1. Cut out the bottom of both buckets.
2. Select one of the bottoms and drill four holes the size of the inside diameter of your PVC in it.
3. Make four round washers.
4. Take some scrap inner tube and make 2 flexible washers of the same size.
   
    

Make the inlet and outlet tubes

• File down the edges of the buckets so they are smooth (the buckets are the inlet and outlet tubes).

Assemble the bags

1. Cut the bags to the right length:
   • Cut one stretches of plastic bag (each of length x=L+4 meters)
   • Cut the tarpaulin to the same length
2. Fold each tarp lengthwise and sew the edges together to make two open-ended bags. Lay the tarp bags out on the flat, smooth area, and pull the doubled-up plastic bag through one tarp and then through the second. Now there should be a bag of two layers of material with two open ends. Note that you will normally need four layers of materials for a practical biodigester to reduce the risk of leakage and piercing by plant roots. 

Make an exit for the methane

1. Choose a location for the methane exit hole. This might be somewhere around the mid-length of the pipe or someplace more convenient.
2. Make a hole through both layers of bag, without harming the plastic layers on the other side. One way to do this is by placing a piece of smooth wood inside the bag and then using a hole cutter to cut a hole through the top layers of plastic and into the wood in the middle (the wood protects the bottom layers of plastic from being cut).
3. Send someone inside the bag with the PVC male adaptor, a plastic washer, and a rubber washer. Again, it may be possible to do this without actually going inside the bag.
4. Attach the connection as below.
   
   

Put the buckets on the bag

1. Fold the end of the bag in a fan formation (paper fan style) and pull it through the assembly of three buckets, then around the sides.
2. Make sure the methane outlet is still at the top of the bag and in the location you would like it to be once you place in the bag in the trench.
3. Make sure there is a pathway for waste to travel through the buckets inside the innermost bag – you can use a blunt-ended stick to reach through the bags in these bucket-tubes. If you can feel the end of the stick below the bottom of the buckets, the path is sufficiently clear.
4. Fold the end of the plastic bags sticking out past the buckets over the edges of the bucket-tubes, and tie them down with some rubber tube. Make sure they are tied down securely. 
   
   

How to size the biodigester

1. Determine your energy needs
   
   1. To provide energy, E (MJ/day), for a certain amount of cooking time:

E = 1.055 * P * t / 1000

P = Btu rating of the stove (e.g., 3000 for a simple stove)

t = Hours of desired cooking time per day

2. To provide energy, E (MJ/day), to replace another fuel:

E = k * V

k= Volumetric (or per unit mass) energy content of fuel replaced

(e.g., propane: 22.8 MJ/L; see Table 3 in the Extended Guide for other fuels)

V = Volume (mass) of previous fuel used daily

2. Find length of the biogas digester

L = 0.0734 * E / (r^2) + 4

E = Energy needs (MJ/day) computed in (1)

r = Radius of digester plastic bag (e.g., 0.4 m is common)

L = Length (m) of plastic bag to be cut

3. Find volume of waste required daily

V = 0.361 * E

E = Energy needs (MJ/day) computed in (1)

V = Volume (gallons) of raw waste to be mixed with water and added daily

For typical 5-gallon buckets: number of buckets of waste = V/5

• : If the total volume of the bag (=3.14*r^2*(L-4)) is less than 5 m³, building a biodigester is not recommended to supply your energy needs.