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Mechanical or Manual Separation Techniques in Waste Sorting System

A significant issue in the operation of the MRF is the choice between mechanical or manual separation techniques. Older, traditional MRFs rely heavily on manual sorting, which is both very expensive and time consuming when handling large volumes of materials. Labor represents one of the highest cost components of the MRF. The annual personnel cost for just five sorters is equivalent to roughly the amortized cost of a million dollars worth of capital equipment (over 20 years at 10 percent interest). There are trade-offs between operating and capital costs when considering whether to employ manual or mechanical separation processes. Despite these trade-offs, because of the high nature of labor costs, most long-term cost analyses will typically show that automated processing is more cost effective than manual processing.

 

Manual sorting can potentially produce higher quality material recovery, but is inefficient because of relatively low processing rates. Manual sorting also yields more rejected materials and misses a considerable portion of the HDPE and PET plastics waste stream due to the inability to target certain container shapes. If a plastic resin cannot be distinguished with the naked eye, it cannot be efficiently manually sorted and will therefore not be targeted.5 It is extremely difficult for a sorter to distinguish between PVC and PET plastics, but these resins can be separated quickly and accurately using automated systems.

 

In comparison to manual sorting, automated sorting has lower labor costs, greater material recovery and faster processing rates. Automation also has the advantages of reducing the health and safety risks that result from workers handling wastes directly. Furthermore, machines can usually be adjusted to target new materials by just adding new sensors, and can consequently take more from the waste stream as new markets develop.7 This is important for accommodating expansions in the NYC recycling program that increase both the volume and range of recyclable material that need to be processed. 


Most of the private MRFs that the city currently employs are extremely manually intensive, making the materials recovery very costly. City must therefore pay contractors to accept its wastes. The proposed MRF for city will be as automated as possible to increase speed of operation, reduce costs and improve quality. Currently, it is not feasible to have a fully automated MRF, since there are certain automated unit operations that are not well proven and may still be unreliable. For example, there are available automated paper sorting technologies, but manual sorting remains the most reliable way to ensure quality separation. It is important to provide flexibility within the MRF to eventually allow automated technologies to replace manual operations and be integrated into the operation system.



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