How to Build a Gray Water Septic System

Why would you need to know how to build a gray water septic system? First of all … what is it? There are two types of waste produced by a household, solid waste, which is obviously from the toilet, and gray water waste which is water from the shower, sinks, washing machine. In normal residence both these waste waters are dumped into one septic tank system which disposes and distributes it in various ways depending on the septic tank design. However, there are cases where a full blown septic system is not required.

You have done your homework well before building a small cabin off grid for your family of four to vacation. You have installed a composting toilet, which uses little to no water, and completely disposes of solid waste. You have combined solar and wind turbine power to supply enough electricity to operate a hot water heater and lights. You can take a hot shower and wash dishes in hot water just like at home. Problem is, how do you get rid of the shower and sink water?

That’s no problem you think. Just run a 2” PVC drain pipe out away from the house and let it drain out onto the ground. It’s relatively clean water with the exception of some soap and maybe a few food scraps from dirty dishes. It poses no harm. Really?

You may be able to get away with that simple answer, but depending on soil type, amount of use and other variables, maybe not. If whatever reason, the water does not totally dissipate, leaving the ground moist or wet, you could develop problems. Any standing moisture draws pests like mosquitoes, flies, bees and wasps … all dangerous to humans. And if the ground is not wet? What about those few scraps of food waste from the dirty dishes? Again, it attracts critters.

What to do? Install a septic system for $20,000? No way! Let’s see how to build a gray water septic system for under a $100 which will totally eliminate your problem.

Lots of Different Systems

There are a lot of different ways to build a gray water system. We’re going to build a very simple, yet effective system. First, let’s do a quick tutorial on how the system works. Gray water drains through a solid pipe to a dispersing tank, where the water is dispersed for evaporation. Simple enough.

Inside Plumbing:

Everything begins with the inside plumbing, which we won’t get into, and where it joins all the drains together and exits the house. Prior to installing your first piece of PVC waste pipe to the inside plumbing be sure you have installed a P-Trap first, which maintains a water barrier that prevents sewer gas from returning to the house. I’d suggest using a P-Trap with a drain plug for easy draining for winterizing the cabin. (You can also pour antifreeze into the drain)

Construction Steps: How to Build a Gray Water Septic System

Material List:

  1. 2” PVC pipe. It comes in 10′ lengths. Buy accordingly

  2. 2” PVC couples. These join the lengths of pipe together

  3. 1 50-55 gallon plastic barrel

  4. Glue and solvent

  5. Shovel

  6. Torpedo level

  7. Drill & 1/2” bit

  8. Gravel

  9. 90 degree fitting

Step 1. Depending on how your inside plumbing was installed the exit end of the drain pipe will either be 1 1/2” or 2” PVC. If it’s 1 1/2” I’d suggest inserting an adapter to increase the size to 2” as it carries more water and is less prone to stop up.

Step 2. Dig a trench at least 14” deep, you want to cover the drain pipe with at least 10” of dirt, and 2′ wide. It depends on your situation how far you want to trench it away from the house, but don’t get lazy, move it away from any possible activity that may be around the house, parties, etc.


Step 3. Determine where you’ll place your place your dispersing tank (plastic barrel) and dig a hole deep enough and wide enough for the barrel to easily fit into the hole with room around it to fill with rock.

Step 4. Take the plastic barrel and drill 1/2” holes sporadically placed around the entire barrel, including the bottom, for the water to drain through. Set the barrel into the pit.

You now have a trench dug to where the buried barrel is located. Cover the trench floor with gravel, 1/2” minus, pee gravel, rock, doesn’t really matter per se’ it’s to slow drainage run off.

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Step 5. Run your 2” PVC drain pipe along the trench and insert it into the top of the barrel, either through the threaded opening or cut a hole in the top of the barrel 2” in diameter and push the pipe through it. Be sure the pipe is inserted at least 8” to 10” into the barrel to prevent it coming out. Caulk around the opening to insure a good seal and to prevent foreign objects from entering.

Step 6. Back fill around the barrel with gravel and/or rock to prevent soil from clogging the drain holes. Another option is to wrap the barrel in landscaping cloth. Back fill with dirt.

If you were running a waste sewer line you would use a 1/2” drop per 10′ to the barrel to insure the solids traveled with the water, but since we’re talking gray water, grade the pipe ½ bubble off. You just want it flowing downhill and use a 90 degree fitting to drop straight into the top of the barrel, something else you wouldn’t do with a waste pipe.

That’s pretty much How to Build a Gray Water Septic System. Because you don’t have to worry about creating bacteria to dissolve the solid waste there’s not a lot of science involved, but by draining the water underground preventing an issue of moisture or standing water above ground, you won’t experience any of the issues such a condition presents.

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