Composting In Schools

High-Tech Bioreactors

Click here to view diagram of bioreactor

A bioreactor is a name given to an enclosed composting vessel. The difference between a bioreactor and a typical composting system is that more parameters of the composting process can be measured and controlled in bioreactors. Even though the degradation of organic wastes has been happening since the introduction of life on our planet, it still is not completely understood. Using a bioreactor enables students and teachers to study and manipulate composting parameters inside a classroom or lab setting.

This is a complicated construction project, and access to a machine shop is highly recommended. For simpler classroom composting systems, try Soda Bottle Bioreactors or Garbage Can Bioreactors .



Building the Bioreactor

1. On a band saw, cut 3" off the end cap. Save this ring as it will be used to make the top cap.

2. Drill a hole in the center of the end cap large enough for a 1" tap. With a 1" tap thread hole for 1" PVC nipple.

3. Using the same drill bit, drill two holes in the side of the 18'' of PVC pipe. The first one should be 7 inches from the edge and the other should be 14 inches from the same edge directly above the first. Using the 1" tap, thread the hole. These are where the 1" plugs will be placed.

4. Using the ring formed from the cut on the end cap, draw the outline of the outer diameter on the plastic plate with the permanent marker. Cut this out with a band saw or jig saw. Glue this to the ring using the epoxy (&laqno;DO THIS OUTSIDE OR IN AN AREA WITH PLENTY OF VENTILATION!) Allow to dry overnight. Drill a hole in the center of this plate large enough for a 1/2" tap. With a 1/2" tap, thread this hole.

5. Place the 18" section of PVC pipe upright on the plastic plate. Using the marker, draw on the plate the inside diameter of the PVC pipe. Cut this plastic plate using the same equipment used in #4. With a drill press, drill 1/4" holes uniformly over the entire plate. {how many?, spacing?}

6. Using a band saw, cut 1" off the 18" section of PVC pipe. This will be used as your support for the plate you made in #5. Cut a section out of this ring so that it will fit snugly inside the remaining 17" of PVC pipe. (It will be an inside ring.) Using the epoxy, glue this ring inside the remaining 17" of PVC pipe so that the bottom edge is flush with the bottom of the pipe.

7. Glue the bottom end cap with epoxy to the 17" section on the side with the plate ring. Use excess epoxy for a tight seal.

8. Using a hand drill with a 1/8" drill bit, drill four holes on each side of the reactor body directly across from one another. These should be arranged so that you can obtain temperature readings from above, the top 1/3, the center, and the bottom 1/3 of the composting material.

Constructing the Exhaust and Condensate Collection System

1. Put Teflon tape on all threaded fittings.

2. Thread a 1/2" x 3" nipple into 1/2" elbow, thread a 1/2'' x 6" nipple into other end of elbow, put a coupler onto this 6" nipple, and to this coupler add a 1/2" x 6" nipple. To this nipple, thread on the 1/2" T at the middle junction and to each side of the T, add the other two 1/2" x 3" nipples.

3. Thread the nipple closest to the elbow into the top of the reactor. The 6" nipples should be in a position perpendicular to the floor. To the 6" nipple facing down, put a condensate collection bottle. (This doesn't have to be air tight). To the 6" nipple, pointing upward, place the 3/4" tubing over the top and the other end will be put outdoors.

Constructing the Leachate Collection and Air Input Systems

1. Drill a hole in the top of the nalgene bottle large enough for the 1" tap. (It's easier if cap is left on the bottle during drilling.) Using a 1" tap, thread the hole.

2. Thread the 1" x 3" nipple into the end of the 1" T, the reducer into the middle port of the T, the 1/2" nozzle into the reducer, the 1" x 6" nipple into the top of the T.

3. Connect the 6" nipple to the bottom of the end cap and the gal. Nalgene bottle to the 3" nipple. (These must be airtight; use silicon caulk.)

A reactor stand is recommended, but its design is up to the individual. The only criteria are that it be near an outside wall for the ventilation of the exhaust gases from the reactor and that it hold the reactor high enough for the leachate bottle to be installed.

You are now ready to begin composting and experimenting!

Science &
in Schools


Cornell Waste Management Institute ©1996
Department of Crop and Soil Sciences
Bradfield Hall, Cornell University
Ithaca, NY 14853