Introduction: Water Your Garden With Gray Laundry Water

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About: By day I'm a mechanical engineer at a university laboratory. In my free time, I do my own projects.

For a long while I've wanted to be able to water our backyard ornamental garden with gray water. We already have a drip irrigation system and, while those are known for using water very efficiently, it still uses quite a lot of water. I researched gray water systems online and in books, but I didn't find any that would work similarly to a drip irrigation system by delivering the water to a large number of plants individually (actually, there are such systems, but they must be professionally installed and cost thousands of dollars).

Typical gray water garden systems deliver water to a few fruit trees or to large areas called "mulch basins" or they have water coming out of a hose that one can move around manually to different plants. What I wanted was a system that would take my gray water and distribute it to all of the places where the drip irrigation system goes. After almost six months of pondering this problem, last summer (2009), I hit upon a very simple solution that appears to work very well. I used it for a couple of months before our rainy season came, and then I switched the washing machine over to another water saving device, my laundry water recycler. Now the rainy season is coming to an end here in N. California, so I'll be reconnecting the irrigation system soon. Read on and you'll see how it works. I look forward to your comments and suggestions.

Step 1: Description of the Watering Technique

Although it took me six months to figure this out, the technique is extremely simple. First, I decided not to use any water from the kitchen for this gray water system, because of the difficulty of filtering out food particles. This system exclusively uses water from our clothes washing machine, which does contain a lot of lint, but that is easy to filter out. NOTE THAT, FOR HEALTH REASONS, IT IS NOT RECOMMENDED TO WATER VEGETABLES WITH GRAY WATER (fruit trees are OK though).

Drip irrigation systems water individual plants with little parts called "emitters" that allow the water to drip out. They are specially designed to not clog with roots or debris, but they require a fair amount of water pressure to operate and everything I have read about gray water systems warns that emitters will become clogged if you try and put gray water through them. However, there is another part, frequently used in drip irrigation systems that is the key to my system. It's called a 'barb x barb" connector and you can see a picture of one here (I'm not trying to endorse this vendor, it's just the first photo I found on the web). The barb x barb is made to connect a 1/4" diameter extension hose to a larger main line. In my system, I just put one of these barb x barbs next to each plant that will be watered. The first photo below shows one of  these "barb emitters" in action.

Each barb x barb has a hole that's maybe 1/16" in diameter, and this is large enough so that it won't clog (at least they haven't clogged after the first few months of use last summer and fall). In my first version of the system, I put the barb emitters at a slight upward angle, like in the photo. There were two problems with this: First, in some parts of the terrain, the pressure became low enough so that having the barbed emitter pointing up led to low or no water flow. Also, one of the nicest things about drip irrigation systems is that it delivers the water underground, so you get less evaporation. Putting the barb emitters straight down, sticking into the soil, would help the gravity/pressure problem and also the evaporation problem. However, just sticking the barb emitter into the soil could lead to clogging with particles or roots.

So, my second version goes as follows (see photos 2-5 below): I insert the barb x barb into the hose pointing straight down, then I wrap the middle section of the barb x barb with adhesive foam weather stripping. I then use a wooden dowel to poke a hole in the soil just under the barb emitter. I take a 3" length of irrigation tubing, fit it snugly over the weather stripping, and insert the tube into the hole in the ground. That way, the barb emitter has an empty space below it that is kept open by the tubing. I have had some of these in place since last fall and soon I'll check them to see if they are still working. Overall, I think the second version is a better idea, but there's something really gratifying about seeing the water squirting out of the above-ground barb emitters.

***UPDATE - the weatherstripping didn't hold up very well over the winter, so I'll need to find another way to plug up the hole. For now, I'm just leaving them - I have decided that pointing the x barb x straight down doesn't work in the long term, because the hole always fills up with soil, even when I put a piece of irrigation tubing inside the hole. Therefore, I recommend just having these "emitters" at an upward angle, as shown in the first photo.

Sometimes I don't have the main irrigation tube very near a plant. In those cases, I just insert a barb emitter nearby, and then insert a piece of 1/4' tubing to bring the water flow right to the plant (which is what barb x barbs are meant to do).

Step 2: Materials

Drip Irrigation Hose
1/4" irrigation tubing
32 Gallon Plastic Garbage Can
barb x barb connectors (one for each plant you want watered)

For each branch in your irrigation system:
-3/4" male to 1/2" female PVC pipe thread adapter
-1/2" male pipe thread to irrigation hose adapter
-3/4" metal nut
-3/4" rubber washer (or an old bicycle inner tube for DIY)
- 3/4" hose clamp for attaching the irrigation tubing


-One or two mesh strainers or any fine screen for a lint filter

If you need to move water more than a couple of feet uphill from you washing machine:
-sump pump
-sump pump hose
-5 gallon bucket

Step 3: The Holding Tank

Water exits the washing machine and is transferred to a holding tank, a 32 gallon plastic trash can, and from there it flows out into your garden with just the force of gravity. In my system, the water from the washing machine travels through a length of sump pump hose that enters the tank through a hole near the top.

In my garden, I have the tank about 2 feet off the ground, sitting on a wooden mount in the corner of my backyard fence. I think the actual height of the tank could even be lower and it would still provide enough pressure to deliver water everywhere in the garden. Having the water flow more slowly may be more like a drip irrigation system (though it will never get that slow), but having the water flow more quickly will clear debris from the tubes better. I don't know what the optimum flow rate is for this system, but I can tell you that my system has three irrigation branches with a total of 55 barb emitters and it takes just a few minutes for 20 gallons to flow out.

Note that plastic garbage cans are susceptible to UV damage from the sun so, for better longevity, try to keep your can in the shade. Mine is in the SW corner of the yard and I have a square piece of wood as a top, so the can gets very little solar exposure.

Each branch will have its own outlet from the can. Each outlet is made in the same way that I made watertight connections to a 32-gallon can in one of my other instructables. The only difference is that in that project, I used 5/8" connectors and in this one, you only need 3/4". So, instead of cutting a 5/8" hole in the can, just cut a 3/4" hole. Also, I have found that you don't really need rubber washers both inside and outside of the can. One washer on the outside, squeezed between the PVC adapter and the can wall, seems to do the trick.

After you've put in the connection, screw in the elbow adapter, applying teflon tape to the threads first, and then use a small hose clamp (also with teflon tape around the adapter) to attach the end of your irrigation tubing to the tank. You may want to also insert a valve in each line. This could be handy if you want to water some parts of your garden more than others, or if you want to store water in the tank before releasing it to the garden.

Step 4: The Plumbing

If, like most people, your washing machine is in your house, at least 6 or 7 feet higher than your garden, then you can run a hose directly from your washing machine to the top of the holding tank. If that's the case, then you can skip steps 1 and 3 below. Instead, you'll want to run a hose (not garden hose, but something of wide diameter, such as washing machine hose, or sump pump hose) to your holding tank. If the distance from the washing machine to the tank is not too long, you can even have the washing machine pump the water uphill a couple of feet.

1. My washing machine is in the basement, about 6 feet below the garden level and maybe ten feet below the top of my holding tank. So, unfortunately, I need to use an electric water pump to move the water some of that distance. I use a submersible sump pump that cost around $60. First, I have the washing machine hose connected to some sump pump hose, out through a hole in the wall of my house. I measured the electricity that the pump uses and it's about 20 Wh for ~40 gallons of water.

2. The water exiting the washing machine hose passes through a piece of fiberglass window screen, which catches most of the lint from the laundry water, to a  gravel filter in a 5-gallon bucket. You can't have too much filtering of the water, because once debris starts accumulating in the system, the performance starts to degrade. I became lazy about the. Filtering for a while and I ended up having two flush out the entire system.

3. The bottom of the gravel filter bucket has an outlet that passes the water through a mesh kitchen  strainer into a second 5-gallon bucket inside of which sits the sump pump. The pump has an automatic float switch, so as the washing machine empties into the sump pump bucket, the pump turns on when the water level gets high enough. The outlet of the sump pump has another length of hose that travels across my yard and up to the top of the holding tank. I have a final sheet of fiberglass mesh that catches any debris still remaining in the water.

Step 5: Planning and Laying Out the System

Before you start building, you should decide where is the best place to put the tank and  map out where in your garden you will put the irrigation hose. This is similar to planning a drip irrigation system, except that you will have to take gravity into account. If at all possible, you want to avoid making the water in the tubes go uphill, although my system has one or two small "humps" and it still seems to work OK.

I made a sketch of the locations of my plants and the paths for my irrigation lines, and then marked where I wanted to put in barb emitters, sometimes giving larger plants two or three if they require more water. This took a while, as I wanted to choose the simplest paths that would pass by as many plants as possible. I then followed my sketch and  laid out the irrigation tubing in the garden and staked it down with U-stakes. Note that each branch of your system should have its own outlet from the holding tank. Initially, I had three branches coming off of one outlet and I found that they competed with each other for water pressure (the branch with the most slope won).

Now it's time to insert the barb emitters into the irrigation line(s). First, decide if you're going to have your barb emitters pointing at an upwards angle (or maybe just horizontal), or if you want them to come straight down (my version 2 technique), or maybe a combination of all three, which is what I currently have.

I have a special tool that pokes a hole in the tubing. The barb x barbs are kind of difficult to push in, so I made a simple tool for that - a piece of wood with a hole for one end of the barb x barb. It gives me more leverage to push them into the wall of the hose (see Step 2).

If you are using version 2 emitters, then you'll have to follow the installation instructions from Step 1 for each barb emitter.

Try out your irrigation system to see if you get good flow to each plant. You may have to alter the terrain a little bit if you have some uphill sections that are lowering the pressure.





Step 6: Using the System

You'll want to purchase some laundry detergent that is suitable for your garden. There is plenty of information on the Web about this topic. The first one I used may have caused an Abutalon plant to become sick (I'm not certain), but all of the other plants in the garden seemed to do fine with it.

So far, the only maintenance necessary is to clean the lint filter every few loads of laundry. Like all irrigation systems, the tubes and barb emitters should be checked periodically for damage or blocking. I need to take a look at my version 2 emitters and see if the hole beneath them stays clear over time.

March 28, 2010
It appears that the adhesive weather stripping in the version 2 emitter holes did not hold up well over time. I think the idea of plugging the top of the hole is a good one, but I'll have to come up with another material. For now, I'm just leaving the top of the hole unplugged.

One issue I have is that I don't know which plants are receiving more water and which less. With emitters that point straight out or upwards, it would be possible to measure the flow rate, but that would be more difficult with the down-pointing ones. I guess I'll just have to keep a close eye on the plants and add more emitters to plants that seem to need more water.