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Our industry is in the midst of a paradigm shift, which is particularly noticeable in California, where state-mandated diversion goals are often overshadowed by even more ambitious municipal goals and requirements. Technological advances are bringing a multitude of diversion alternatives from demonstration phases into field-pilot programs and on up to the initial full-scale commercial enterprises.

With any conversion technology, it is critical that the feedstock consistently meets composition, quality, size and density requirements. As these technologies scale up to receive waste generated by residents and businesses, they often require a significant preprocessing step to ensure that they operate consistently with predictable yields and residues requiring further management.

At the most basic level, three activities need to be coordinated in order to produce a feedstock that is consistent in size, composition, moisture and other qualities. These three activities are collection, preprocessing and conversion.
 

Facility evolution

The Integrated Waste Management Hierarchy adopted by the U.S. Environmental Protection Agency (EPA) is designed to show the most environmentally preferable options for waste management. The hierarchy places emphasis on reducing, reusing, and recycling the majority of wastes.

In order to recycle and reuse as much material as possible and encourage energy recovery over landfilling, waste management infrastructure has adapted to meet customer behavior and end user requirements. As a result, modern material recovery facilities (MRFs) and transfer stations have increasingly overlapping functions.

Traditional transfer stations are primarily used for transferring material from collection trucks to transfer trailers for transport to a landfill or other solid waste management facility. They often have a pit and tunnel for top loading of walking floor trailers. They have limited sorting capability, consisting primarily of manual on-floor sorting for metals, large items or cardboard, which are loaded loose into roll-off containers.

Modern transfer stations have an increased emphasis on diversion.

Technologically advanced transfer stations may have a simple conveyor sort line with a fines screen and manual pick stations for removing metal, wood and cardboard. They often have separate areas for multiple material streams, including a staging area for inspection and grading, bins, drums, bag breakers, presorting conveyors, pick stations, postsort quality control, screening (spinning shafts with stars or discs, trommels and shakers), optical sorting (near infrared, color camera, X-ray), balers, compactors, shredders and grinders. In this way, materials can be transferred to facilities that can recycle, reuse or otherwise derive value from them.

Traditional material recovery facilities (MRFs) perform recovery of material from recyclables such as paper, plastic, glass, aluminum and tin/steel which have been collected from residences and businesses and tipped together at the facility. MRFs can be dual stream—for example, paper and containers set out separately by the generators—or single stream—all accepted materials are commingled when set out by the generators.

In either type of MRF, incline feed conveyors are used to presort the material, and then mechanical screening is conducted to remove and sort paper and cardboard based on material size. Metals are sorted using magnets and eddy currents and plastics are separated using manual positive sorts.

In even the most technologically advanced facilities, manual sorting is conducted along the way for purposes of quality control. The sorted materials are stored in bunkers until they can be conveyed to high density auto-tie balers prior to being shipped to domestic or overseas end users.

Modern MRFs have increased emphasis on handling different material types through different channels. Due to flexible equipment design, they may be able to process commercial and multifamily materials separately from materials collected from single-family homes.

These modern MRFs have upgraded equipment including optical sorting for plastics and paper, and often have preprocessing capability for organics and energy recovery. The systems can be quite complex and include integrated computer software controls to monitor and measure a vast array of functions, which include but are not limited to metal detectors, air systems, cameras, size reducers (shredders), scales, balers, compactors and various throughput measurements.

Over time, the evolution of transfer stations and MRFs has progressed and overlapped toward the generation of facilities that can act as preprocessors to make feedstocks for a myriad of end users. At a preprocessing facility, recyclable, compostable or otherwise recoverable materials are separated, and residues needing to be landfilled are minimized. This reduces the net disposal costs as well as transportation costs to the disposal destination. They may also be otherwise pretreated or screened in order to meet end-user requirements.

To be usable for energy recovery, a preprocessor may choose to follow guidelines set by the U.S. EPA Non-Hazardous Secondary Materials (NHSM) Regulations, found in 40 CFR 241.3 (b)(4).

A fully permitted solid waste facility is capable of receiving all streams of nonhazardous materials and efficiently and safely creating raw materials which meet or exceed end-user requirements with minimal residue.

Under the NHSM regulation, in order for the processed materials to be used as fuel for energy production, facilities must meet:

• Processing criteria – prove that feedstocks are selected and evaluated to meet specific customer requirements and that ongoing product testing occurs; and,
• Legitimacy criteria – show that the material is a useful product and not simply disposed as waste, has a heating value that exceeds 5,000 Btu, and that contaminant levels are comparable to the fuel being displaced.

In order to expand to be a preprocessing facility, existing transfer stations and MRFs need to have fully permitted land preferably near the source of waste. They likely need to modify their buildings and equipment to process the incoming feedstocks to the desired end-market specifications.

The buildings and equipment represent a potential marginal capital investment of $5 million to $15 million, with overall capital investment in the range of $20 million to $50 million. A full greenfield project could be in the range of $50 million to $75 million.
 

Feedstocks and products

At a modern mixed waste processing facility often referred to as a “dirty MRF,” the specifications of both the incoming feedstocks and the desired products must be known. These four incoming and outgoing streams are common:

• Wet (food, green, organic) – Anaerobic digestion or Composting facilities;
• Dry (paper, plastic, wood, metals) – Users of recyclables or refuse-derived fuel for energy recovery;
• Inert(dirt, rocks, concrete, asphalt) – Landfill cover or construction aggregate; and
• Hazardous/Specialty – Specialized hazardous waste destruction facilities or residual heat recovery.

Regulatory compliance

In some parts of the country there is a movement toward establishing numerical standards by which a mixed waste processing facility (MWPF) can be evaluated and determine if it is recovering sufficient quantities of recyclable materials to qualify as being “high performing.”

It is likely that a designated facility will “count” both recycling and diversion level for the local jurisdiction providing materials thereto. For example, a business that is required to recycle and sends its unsorted waste to a high-performing mixed waste processing facility (HPMWPF) would be in compliance with local regulations. It is possible that two standards for evaluating whether a facility is a HPMWPF could include:

1) Aggregate selected materials recovery rate – This is a minimum recovery rate for selected recyclable materials. It is a percentage of selected materials recovered from the total amount of selected materials accepted.

2) Aggregate unrecovered recyclables rate – This is a maximum level of unrecovered recyclables in the residuals. It is a percentage of aggregate unrecovered recyclables that are still in recoverable condition present in the MWPF’s residue.

Some regulatory bodies may establish standards for the above two rates and then monitor them with independent, accredited evaluators, with an initial certification and ongoing annual evaluations. The criteria for the evaluation of these standards should be reviewed periodically in concert with stakeholder input and analysis of the technical capability of various facilities, as well as market conditions that influence the economic practicality of separating material types.
 

Conclusion

Reducing MSW generation is the most effective way to address waste management costs and prevent the use of virgin materials. However, reusing materials in the MSW generated by our economic engines is the second-best method. Capturing the material value of MSW through recycling should be considered next, and after that organic materials should be removed for composting or anaerobic digestion. The remaining materials are defined as residuals.

Energy recovery can make most of these residuals into a resource rather than a waste, and provide an even greater value from those residuals if they can be appropriately and consistently preprocessed to achieve maximum energy recovery. Even if the residuals go to landfill, the preprocessing is still beneficial as it removes greenhouse gas-causing organics and prevents recyclable materials from being wasted.

As solid waste management systems continue to integrate and move up the hierarchy, modern transfer stations, MRFs and preprocessing facilities can bridge the gap between generation and end users, and are critical to deriving the greatest value from the materials.

The author is vice president of Potential Industries Inc., a Wilmington, Calif.-based exporter and end user of recovered fiber, and can be reached at [email protected]. Potential Industries also handles segregated, dry recyclable materials from residential collection programs and commercial and industrial suppliers at numerous facilities in Southern California.

Visual Aid
To see the Integrated Waste Management Hierarchy and essential preprocessing components, visit www.REWmag.com/rew0214/preprocessing.aspx.