree on what it is. It does include water collected from washing machines, bathroom sinks, and showers. Some people also add water taken from the sink, while others don't.

Water from the toilet is called black water and some include sink water in this category.

Why can't we agree on a definition? Because politics is part of the story. Every government, federal, state, provincial and municipal wants to have its own definition. The government is not smart enough to standardize.

In this post, I will consider gray water to exclude the sink and call it "sink water."

Yuk-factor

Imagine eating vegetables watered with the same water you bathe in? Bah! For some reason we find this unacceptable.

But consider this. What gives that tomato its great taste? worm poop. Rotting vegetation. bird droppings. Dead mice. Thousands of different dead insects.

Gray water and the law

Each level of government has different rules about gray water. In some places it is illegal to use it in the garden - silly, I know, but that's the way it is. In other places you are not even allowed to collect rainwater for use in the garden.

In other areas, you can use gray water, but only after it has been treated. This seems to be common practice here in Canada where the emphasis is on adding large gray water cleaning systems to homes. The only problem is that they are too expensive, especially since our water is still cheap. So nobody uses them. But I doubt the police will check how you use gray water.

In some places you are free to use gray water however you like, but most jurisdictions have some rules that you should check before using it. Or at least, don't tell your neighbors what you're doing. 🙂

Understand your local laws.

Concerns about gray water

There are two main concerns about using gray water in the garden; chemicals and bacteria.

Gray water contains numerous chemicals, but the most important are the cleaning products added during the washing process.

Bacteria are a concern because they can cause infections and disease. They are unlikely to affect the health of the plant, but if these bacteria return to people or pets, they could potentially cause problems. Remember that our body is covered with bacteria and fungi. As we wash, some of this is washed away in the gray water. When you wash some fruit, you add microbes from the fruit. Even stagnant water collects microbes from the air.

A major concern for bacteria stems from the fact that stool that enters gray water can cause disease.

A third concern is the pH of greywater.

Chemicals in gray water

Certain chemicals are harmful to plants. Those concerned about gray water are sodium, boron, and bleach.

Sodium and boron are commonly found in soaps and detergents, and even small amounts are toxic to plants.

A lot of online advice suggests that you use gray water in the garden, you should only use soap and no detergents. This advice has since made no sensesoaps like dish soap are actually laundry detergents.

Both soaps and detergents can contain sodium, which is harmful to plants. The best option for a cleaning product is one that does not contain sodium or boron. It can be soap or detergent.

Please note that we don't normally use a lot of cleaning products. Think about how much you use when washing your hands compared to the amount of water. These chemicals are quite diluted.

Consistent use ofgray water does show an increase in boron and surfactantsin the ground. Excess sodium tends to wash away with rain, but can be a real problem in some soil types in arid regions.

Most other chemicals in cleaning products are organic in nature,meaning they contain carbon - this has nothing to do with organic gardening. This means that bacteria in the soil break them down and convert them into CO2 and water. In low concentrations they are not a problem.

Because chemicals can damage plant leaves, the gray water should be applied to the soil and not directly to the plant.

Bacteria in gray water

All gray water contains a wide variety of bacteria. Most of these are not harmful to animals or plants. A few can make us sick, but are unlikely to harm plants.

Whensoil treated with clean water and treated gray water was analyzed, various pathogens and fecal indicator bacteria, includingEscherichia coli,Klebsiella pneumoniae,Salmonella enterica,Pseudomonas aeruginosa,Enterococcus faecalis, InShigellaspp., were found. The reality is that these pathogens are already in your yard, even if you don't use gray water.

While gray water stands, the bacteria will begin to multiply, making a potential disease problem worse. Many gray water regulations require you to use the gray water within 24 hours if it is not treated.

If gray water is added to ornamental beds, there is no health risk. Any added bacteria must survive their new environment and compete with the bacteria already there. Human pathogens don't live that long in the soil. Since we do not eat ornamental plants, we are not at risk.

Whenlettuce, carrots and peppers in a greenhouse were treated with gray waterwith high levels of both fecal coliform and fecal streptococci (mean 4 × 10⁵/100 ml and 2,000/100 ml), “no significant difference in contaminant levels was observed between crops irrigated with tap water, untreated gray water or treated gray water. Contamination levels for all crops were low and pose no significant health risk.”

Adding gray water directly to the soil does not create a problem with pathogens in plants such as tomatoes or beans that fruit above the soil. Pathogens do not enter the roots and migrate to the fruit.

There is a potential problem with pathogens attaching to root crops. When you eat the root vegetable, you can ingest some pathogenic bacteria unless you wash and cook it before eating. If you eat the crop directly in the garden, or prepare it fresh, there is a very small potential problem.

But consider this. Soil contains thousands of different types of bacteria and we don't even know what they all are.We do know that pathogens are present. Humans have been pulling carrots from the ground for thousands of years, wiping them on dirty pants and eating them raw. We're still alive.

Authorities can't take any chances, so they tell you not to use gray water on root crops, but the reality is that the risk of doing this is very low and it's common in many parts of the world.

The pH of gray water

Gray water tends to be more alkaline, mainly because of the soaps and detergents in the water, but that's not always true. In astudy the clean spring water had a pH of 7.7 and the gray water had a pH of 8.2.In another studygray water had a pH of 6.7 with clean water of 7.6.

Several studies, but not all, show an increase in soil pH when gray water alone is used. The degree of change depends on the pH of the gray water and on the type of soil being tested. Changes in pH will be less dramatic if rainfall is a significant contributor to watering. Higher microbial activity in the soil will also mitigate such pH changes.

What about the sink water?

The problem with this is that this water contains more bacteria and food particles than normal gray water. The pieces of food make bacteria grow faster, which means there is more chance of pathogens.

Where do the bacteria come from? Some come from your hands, but these are also found in other types of gray water, so we can dismiss them as a problem. The rest comes from the food we wash. Running water over a carrot or apple before eating it adds various microbes to the water. But these will usually be plant microbes - not human ones, so they are less likely to be human pathogens.

When you water a vegetable crop from above, it also dumps massive amounts of plant microbes onto the soil. Is this anything other than washing some lettuce leaves in the sink and dumping the resulting water on the floor?

The other problem with this water is that it contains fats, oils and fats. These decompose very slowly in the soil and too much in the garden can be a problem. A lot depends on how much grease you throw down your sink and how big your garden is.

Concerns about gray water in the sink are greatly exaggerated, provided the water is used right away. Storing this water without cleaning it first is a bigger problem.

Other myths about gray water

Washing machine water should not be used

Some people recommend not using water from washing machines because the plastic microfiber of the synthetic clothing causes drainage problems in the soil. It is true that microplastics are found everywhere and the best thing to do is to keep them out of the environment. But our sewage cleaning systems don't filter them out very well and they end up in rivers and oceans where they probably do more damage than the soil.

They break down very slowly in the soil and plants will not absorb them. They don't end up in your food.

I doubt they will change soil drainage conditions, except perhaps if used for a very long period of time. The plastic is eaten by, for example, earthworms, which are then eaten by larger animals. We still don't know what damage they do to animals once.

Microfibers are a problem - we just don't know how big the problem is.

Biodegradable soap is okay

For many people, "biodegradable" means it's completely safe. That's just not true. Biodegradable means that microbes break it down until it is finally down to its basic elements. But when broken down, it can actually be turned into harmful chemicals.

Almost all soaps and detergents are biodegradable - even if it is not stated on the packaging. Even crude oil is biodegradable. Products that are biodegradable and high in sodium are worse for the soil than non-biodegradable products with less sodium.

Soda is a safe soap

Washing soda isn't even a soap - it's a salt.

Washing soda has been used for many years and is considered by many to be a very safe product, and it is. The problem is that it is sodium carbonate (sodium, carbon and oxygen) and sodium is very toxic to plants. This is therefore not a good product for collecting gray water.

Do not spread it on the surface of the soil

One person commented that you shouldn't spread it on the surface of the soil because you will get flies.

Some regulations require that the gray water be applied below the soil surface. I think this reduces the chance of some gray water splashing on plants, but that's hardly a problem. Mulch will also prevent this.

It will not create flies. They may come for a drink if you spread it on the surface, but so do bees and butterflies.

Gray water does not contain feces

A common definition of gray water is "all waste water that isgenerated in household or office building sources with no fecal contamination“.

That is not true. Shower water and washing machine water both contain feces from our bodies and our dirty clothes. In astudy, fecal coliforms (CFU 100 ml–1) had a value of <1 in clean water and 1,000,000 in gray water. You're not as clean as you think.

The same study found that faecal coliforms couldn't survive for long in the soil, so it's not really a big deal.

Do not use it on acid-loving plants

It is generally accepted that gray water is alkaline and therefore should not be used for acid-loving plants, such as rhododendrons and blueberries.

Gray water isn't always alkaline, and even if it is, it doesn't mean it makes the soil more alkaline. If it doesn't make the soil alkaline, it's fine for acid-loving plants. Be careful andconsider measuring the pH of your soil.

How does gray water affect plant growth?

Gray water contains nutrients that plants need. Many soaps contain phosphorus. Organic matter will contribute nitrogen, sulfur, calcium, magnesium, iron and other nutrients. Soaps that do not contain sodium most likely contain potassium. Gray water is a fertilizer.

How does this affect plant growth?

A study that looked at this problem found that gray waterimproved growth and productivity of carrots, peppers, spinach and beets compared to clean water. In some cases, growth in gray water was as good or even better than in fertilized water. This test was done in jars.

Field tests in Texas showed that gray water (wash water) increased productionin peppers, chili and tomatoes compared to clean water. Gray water did not increase salt accumulation, but it did increase pH.

Greenhouse tests showed that tomatoes grow better in gray waterthan in tap water, and those grown in gray water had higher levels of P, K, Ca, Mg, Na, Fe, and B.

Other studies have found no increase in productivity, but none reported a decrease in productivity.

Should gray water be used?

From both a healthy gardening perspective and an environmental perspective, the answer is clearly yes.

Gray water, as well as sink water, should be used on ornamental beds and lawns. Gray water can certainly be used on non-root crops and can probably be used on root crops without concern. It is certainly safe on root crops that are getting cooked.

Follow these suggestions.

1. Do not store gray water for more than 24 hours.
2. Do not use gray water on vegetables if someone in the household has a serious disease-causing infection.
3. Use cleaning products with a lower sodium and boron content.
4. Use smaller amounts of cleaning products.
5. Spread the gray water all over your yard - don't keep dumping it in the same spot.

References:

1. 1. Use of gray water in agriculture for agricultural irrigation in urban areas;https://ocw.un-ihe.org/pluginfile.php/682/mod_folder/content/0/14_Greywater_reuse_South_Africa_Salukazana_Donsheng_conf_2007_.pdf?forcedownload=1