This project proposed to evaluate the speed and effectiveness of composting using a continuously rotating drum propelled by an elevated water supply.
A rotating-drum composting system has been developed using a drum fashioned from a piece of culvert pipe 3 feet in diameter and 5 feet long and using a chain-driven rotating mechanism.
The main purpose of the project was to eliminate the time-consuming task of turning a compost pile by composting in a rotating vessel. The project sought to make the rotary composting system in a size suitable for small farms and so it could be assembled by farmers using off-the-shelf components at a relatively low cost. Another goal was for the system to produce usable compost fairly quickly with no or low energy input.
The major obstacle proved to be developing a suitable, reliable and low-cost way to rotate the drum. After many unsuccessful attempts, a chain-drive system was developed using a 1/100 hp gear motor. All of the components are either off the shelf or can be readily fashioned.
“The basic principle of using a rotating drum appears to work well in producing compost quickly,” says project coordinator Jack Caldicott, “with the quality being comparable to that produced by existing 4-foot cubes, but without the need to turn.”
After considerable searching, Caldicott found and purchased a 36-inch diameter double-walled plastic culvert pipe for use as the compost drum. The outside is corrugated and the inside smooth, providing air pockets and a measure of insulation to help retain heat during composting. A 5-foot-long section was fashioned into a drum using a plywood disc at one end and a removable plywood door at the other with a flap two-thirds up the door to allow for loading the compost material. Drum volume is 1.3 cubic yards, and the estimated weight of the loaded drum is 500 pounds. The drum was mounted on four 10-inch diameter heavy-duty wheels attached to a pallet to allow it to rotate. The corrugation provided automatic guides for the wheels.
The initial tests were done to make sure the volume was adequate to achieve good composting action. Using a mixture of horse manure, bedding material and garden waste, all shredded before loading, brought the temperature to 160 degrees F in 24 to 36 hours. The temperature remained at 140-160 for three to four days. The loaded drum proved too heavy to turn by hand, so a boat winch was used to rotate the drum daily. Caldicott tried several versions of the wheels and wheel-mounting arrangement before settling on the 10-inch-diameter heavy-duty wheels, each with a rating of 150 pounds and mounted on wooden blocks screwed to 2 by 4s.
Caldicott says the compost produced was comparable to his pre-drum compost.
“Compost made in the drum was used to grow a variety of vegetables through the growing seasons of 2001 and 2002,” he says. “Although the results are subjective, vegetables produced with drum-made compost did every bit as good as compost made with my conventional 4-foot bins.”
What’s more, he says, the compost from the drum is visually more consistent.
His second objective, using water as a drive mechanism to rotate the drum, was found to be impractical because of the considerable force required to rotate the loaded drum. He tried many alternatives, and their failure proved to be his most frustrating and time-consuming problem.
Caldicott eventually succeeded in creating a chain drive around the drum, using an industrial-grade chain and inserting screws from the inside at 4-inch intervals to act as teeth. Ten-tooth and 39-tooth sprockets were mounted on a 5/8-inch shaft with the drum chain passing around the 10-tooth sprocket. The 39-tooth sprocket is driven by another 10-tooth sprocket mounted on a half-inch shaft with a “V” groove sheave a foot in diameter. A 110-volt AC, 1 rpm, 1/100 hp gear motor is fitted with a 2-inch-diameter “V” sheave that drives the 12-inch sheave with a “V” belt.
“The result of all of this gearing down,” says Caldicott, “is that the drum completes one revolution in approximately six hours.”
A simple chain-tensioning device keeps the chain tight to ensure that it does not come off the drum teeth. A similar “V” belt drive is used to eliminate slippage and allow the belt to be removed so the empty drum can be turned to the right position for loading.
Caldicott says that a DC motor similar to the 1-rpm, 110-volt motor could be used, driven by a 12-volt, solar-powered marine battery.
“This arrangement, although more expensive, would have the distinct advantage of not requiring any external power source,” he says.
For the tests, he rotated the drum once a day for six to eight hours, using a simple household timer for turning the motor on and off.
Caldicott says he can produce usable compost in 10-14 days. After the drum is filled, the compost reaches 160 degrees in 24 to 72 hours, remaining in the 130-160 range for about three days. Over the next five to nine days, the temperature falls gradually to 80 degrees, at which time the drum is emptied and ready for the next batch.
Owing to the problems encountered, outreach had not been conducted at the time of this report. However, the project has been shown to several small-scale farmers who expressed considerable interest. With his prototype done, Caldicott plans to build another for the Clallam County Sustainable Farming Program demonstration farm. The composting system should be done in early 2003.
POTENTIAL BENEFITS OR IMPACTS ON AGRICULTURE
While the benefits of compost are well established, small-scale production is arduous and time-consuming. A rotating composter for small farms eliminates one part of process – having to turn the compost.
“I now use the rotary composter exclusively for making my own compost as it eliminates the need for turning and produces compost comparable to that from my 4-foot by 4-foot bins,” says Caldicott, who operates Jacyn Lavender and Vegetable Farm with his wife, growing lavender, cut flowers and vegetables for sale at farmers markets and over the Internet.
FARMER ADOPTION AND DIRECT IMPACT
None of the farmers familiar with the project has attempted to emulate the rotating drum practice, but one farmer said, “If you get it working, I want one.”
REACTIONS FROM FARMERS
One commented that he would like something about twice as big as Caldicott’s prototype.
WHAT WOULD I DO DIFFERENTLY
Caldicott says he would be more scientific, doing more calculations up front and seeking advice from someone experienced in drive mechanisms and with knowledge of where to obtain the parts.
Ideally, he says, the process of shredding the compost material and loading it would be automated. The most time-consuming aspect of making compost remains shredding the material into a size suitable for rapid composting. Current small-scale shredders, he says, leave much to be desired.
Caldicott says that although the composter operates reliably, it does have some limitations. A drum larger in diameter would give more mass and probably provide more compost more quickly. Caldicott says he could not find such a drum, but he says the drive mechanism could be scaled easily to a larger drum
Finally, he says, better insulation, particularly in the drum ends, would help ensure more thorough composting at the ends.