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Wednesday, August 13, 2014

Gray Water Irrigation System

The construction of the gray water irrigation system here at the house is functioning on an experimental basis and is almost completed. We have departed from the standard or recommended design for the distribution of the water into the garden with this system. We did this primarily because we do not want to create the larger pits filled with mulch or gravel for the water that percolate into the ground as is usually done.  We want to use flexible irrigation hoses with holes in the line near each one of the plants with small gravel or mulch basins near each hole in the flexible water hose.  We think this will be more efficient, but the holes may get clogged. We still consider this approach very experimental. I will report on the success of this method once we have it working with a little time under our belt operating it.

The gray water that we are capturing is the water from the laundry and one bathtub/shower and the bathroom sink.  We may add one other shower to this system, but that one is used very infrequently and it may not be worth running a pipe that far (Note: I may add that shower as an alternative to my taking a shower in the bathtub). Also, we do not think that we can effectively use that additional water as it might exceed the capacity of the system. Also, we are not adding the gray water from the master bathroom because it is on the second floor and we would have to open up too many walls to separate the gray water from the black water.
We have decided to bathe downstairs so that we can capture this gray water.  This will be primarily during the summer months when we have the largest need for water in the garden and very little rainwater.
(Note: this comment was added after the system had been installed and was running for a while.  Now that the system is operational, I am running the drain line from the shower to the gray water systems.  This will not increase the load on the system as I will be not taking a shower in the bathtub and, instead, showering in the shower stall.  Total cost for the additional run turned out to be about $15. 00 for the convenience of taking the shower there.)


We do about six loads of washing a week, at 15 gallons per load. We have a front loading high efficiency washing machine. Figure about 90 gallons a week for this.  For this system, however, it will be far more meaningful to talk about daily usage, since we do not want to flood the garden with too much gray water at one time.  We normally do no more than two loads in the washing machine a day (2 loads = 30 gallons).
There are two of us in the house and we either take a shower or a bath. Figure about 20 gallons for a shower (my showers are usually only 5 minutes long and should only use about 10 gallons) and about 30 gallons for a bath. I generally take the quick shower, my wife tends to take the bath.  So, figuring that on a typical day we each bathe, that is another 50 gallons of water or less. I have to assume for computing the capacity that this is on the same day as the 30 gallons or water for the washing machine.
In summary, for a typical day when the laundry is done we might use:
Washing machine                  2 loads                   30 gallons
Bath                                        1                            30 gallons
Shower                                   1                            20 gallons

Total                                                                     80 gallons

Now, we know that this is not going to be all used at one time, so we are going to build the system to handle a maximum of about 40 gallons in one hour.

Gray Water Storage Capacity

It is not a good idea to store gray water for more than a day.  We want to merely store that larger quantity of water for, say, at most an hour or so to let it permeate the ground, instead of running off on the surface if it is discharged too rapidly.  We are building the system to handle about 40 gallons an hour, but do not expect to be using 40 gallons in one hour very frequently (at most 3 times a week), and most other times there will be no or very little water going through the system.

The Garden

We have chosen a large succulent and cactus garden that we have constructed that we currently hand water.  The Garden is about 320 sq. ft. in size.  It is located downhill from the house and about 60 feet away from the house.

The Storage Tank

We have departed from the typical storage tank concept and decided to use the drainage pipe itself as the storage device.  There are approximately 55 feet of 4 inch diameter drainage pipe once the water leaves the house.  There is also about 20 feet of 2 inch pipe running along the foundation leading down to the 4 inch pipe.  The storage capacity in the pipe is as follows:

55 ft. of 4 inch pipe  = 4.80 cu. ft.
20 ft. of 2 inch pipe  =   .44 cu. ft.
Total capacity              5.24 cu. ft. = 39.20 gallons

Water Distribution to the Plants

At the base of the pipe just as it approaches the garden from the house we have installed a large irrigation box. You can see the green cover plate for that box in the picture above. Immediately before entering the irrigation box the pipe changes back from a 4 inch pipe to a 2 inch pipe. In the box we have an inspection window in the pipe made from clear plastic tubing the same diameter as the pipe.   Then, following the window there is a manifold.  The header is made of 2 inch plastic pipe. We have drilled  5  holes, each  3/4 inches in diameter into the manifold and inserted nipples and connected the black flexible 1/2 inch irrigation tubing to the nipples.  The flexible tubing weaves between the plants underground (Note: since I am still experimenting the tubing is above ground as you can see in the picture above). Also note that there are a number of bends in the pipe within the irrigation box so that I could fit it all in there.  Obviously, if you have a different setup you may be able to do this without all the elbows I had to install.

The flexible tubing has holes near each of the plants to be watered.

The amount of water that will be distributed to the plants on a square foot basis is computed as follows:
If you assume that he water is distributed evenly over the 320 sq. ft. area of the garden, then, on laundry day (80 gallons of water)  the amount of water per sq. ft. will be equal approximately 10.5 cu. ft./320sq. ft. = .033 cu. ft../sq. ft. To compute the equivalent rainfall this must be converted to inches. This is 57 cu. in./sq. ft., which translates into the equivalent of .4 in. of rainfall. Since this distribution method will not distribute the water evenly, we are estimating that the equivalent for the areas that are actually receiving the water will be more like the equivalent of 1/2 in. of rainfall.  Ideally this level will be achieved at least three times a week, with smaller amounts on days when only the bathing occurs.

Percolation of the Water into the Garden

There are two inspection windows made of clear plastic 2 inch pipe. One window is attached to the manifold down by the garden (see above picture) inside the irrigation box that we just discussed above, and the other is just at the point where the pipe leaves the building (see picture below).

This is exactly the area between which there are about 40 gallons of storage capacity.  With the bathtub full of 40 gallons of water I can now test to see how long it takes for the storage pipe to empty (I considered installing a valve in the irrigation box that would allow me to store all the water above the valve, but opted not to do it for fear that I might forget to open the valve when required and thereby cause a backup into my house.)
(Note: Now that the system is in I think it might have been better to put in a bib valve so that I might take water from the garden hose to connect to the system so that I might use the irrigation portion of the system without having to do laundry or take a shower.  The only alternative I have without changing the system is to put water into the bathtub and let it flush through the system.)

Gaging from the time it takes to percolate into the ground with a limited number of holes I have now computed that it takes about 10 holes to percolate 40 at gallons per hour. I have installed about 40 holes, so it takes about 15 minutes to distribute 40 gallons of water.

The flexible irrigation pipe is exposed for testing purposes.  When testing is completed the hose will be buried and mulch or gravel will be inserted around all the outlet holes near the plants.


The cost of the project has been about $200 in materials.  It took a few days to install.

Other Considerations

I have not yet installed either a bypass valve or an overflow for the system.  These matters concern me and I will be addressing them.  The main problem with the bypass valve is that I must install one that is electrically controlled as the place where the valve must be located is in the crawl space below the house. More on this later.

last modified 8/17/2014