Art vs. science: facing the realities of modern industrial composting
Even in this enlightened era of organics recycling, it’s not unusual to read an article or hear a speaker discussing the “art” of composting.
Once upon a time, composting really was an art, practiced by enthusiasts on a residential or community scale. These folks came from all walks of life. Trial and error, intuition and creativity saw many types of biodegradable wastes successfully recycled as compost.
So much so, that the last decade of the 20th century sparked a thought transition in mainstream waste management that inspired larger, more sophisticated composting operations. Many of these facilities were in the municipal sector, paid for by taxpayers.
Unfortunately, with engineering as the guiding force behind design and operation of many of these big facilities, the artistry that worked so well for smaller operations was not always successful when attempting scale-up for the waste management mainstream. Some of the early technologies impeded the biological process responsible for making composting “happen.”
Some of those early high-volume facilities failed, proven too unreliable, problematic and/or expensive for managing waste streams on a municipal or industrial scale.
But developing parallel to these engineering-centric operations were modern industrial composting facilities that put science – specifically, biology – in the driver’s seat. This paradigm shift, built on the science-based research of some notable universities in the 1970s, changed the industry.
As a result, some of the largest composting operations today are indoor facilities using advanced systems and technologies that put science in charge. Science offers a more predictable and reliable approach for organics waste management in the new millennium.
No art, just science for modern industrial composting
There is no art involved in composting if one understands the science.
When walking through a natural setting, scrape back the decaying leaves to reveal the underlying humic layer. That layer consists of decomposed organic matter created from the organics that landed on the forest floor and were consumed and degraded by the creatures living there.
In nature, biodegradation takes place when things like microbes and earthworms feed on dead animals, droppings, fallen leaves and other organic materials. Enzymes are released during feeding, breaking molecular bonds.
Complex compounds become simple compounds. Simple compounds become molecules. Molecules become elements that are (eventually) reduced to atoms.
At some point between the complex compounds and the atoms, organic matter reaches a state of decomposition where toxins are neutralized and identifiable remains of flora or fauna no longer exist. Organic matter at this stage of decomposition has an even consistency, earthy aroma and is packed with soil-beneficial microorganisms, as well as macro and micronutrients.
When manufactured by nature, this residual is humus. When manufactured by humans, it’s compost.
Running hot and cold
In the forest, nature is continually recycling, preparing the soil to support the next generation of saplings and maintain the existing tree canopy. The natural system is an effective one, but it is s-l-o-w and only works when the detritus degrades in place.
For example, a pile of yard waste, if left to its own devices, can take several years to degrade to the point where the twigs and small limbs have disappeared and the resulting organic matter can be raked up for use elsewhere in the yard or garden.
Even cut grass, when piled into a heap, can take a while to decompose if the job is left entirely to nature.
During biodegradation, the feeding activity of aerobic microbes releases heat, water and carbon dioxide (CO2). These by-products of decomposition can be seen rising from a composting mass in the form of steam. The pile will be warm to the touch.
When unchecked, the heat build-up can exceed the tolerance level for the aerobic microorganisms that make composting “happen.” They die, and the system crashes. There will be little composting activity until aerobic populations regrow and temperatures return to the ideal zone. But as before, the heat will continue to build. The pile becomes too hot, kills off the microbes, and the composting process must restart yet again.
Eventually, the microbes will decompose the entire pile, but not before running through this hot-cold cycle many times.
Temperatures and moisture levels outside the aerobic comfort zone also encourage the proliferation of the wrong kind of microorganism – anaerobic – the microbes associated with odor generation. That’s why some sludges, putrefied meats and other wastes smell as they decay … they’ve gone anaerobic.
Anaerobic systems have their uses, but producing a pleasant-to-use, easy-to-manage, eminently marketable product isn’t one of them.
The objective of any modern industrial composting system is to create and maintain a processing environment that encourages and maintains aerobic microbial populations. The more successful a system is at achieving that goal, the faster the rate of decomposition.
A successful composting system is just that – a system. Design, technology, and management combine to deliver a workable, reliable method for controlling decomposition.
Controlled decomposition makes organic recycling possible
In a forest setting, the time it takes for organic matter to degrade isn’t an issue. But that’s not the case in developed areas where human activity has not only cut down forests, but also stripped away topsoil through agriculture, construction, and erosion.
Here, nature’s recycling system is broken. The natural soil replenishment system no longer exists. Fortunately, while people may have disrupted that system, people also have the ability to fix it by recycling organic wastes.
However, for both aesthetic and health reasons, urban organics cannot be allowed to simply pile up while nature takes its course. Waste generation rates and volumes are too high, space too limited, and the putrefaction factor too great.
To recycle this organic matter back to depleted farmlands, parks, sports fields and lawns, the material must be stabilized and converted to a form that allows for easy, inoffensive transport and application.
And that’s exactly what modern industrial composting does.
Food scraps, natural-fiber textiles, yard waste, agricultural waste and many other by-products of human habitation can be safely and quickly recycled back to the soil as compost products. In fact, as much as 70 percent of the global waste stream is compostable.
High-rate composting is a very efficient waste management technology, as different from outdoor windrow composting as the horse and dray is to a tractor-trailer rig. Environmentally, it’s far better than disposal options that bury or burn organic matter. In many communities, composting may be cheaper than disposal, too.
Composting at an industrial scale can be more cost-effective than anaerobic digestion or waste-to-energy combustion (WTE), since revenue from the sale of energy may not cover capital and operational costs. The profitability of composting, on the other hand, is proven through decades of commercial success without subsidies.
How scientific, high-rate composting works
Humans can manage the natural degradation process to speed it up and ensure all materials within the composting mass are degrading at the same rate. This makes recycling via industrial composting a practical option for mainstream organic waste management.
When done correctly, a high level of degradation control results in a safe, easy-to-handle product with real market value.
In modern, high-rate composting, critical design and management objectives focus on the creation and maintenance of an ideal environment for the specific microorganisms responsible for biodegradation. Physical plant, equipment, management protocols and composting technologies combine to achieve the optimum balance of —
- Food for the microbes responsible for biodegradation — provided by a blend of organic wastes
- Water — inherent in the waste material or moisture added/extracted to achieve desired moisture levels
- Porosity — to allow free flow of air through the composting mass to facilitate heat extraction and provide transportation channels for moisture and microbes
- Homogeneity – resulting from a thorough mixing of all feedstocks to ensure even texture, particle size and materials distribution without lumps, clumps or swirls
- Temperatures — maintained within an ideal zone for the specific aerobic microbes responsible for biodegradation
- Oxygen — typically delivered during the same turning or aeration that moderates temperatures
Key stages of high-rate compost production
On any given day, the main objective of a modern industrial composting facility is to intake multiple loads of disparate waste materials, prepare them for processing, move admixture systematically through the plant, and load outbound trucks with a uniform, stable soil amendment.
If the initial feedstock blend isn’t right, the process suffers. If processed incorrectly, the batch won’t meet state and federal requirements for vector and pathogen control. If fresh compost is not finished properly, the end product lacks market value. If care isn’t taken at every stage to mitigate odors and eliminate leachate, regulatory and public relations headaches follow.
To meet processing goals, a facility and its operations must be designed (and managed) to maximize efficiencies and minimize breakdowns, delays and human error. The more successful operations do this well. The less successful operations do not.
And, yes, it is that simple.
Composting done right – at high volumes – is not easy. That’s why so many of the more successful industrial-scale operations use both science and engineering to preempt or minimize the most common ills plaguing composting facilities.
Scientific principles must take the lead every step of the way.
PHASE 1 — BLENDING
Blending is the first step toward successful composting. If the blend is wrong, nothing else will go right. Regulations will sometimes focus on the C:N (carbon to nitrogen) ratio of a blend, but that’s just one consideration when prepping materials for composting.
The ideal blend will target and meet criteria that include:
- Homogeneity. All ingredients should be evenly disbursed throughout the blended batch. No pockets of dry amendment. No clumps of wet ingredients. It takes a blend of both wet and dry, carbon and nitrogen, to compost effectively. Unfortunately, if the distribution is haphazard, the rate of decomposition will suffer. In many cases, blending is sufficient to neutralize odors from intake of highly putrescible materials. But if that blend is inconsistent, pockets of odorous compounds can persist long after degradation of surrounding material has begun.
- Uniform particle size. Shredding and chopping to reduce all blend constituents to approximately the same size will help equalize the rate of decay, speeding up the composting process. Reducing the size of larger particulates exposes more surface area to the microbes.
- Porosity. Pore space provides transportation pathways for air, moisture and microbes. If pore space is not uniform, temperatures can be uneven, some spots can be drier than others and microbes may not have equal access to all areas of the composting admixture. This can slow the process and/or result in a compost batch that does not meet regulatory standards for pathogen kill or customer quality standards.
- Moisture. Like humans, microbes require water. Also like humans, too much or too little water can wreak havoc. The ideal moisture level for composting should be in the 50-60 percent range after blending.
- C:N ratio. If raw oatmeal were 100 percent carbon and juiced wheat grass 100 percent nitrogen (they’re not), it would take 30 cups of oatmeal for every cup of wheat grass juice to build a breakfast balanced for composting. Different wastes have different C:N ratios. It is important to determine the C:N ratio of each ingredient, then build the batch recipe. Computer modeling or manual calculations will ensure the batch recipe is as close to the perfect 30:1 as possible. Failing to hit the target will negatively influence the process. If carbon is too high, the process slows. If the batch smells, there’s too much nitrogen. Understanding the science behind C:N ratios and its impact on achieving blending goals for every batch, as well as profitability, is essential to modern industrial composting. When feedstock types and volumes available for blending can fluctuate daily, the ability to modify recipes on-the-fly is a critical skill for the people responsible for blending batch formulations. (Read the post “Calculating C:N ratios.”)
PHASE 2 – PRIMARY PROCESSING
The basics of composting are the same no matter the method:
- Achieve critical mass. A pile of insufficient mass won’t hold heat. A minimum in-vessel system should use containers holding at least 1 cubic yard (3 ft. x 3 ft. x 3 ft.). Typically, municipal and commercial facilities are large, so meeting minimum mass requirements is not a problem. But for smaller facilities with insufficient intake volumes to make up at least one processing batch per day, putrescibles can be temporarily stockpiled on an impermeable surface using dry, finished compost as the foundation layer and cover. The bottom layer acts as an absorbent to eliminate leachate while the cover functions as a biofilter to capture/degrade airborne pollutants and mitigate odors.
- Placing the blended admixture. Allowing the pile to compact will impede air flow. Don’t pack admixture into a vessel or use hydraulic compactors or do anything else that will eliminate the pore space created during blending. Don’t allow loaders to drive over the admixture once it has been placed.
- Air flow and moisture. Too much air and moisture can be just as bad as not enough. Constantly monitor. Adjust air flow and add water, as needed. In an industrial-scale operation, this could require automated air delivery and watering systems with sensors buried within the composting mass. At smaller facilities, manual temperature probes and a hose may get the job done.
- Temperatures. Keep temperatures within the zone. Folks who don’t understand the science tend to jump for joy when composting temperatures shoot through the roof. But a “hot” pile is not a good thing if it exceeds the tolerance level for aerobic microbes and kills the process. Stringent temperature control is a critical management objective during composting. Maintaining temperatures within the desired range eliminates hot-cold cycling to ensure rapid biodegradation. Cornell University says 149 degrees Fahrenheit/F (65 degrees Celsius/C) is the ceiling for composting and recommends 104-140 F (40-60 C) during the thermophilic phase of rapid decomposition. Both the U.S. and E.U. have specific time/temperature requirements to ensure pathogen kill and the destruction of weed seeds.
Relying on periodic manual temperature readings can delay detection of heat buildup during critical phases of composting, adding to the processing time required to meet regulatory pathogen kill requirements. The use of electronic sensors can provide immediate feedback, allowing workers to adjust fans or turn the pile to maintain ideal conditions.
Computerized control systems automate and adjust air delivery 24 hours a day, 7 days a week, even when workers are not on site. This can shave days off the duration of the thermophilic phase and move fresh compost to curing much faster. When composting high volumes, the ability to process more in the same space is a competitive advantage and a boon to profitability.
Automating to prevent excess heat build-up also diminishes the likelihood of spontaneous combustion.
Climate control. Many modern industrial composting facilities have transitioned to covered or indoor operations to combat the biggest enemy of process control — the weather. It’s difficult to efficiently manage a process that’s exposed to dramatic fluctuations in ambient conditions (wind, temperature, precipitation). But by wresting control from Mother Nature, a predictable, robust composting process is possible year-round, even in wet and/or cold climates.
In a tightly-controlled, aerated system, a temperature drop usually indicates the exhaustion of food supplies, marking the end of primary composting. For passive static or windrow systems, however – especially in the first few days — it’s more likely a signal to turn the pile to moderate the temperature, restore oxygen supplies and/or supply more food.
PHASE 3 – CURING
As the food supply wanes, so do thermophilic microbial populations. Feeding activity slows. Temperatures drop. For passive static piles and windrows, moving or turning no longer raises temperatures. While some slight odor may linger, the pile is uniform in color and density. The original feedstocks have decayed and disappeared.
This period of stasis heralds the beginning of the mesophilic or curing phase of composting. Mesophilic organisms thrive in temperatures between 70 and 90 degrees F (~21–32 degrees C).
Moving the compost from primary processing to a curing pad refreshes oxygen and breaks up clumps which may have formed as the composting mass settled. Most often, fresh compost is placed in banks or windrows and allowed to sit for weeks or months until fully stable.
However, like primary composting, aerated curing with attention to temperature and moisture levels can speed up this biodegradation phase, too.
Stability vs. maturity. The goal of curing is to arrive at a state where compost is both stable and mature … and these terms are not interchangeable.
When degradation slows, a compost can stabilize. But that stability can result from a lack of air, water or food. Turn or water the pile, giving the microbes a drink, a shot of oxygen or exposure to more food, and robust microbial activity can resume.
This is one of the reasons a dried or dehydrated product will heat up or smell once it has become wet. The product was artificially stabilized by driving out moisture before it reached maturity. Dehydration is not the same as composting.
A compost product can be stable, but still immature. This means the compost has not yet reached a state that ensures the breakdown of target compounds that existed in the original feedstocks or the intermediate compounds formed during biodegradation. These compounds have been known to cause damage to sensitive plants, and they can smell.
Conversely, a mature compost exhibits the same signs of stability, but this state is due to the absence of compounds requiring further degradation. Adding water won’t generate heat, harm plants or give off offensive odors.
Compost can be tested for maturity. While no single method is a hands-down winner, a C:N ratio of 25 or less can indicate a mature compost. (Some may prefer a target C:N ratio target of 10:1.)
Moisture content target for finished compost is 40-50 percent, though drier product can be preferred by some customers to reduce weight and transportation costs.
Other maturity tests gauge:
- Oxygen uptake – an indicator of microbial activity
- Germination – an indicator of the presence of plant-toxic compounds
- Compost temperature relative to ambient – an indicator of microbial activity
- Weight reduction – an indicator of water and CO2 loss, the by-products of microbial activity
The final “test” is, in reality, an assumption: if the compost has cured for at least six months under conditions that promote mesophilic composting, the compost is assumed to be mature.
PHASE 4 – PRODUCT FINISHING
Typically, once a fresh compost has passed all regulatory time/temperature requirements, the material can be and is often sold.
Since most farmers are not bothered by immaturity or lingering odors, the primary market for fresh compost is growers who appreciate the lower price and slightly higher nutrient content of immature compost. An immature compost can also be used for daily landfill cover and biofilter construction.
These markets will often accept the product “as-is” without the need for additional materials handling.
But the markets with the most revenue potential want a mature product that’s loose and friable and of consistent quality.
Screening either before or after curing (or both) results in the ideal soil amendment for high-end markets, one that is dark, evenly textured and smells like soil. Some specialty products, like compost intended for use on active playing fields, can require one or more additional screenings to further reduce particle size.
The benefits of modern industrial composting
When processing hundreds of thousands of tons of putrescible wastes every year, scientific composting offers a number of advantages:
- Less space, higher throughput — more primitive degradation methods require more space to process the same volume as a high-rate system, because facility throughput is much slower. A high-rate, indoor facility can process as much as 10 times the volume of a windrow operation in the same space. The ability to process more within a smaller footprint can be a critical factor to economic success.
- Site closer to high-value markets — suitable sites close to urban populations can be in short supply, and those properties tend to be smaller and can carry price tags in the millions of dollars per acre. A high-rate system can fit on small sites and operate totally indoors with a biofilter or other system add-on to mitigate odors. When combined with berms and other visual barriers, these facilities can fit nicely into urban and suburban environments with industrial zoning.
- Reliability — even a small processing glitch can cause big problems when daily throughput volumes are in the thousands of tons. Managers who understand the science behind the formulation of blending recipes are better equipped to deal with the vagaries of feedstock intake. They can order immediate adjustments to achieve desired moisture and C:N ratios for every processing batch. Like all manufacturing, the inherent systemization of high-rate composting also offers predictability, making it more reliable as a compost production method. Reliable throughput means a facility can tightly schedule both waste intake and compost output with fewer production surprises, promising dependable service to both waste generators and compost customers.
- Economical – high-rate, indoor composting facilities can require a higher upfront investment than open-air operations with the same throughput capacity. But the fact that science-enhanced operations can achieve long-term, commercial success while meeting or exceeding regulatory expectations proves the right combination of design, management and technology makes economic sense. Financial success may be realized through multiple factors, including economies of scale, higher throughput rates, efficiency, dependability, automation, lower maintenance costs, fewer time- and dollar-sucking nuisance issues, and high-value compost products.
- Environmentally superior – Per dollar invested, tightly-built and well-managed composting operations offer a high level of environmental protection:
- Poorly-designed and/or managed composting operations can generate leachate. Organics degrading in a landfill generate leachate that must be managed with add-on capture and disposal systems. Aerated composting systems, combined with good design and sound management protocols, eliminate leachate generation as a management issue.
- Indoor operations allow air from throughout the facility to be scrubbed or treated prior to venting.
- Compared to composting, landfills generate 25 times the amount of greenhouse gas. That’s because composting done right does not produce methane (CH4), a greenhouse gas associated with global warming. Generated during the anaerobic decay of landfilled organics, methane can be managed through the addition of expensive recovery systems. But would it not be better to avoid generating that methane in the first place? Carbon dioxide is generated during both composting and anaerobic decay. However, under these circumstances, CO2 is not considered a greenhouse gas, since it is part of the natural carbon cycle of growth and decay.
- Despite the buzz generated by WTE, in truth, the net calorific value of organic material is relatively low – only 4 MJ/kg compared to 35 for plastics – and not much higher than glass and metal, both with a zero value. (The organic exception is paper, with a net calorific value of 16.) Organic waste tends to be wet, and wet things are hard to burn. Fresh food waste, for example, is 80-90 percent water, and that moisture must be removed before the material will combust. Drying typically demands energy input. So unlike composting, which requires moisture, the water content of organics exacerbates the environmental and economic negatives of combustive disposal.
A new approach for the 21st Century
For millennia, urban populations dumped garbage into unlined landfills and channeled raw sewage to open pits and surface water outfalls – but not anymore.
Until recently, filling valuable landfill space with yard waste or destroying that organic matter in incinerators was thought to be an acceptable practice – but not anymore.
Rather than burying or burning, composting is now viewed as a Best Management Practice (BMP) for biodegradable wastes. But is it smart to continue the use of antiquated composting methods that are the technology equivalent of a mule-drawn hand plow?
As humans move toward a future where urban populations grow unabated and arable land vanishes in the wake, the survival instinct drives societies toward better stewardship of all resources. With that awareness comes a need for biologically-based waste treatment facilities with some real power under the hood – and that’s high-rate composting.
Fast, efficient and cost-competitive, modern industrial composting stands alone as a waste management technology capable of restoring the critical soil functions needed to support life — the only sensible infrastructure investment for managing organics and recycling organic matter.