Operations around the country use a variety of methods to turn organic waste into a marketable product that saves valuable landfill airspace.

By Daniel P. Duffy

Of the entire wastestream in the United States, fully two-thirds consists of organic materials. Some organic materials, such as newspaper, office paper, and corrugated cardboard, are recycled directly. Others, such as yardwaste and foodwaste, still tend to be landfilled. This provides the potential for significant increases in the rate of composting nationwide.

Composting Techniques

Compost contains sources of carbon and nitrogen. Generally speaking, carbon sources are brown in color (woodwaste and leaf waste), while green grass and food scraps provide a source of nitrogen. The ratio of carbon to nitrogen within the composting material affects process time, final quality, and the seriousness of the composting odor. While both sources are important, the ideal ratio is 30 parts carbon to 1 part nitrogen. Materials that can be composted include fruit and vegetable scraps, egg shells, coffee grounds and filters, tea bags, fireplace ash, leaves, grass, yard clippings, vacuum cleaner lint, wool and cotton rags, sawdust, and non-recyclable paper. And although the following are organic in nature, they are not suitable for composting: meats, dairy foods, fats, oils (including peanut butter and mayonnaise), grease, pet excrement, fish scraps, diseased plants, and bones.

Breaking down organic material during composting is a two-step process. The first step is to reduce the material into a size that can be composted more efficiently. Large wood branches are not easily composted, but shredded wood chips are. The second step is chemical decomposition performed by various invertebrates and microorganisms, which perform most efficiently at optimum temperature, oxygen, and moisture contents. The addition of water and air (either by regularly turning the compost or by injecting air) is therefore necessary.

Composting produces heat generated by the microbes that can raise the temperature of the compost material to 90°F–140°F. Lower temperatures result in slower decomposition rates, so these high temperatures are necessary for rapid composting. The high temperatures also kill weed seeds, insect larvae, and potentially harmful bacteria.

Simple mulching with a lawn mower is a form of composting in which residential, commercial, and institutional establishments leave cut grass on their lawns. Individual homeowners can compost food scraps and yard trimmings on their property. For somewhat larger operations, yard trimmings and other organic wastes from whole neighborhoods can be collected and composted at central facilities. Larger institutions (universities, schools, hospitals, etc.) can process their own food scraps, paper, and yard trimmings at onsite composting facilities. Even larger composting programs can be run by commercial generators of organic wastes (stores, supermarkets, schools, farms, etc.). Municipalities may integrate composting into their overall MSW management strategy. This can be accomplished either through source separation (homeowners setting aside organic waste for curbside collection or hauling the waste to a regional facility), or by separating out organics from the collected waste at a material recovery facility (MRF).

Types of Facilities

Composting facilities use one of four methods to compost large quantities of organic materials. Each of these systems is designed to introduce air into and maintain moisture content in the composting material:

Aerated static piles: Air ducts and perforated pipe systems are used to force air into the composting material. Air also can be drawn up from the pile, provided air pathways allow for aeration to occur. Though less effective than a forced-air system, air-drawing systems allow for better odor and air pollution control.

Windrows: A windrow is a long pile of composted materials built on flat surfaces. Air is introduced into the windrows mechanically by turning the material at regular intervals. This is done either with a simple frontloader or with a machine called a windrow turner. Failure to turn the piles sufficiently will result in the process going anaerobic from lack of oxygen. In addition to failure of the composting process, strong, unpleasant odors will be produced.

Turned-aerated piles: These types of composting facilities combine both of the above technologies for more consistent process control and product quality.

In-vessels: Composting is done in an enclosed structure that allows for consistent air and moisture control, along with mechanical turning of the material. This allows for rapid decomposition and composting, but large-scale facilities can be prohibitively expensive compared with windrows or aerated piles.

Case Studies

Large-scale composting operations are performed nationwide with each state devoting a large percentage of its overall wastestream to composting organics. However, each region has its own operational approach, which is dictated by climate, population density, potential markets, and available waste management infrastructure. The following examples are provided to illustrate these differences.

Mecklenburg County, NC

Steve Elliot, manager of the Mecklenburg County composting program, provides the following information on composting in North Carolina. Approximately 60% of the organic materials diverted to composting comes from curbside collection in the city of Charlotte. The remainder comes from landscapers, residents outside the city limits, and small towns around the county. Collections are preformed year-round; there are no seasonal reductions in composting due to North Carolina’s consistently warm climate. However, there are seasonal highs in the spring (due to an increase in garden waste and yardwaste) and the fall (from the addition of fallen leaves). Daily tonnage varies from 200 tpd to over 500 tpd.

Curbside collection is the rule in Charlotte and in some neighboring small towns. Residential users set out yardwaste on the curb in open-top garbage cans, clear plastic bags, or small bundles. It is debagged at the curb and put into rearloader trucks. This obviates the need for onsite debagging and disposal of the bags. In addition to curbside pickups, there are four yardwaste drop-off centers in the county; some are open five days a week, while others are open six days a week. All are open year-round.

When debris arrives at the composting facility it is offloaded and processed through tub grinders to achieve uniform particle size and reduce larger organics (such as tree limbs) to a more easily compostable size. The ground-up debris is piled into windrows, which initially are 20 feet wide, 8 feet high, and from 400 to 800 feet long. Moisture content is adjusted to 50% to 60%, either by turning during rain or using a water truck. Material processed during the growing season has a carbon-to nitrogen ratio of 30:1, which is perfect. Material processed during the fall and winter has a ratio of 70:1, which is adjusted to 30:1 by adding urea or ammonia nitrate as needed. Windrows are turned at least once weekly.

In addition to sampling for carbon/nitrogen ratios, temperatures are taken twice weekly. Windrow temperatures average between 145°F and 150°F during the active composting process. It takes about seven months to make a finished product. After the process is completed, the final product is sifted through a three-eighths-inch screen.

Unlike in most composting operations, Mecklenburg County is able to market its composted material. It is able to sell all the compost that it can make; none is given away. The current price is $23 a cubic yard. Only about half of the yardwaste that arrives is composted because of space restraints; the other half is sold as mulch and boiler fuel. Selling the compost allows the county to break even; there is no “creative accounting.” The composting operation is managed as a local government body but run as a business. The operation is self-sufficient and tax dollars are not needed.

The composting operation employs 16 full-time people to staff four facilities. Equipment consists of five dump trucks, one tractor trailer, four tub grinders, six wheel loaders, one trackhoe, two trommel screens, one star screen, one windrow turner, and miscellaneous support equipment. The county plans to expand the operation’s main facility so it can make more compost and reduce the amount of boiler fuel sold. Selling the extra compost will present a marketing challenge.

This program is unique because it is a public/private partnership. It is basically run as a self-supporting nonprofit entity. A local private firm markets half of the compost produced by the program. This local company purchases the compost at a contracted rate and in turn markets the product to landscapers, garden stores, big-box stores, and landscape supply yards. The product is offered in several different blends, either bagged or bulk.

Bismarck, ND

Galen Bren of the City of Bismarck’s Department of Public Works says that the participants in the city’s composting program can either have their organic waste picked up at the curb or deposit it in special Dumpsters. The Dumpsters are checked daily as part of normal route operations and picked up as needed. Curbside pickups are preformed as part of the regular weekly waste collection. Residential participants in the composting program separate the organic material into plastic bags, which are disposed of at the composting facility.

Given the potentially harsh winters in North Dakota, the program is seasonal and runs from March or April to October or November each year. Collection programs and composting operations cease during the winter months.

Composting is performed in windrows operated by a self-contained turner. Grass is added as needed to increase the nitrogen level of the composting material. Water is added if the compost’s moisture content becomes too dry. If increased ventilation is required, wood chippings are added to the mix. The final compost material is gathered with a scraper and sifted to remove large objects. Two dump trucks and three to four personnel per eight-hour shift are dedicated to the composting operation.

This is a relatively new program that seeks to provide finished compost to the public free of charge. The program is a nonprofit effort with its main benefit the saving of landfill disposal airspace, thus lengthening the operational lifetime of existing landfills and pushing back the day when expensive landfill construction is required.

Not only is Bismarck colder than most American cities, its environs are sparsely populated. This presents a serious challenge to future expansion of the program beyond the city since transportation costs increase dramatically with lower population densities. In the meantime, the program will concentrate on raising public awareness and increasing public participation, especially the use of the composting Dumpsters.

Hot Springs, AR

According to Ron Wacaster, utilities facilities operations manager of the City of Hot Springs, there are three major sources of organic materials treated by his composting program. Unlike in many mass-composting operations, Hot Springs includes a large quantity of municipal sludge from the city’s main wastewater treatment plant (which is located next door to the composting facility). The second source is the standard brush and yardwaste from residents and commercial sources (landscapers, construction firms, farms, etc.). Finally, the presence of a thoroughbred racetrack allows for a significant organic source in the form of bedding straw and associated waste material. This unique source of organics sets this composting operation apart from others.

With a relatively mild climate, collection and composting operations are performed year-round. Organic waste is collected by truck and brought to the composting facility by the public. There are no tipping fees and those who deposit at the facility may do so without charge. The racetrack makes regular bulk deliveries and sees the free disposal as the best means of getting rid of its accumulated stable waste.

The operation utilizes a wide variety of equipment, including a Caterpillar 928 loader (3.5-cubic-yard bucket), a Caterpillar 938G loader (8-cubic-yard bucket), a pair of 10-cubic-yard dump trucks, and a four-wheel-drive backhoe. Arriving organic waste is first passed through a rotating trommel screener to remove large objects. The operation employs one part-time and two full-time employees.

The operation is included in the city’s sludge-management budget, with over $128,000 budgeted to cover salaries, fuels, equipment rental, and other expenses. The finished compost is marketed to the public and sold directly to individuals and commercial compost users.

The composting operation recently has switched from windrow composting to aerated static piles. Aeration piping is provided by a perforated 4-inch-diameter hose laid under the piles that forces air into the composting mass. Runoff from the piles is collected and piped to the adjacent wastewater treatment plant. While suited to the needs of organic material with a high concentration of sludge and animal waste, the curing phase takes longer for aerated piles. The biggest challenge facing the program is acquiring the additional space needed for the new process.

Rapid City, SD

The composting programs run by Jerry Wright, superintendent of Solid Waste Operations for Rapid City, service the city and its surrounding areas. Unlike other composting programs, the centerpiece of Rapid City’s effort is a new MRF. This makes composting a part of an overall waste management process instead of being an isolated program dependent on source separation and voluntary disposal.

The MRF is augmented by two Dano rotating drums capable of processing almost 19 tph. Attached to the end of each drum is a two-level trommel-type screen. The first level has 5-inch openings that allow for initial separation of large objects (which are then separated for further processing and recycling). Smaller objects fall through the second screen with 1.75-inch openings. What remains after the second separation gets sent to the composting hall of the MRF. Use of the Dano drums allows for efficient volume reduction without having to shred the MSW prior to composting.

Once the composting fraction has been separated out of the wastestream it is sent to bays at the MRF. Each bay is divided into four zones, each of which is aerated by blowers that create a total air flow of 100,000 ft³/min. Air pulled from the bays is sent either to an acid scrubber or to a 24,000-square-foot biofilter made of wood chips.

After about a month in the aeration bays, the material is loaded and sent to an adjacent curing building. Curing consists of more positive aeration without treatment of the exhaust air. Final finishing is processed through a Bivitec three-eighths-inch screen and a Triple S de-stoner to remove larger aggregate and cobbles. That the entire process takes place indoors under controlled environmental conditions allows the operation to continue year-round without being impacted by the region’s severe winter climate. The MRF employs 20 personnel, with three assigned to composting operations.

Though the operation is economically self-sufficient, the main financial advantage is total waste reduction (estimated at a little over 50%). This will extend the existing landfill’s operational lifetime by 26 years, resulting in significant capital cost savings. Projected revenue from the sale of compost is estimated to be $75,000 per year. The additional sales of other recyclables separated by the MRF will be two to three times the sale of compost.

Lehigh County, PA

According to Kurt Fenstermacher, solid waste and recycling manager, the Lehigh County Office of Solid Waste operates a yardwaste recycling system that services 24 of the county’s 25 municipalities. Most of the organic waste arrives via disposal from private individuals, waste haulers, and landscaping companies. The remainder comes from the county’s own parks and game preserve management programs. Most of the organic waste is composed of leaves, grass, and brush. During 2003, the county processed almost 21,000 tons of organic waste. The program saved the county over $1,100,000 in cost avoidance (mostly landfill airspace savings).

Unlike many mass-composting operations, Lehigh County has a decentralized system consisting of a network of 11 municipal and county sites in a cooperative system. The cooperation takes the form of a division of labor between the county and the municipalities. The municipalities are responsible for collecting and transporting yardwaste from individual residences. The county provides both the staff and equipment for all stages of composting. But instead of having the waste come to a central site managed by county employees, the county employees move the specialized equipment (two grinders, a windrow turner, two screens, and three pay loaders) from site to site and municipal employees assist in the processing and transportation of the materials within and among the sites.

Lehigh County operates a year-round composting operation of windrows that primarily handle leaves, brush, branches, and grass clippings. New windrows are established at the end of each month and season. Old windrows are completed (the final compost product removed for market) simultaneously with new windrow construction. The finished compost is screened prior to sale. The county sells the finished mulch and compost to local distributors and retailers. The county also uses a significant amount of the compost in its gardening projects.

Daniel P. Duffy, P.E., is an environmental engineer for Rumpke Waste Inc. in Cincinnati, OH.

MSW - January/February 2005