Advanced BioFuels USA

by Jim Miller and Craig Coker (BioCycle Magazine)  Much of the reject stream becomes a mix of film plastic, food-soiled cardboard and paper, and smaller pieces of HDPE, PET, PP, and metals. Part II  — Preprocessing/depackaging systems are fairly simple in concept, using the “switch” to act on the “code” of the incoming waste stream. There are two outputs, as noted in Part I of this article series: the extract, or recovered organics, which can be further processed by anaerobic digestion (AD) and/or composting, and the rejects, which are film plastic, HDPE (high density polyethylene), PET (polyethylene terephthalate), PP (polypropylene), cardboard, paperboard, aluminum, and steel. Frozen foods can easily end up in the reject stream (and have damaged separator paddles in one installation), so thawing the frozen foods first demands extra labor and space.

As no preprocessing system is 100% efficient, small pieces of packaging will end up in the recovered organics. But it’s possible to reduce contaminants to less than 2% through proper source separation, presorting, and screening. One operator in the Midwest installed a density separator downstream of the depackager to try to remove additional contaminants and found that when they processed Snickers^® Ice Cream Bars, the peanuts ended up rejected by the density separator.

For waste streams that have high percentages of recyclable content, preprocessing systems may be designed to recover the recyclable materials. Optional wash tanks are available for facilities that wish to clean and bale conventional recyclables for sale, but that leads to a wastewater management issue (see infrastructure considerations below). Several operators of depackagers reported having to open cardboard boxes of packaged foods by hand so that the cardboard would remain clean enough to recycle (i.e. not smeared with food).

Much of the reject stream becomes a mix of film plastic, food-soiled cardboard and paper, and smaller pieces of HDPE, PET, PP, and metals. Where materials recovery is not practical or feasible, facilities load this mix into an open-top dumpster for transport to a landfill; for others, the mix can be baled and shipped to a mass-burn waste-to-energy plant for incineration. Missouri Organics Recycling in Kansas City (MO) blends the reject mix with dry shredded pallets as a “refuse-derived fuel” for a nearby cement kiln. One of the authors got a quote from a mass-burn plant to take baled rejects from a proposed preprocessing system at a tip fee of $95/ton, plus 150-mile one-way transport.

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One common theme the authors have heard from those operating preprocessing/depackaging systems is the constant need for cleaning.   READ MORE