Construction

This is the basic digester plan. I used a 1 gallon carboy (to contain the slurry), rubber stopper, plastic tubing, stainless steel and copper tee fittings, a heavy plastic bag, a brass stop cock and copper tubing.
Initial construction	In this photograph you see the assembled digester. A large canning pot was used to hold the hot water bath. The brass stop cock and copper tubing were assembled and mounted to the base, and a burner tip was created by compressing the tubing around a small sewing needle.
Rubber stopper	One section of tubing went through a one hole rubber stopper and into the carboy. The stainless steel tee had tubing on either side. The tubing was routed through a board mounted on the pot to reduce strain on the rubber stopper. One side of the tubing went to the manometer, the other to the burner and storage bag.
Manometer	To measure and maintain pressure in the system, I created a manometer with some plastic hose. After experimenting with different lengths, I settled on an 8 inch loop filled with water. As the pressure in the system increased, water would rise on the right hand side of the manometer. If too much pressure built up, the water would be forced out of the manometer thereby relieving the pressure and preventing other parts of the system from exploding.
Burner tip	I tapered one end of the copper tubing, then flattened the end around a sewing needle. This created a small orifice which acted as a burner tip.
Creating the hot water bath	Methane production is best at 90-100 degrees Fahrenheit. To keep the mixture at a high enough temperature, I used an aquarium heater and a water bath. I partially submerged the jar in the water, resting it on top of some canning rings to keep the slurry level at or above the water level. I installed the aquarium heater in the pot, and set it on its highest setting. The temperature stayed at about 85 degrees, not optimal, but high enough to faciliate methane production. I also used some foam packing material to retain the heat and reduce water loss due to evaporation. I monitored the water level every day to keep it at the appropriate depth for the aquarium heater.
Slurry in water bath with aquarium heater	Here's a picture of the aquarium heater installed.
Filling the manometer	Filling the manometer took some practice. If the inside diameter of the tubing had been a little larger, the water would have poured in. The tubing used was 1/4 inch I.D., so it had to be filled it with a syringe.
Manometer water level	Here's the manometer filled to the appropriate level. When both sides were level, the pressure was equalized. When the left side went down and the right went up it indicated that the system was pressurized.
Collection bag	I added a collection bag to store the gas as it was produced. My bag was actually an empty heavy-duty plastic bag (once used to store rawhide dog chews), with copper tubing clamped onto the end and sealed with silicone. A Mylar ballon or an empty milk bag would probably have made better a collection bag, as suggested by some of the other project sites listed in the Resources section.
Fully assembled	This is the digester fully assembled.