Operator Strategies : Sorting Out the Plastic

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Courtesy of BioCycle Magazine


Ideas and products in the Better Bugs category attempt to affect the biological components and biochemistry of processes by altering the mix of microorganisms or enzymes present. After all, biology is the foundation of many, if not most, organics recycling systems.

Pyrophiles: The first example of Better Bugs involves pyrophilic organisms, bugs that can take the heat. This example is not actually a new product but the consequence of one. A relatively recent vertical silo-type in-vessel system is available that maintains the upper zone of the vessel at extremely high composting temperatures (160°F to 185°F or 70° to 85°C). These temperatures are well above the 140°F level considered the upper limit for healthy composting organisms.

However, the developers of this silo system suggest that this is advantageous, offering the following explanation: High temperatures enhance the decomposition of certain compounds, including fats, oils and waxes; the zone is inhabited by extreme-heat tolerant pyrophilic organisms; and these organisms contribute to the decomposition of the feedstocks. The pyrophilic zone is only a segment of the vessel. Like most silo-type reactors, a temperature gradient exists within the vessel. The coolest temperatures occur at the bottom where air is introduced and compost is removed. The highest temperatures are at the top where air exits and fresh feedstocks enter. Thus, after passing through the pyrophilic zone, materials are exposed to more conventional thermophilic temperatures and organisms as they move down the reactor.

Wood Workers: Woody materials are among the most abundant feedstocks and the most difficult to decompose. It simply takes a long time (more than a year) to transform woody feedstocks, with their lignin-bound carbon, into compost (not to be confused with raw wood mulch). In general, decomposing wood requires the help of fungi that can produce enzymes capable of degrading lignin. However, several companies are selling a special combination of microorganisms that reportedly turn woody materials into compost (or composted mulch) much faster then we are normally accustomed to waiting.

This set of bugs (most are bacteria in the Bacillus family) are promoted as having the ability to produce unique enzymes that are long lasting and particularly effective at breaking down carbon compounds. The explanation for their proficiency is that under stressed conditions, such as low oxygen levels, the selected bugs produce the desired enzymes. The enzymes make oxygen molecules available from the surrounding compounds when free oxygen from the air spaces is scarce.

While these microorganisms are purported to be more effective than the otherwise natural populations, they are not magical. It still takes on the order of two months to produce compost, and typical management practices are required, such as grinding wood to six-inches or minus, adding nitrogen (e.g. urea) and maintaining adequate moisture content. In addition, the solution of bugs plus water must be thoroughly mixed into the ground wood. A typical procedure is to grind, or double-grind, the wood feedstock and spray the bugs on the ground wood as it passes on the outlet conveyor. The ground wood is then stacked in passively aerated static piles to decompose until temperatures subside to near ambient levels. (Temperature monitoring also would be a good idea as the large pile size raises the possibility of spontaneous combustion.)

Several companies in various U.S. locations offer the same product, or offer their services using the product, as licensees of the product’s developer. Although these Better Bugs work on nearly any organic feedstock, the initial target applications appear to be yard trimmings, logging and landclearing debris, and other woody materials, in addition to bioremediation.

Composting Inoculants: Inoculants for composting are alive and kicking. Inoculation adds particular organisms at the start or during the process to make composting faster or less troublesome or to alter the compost product. The concept is not new. Based largely on research conducted over 30 years ago, current composting wisdom suggests that inoculants generally provide no advantage to the composting process. Both the feedstocks and the environment contain enough organisms to do the work just as fast and just as well, although exceptions occur with feedstocks, like food, that have been previously sterilized. At least, this is the conventional wisdom.

Despite convention, inoculation products for composting seem to be increasing and gaining interest. They are integral to several composting “philosophies,” including Advanced Composting Systems, Biodynamic Composting, Controlled Microbial Composting and Kyuesi Nature Farming. Furthermore, some credible composting facility operators say that they have observed differences when inoculants are used, although they may not consistently use them.

Given that research has shown no advantage, why are inoculants still being marketed and used? Proponents cite several possible reasons. First, the key research arguing against inoculation occurred over 30 years ago. In the meantime, proponents claim, better products and better ways of handling them have made inoculation more effective. In particular, turning machines that add water (with inoculants) while turning have improved distribution of the inoculated organisms plus improved conditions for their proliferation. There also is better guidance about composting recipes and conditions that sustain the inoculated organisms (e.g. including clay soil). Other proponents respond that the primary effect of inoculants is to improve the quality/characteristics of the compost product (e.g. texture, nutrient content, maturity), not the composting process. They argue that past research focused on the process and the product effects were neglected. Some advocates expect composting inoculants to become as well accepted as inoculants for farm silage, which also were once deemed unnecessary.

Even if composting inoculants have a positive effect, they certainly are not a requisite to producing good compost. Therefore, outside of the philosophical reasons for using inoculants, their prolonged use will depend on whether the perceived benefits are greater than the purchase and handling costs.

The more recent development concerning inoculation is adding specific organisms to compost, rather than the feedstocks. In most cases, compost is inoculated with specific beneficial organisms to improve its disease suppressive qualities, or make disease suppression more consistent. Once the compost cools to mesophilic temperatures and matures, it can serve as a carrier for disease suppressing microorganisms and also provides the organisms nutrients and a proper micro-environment.

Many of the added organisms are proprietary. Because of regulations that govern plant protection and microbial products, bringing inoculated compost to market takes time. Efforts to develop such products are continuing. They should emerge on the market within the next few years.


Better Mousetraps encompass products, practices, and systems that offer mechanical, chemical or structural innovations. They may include biological components but, unlike the Better Bugs, the innovations lie outside the biological processes. There are many Better Mousetraps promising superior, faster, or cheaper processing of organic residuals. Better Mousetraps are diverse and can be complicated. Some qualify as “black boxes”, systems that are somewhat mysterious because the supplier cannot or won’t reveal details of how they work. Therefore it is difficult to judge whether they are legitimate. The following examples are a small and selected sampling of current Better Mousetraps.

Micronized Compost: An example of a recent compost product innovation is micronized compost. It is a finely ground compost with a consistency similar to talcum powder. It is sold in bags, buckets and even jars. The small particle size of the compost makes it possible to carry and apply it with water. Thus, compost can be delivered through irrigation systems. The product also can be used to make compost teas at the application site, saving the cost of transporting water. Micronized compost has been called soluble compost, but it more likely a suspension of fine particles rather than a true solution, so periodic agitation is necessary.

The Better Mousetrap aspect of micronized compost is how it is made. The challenge is to grind compost to a small particle size without creating excessive heat that would kill the microorganisms. The manufacturing process is a guarded black box, but it appears that size reduction occurs with high velocity air movement, which also carries away heat. Suppliers of micronized compost cite laboratory tests showing that the product contains a greater density of microorganisms than the unprocessed product due to the increased surface area. Processing compost into micronized particles adds to the cost of the product, but it opens up new applications and market niches.

Pellets, Compost and Otherwise: While the micronizing process breaks compost particles apart to create a consistent product, pelletizing achieves uniform consistency by assembling small particles into larger ones. Because a consistent texture aids in transportation and application, pelletized compost is an emerging product, made possible by Better Mousetraps in forming pellets.

Various materials, from sawdust to biosolids, have been pelletized for many years, but the practice is relatively new for compost. Again, the challenge is to form compost pellets without heating the material to microbe-killing temperatures. The challenge is made more difficult because the compost must be reduced to small particles and dried before being formed into pellets. Typically, pellets are formed by extruding, compressing, rolling, tumbling or some other method of aggregating particles together, usually with the aid of a binder like molasses. Extruding and compressing create a lot of heat so some other form of aggregation, like tumbling, is more appropriate for forming compost pellets.

In addition to compost, there are other emerging pelletized soil fertility products derived from organic residues. In part, development is being driven by an overabundance of manure, poultry litter and biosolids. The Better Mousetraps associated with these products are in the various combinations of operations and processes used to manufacture them, including pelletizing, size reduction, drying, blending, and/or chemical stabilization.

Drums, Plus Boxes And Tunnels: For many years, rotating drum composting reactors have been used by backyard composters and at large mixed waste composting facilities. More recently, there has been a proliferation of rotating drum systems that suit applications between these extremes, such as farms and institutions that generate food residuals. At least four moderate-sized drum systems are available commercially. The drums represent Better Mousetraps because of their scale of operation and suitability to composting feedstocks at the point of generation. The development of these rotating drum systems follows the growing interest in composting manure and food residuals in contained environments that provide closer odor control.

Similarly, other contained composting systems, including aerated boxes and tunnels, also qualify as Better Mousetraps. Like drums, they contain the composting environment and its associated odors and make on-site composting of difficult feedstocks a possibility.

Most, but not all, rotating drum systems are characterized by very short retention times, typically less than five days. Some proponents claim this is sufficient time to produce usable compost, and some supply the results of maturity tests to support their claims. However, maturity tests can be misleading and much depends on how the compost is to be used. In general, such short retention times rarely produce a stable product suitable for general horticultural use. The composting periods typical of other aerated containers are longer than those associated with drums but often, they still fall short of producing mature compost. A secondary stage of composting or an extended curing phase is normally used. But, the drums and containers do produce a material that has been substantially decomposed and, therefore, has a much lower odor potential. Odor research has demonstrated that most odorous compounds are released early in the composting process. The material produced by a rotating drum or aerated container system can be finished by a less intensive and less expensive composting method like windrows or aerated piles with less risk of odors.

Oxygen Injection: One of the newer commercial rotating drum systems offers another innovative Better Mousetrap, oxygen injection. Rather than relying on air to deliver oxygen, this particular system, which is just entering the market, uses a pressure swing adsorption (PSA) oxygen generator to supply nearly pure oxygen to the materials in the reactor. Oxygen injection does not necessarily require a rotating drum, but an enclosed environmentally-controlled reactor is necessary to maintain the high oxygen atmosphere.

Does supplying nearly pure oxygen speed up the process? In some respects, the practice is doubtful. For forced aerated composting methods, it is well accepted that air flow rates are determined by the need for cooling rather than supplying oxygen to the process. In other words, if enough air is provided to keep the materials from overheating, there is more than enough oxygen available for aerobic decomposition. Thus, if the reactor must be cooled by exchanging air, injecting oxygen is unnecessary and the oxygen may be flushed out. It is possible that injecting oxygen has other effects such as improving diffusion of oxygen into the mass of materials (by increasing the oxygen gradient) or by altering the microbial population. In any case, the system’s developers say they are seeing faster composting rates plus other advantages in their pilot projects.

Passive Aeration: Recent developments in the composting industry suggest a renewed interest in passive aeration. Several new composting systems have chosen to supply air without fans, incorporating different twists that seek to make passive air exchange more reliable. Many innovations are evident in backyard and other small-scale applications, but even large-scale systems are trying Better Mousetraps for passive aeration. For instance, the vertical silo system discussed earlier is a passively aerated vessel. The high, pyrophilic temperatures at the top of the reactor increase the driving force for thermal convection. Another relatively new passively aerated silo system contains composting materials in a series of narrow wire mesh cages. Each cage is about four feet wide and separated from adjacent cages by an air channel. Therefore, material in the cages is at most only two feet from a fresh air source.

Although the Passively Aerated Windrow Systems (PAWS) of composting has been in use for many years, the basic approach continues to evolve. The original procedure used perforated pipes across the base of the windrow to draw in air by thermal convection. An adaptation of that approach replaced the pipes with a hollow concrete slab with slots on top to distribute air to the overlying materials. A more recent and simplifying modification eliminates the pipes and concrete slab, and relies only on a base of coarse organic materials, like wood chips, to distribute fresh air to the pile. This modification has been termed the Naturally Aerated Static Pile (NASP) method.

While passive aeration seems to be “hot,” the quest for Better Mousetraps is not limited to passive mechanisms. Innovations in forced aeration are occurring also, including better air distribution methods (plenums and orifices), improved control strategies and devices, cheaper valves, air recirculation and biofiltration. Similarly, Better Mousetraps continue to be developed for windrow turners and agitated beds with innovations that affect turning effectiveness, ease of transportation, the ability to add water, automated operation, and operator comfort.

Composting Appliances: In Japan, consumer products companies offer composting appliances intended to turn household food scraps, mixed with sawdust, into a usable soil amendment. Kitchen composting units have yet to rock the US consumer market (although there is a food grinder promoted as a kitchen composting device). Some experts familiar with the Japanese composting appliances say these units are little more than grinders and dryers. After the processed mixture of food and sawdust becomes wet, it develops odors and flies just like raw food scraps. This is not surprising since converting food into something close to compost is difficult in the tight space and short time frame afforded by a household appliance. Nevertheless, the idea is appealing, given that home composting is the most cost-effective means of recycling household organic materials. Perhaps there is some merit to an appliance that can partially stabilize food and other household organic residues and make them less “icky” and less laborious to add to the backyard compost pile.

 Many More Mousetraps: Many interesting attempts at creating Better Mousetraps for recycling organic residuals exist. A few additional examples include: aerobic fermentation (an oxymoron?) units that produce a stable soil amendment in a few days; a reactor that stabilizes or partially stabilizes manure using an electric current; steam treatment for preliminary breakdown and volume reduction; novel separation and size reduction techniques; and a variety of chemical treatment processes that stabilize or temporarily stabilize organic materials.


The Better Bugs and Better Mousetraps described in this article are interesting, but they are not necessarily better. It remains to be seen whether they will succeed technologically and economically.

The search for Better Bugs and Better Mousetraps is never exhausted. I have omitted many of the innovations that I am aware of and there are many more innovations of which I am unaware. Furthermore, because the search for Better Bugs and Better Mousetraps never stops, this article will be outdated soon. New Bugs and Mousetraps will emerge at conferences, in news releases, and in the pages of trade magazines and journals. Columns like BioCycle’s Industry News are a good place to hunt for them. We’ll be back in coming issues to report another round of developments.

By Robert Rynk

Sorting Out the Plastic

Plastics are, without a doubt, the organics recyclers’ Catch-22. They remain, for many types of generators, a convenient collection container, and for others, like grocery stores and restaurants, something too costly to sort out. So processors are faced with the tough choice of not accepting as much organics as might be available from a source, or dealing with the plastic at their recycling or composting facility. Use of plastic products made from biodegradable polymers is a good step in the right direction, especially for programs diverting organics from households, restaurants and institutional food service programs. In the case of grocery stores, however, the problem has more to do with compostable food prepackaged in plastic, such as precut vegetables, salads and breads.

Some processors, especially composters, will not touch plastics with a ten-foot pole, either because they don’t want to take a chance at compromising the quality of their finished product, aren’t able to invest in equipment to separate out plastic, and/or don’t want to deal with the litter and other nuisance factors. Other processors, especially ones who want to service generators or municipal programs that have plastic tied up in some way with their organics, are learning how to optimize recovery and minimize contamination and other negative effects.


The Sumter County, Florida materials recovery and municipal solid waste composting facility receives almost all of its feedstocks, about 80 tons/day, in plastic bags (actually, often several layers of plastic bags, e.g., a bathroom bag inside a kitchen bag inside of a 30 gallon trash bag). “Film plastics, by weight, are six percent of what we get in,” says Terry Hurst, operator of the Sumter County facility. “Other plastics, not film but also not recyclable, account for three percent by weight.” The facility also gets in about three percent HDPE and PET, and one percent of the other grades of recyclable plastics.

The front-end of the plant has a materials recovery facility (MRF), where recyclables are sorted from the mixed waste stream. After sorting, remaining materials are loaded into a rotating drum to begin the composting process. Feedstocks move through the drum over a three-day period, then are passed through a trommel screen. The unders are composted, then go through a final screening.

There are three primary points in the overall processing layout where plastics are removed. The first is as part of the MRF. “We start dealing with plastic at the first manual sorting station, which is the oversized bulky waste sorting station,” explains Hurst. “Cardboard, noncompostables like carpeting, stringy plastic (e.g. hothouse plastic from growers) and other materials are pulled out. Immediately following that station, the garbage bags go into a Bulk Handling System bag breaker. It is essentially two drums with spikes that rotate. The spikes pull the bags apart and drop the contents onto a conveyor that goes to the main film sorting station.”

Hurst has found that the bag breaker is efficient opening the 30-gallon bags, but the smaller bags tied up and stuffed inside of the larger bags do not always get opened. “When those bags reach the film sorting station, workers have to open them by hand,” he says.

There are four to five people who work at the film sorting station, pulling only film plastic. They remove in excess of 1,000 lbs/hour. “This is the point where the majority of the film plastics get pulled out,” notes Hurst. “The plastics are baled in a vertical baler and disposed off-site.”

Past that station, material passes through an overhead rotating magnet, a disc screen, and then goes to the main sorting line. There are 14 sorting stations, seven on each side, and recyclables are sorted according to market demand. At this time, for example, the market is favoring sorted PET (#1) and HDPE (#2) plastics, and there isn’t a market for grades three through seven. Other times, #1 and #2 plastics can be commingled, along with the other grades.

Hurst estimates that 60 to 70 percent of the film plastic is pulled out between the film plastic and main sorting stations. “A fair amount still gets into the digester because we see it when that material is screened before the windrow composting phase.” Sumter County has a Powerscreen trommel with two-inch screens. Overs from the screen drop into a 100 cy walking floor trailer and are taken to off-site disposal. “We’re getting the bulk of the plastic out with that trommel,” he adds. “The tumbling action in the digester does cause some breakdown in particle size of the film plastic, but after screening, the amount left in the compostable fraction is pretty minute. It’s about one percent by weight going into the windrows.”


The final plastics removal point is a trommel with a three-eighth-inch screen size, used for screening the finished compost. When it comes to removing plastic at this point, a critical “moment” in terms of compost marketing, Hurst has learned that the most important element is optimizing the composting process itself. It starts with getting the right mix of MSW, nitrogen source (biosolids are used primarily) and moisture content. “Everything is relative,” he says. “If we do a good job with the compost recipe, adding the right amount of nitrogen, and don’t overload the digester, we will get accelerated breakdown of the organic material, which improves separation after the first screening. If for some reason the recipe is mixed up, e.g. if there is too much moisture or not enough nitrogen source, there is a visible effect on the product coming out of the digester and thus the effectiveness of the screening. As such, we check the recipe daily in each of the three compartments of the digester. Samples are taken from each compartment and checked for moisture. Basically, our recipe is that for every two tons of sorted MSW, we put in one ton of biosolids or other nitrogen source to bring the C:N ratio down from 50:1 to 25 to 30:1. If we see the moisture level isn’t right, adjustments will be made to the recipe going into the digester.”

Moisture content also is closely monitored during the 15 days of windrow composting. “If material doesn’t go into the windrows at the right moisture content,” he adds, “we will see more organic matter coming out with the plastics on the three-eighth-inch screening. We also can pretty much tell the recipe is right if temperatures are where we need them for accelerated decomposition.”

Moisture content is measured prior to screening as well. If batches are too wet, operators have two options, run the Scarab turner through the windrow to dry it out, or feed the trommel screen more slowly. If the moisture content is too high, it will clog the screen, and more organic matter goes out with the overs, even with the cleanup brush running along the outside of the trommel. Screened compost is used for county public works and landscaping projects.

Ultimately, says Hurst, Sumter County would like to start a program that encourages use of paper bags instead of plastic. “Plastic is entirely incompatible with our system and paper would suit us ideally. For a start, we are looking into obtaining some funding to work with the main food vendors in the county to get stores to bag groceries in paper instead of plastic.”


Across the country in California, composting consultant Matthew Cotton of Integrated Waste Management in Nevada City, California, has found that composting facilities servicing institutional, commercial and/or industrial generators need to be set up to deal with plastics, no matter how much source separation is worked into the diversion program.

“Educating generators and collectors is the first and best line of defense,” he says. “Still, you’re going to get some contaminants.” For example, although residents served by the City and County of San Francisco’s organics collection program are instructed to use paper bags or newspapers, workers still must go through material and pick out whatever plastic they find.

In Cotton’s experience, a trommel is an effective tool for plastics removal, especially if it has a cyclone, a pneumatic separator that vacuums off lighter material. “Even the most sophisticated composting sites don’t spend lots of money on advanced plastic removal technology,” adds Cotton. “They usually find that trommeling is enough. The only problem is that whatever you’re sifting out is removed by size, so you’ll get overs mixed with the piles of plastics. Most sites wind up landfilling that material, although some use it as alternative daily landfill cover.”


Another option for some composters with plastic bag woes, especially those handling food residuals and yard trimmings, is to wholeheartedly accept bags, as long as they’re biodegradable. Alan Chappell of Environmental Concepts and Designs in Battlefield, Missouri has tested and worked with biodegradable bags at a number of operations over the past three years (see “School District Supplies Organics To Commercial Composter,” page 57). For example, he foresees them having an expanded role for a composting client that will take large volumes of yard trimmings from Sedgewick County, Kansas when they are banned from disposal in October, 2001. “Hopefully, through education and being able to offer biodegradable bags on site, we’ll have a lot more control in keeping plastic out,” says Chappell.

He adds there haven’t been problems with biodegradable bags degrading before reaching the composting site, or not breaking down during composting. “They get put through without any change in materials handling,” he explains. “The turner breaks the bags open as part of normal operation; they don’t need to be separately cut, shredded or processed in any way. In 30 days, you virtually can’t find them.”

Certification according to the relatively new American Society of Testing Materials standards (see sidebar) is essential for Chappell to have confidence in a biodegradable bag. “I don’t have to rerun a test to ensure that we’re not bringing residue into someone’s facility,” he says. “If the product is certified, that means it breaks down and leaves nothing behind.”

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