How to build a Solar CITIES 5 gallon water jug digester (aka "Water Jugestor" or "Biojugestor" or "Baby Dragon").

So you want to build a biodigester eh?
With the Solar CITIES 5 gallon water jug biodigester or "biojugestor" you can build a functioning mini biogas system that fits on a classroom table and makes the best entry level STEM science activity for learning about biofuels and the zero waste "circular economy" one can think of.
Inexpensive and elegant and simple to construct, our 5 gallon water jug digestor teaches all the principles needed to scale up to truly useful biodigester systems -- pickle barrels, 200 liter (55 gallon) drum digesters, IBC tank digesters, 1000 gallon water tank digesters and on up; once you've learned the principles and built a biojugestor in your classroom or home, you are on your way to creating high utility functioning food-waste-to-fuel-and-fertilizer systems for home, school or community.  But it starts here, with this entry level and easy to make "BABY DRAGON".

We suggest you work in teams of up to 4 people each.  
Each person will need thick gardening gloves (the kind with leather protection) and eye protection since we will be working with power tools.

What do you need?

Here is a materials list for each team:


1)  A rigid 5 gallon water jug WITH LID. 
The water jug can be used as long as the lid can still seal water tight around the edges. Don't worry if it has the hole in it as we will be driling it out and putting in a bulkhead fitting through the old lid.
DO NOT use a flimsy 5 gallon water jug -- they don't seal well and they can be dangerous to cut holes in because the thin plastic gets caught in the hole saw.
Use a rigid thick plastic jug that HAS A HANDLE on it. 
Like this:

Water jugs with handles

The water jug is your baby fire breathing dragon's STOMACH.

Water Jug

2) You need a 2 foot piece of 2 inch PVC schedule 40 pipe

2 inch pipe
This will be the THROAT of the baby dragon


3) You need a 2 inch to 4 inch PVC adapter

4 inch to 2 inch adapter

This will be the MOUTH of the baby dragon

4) You  need a 2  foot piece of 1 inch PVC schedule 40 pipe

1 inch pipe

This will be the URETER of the baby dragon through which it will "pee" nutritious liquid fertilizer.

To help the dragon "pee" in the right place and not make a mess you will need:

5) A 1 inch Tee


1 inch T
6)  a 1 inch elbow

1 inch elbow

7) You will need a two foot piece of 1/2 inch PVC schedule 40 pipe.

half inch pipe

This will be the baby dragon's ANUS where it will "pass gas".
To allow us to capture the gas and regulate it you will also need

8) A 1/2 inch slip to slip PVC ball valve (or equivalent).

half inch ball valve

9) A half inch elbow.


half inch elbow

10) A half inch slip to thread adapter

slip to thread

11) A half inch threade to barb nipple


barbed hose adapter

12)  10 feet of  5/8 inch outer diameter (1/2 inch inner diameter) clear PVC hose.

5/8 inch clear PVC tubing

13) a 5/8 inch barbed splice adapter

barbed connector

These will let you get the gas to a gas storage system.

Now, HOW to get the pipes INTO the stomach/tank?

You will need 2 uniseals a and one bulkhead fitting.

14)  You need a 2 inch uniseal for the two inch feed pipe

2 inch uniseal

15) You need a 1 inch uniseal for the 1 inch slurry (fertilizer) outflow pipe

1 inch uniseal

16)  You need a 1/2 inch bulkhead fitting for the biogas out pipe.

half inch bulkhead

Then you need a set of hole saws to cut the holes in the tank for the unseals.

17) You need a 76mm or  3 inch hole saw for the 2 inch uniseal

3 inch hole saw

18) You need a 44mm  1.75 inch hole saw for the 1 inch uniseal.

19) You need a 29 mm or 1 inch to 1 1/8" hole saw for the 1/2 inch bulkhead fitting.

This multi saw kit would seem to be the  cheapest way to get both but DON"T BUY IT.  THE BLADES ARE FLIMSY AND WON"T DRILL STRAIGHT HOLES:

Multi Blade Hole Saw


Instead, please get a DEDICATED HOLE 1 3/4 hole saw and an DEDICATED 1 Inch hole saw. It will cost more, but save a LOT of hassle! You have been warned! We learned the hard way!


20) You will need a cordless drill to use the hole saws (well, technically you could grind the hole saws into the plastic like we did at Dublin Castle in Ireland when the battery for the drill died, but it is easiest if you have a drill!).

cordless drill

To ensure that the uniseals and bulkhead fittings do not leak you will need some silicone.

21) silicone


To cut the PVC pipes you will need a hacksaw or a PVC cutter. We recommend both.

22) Hacksaw


23) PVC cutter capable of cutting 2 inch pipe

PVC cutter

To get the pipes in the uniseals, just have some dish soap or rubbing alcohol handy. Either will work but alcohol is less slippery and easier to use. Dish soap is much cheaper.

24) Denatured alcohol




Hole saws are sharp and in a power tool can be truly dangerous (I have cut my wrist once and my left hand extensor tendon on my thumb, both times because I gave my gloves to somebody else, thinking I would be okay.  But accidents happen.  WEAR GLOVES!!!
Adult supervision is required for all student use of power tools.

 24) Leather safety Gloves

Leather safety gloves

25) Safety glasses 

Safety glasses

That's what you need to do the build of the digester itself.

The build is fairly simple.  

Drill the 3 inch hole on one side of the tank and the 1.75 inch hole on the other. 
Put silicone on the unseals and pop them into place.
Drill the 1 inch hole in the lid of the water jug (this may be tricky if the lid already has a hole in it. If it does, cover the hole with a piece of masking tape or duct tape and then do your drilling.)
Put silicone on the threads of the bulkhead fitting and insert it from the bottom of the lid with the threaded part facing up, put on the locking nut and tighten.

Cut out a 4 inch rectangular section on one end of the feed 2 inch feed pipe so food can slide into the bottom of the jug. 

Sand edges of the pipe with sand paper or by rubbing on cement to wear them down.
Either soap the pipe or dress it with rubbing alcohol and do the same with the inner surface of the uniseal. Push the pipe through the uniseal, twisting as you go, until it touches the bottom of the tank.

Align the one each pipe with the tank on the outside and mark the pipe where it would be halfway into the tank if the bottom of the pipe was touching the bottom of the tank.  Cut out a hole of about 1/2 inch at that location. Sand the bottom of the pipe or rub it on concrete to smooth it.
Wet the pipe with soap or rubbing alcohol and push through the uniseal, twisting as you go, until it touches the bottom of the tank.

Cut three 2 inch long pieces of half inch pipe and fit them one into the bulkhead fitting.  On the other end fit the ball valve, then put another piece of half inch pipe on the ball valve and put an elbow on that piece. Fit another piece of pipe to the elbow and slip on the half inch slip to threaded adapter and then screw in the barbed hose fitting. On this place the 10 foot clear hose and on the free end place the brass barbed connector.

Cut the top of the portruding 1 inch pipe two inches below the top of the 2 inch pipe (the feed pipe should be higher than the outflow pipe).
Put the 2 inch to 4 inch adapter on the top of the 2 inch feed pipe (it will be your funnel).

That completes this phase of the build!

This video animation shows what it should look like: