April 1, 1995

7 Min Read
Which Waste Processing System Is Best For You?

Marc J. Rogoff and John F. WIlliams

Every move or de-cision made by recycling managers likely is driven by pressure to meet state and local government mandates.

Whether you are starting a recycling program, or expanding a current one, you must consider the short- and long-term objectives of the program. After establishing current per capita waste generation and material recovery and recycling rates, you can match the available technological options with the program needs.

The percentage of a material you want to recycle will in-fluence which technology suits your community.

Mixed Waste Processing Mixed waste processing amounts to a second chance to recover recyclables that have not been captured by source separation. It also can be developed as a part of composting or waste-to-energy programs. Generally capital-intensive, a mixed waste processing system can sort materials by machine, hand or a combination of both.

A system that uses less machinery minimizes capital costs, but requires more labor. Private scrap metal and recycling operations typically use this process since it allows a quick response to marketplace fluctuations. For example, the business can reduce staff and stop processing the material when its markets are down; once the material's market rebounds, start-up costs are minimal.

A typical large-scale system requires employees on the tipping floor to hand-segregate incoming wastes. Large items such as white goods or corrugated paper should be removed to reduce burden depth and handling problems on the processing lines. This process also prevents contaminating the corrugated paper.

A loader is used to feed the remaining wastes to the conveyor lines and, if necessary, materials can still be hand-picked.

High-volume plants often require a bag breaker so that the waste can be inspected. A screening process can separate the pre-sized material from the oversized material and will decrease the burden depth on the conveyor.

Next, the ferrous materials are magnetically separated from the waste stream. The undersized fraction includes cans and bi-metal products; the oversized fraction in-cludes heavy and more valuable ferrous products.

Conveyors then carry both streams past the picking stations for separation. The oversized fraction contains film plastic and PET containers as well as most of the paper, cloth and wood. The undersized fraction contains most of the aluminum cans, glass containers, yard waste, dirt and grit and food wastes. In the end, the separated materials are dropped down chutes for shredding, baling, screening, washing or magnetic cleanup. All residue is landfilled or delivered elsewhere for further processing.

A high-technology material recovery system relies on mechanical equipment to process and recover magnetic ferrous materials, film plastics and paper.

Mechanical equipment can be used to receive and store materials; separate and reduce the size of a material; and classify and recover materials.

Mixed Waste Composting Mixed waste composting and processing systems are similar. Most mixed waste composting facilities use a mechanically-oriented front end system.

Mixed waste composting biologically decomposes the organic portion of the waste stream under controlled conditions. The process produces a dark, humus-like material with a crumbly texture.

Composting can take place under either aerobic (oxygen rich) or anaerobic (no oxygen) conditions. Anaero-bic composting is mechanically moved in a low-temperature, sealed vessel. This process, which requires more time than aerobic composting, also forms methane gas during the biodegradation process. In some third world countries, manure is anaerobically digested and the methane is used for electrical power production.

Aerobic composting, which is the most common method in the United States, requires moisture, oxygen and temperatures as high as 150 de-grees. This process, which decomposes the materials quicker than an anaerobic process, produces carbon dioxide as a by-product.

Most composting systems include four basic steps: * Pre-processing the feedstock. This includes mechanical and/or manually sorting solid waste into or-ganic and inorganic fractions; re-ducing the size of the feedstock; ad-justing the moisture content of the feedstock; and adding nutrients and bulking agents.

* Composting, or digesting the feedstock. Much of the biological ac-tion takes place during this critical step.

* Curing, which completes the biological activity.

* Postprocessing (which may in-volve screening the compost) completes the process and prepares the product for market.

The entire composting process can be completed within six to 15 weeks.

Yard Waste Composting Nationally, yard waste accounts for 15 to 20 percent of the waste stream. The popularity of composting leaves and grass clippings at home has led to the growth of yard waste composting facilities.

Yard waste processing ranges from simply forming the collected leaves into windrows and turning annually to varying combinations of bag re-moval, shredding, windrows with forced aeration and temperature monitoring and control.

The technology that will be used is determined by the quantity of compostable materials, the area available and financial resources.

Many smaller municipalities use public works equipment to collect leaf waste and to form windrows. If the public works department lacks equipment, it may turn the windrow once a year or it may work with the county or other municipalities to share resources.

Grass compost needs to be turned more frequently and produces odors. In addition, it must be sampled for contaminants from lawn fertilizer, pesticides and weed killers. Any traces of lawn chemicals could hamper marketing and limit the uses for the compost.

The yard waste composting process must be controlled in order to maintain the optimum rate of de-composition. This can be accomplished with a feedback control circuit between temperature monitors and blowers and adding moisture regularly. Since leaves are high in carbon, some municipalities add a nitrogen-bearing material to achieve a C/N ratio for rapid composting.

Composting programs can use a low-technology static pile, the win-drow method, a forced aeration method or a variation of the three.

Windrows are the most prevalent method of composting. With this method, the leaves are delivered to the site in bulk and formed into windrows with a self-propelled turning and aeration machine and/or a front end loader.

Although windrow's length is limited by the site's size, the piles are approximately six to 10 feet high and 12 to 25 feet wide. Windrows should be turned and reformed by a front end loader at least twice a year.

This approach is relatively inexpensive and produces compost in a practical amount of time. However, it also requires a large area to complete the composting process.

Unlike the windrow method, static piles do not require turning and have air forced into them by perforated pipes and a forced-draft fan or an induced-draft fan. Front end loaders form, manipulate and load the static windrows.

Static piles require less space due to the large windrows. In addition, forced aeration speeds the composting process. However, this method also has higher start-up costs due to the capital costs and site preparation. Pipe breakage and pump maintenance also contribute to higher op-eration costs.

Wood wastes can be processed in a tub grinder, shredder or portable chipping unit. The material can be sold as mulch for agricultural and landscape applications. Only separate, metal-free yard wastes should be processed as mulch.

Composting and mulch production equipment needs vary. Compost turning equipment costs around $145,000 for larger scale self-propelled machines that operate by straddling the windrows while traveling over them. This equipment uses metal teeth to break up, aerate and redeposit the material in piles.

Smaller versions, which can be mounted on front end loaders, are available for $10,000 to $60,000. The front end loader is basic equipment for windrow composting. Ap-proximate costs range from $50,000 to $100,000 plus $10,000 for the claw attachment which forms win-drows, turns them and moves the finished product. Either track or wheel front end loaders can be used.

Forced aeration equipment in-cludes piping and blowers, which cost approximately $250 for a one- horsepower blower. At least one blower will be required per windrow. Six inch diameter plastic pipe costs about $9 per linear foot installed.

Two tub grinders are available: self-propelled, which can service different sites, and stationary.

C&D Waste Recycling By including construction and demolition wastes (C&D) in community recycling programs, it may be possible to increase the "denominator" on which recycling rates are based. Florida and New Jersey, for example, credit mandated recycling goals for C&D recycling.

C&D waste processing consists of mechanically shredding, grinding or screening. Shredding is used to convert stumps and other wood waste into mulch. Asphalt and concrete are generally processed in grinding machines for size reduction. Screen-ing is used to separate and size wood wastes and hard materials. Magnets are used to recover ferrous materials such as nails and reinforcement bar embedded in the wood and concrete materials.

C&D waste processing systems re-quire conveyors, dust control equipment and rolling stock. The facilities can be enclosed or open. Tipping floors and load-out areas are integral parts of the systems. Some systems are automated while others are transportable.

Many C&D recycling operations are "in-house," or performed by C&D contractors. In addition, some vendors offer full C&D recycling services (design, build, own and operate).

In July, this series will discuss marketing recyclables.

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