Adding forced aeration to a composting process holds the promise of faster stabilization and reduced emissions of odors and VOCs. When ASP’s were introduced in the 1980’s, researchers found that covering the pile with an insulative biolayer raised the temperature of the outermost raw material so it too could pass the time/temperature pathogen reduction requirements. As a result, the addition of a biolayer cover was written into the EPA regulations governing the static composting of biosolids, and subsequently adopted into many state composting regulations concerning pathogen control. As biolayer covers became widespread, air quality regulators began to characterize them as pollution control devices; they appear in many air quality permits. During this evolution, equipment manufacturers such as ECS introduced a series of fabric covers to the market with claims of improving both the composting process and thermal and environmental control (especially control of odor and VOC’s). In this whitepaper we analyze how the physical properties of various covers effect the composting process, work as barriers to air emissions, and impact the operations and economics of a Covered ASP (CASP) composting facility.
The most common approaches to putting the “C” in a large scale CASP are shown in Figure 1.
Of these four, impermeable and fleece covers are increasingly less common. We include them in this discussion because their physical properties illustrate how covers do, or don’t, effect the composting process.
From 2002 to 2012 ECS tested, designed, patented and sold large CASP systems whose covers were made of impermeable fabric. We accumulated a lot of data and experience with them. The covers are essentially large, heavy duty, UV stabilized, truck tarps with small, distributed aeration orifices. We paired them with negative aeration systems to draw air through the orifices and cause a negative pressure under the cover. This sucked the cover down on the pile and effectively eliminated fugitive emissions. While they provided excellent containment, fabric covers in general don’t provide significant levels of insulation and they pose operational challenges.
In addition, other companies have adapted polyethylene silage bags for composting.
Micro-porous covers allow movement of air while forming a waterproof barrier. They are often made with expanded polytetrafluoroethylene (ePTFE) membrane sandwiched between durable polyester fabric layers for stability, and typically have pore sizes between 0.02 and 40 microns. This pore size blocks rain from entering the pile, therefore regulatory agencies may allow runoff to be managed as stormwater rather than leachate.
Some air and gases can travel through the membrane from high pressure to low, including water vapor, but most of the moisture rising from the compost pile condenses on the cool underside of the fabric cover. Some of the odors get trapped in this moisture layer and that provides a scrubbing effect, as volatile organic compounds are more easily absorbed into a cool liquid film. Excess moisture that condenses on the underside of the cover flows toward the ground. Micro-porous systems most often use positive single direction aeration with on-off timer or temperature controls. The CFM/CY of material is typically quite low because of the fabric impermeability (try breathing through the fabric of your winter jacket?). Negative aeration is not commonly used due to very high resistance pulling air through the micro-porous fabric.
While microporous covers are accepted for meeting pathogen kill, the fabric provides little insulation. This can enable pathogens to survive on the surface.
Micro-porous covers are considerably more expensive than other cover types because of the fabric production costs and semi-frequent (often ~6-8 year) replacement cycle.
In addition, its worth noting that microporous covers contain per- and polyfluoroalkyl substances (PFAS), which exist in the production, use, and disposal of PTFE based covers. PFAS are persistent harmful chemicals that are becoming increasingly regulated. As of this publication (2022), we are unaware of a commercially applied PFAS free microporous cover option.
Fleece covers shed rainfall via capillary action within the fabric, which is generally 1/16” thick non-woven polyester. Since fleece is lightweight, it is generally used to cover passively aerated piles or turned windrows during times of rainy weather, then removed during sunny weather, to provide moisture control to the pile. Fleece is not usually used for odor or VOC control, though there is some scrubbing effect due to the moisture in and immediately under the fleece, similar to the micro-porous covers.
Tarp-like fabric can also be used with engineered aeration holes up to 1/16” in diameter. These can be customized for a particular facility and feedstock to keep most of the rainwater out while allowing adequate air flow with adequately designed aeration systems. Macro-porous covers will work in negative as well as positive aeration systems.
Air flows more easily with macro-porous covers. For example, pressure drop might be 0.5” W.C. across a macro-porous cover versus 2 to 3” W.C. across a micro-porous cover at typical aeration rates. But pressure drop differences are much greater at adequate, peak aeration, as discussed in section 4.
Due to challenges with performance (ripping, efficacy), these covers have been become less common in recent years.
Biolayer covers are a layer of relatively stable, pathogen free, organic materials of sufficient porosity, such as post Process to Further Reduce Pathogens (PFRP) compost, unscreened compost or screened overs. This acts as an insulation layer for pathogen destruction as well as a modest surface biofilter. They need to be at least 6 inches deep to function but are often required to be 12 inches in very cold climates and by some regulators. The scrubbing effect varies primarily based on surface temperature and moisture and can range from 10% to 70% VOC reduction. Managing biolayer cover temperature using surface irrigation offers a significant VOC and odor reduction tool.
At a commercial composting facility the biolayer material is typically found/produced on site at no additional cost. In addition, the biolayer cover can be mixed into the compost at the end of primary composting, avoiding the operational cost of removing a cover. Biolayer covers are not required for secondary composting or any stage that has already met PFRP.
Air flow is not noticeably restricted compared to fabric covers and therefore does not increase fan power requirements.
All fabric covers are heavier when wet, covered with snow, and can freeze to the ground in cold conditions. All fabric covers are susceptible to cuts, tears and rips; and depending on the material may or may not be easily repaired. In addition, fabric covers require operator time to deploy at the start of a batch and remove once finished.
Fleece covers usually last 4 to 10 years, perhaps longer with careful management in low-UV locations, but high UV exposure shortens the lifespan. Thus, the covers may deteriorate rapidly in sunny locations. Generally, this type of cover is put on and taken off for storm events or rainy seasons. Retired covers are disposed of in a landfill as re-use opportunities are very limited.
Microporous covers are usually used more continuously as a scrubbing and odor barrier layer. They typically last 5 to 8 years and must also be landfilled at the end of their lifespan.
Biolayer covers usually consist of post PFRP compost or overs found on site at no additional cost and do not require repairs or end of life disposal costs. Since the material comes from the compost process and is integrated into the compost after the primary composting phase the biolayer becomes part of the finished product.
In large installations, the composter must use machines for moving fabric covers on and off the piles. Such machines include sidewinders, straddle winders, winches, or tractor-pulled winders. Figure 2 illustrates the machinery and operation effort for fabric covers.
Bio-layer covers are added on top of piles as they are formed, usually by applying post PFRP compost from a nearby zone or unscreened overs. The operator would scoop this material using a front-end loader, place it on top of the pile-in construction and use the bucket edge to spread out material. This can commonly add 10-30% additional time to the loader operators effort to build a pile depending upon the distance to the biolayer cover media. Figures 3 and 4 illustrate this process.
Table 1 compares typical capital and operational cost of the various cover types used for large commercial composting facilities. Cost of disposal, replacement, and sealing of covers is not included. Microporous covers are considerably more expensive compared to biolayer, fleece, or impermeable covers.
|Capital Expenses |
($/TPY of capacity)
(includes cover cost)
ECS has provided technical services and tools for large-scale composting since 1999. Early in our company history and in response to consumer demand, ECS sold over 400,000 square feet of fabric pile covers between 2004 and 2014. In 2013, ECS began researching all types of fabrics and measuring their fundamental effects on the composting process. We discovered covers tend to limit airflow. Moreover, any benefits from VOC capture efficacy from using the cover were outweighed by the increase in VOC generation due to inhibited process conditions. “[Fabric] Covering, without following composting fundamentals, can lead to emission of more odors than would have otherwise occurred.” (Rynk p582)
Efficient composting results in high stabilization rates and low air emissions for odors and VOCs. This is achieved by having the right initial compost mix and an optimized composting process. A BMP compost mix has:
An optimized composting process needs:
The microorganisms processing the organic matter in the compost need the right range of temperatures, water, and air. As shown in figure 5, compost particles are covered in a layer of moisture and surrounded by air. Aeration brings in fresh oxygen and cooler temperatures, but dries out the mix, so the initial moisture needs to be between 45% and 60%. The compost should then be re-mixed and re-wetted as needed every 10 to 20 days to re-establish porosity and mix uniformity.
In systems without adequate aeration, composters are often concerned about keeping the piles dry to avoid rewetting the finished compost and releasing odor. With adequate aeration, not only is moisture released as part of the biological process, there is significantly less ‘goo’ with potential for odor since it has biodegraded during the process.
Piles should not be higher than nine feet or so, otherwise the pile compacts under its own weight and the porosity becomes too low. The pile must be relatively homogenous to avoid short-circuiting of air paths and uneven aeration within the pile.
Oxygen levels in the pore space should be maintained at greater than 12-15 %. This allows oxygen saturation in the liquid film to be at least 3 ppm, preferably greater than 3.5 ppm. If ambient oxygen drops and/or temperature increase, it becomes more difficult for oxygen to absorb into the liquid film and anaerobic microorganisms start to proliferate. As shown in Table 2, with adequate aeration there is large range of acceptable temperatures for aerobic conditions. Many facilities cycle their fans on and off, but oxygen levels in active piles can quickly deplete, limiting the film level absorption. Therefore, ECS recommends continuous, adequate aeration for maintaining efficient aerobic conditions, which we have found produces the fastest bio-oxidation rates.
Having a cooler, mesophilic phase at the beginning of the composting process is the key to neutralizing odors. Mesophilic bacteria, which thrive in the 20-45°C range, are needed to neutralize the pH. Two to three days of mesophilic temperatures (as shown in the upper charts of Figure 6) can change the pH from 5 to 7.5. Having a mesophilic phase greatly reduces odors and VOCs, as shown in the lower charts of Figure 6. Once pH is neutralized, composting in the thermophilic range is highly efficient (thermophilic bacteria thrive in the 41-75°C range).
As described above, very high aeration rates are needed for the first two or three days to keep the pile cool enough for mesophilic bacteria, which in turn neutralize the pH and therefore greatly reduce odor and VOC production. This typically requires 3-6CFM of air per CY of material. This is especially important with high-energy feedstocks, such as those including food waste. Even with lower-energy feedstocks, such as yard-waste only systems, high aeration rates are needed to maintain oxygenation levels at least 3 ppm in the liquid film.
Microporous covers quickly run into two challenges:
First, the pressures become very high. As more air is forced through tiny pores in the cover, pressure drop across the cover is increased, in approximately a linear relationship to aeration rates. When water condenses on the underside of the fabric, which is desirable for scrubbing, pressure drop is approximately doubled. The aeration needed to stabilize odors may typically require nine times greater airflow than needed for adequate oxygen. At these high rates, pressure drop across the fabric cover becomes very large, perhaps as significant as the rest of the system combined. Figure 7 shows results of pressure drop tests for microporous fabrics.
Note, we tested high flow rates. If we extrapolate these curves to 5CFM/CY, we find an average of ~8.0” WC wet, and 2.0” WC dry.
Biolayer covers, which add another foot to an eight- or nine-foot pile height, create only a small amount of pressure drop. Fleece covers behave similarly, with some pressure drop but much less than microporous fabrics. Figures 8 and 9 show the typical difference between inadequate and adequate blowers, assuming biolayer covers. Blowers need not be oversized, since blowers are connected to multiple aeration floors with a manifold, and typically only one pile needs high aeration rates at any given time.
In order to use fabric covers to prevent excess odor and VOC emissions, they must be adequately sealed around the piles. Yet covers must be removed to agitate and redistribute the material. Without adequate aeration, the VOCS that have formed are released anytime the cover is moved and the compost is agitated. This is when odor issues tend to occur. A quick google search of ‘compost lawsuits’ pulls up examples of this reality (at the time of this writing, 2022, the top results were systems with microporous covers).
Adequate sealing of fabric around the piles can be labor intensive or require specialized sealing equipment (for example, tubes encircling the cover in which water is pumped in and out). Let’s assume, conservatively, that pressure drop is only 2 inches water across the fabric and that the compost pile is fairly small, 20 feet by 70 feet. This causes 14,900 pounds of lift. Adding wind lift and a factor of safety means about 10 tons of weight is needed, evenly distributed around the edge of the pile. Sand is about 2,700 pounds per cubic yard, so that means 7-8 cubic yards of sandbags are needed for each pile and cover, which quickly becomes impractical. Not to mention, trapped VOC’s are released to atmosphere when the cover is removed.
The primary roles of covers for large ASP systems include surface insulation and mitigation of VOC and odor emissions. However, ECS believes the first step toward successful composting should be to optimize the process, which greatly reduces the odors formed in the first place. For low pH feedstocks like food waste, this requires a short period of very high aeration at the beginning of the process to allow mesophilic bacteria to stabilize the pH. For almost all other feedstocks, this requires high aeration to cool the pile and provide optimized oxygen saturation at the liquid film level. In most situations, microporous fabric covers impede adequate aeration due to the large pressure loss across the fabric at these high aeration rates.
Composting with optimized process conditions minimizes odor and VOC generation. This proves to be a much more effective strategy than operating with poor process conditions, which generates high levels of odor and VOC, and attempting to mitigate this with a filter. A biolayer cover, using compost made on site, also has significant scrubbing capabilities, allows high aeration rates, and costs less overall.
Coker, Craig; Gibson, Tom. Design Consideration In Covered Aerated Static Pile Composting [Part III]. BioCycle. May 2013, Vol. 54, No. 5, p. 21
Rynk, Robert et. al. The Composting Handbook: A how-to and why manual for farm, municipal, institutional, and commercial composters. 2022. Compost Research & Education Foundation.