Industrial, high-rate composting: exploiting the power of microbes
Thirty years ago, beyond the entry sign announcing the location of a composting operation, it wasn’t unusual to see a former cow pasture crowded with long rows of rotting yard waste.
Start-up for these primitive facilities was (and still is) relatively cheap. A windrow operation is viewed as simple and attracts owners whose primary goal is to get a facility up and running without investing much in capital.
However, in all but the most arid climates, the Great Outdoors is not that great for the microbes responsible for composting’s biodegradation. Aerobic microbes — the stars of every bona fide composting operation — will only reach peak performance levels if they are protected from the elements, provided with an ample food supply and a bit of water, and live in an environment equivalent to a microbial Goldilocks Zone.
Bring all of these conditions together in one place, and composting doesn’t just happen. It goes gangbusters.
Today’s industrial composting plants and advanced biodegradation systems are designed to do just that, because the realities of high-volume organics recycling often demand more than the typical windrow can provide.
Science-based recycling systems — exploiting the power of microbes
When serving metropolitan areas, composting operations can be expected to recycle everything from fecal-laden yard waste to industrial by-products — in high volumes. These facilities intake and process hundreds of tons each day. The larger operations may be processing 100,000 tons or more per year.
Odors emanating from some of these feedstocks can be unpleasant. The materials can be very wet. A few will carry chemical residues that require an advanced degradation technology to render them safe for reuse as ingredients in soil amendments.
That’s why more modern plants, those tasked with managing multiple types of organics from large geographic regions, are indoor operations. Some may still turn under that roof, but others have kicked it up a notch by employing more advanced systems (i.e., aerated static pile or ASP) instead of turning.
While a windrow tends to plod along, controlled aeration accelerates composting, turning stodgy microbes into sleek degradation athletes. With high stamina and a voracious appetite for all things organic, these Olympians of the microscopic world bring speed, reliability and high performance to an otherwise lackadaisical process.
The industry’s transition from windrow to ASP turbocharged composting, exploiting the power of microbes and giving it the efficiency and predictability required to successfully compete with landfills and incinerators. But this metamorphosis did not result from genetic manipulation, chemical additives or fairy dust — it was simple biology.
That’s it. Not engineering. Not artistry. Just biology, specifically, exploiting the power of microbes.
Prior to some notable research by scientists beginning in the 1950s, folks may have known how to keep a compost pile chugging, but not why their management efforts worked. But once researchers figured out the why, they were able to control the process by giving aerobic microbes exactly what they needed to survive and thrive (air, water, food, temperature) in the right amounts and within ideal ranges.
They discovered composting’s Goldilocks Zone.
By the 1990s, this academic exercise had captured the eye of the commercial sector. With some tweaking to improve efficiency and profitability at scale, a robust, predictable process emerged, one with the ability to cost-effectively recycle high volumes of organics.
But back to those microbes…
After many trials and several errors, industrial composting moved into the waste management mainstream. But to make biology work as the power behind the progress, both designers and facility operators had to grasp, embrace and deploy a few scientific principles.
At the core was a rudimentary understanding of the two broad categories of biodegradation processes – aerobic and anaerobic. Each identifier reflects the environment in which the microbes live.
Aerobic organisms require air and water, but like people, they cannot breathe under water. Conversely, anaerobic microbes are like fish – they’ll die when exposed to air.
Anaerobes live and thrive in much wetter conditions than can be tolerated by aerobes. Both prefer a moderate temperature zone. Anaerobes will die off at around 150 degrees Fahrenheit (F) or 65.6 degrees Celsius (C). While aerobes can tolerate more extreme temperatures, the most active phase of aerobic composting takes place between 55 and 155 degrees F (12.8 to 68 C), with a preferred range of about 122-140 degrees F (50 to 60 C).
Anaerobic fermentation generates methane, which can be a good thing if captured and used for heating, cooking and generating electricity. If not, then it’s a bad thing, a potent greenhouse gas. When anaerobes are at work, certain compounds are created during intermediate degradation stages that result in unpleasant odors. That is why some wet, decaying materials carry an offensive stench — the rotting organic matter has “gone anaerobic.”
But an aerobic process neutralizes odors by creating drier conditions, killing odor-causing anaerobes. Methane is not generated during a well-managed aerobic composting process, and the resulting carbon dioxide emissions are considered carbon-neutral since the gas generation volume is the same as if the materials degraded naturally.
Beneficial bacteria and fungi are among the aerobic microbes that make compost “happen.” About 2,000 species of bacteria and 50 species of fungi are ably aided in their degradation efforts by a zoo of macro-organisms like beetles and worms. However, aerobes are the worker bees of the compost pile. They break down organic matter at the chemical level as opposed to the physical rending of the macros.
Feeding on organic waste, aerobes power the engine that drives moisture from the composting mass, degrades pollutants, and eliminates odors. The enzymatic action associated with aerobic digestion breaks molecular bonds, releasing by-products (heat, water, carbon dioxide) in the form of steam. Once these microbes have consumed all available food, they die fat and happy, their microscopic bodies becoming part of the residual mass.
In a controlled composting process, a temperature drop signals a decline in food supplies and a correlating reduction in microbial populations. Degradation slows, but still continues at the lower temperatures associated with compost curing.
If left to time and nature, organic matter will continue its disintegration until nothing remains. But long before that happens, biodegradation enters a phase where the residual is relatively stable, while still microbiologically active and chock-full of both macro and micronutrients. With its soil-like aroma and appearance, the material is pleasant and easy to use – a critical requirement for any product intended for widespread general use — and really, really good for rebuilding depleted topsoil.
This stuff, of course, is compost.
Microbes just keep going and going and…
When talking microbes, conversion of waste to valuable product is only half the job. Once that compost has been added to soil, the little critters take on even more tasks:
- DEGRADATION OF POLLUTANTS – microbes break down synthetic compounds to neutralize the impact of things like petroleum products and fertilizers/chemicals that can negatively impact both soil and runoff quality.
- IMPROVE NUTRIENT UPTAKE – microbes convert nutrients to plant-available form, making more food available to plants and reducing the need for synthetics.
- IMPROVE DISEASE RESISTANCE — microbial activity is responsible for the plant disease suppression associated with compost use.
The influence of science on facility design
The biggest problem with outdoor operations is not weather, per se, but the fact that weather cannot be controlled.
If a composting mass needs moisture, rainfall can be a welcome addition. While it’s common for the sides of a compost pile to “crust,” discouraging rain infiltration, piles can be flattened and then concaved on top to capture rainfall for slow infiltration over time. In this regard, rainfall can be a compost manufacturer’s friend.
But excess rainwater rolling down the crusted sides of a pile will settle into pools of “black liquor” (a.k.a. leachate) at the base. Leachate and associated runoff contaminate ground and surface waters, attract flies and harbor unpleasant odors. If the pile gets too wet too soon, pathogens rebloom. When composting outdoors, a heavy rainfall can set the stage for nuisance complaints and regulatory intervention.
Conversely, maintaining acceptable processing conditions outdoors during dry spells requires sprinkler systems or a hose brigade if the microbes and the process are to be saved.
Add complications like high winds and ice storms to the mix, and the operation of an outdoor facility becomes more about battling Mother Nature than recycling organics.
Having to reprocess ruined piles and windrows adds cost and retards throughput. When hundreds of tons of waste arrive at the gate each day, a stuttering throughput rate can cause massive pile ups that compound and exacerbate the weaknesses of outdoor facilities.
Exploiting the power of microbes means protecting the creatures from the vagaries of weather is a top priority for modern facility designers. Solutions can range from a shed roof to encapsulation to full facility enclosure. Each rung on the containment ladder offers an elevated level of environmental control and protection, as well as fewer operational complications.
On that list are the elimination of materials handling woes related to weather delays and the ability to capture inside air and processing off-gases for biofiltration. Indoor facilities can also make a composting operation more palatable to the locals by providing visual camouflage and sound buffering.
Making biology work for day-to-day operations
Putting a roof over a composting operation may remove many headaches from the manager’s plate, but design is only as effective as the people running the place. Any composting facility — from the most basic to the most sophisticated — can still run into trouble if mismanaged.
Exploiting the power of microbes requires a multi-faceted strategy.
Feedstocks like food waste and biosolids can be wet and odor-laden when they arrive at a composting facility. One of the top priorities for modern composting operations is to get these types of materials blended with dry amendment and aerated as soon as possible to kill off anaerobes and encourage the proliferation of aerobes.
But if the blend isn’t right, a batch can be doomed before the admixture ever hits the composting pad or aeration floor. Wet or dry pockets impact microbial movement throughout the composting mass. An irregular texture means patchy distribution of target compounds and uneven exposure to the microbes. Pockets of untouched raw waste can survive an otherwise successful process, leading to regeneration of odors and reblooming of pathogens.
Particle size needs to be consistent to achieve an even degradation rate for all blend ingredients. Material placed on the composting pad should not be compacted. Aeration pipes must be free of debris. Windrows may need more turnings than required by regulations to keep the process humming.
Many items on the list of best management practices (BMPs) are common to all composting operations, from backyard to industrial. Many items on the DO list relate to the creation and maintenance of an ideal environment for the microbes responsible for biodegradation. The DON’Ts focus on discouraging of the kind of microbes that cause and perpetuate odors.
But no matter the design or process, people are ultimately responsible for making the science work as it should, keeping those all-important “bugs” happy and ensuring a trouble-free operation.
- A bibliography that includes references to early municipal composting research: https://www.nal.usda.gov/afsic/tracing-evolution-organic-sustainable-agriculture-tesa1945
- About the science of composting: https://web.extension.illinois.edu/homecompost/science.cfm
- About composting’s organisms: http://compost.css.cornell.edu/microorg.html
- About plant disease suppression: https://link.springer.com/article/10.1007/s12600-011-0177-1