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Recursive recycling is a technique where a function, in order to accomplish a task, calls itself with some part of the task or output from a previous step. In municipal solid waste and waste reclamation processing it is the process of extracting and converting materials from recycled materials derived from the previous step until all subsequent levels of output are extracted or used.
Level 1 recursive
Solid waste or municipal solid waste can be treated, sanitized and separated under steam in a pressure vessel (waste autoclave). Following the processing under steam and removal of toxic materials via condensate filtering, usable recyclables are immediately extracted for reuse (plastics, ferrous metals, aluminum, glass, wood, etc.).
Level 2 recursive
Organic materials from the original waste stream are converted to a fiber using steam at 60 psi and 160 °C. The converted organics (sanitary fiber) is size reduced by 85% and can be used to produce bio-fuels using acidic-hydrolysis or enzymatic-hydrolysis as ethanol or may be used as refuse derived fuel.
Level 3 recursive
Level 4 recursive
Finally, the non-toxic ash from the combusted fiber can be collected and used as a filler for preparation in super concrete and then reused in combination with similar materials (gravel, stones, pottery, glass) to form aggregate for construction materials.
In true recursive recycling and conservation processing the ability to divert all materials in the waste stream from landfill at greater than 99 percent is a concept based on outputs used to provide the next level of processing, reuse, conservation and market delivery of the derivatives.
The concept of recursive recycling has been proved up in small scale facilities (thermal hydrolysis, plasma, etc.) but has not been widely accepted because of the financial impact it may have on existing protocols in waste management. One pilot facility operated commercially in Wales for approximately six years. However, the core equipment was moved to another location while the original facility was scheduled for retrofit. No further information about the facility's capacity or the equipment movement has been made available via open source release.
Since that pilot commercial facility stopped operating, the concept of recursive recycling has not met with as much success as originally anticipated by environmentalists and conservationists. There are a number of companies operating autoclaves with limited success across the globe (the autoclave operating in Anaheim California was de-commissioned in c. late 2007-early 2008) but the full concept of waste treatment using thermal hydrolysis technology has not been fully realized because of several misconceptions in the autoclave and related steam treatment technologies. There is available engineering background to demonstrate successful testing that can be validated in physical production facilities but because of a lack of participation and general knowledge is a closely held secret, the application of technologies to achieve full recursive levels has not been accepted.
A number of companies are working on technology to support this concept. That may bring about change to conservation and recycling when associated advances prove out as successful. However, given the current state related areas of technology the growth to full-scale production appears to be limited because the technology and demonstrating it is available in larger capacities has not been demonstrated commercially.