The Compost Manufacturing
Process

The production of compost is both a mechanical and a biological process. The raw materials must first be separated, collected, and shredded by mechanical means before the biological decomposition process can begin. In some cases, the decomposition process itself is aided by mechanical agitation or aeration of the materials. After decomposition, the finished compost is mechanically screened and bagged for distribution.

There are several methods for producing compost on a large scale. The methane digester method places the raw materials in a large, sealed container to exclude oxygen. The resulting oxygen-starved decomposition not only produces compost, but also methane gas, which can be used for cooking or heating. The aerated pile method places the raw materials in piles or trenches containing perforated pipes that circulate air. The resulting oxygen-rich decomposition produces a great amount of heat, which kills most harmful bacteria. The windrow method places the raw materials in long piles, called windrows, where they are allowed to decompose naturally over a period of several weeks or months. It is the least expensive method of all. Here is a typical sequence of operations used to convert municipal yard wastes into compost using the windrow method.

Separating

1 Yard wastes are deposited in separate containers by homeowners, and the containers are placed at the curb for pickup on the regular refuse collection day. Homeowners are instructed that only certain yard wastes are acceptable for collection. These include grass clippings, leaves, weeds, and small prunings from shrubs and trees. Short pieces of tree limbs up to about 6 in (15 cm) in diameter are also acceptable. Homeowners are also instructed that certain other yard wastes are not acceptable. These include rocks, sod, animal excrement, and excessive amounts of dirt. Palm fronds are prohibited because the frond spikes do not decompose and carry a poison. Food scraps, fruits, and vegetables are also prohibited because they can attract rodents, carry unwanted seeds, and contribute to odors.
2 The yard wastes are collected by separate refuse trucks and are transported to the processing center where they are dumped in piles. The piles are visually inspected, and any oversized or unacceptable materials are manually removed.

Grinding

3 A large, wheeled machine called a front loader picks up material from the piles and dumps it into a tub grinder. The tub grinder has a stationary vertical cylindrical outer shell with a rotating cylindrical inner shell. As the material passes between the two shells, it is ground into smaller pieces and thoroughly mixed. The ground material falls out the bottom and through a screen where the larger pieces are screened out. The remaining material is transported by a conveyor belt to a holding pile.
4 The larger pieces are sold to landscaping companies for use as mulch or ground-cover without further processing. The rest is loaded into large dump trucks and transported to the composting area where it is dumped in long rows, called windrows. Each row is about 6-10 ft (2-3 m) high and several hundred feet (m) long with a triangular cross section. A flat space about 10 ft (3 m) wide is left between each row to allow vehicles to move along the length.

Composting

5 The composting area may cover several acres (hectares). After a windrow is laid in place, the material is dampened by a tank truck that moves along the row spraying water. The water aids in the composting process and helps minimize wind-blown dust.
6 Every few weeks, a special machine straddles each windrow and moves along its length to turn and agitate the material. This breaks down the material into even smaller pieces and exposes it to oxygen, which aids in the decomposition process. After the windrow is turned, it is sprayed with water again. This process continues for two or three months. In hot, dry weather, the windrows may have to be watered more often. During decomposition, the internal temperature of the pile may reach 130° F (54° C), which helps kill many of the weed seeds that might be present.

Curing

7 The raw compost is scooped up with a front loader and moved to a large conical pile where it is allowed to finish the decomposition process over a period of several weeks. This process is called curing and it allows the carbon and nitrogen in the compost to adjust to their final levels.

Screening

8 After the compost has cured, it is scooped up with a front loader and dumped into the hopper of a rotary screen. This device consists of a large cylindrical screen rotating on an axis that is slightly inclined above the horizontal. The openings in
Diogram depicting the commercial processing of yard waste into compost.
the screen are about 0.5 in (1 cm) in diameter. The compost is fed into the raised end of the rotating screen from the hopper by a conveyor belt. As the compost tumbles its way down the length of the rotating screen, the smaller material falls through the screen and is moved to a storage pile by a conveyor belt. The larger material that cannot pass through the screen falls out the lower end of the cylinder and is either returned to the compost piles for further decomposition or is sold as wood chips.

Distributing

9 Much of the finished compost is loaded into large dump trucks and sold in bulk to landscaping companies, municipalities, nurseries, and other commercial customers. Some of it is sealed in 40 lb (18 kg) plastic bags for retail sale to homeowners. Using the windrow method, a typical suburban yard waste processing facility can produce as much as 100,000 tons (91,000 metric tons) of compost a year.

The Compost Manufacturing Process