As composting stretches its roots deeper into the U.S. solid waste management infrastructure, a consistent approach to facility operation and process control has become essential. No matter the technology or techniques used, a rich, top-quality end product is the goal.
Modern composting relies upon a variety of processing technologies, systems and operating methods. The decomposition rate is determined by the material type, local climate, system configuration and operating procedures. Aerobic composting will produce an end-product in one to six months, depending upon the method used and market requirements.
Preparing Feedstock Household hazardous waste (HHW) should be removed from the feedstock before it comes in contact with the compostable material. Otherwise, HHW may create dangerous working conditions at the composting facility, inhibit the biological process and contaminate the finished product. To segregate these materials, facilities can use mechanical screening or hand-picking methods.
The recovered mixed organic material should be processed into uniform (one- or two-inch) particles which, because of their small size and large surface area, promote microbial activity and rapidly decompose. Uniform particle size also maximizes product stability and reduces the contaminants. To prepare feedstock, various machinery such as hammermills, shear shredders, rotary drums and screens can be used.
Once the materials are reduced, maintaining a proper balance between carbon and nitrogen helps ensure rapid decomposition without causing excessive release of ammonia. However, because facilities have little control over the materials they receive, creating the optimum balance between carbon and nitrogen can be difficult. Variations in quantities of a specific material can greatly affect the compost's carbon-to-nitrogen ratio (C:N).
Paper, newsprint and wood waste are all carbon-rich and have a major effect on the C:N. To avoid excess carbon, reduce the volume of wood or newsprint collected for composting; nitrogen also can be added.
The optimum C:N is approximately 30:1. The C:N at the compost process end will indicate product stability; stable compost's ratio is around 10:1.
To encourage biological activity during decomposition, moisture must be added during feedstock preparation. Again, a proper balance must be achieved. If the material is too dry, decomposition may halt or never even begin, while material that is too wet can inhibit aeration and cause odors.
Finally, the recovered organic materials must be mixed to ensure uniformity and to blend with other elements such as sludge or yard waste. If a facility uses mixing drums with retention periods greater than seven hours, microbial activity may begin during this stage, which naturally leads into the active composting phase: high-rate decomposition.
The Music Of Decomposing Materials During high-rate decomposition, microbial activity increases as the bacterial colonies consume nutrients, grow and reproduce. As long as surplus nutrients, adequate moisture and oxygen and an appropriate temperature exist, the microbes will thrive.
The next step is stabilization, which continues the active composting process that began during high-rate decomposition - but at a declining pace. Compost is considered stable when it can be stored or applied to soil without producing adverse effects such as generating odors, attracting pests or harming plants.
Compost stability is facilitated by curing, or aging, under aerobic conditions. This allows the product to mature; microbial activity during this process is long-term and low-level as the organisms slowly convert organic material into inorganic products (carbon dioxide) and soil-like material (humus).
Creating A Controlled Environment Diligent process control is the key to successful operations. Organic materials will break down in three phases: high-rate decomposition, stabilization and curing. During each phase, pH, aeration, temperature, turning, moisture and pathogens must be closely and consistently monitored. The methods are identical during each phase, but the operating boundaries may vary according to the operator's goal.
* Acidity. The pH indicates the type and intensity of microbial activity and the composting material's corrosiveness. A low pH (acidic) may signal anaerobic conditions, which increases odors and produces toxic byproducts. A pH below five will increase metal solubility, which can contaminate the compost and corrode equipment. However, during the early decomposition stages, a slightly acidic compost can be beneficial because a lower pH reduces ammonia releases.
In the latter phases of high-rate decomposition, the pH should be approximately eight or lower as the compost reaches a natural balance. The pH during the remaining stages may fluctuate between six and eight, as long as aerobic conditions continue.
* Aeration and Temperature. Air is typically introduced into the compost during high-rate decomposition and stabilization to maintain optimum temperature and oxygen.
During the first several hours of decomposition, forced aeration via blowers helps to establish and maintain aerobic conditions, to remove carbon dioxide, moisture and volatile organic compounds from the pile, and to minimize pH depression caused by organic acids. Although the main purpose of aeration is to remove heat, microbial activity isn't vigorous enough at this early stage to raise temperatures.
At the beginning of high-rate decomposition, when easily digestible material is abundant, temperatures will rise quickly. Temperature must be controlled during this stage and during stabilization to help microbes efficiently consume organic matter and reduce processing time. Optimum temperatures range between 40 and 50 degrees Celsius.
Temperature is controlled by removing excess heat via forced aeration and, to a lesser extent, by turning the material and adjusting pile size.
As digestible material is depleted and decomposition progresses, remaining compounds break down more slowly, heat generation slows and the need for temperature control tapers.
* Moisture. Correct moisture sustains biological activity. If the moisture content falls below 45 percent, microbial activity slows or stops. Moisture above 60 percent may block air transfer to the microbes, causing anaerobic conditions.
Moisture is controlled by adding water to the material and mixing it thoroughly and uniformly by periodic mechanical turning or constant agitation in the composting vessel. Excess moisture is removed through evaporation by increasing aeration and turning frequency.
During curing, moisture content should be held at 50 percent to sustain a low level of microbial activity; before refining, moisture content may be allowed to fall to 40-45 percent. Finished, cured compost should contain approximately 40 percent moisture, unless markets specify otherwise. Compost that is too dry can be difficult to remoisten and can create excessive dust.
* Pathogen Destruction. When a high enough temperature is maintained, pathogens, or disease-causing organisms, are destroyed. Most operators will destroy pathogens during high-rate decomposition, when more energy and nutrients are available. The typical pathogen-destruction method is a time-and-temperature process where compost is allowed to reach temperatures above 55 degrees Celsius for three to 15 days.
* Turning and Mixing. Compost must be turned and mixed throughout high-rate decomposition and stabilization to blend water; to break up clumps that can become anaerobic, too wet or too dry; to kill fly larvae near the material's surface; and to evenly distribute the materials to produce a uniform product, according to market specifications.
* Refining, Storing And Packaging. When fully stabilized but before marketing, compost is usually "refined" by using screens to remove stones, glass and film and hard plastics. Oversized organic material like wood can be shredded or returned to the composting process or marketed as mulch.
Once the product is refined and packaged, on- or off-site storage space should accommodate at least four month's production to allow for seasonal marketing patterns.
In 1990, a year before Iowa's yard waste ban became official, Bluestem Solid Waste Agen-cy set up shop in Cedar Rapids, Io-wa, to gain a foot-hold in the compost industry, said Dave Hogan, executive director.
The medium-tech composting operation mainly draws its waste from a local paper mill, processing 35,000 tons of mill sludge annually, plus 6,000 tons of fermentation and a small percentage of waste from grain, product and plant spill cleanups. To date, the operation has produced and sold 650,000 bags of compost.
A 30-mill. PVC liner gravel surface was added to Bluestem's eight-acre composting pad last year. The operation runs a SCAT 4832, two dump trucks and three unloaders. To avoid contamination, a Power Screen 830 was re-cently purchased to replace a smaller screen that the operation has outgrown.
Luckily, Hogan said, odor has not been a problem since last year, when added nitrogen quantities were reduced. Because Bluestem is an open site, "we must match the nitro to our capability," he said.
A $15-per-ton tipping fee and cooperative marketing makes the Bluestem operation economically viable. Great River Regional Waste Authority markets Bluestem's material throughout the region.
Economically, Bluestem should continue to thrive, especially now that a new paper mill has opened in the area. The company hopes to permit 20 to 25 acres to handle the increased material.
Gradual expansion and starting slowly are Hogan's recipe for success. "[Start with] yard waste," he said. "Learn the dos and don'ts. Mixed waste is more difficult."
When creating a backyard compost program, "You need to push the right buttons," according to Joe Keyser, an en-vironment education specialist in Mont-gomery County, Md., and "America's Compost King." How? By starting with a survey and learning why people do what they do, or why they don't, when it comes to grasscycling and composting.
For Keyser, his staff of three Depart-ment of Environmental Protection employees and a group of volunteers, this process began in December 1993, when county residents were hit with new regulations prohibiting the landfilling of yard waste.
Meanwhile, the county had set a goal of 50 percent recycling by the year 2000. With 18 percent of the waste coming from yard trimmings, composting and grasscycling appeared to be the answer.
A 1991 survey showed that most residents disposed of yard trimmings with their trash. Only 34 percent did grasscycling and 35 percent did composting. In 1993, the numbers increased to 59 percent and 43 percent respectively, but curbside yard trimmings setout was low.
To focus attention on grass clippings, Keyser began a "carrot-and-stick ap-proach." The county would collect clippings on recycling day - the "stick" - but only if they were packaged in a bin or paper bag marked with a special tag. The "carrot?" Keyser mailed 10 free tags to each of the county's 178,000 single-family homes.
Keyser's group also did a phone survey. He learned that people had been "brainwashed," so to speak, about what makes a healthy lawn. Most believed that leaving grass clippings on the lawn was unhealthy.
To educate the public on how and why to compost and grasscycle, Keyser began an extensive campaign that included direct mail, print, television, transit and radio advertising, workshops and school programs. By the end of 1994, 70 percent of the county was grasscycling.
In 1995, said Keyser, composting "came of age." The county sold composting bins for $5 that hold 20-30 bags of leaves. To date, Keyser said the county has sold 18,000 bins; 95 percent are still being used.
In 1994, 50,000 tons of waste was source-reduced through composting, 27,000 tons of which were grass clippings recycled at home. Last year, 53,000 tons were source-reduced - 60 percent home-composted.
Meanwhile, the compost facility processed nearly 52,000 tons of yard waste in 1994 and more than 57,000 in 1995. Thus, source reduction diverted 49 percent of yard waste from the compost facility.
The 50-percent waste recycling goal is clearly within Montgomery County's reach
* Athey Products Corp. Model 7-20 compost turner/ loader. Contact: Lowell Petty, P.O. Box 669, Raleigh, N.C. 27602. (919) 556-5171. Fax: (919) 556-0122.
* Diamond Z. Tub grinders. Contact: 1102 Franklin Blvd., Nampa, Idaho 83687. (208) 467-6229. Fax: (208) 467-6390.
* Fabric Building Systems. Fabric-tensioned structures for compost facilities. Contact: Tom Ruprecht, 4919 80th Ave., Cir. E., Sarasota, Fla. 34243-4916. (941) 351-6096. Fax: (941) 351-1020.
* Fuel Harvesters Equip-ment Inc. Tub grinders, turners; plastic separators. Contact: Don Green, P.O. Box 7908, Midland, Texas 79708. (915) 694-9988. Fax: (915) 694-9985.
* Jones Manufacturing Co. Tub grinders and hydroforks. Contact: Don Reis, P.O. Box 38, Beemer, Neb. 68716. (402) 528-3861. Fax: (402) 528-3239. E-mail: [email protected] serve.com
* Knight Industrial Divi-sion. Organic compost mixers; trommel screens; spreaders. Contact: Steve Pesik, 1501 W. Seventh Ave., Brodhead, Wis. 53520. (608) 897-2131. Fax: (608) 897-2561. Reference: S&R Compost Services Inc., S 46 W29633 Genesse Rd., Wau-kesha, Wis. 53188. (414) 968-2571. Fax: (414) 968-9438.
* Morbark. Wood/organic waste processing equipment, tub grinders, horizontal grind-ers. Contact: Ken Wagner, P.O. Box 1000, Winn, Mich. 48896. (800) 233-6065. Fax: (517) 866-2280. E-mail: [email protected] Compuserve.com
* Morgan Scientific Inc. Por-table oxygen, carbon dioxide and temperature monitors. Contact: Mike Callahan, 151 Essex St., Haverhill, Mass. 01832. (508) 521-4440. Fax: (508) 521-4445. E-mail: [email protected] morgansci.com
* NaturTech Composting Systems Inc. In-vessel composters. Contact: Jim McNelly, P.O. Box 7444, Saint Cloud, Minn. 56302. (612) 253-6255. Fax: (612) 253-4976. E-mail: [email protected] Ref-erence: Gary Poltz, City Man-ager, Hutchinson, Minn. (320) 587-5151.
* Northshore Mfg. Inc. Built-rite materials handlers. Con-tact: Highway 61, W., P.O. Box 273, Two Harbors, Minn. 55616. (218) 834-5555. Fax: (218) 834-5556.
* Otto Industries Inc. 68- and 95-gallon compost carts. Contact: Ron Cimmino, 12700 General Dr., Charlotte, N.C. 28273. (800) 795-6886. Fax: (704) 588-5250.
* Rexworks Inc. Recycling grinders. Contact: R.C. Car-one, 445 W. Oklahoma Ave., Milwaukee, Wis. 53207. (414) 747-7200. Fax: (414) 747-7345.
* Peterson Pacific Corp. Tub grinders. Contact: 29408 Air-port Rd., P.O. Box 40490, Eu-gene, Ore. 97402. (503) 689-6520. Fax: (503) 689-0804.
* Scat Engineering. Compost turners and mixers. Contact: Pat Lenane, P.O. Box 266, Delhi, Iowa 52223. (800) 843-7228. Fax: (319) 922-2700.
* Toter Inc. 32-gallon to one-cubic-yard roll-out containers. Contact: Sheri Queen, 841 Meacham Rd., Statesville, N.C. 28677. (704) 872-8171. Fax: (704) 878-0734.
* Wildcat Mfg. Composters and trommel screens. Con-tact: Charles Melhoff, High-way 81, P.O. Box 1100, Free-man, S.D. 57029. (605) 925- 4512.