June 15, 2004 | General

Evaluating Compost And Biofilter Aeration Performance

BioCycle June 2004, Vol. 45, No. 6, p. 20
In a perfect aeration world, every microbe would be supplied with just the right amount of oxygen necessary to optimize its metabolism of the waste material. In reality, air always flows through the bed following the paths of least resistance.
Don Mathsen

Forced aeration of bulk material by means of a blower, aeration floor and associated ductwork is at the heart of an ever-growing number of compost processes. Aerated static piles, in-vessel compost systems, and odor control biofilters all require either the continuous, or intermittent, movement of air through a biologically-active bed of material. Within these processes, the effectiveness and cost of the aeration function are major elements defining the performance of the overall system.
To achieve both optimal performance and economic value in an aeration system, an examination of the fundamental parameters at work is useful. Key questions regarding aeration fundamentals are: What are the ideal conditions for optimal aeration of bulk materials? What are the basic air flow principles at work in achieving these conditions? This discussion focuses on methods of delivering air to, or drawing air from, an “aeration floor” on which the biologically-active material rests. However, the air flow principles that apply to this arrangement hold true for any configuration used to either push (positive aeration) or draw (negative aeration) air through a static bed of bulk material.
Much of the development in aeration floors has come from the commercial biofilter industry that originated in Western Europe and has now attained “best available control technology” (BACT) status throughout much of the U.S. for odor control applications. The lessons learned and the more refined aeration methods developed for biofilter applications can now be applied to aerated compost processes.
In an ideally aerated, aerobic bed of biologically-active material, every microbe would be supplied with just the right amount of oxygen necessary to optimize its metabolism of the waste material. At all times, every microbe would live in just the right moisture environment to transfer nutrients to, and waste products from, its own metabolism. Sufficient air flow would be passing through every interstitial space of the bulk media to maintain the optimum temperature required by the dominant species at any given phase of microbial activity.
In reality, air will always flow through the bed following the paths of least resistance. The total resistance of all interstitial pathways in the direction of flow determines air flow at any point in the bed. The more uniform the porosity and density of the bulk material, the more uniformly distributed are the paths of least resistance. The design challenge is to have these air paths so well distributed throughout the entire bed of material as to enable the entire population of microbes to be exposed to approximately the same aeration conditions. Achieving this distribution depends on two factors: 1) The initial preparation and placement of the bulk material; and 2) The effectiveness of the aeration system.
Aeration flow paths impact the management of oxygen, moisture and temperature within a bed of biologically-active material. Ideally, control of each of these parameters as independent variables would be desirable. In reality, all are integrally linked and become dependent on each other, based on the rate and distribution of air through the bulk media as determined by the paths of least resistance to flow. The key, therefore, is to attain air velocities throughout the bed that are, on the average, the same in both direction and magnitude at every point within the material bed. The key terms are uniform average and steady average flows.
Achieving uniform and steady air flows within the pile/bay or biofilter starts with having a uniform particle size that is placed with sufficient care to assure uniform bulk density. These characteristics are a function of proper material preparation and placement. In the case of a compost facility, this relates to upstream processing through grinding, shredding, and mixing operations. For biofilters, the selection, processing and placement of the media determine these bed characteristics. The subsequent discussion on air flow patterns and various types of aeration systems assumes that proper preplacement and preprocessing of the bed material exists.
The path of air through a bed of uniformly porous material is always a very circuitous route (Figure 1). The actual air path is determined by the resistance to flow created by the sum of the pressure drops through all interstitial spaces in the direction of flow. With uniform particle size and uniform bulk density, the mean vertical pressure drop throughout the entire containment area essentially will be constant and a constant uniform average flow will be approached across every horizontal cross-section of the material bed.
A similar objective is to attain steady average flow in the vertical direction. Localized air velocities are changing constantly in the vertical direction as air makes its way through varying interstitial passageways. Therefore, only steady average vertical velocities can be defined, typically using the measurement “free vertical velocity” – determined by dividing the total air flow supplied to, or drawn from, the bed divided by the total horizontal cross-sectional area of the bed. While recognizing that only an average uniform air flow can be achieved laterally across a bed and only average steady air flow can be achieved in the vertical direction, these parameters become the targets of the aeration system design in order to establish and maintain air flow equilibrium within an aerated bed.
A biofilter or compost bed that has lost its uniform internal air flow is costing its owner money. A disturbed bed (Figure 2) – one that has undergone a change in its internal equilibrium – has formed dominant internal preferential air pathways, or channels, through the bulk material. Once it starts, channeling accelerates and aeration throughout the bed is no longer uniform and process performance, be it a biofilter or a compost system, deteriorates rapidly until reconstitution of the bed is performed. This holds true if air flow is delivered under negative or positive pressure. When excessive channeling occurs, physical removal or restructuring of the material is the only means of restoring the desired air flow distribution and performance.
Assuming proper preparation of the bulk materials has occurred, the aeration floor system must deliver uniform average air velocity vectors at the entrance face of the bed in order to establish uniform average air velocity throughout the bed. If either the introductory velocity vectors or media porosity is not uniform, oxygen feed rates to the microbes are variable and different metabolic rates of digestion occur. This leads to preferential and variable rates of decomposition of the host material, thus creating interstitial air pockets that are growing or collapsing at different rates. In addition, the mass transfer between the moisture in the bed and the air stream will not be uniform, further impacting metabolic activity. It is the cumulative effect of these changing pockets that determines the air distribution paths within the bed and can lead to unbalanced flow.
Temperature variations (either the removal or supply of heat) also are caused by the lack of uniformity in either the introductory velocity vectors or media porosity. As temperature increases or decreases, microbe populations respond by changes in both activity levels and dominant species. Both responses lead to further variations in the breakdown rate of the host material, which results in localized changes in resistance to air flow. Given the dependence of each of the significant functions of the microbes on each other, all driven by aeration levels, the prevention or minimization of channeling is one of the most significant factors in maintaining optimal microbial performance in an aerobic, biologically-active bed of materials.
While there is always channeling occurring at the level of the individual media particles, once channeling occurs at the macroscopic level, the bed has departed from optimal performance and eventually must be physically reconstituted or replaced. In a biofilter, this condition is noticeable by a measurable decrease in odor-removing performance. In a compost process, macroscopic channeling can occur in a very short time during the thermophilic phase due to the rate and volume of physical changes taking place. The creation of pore spaces via microbe consumption of the mass and the collapsing of those pore spaces thereby resulting in formation of a plug – plus the removal of water vapor, all contribute to a dynamic condition within the bed, and all contributing to channel formation. In this case, the effectiveness of the aeration system in maintaining uniform air distribution through the bed impacts both the processing time and the time between reconstituting or turning the bed.
Air must be distributed into a porous bed through an opening in some type of structural member (i.e. screen, pipe orifice, perforated floor, etc.). As the air leaves each opening and begins to dissipate throughout the porous bed (Figure 3), air velocities will initially increase at the discharge surface of the orifice as air is forced into the smaller interstitial spaces. As the air velocity vectors stemming from the orifice dissipate throughout the bed, the average interstitial velocities will decrease until equilibrium is reached at each point in the bed relative to air flow and the resistance to flow along each flow path. Minimizing these velocity changes – and making the critical impact zone of these velocity changes as small as possible – is at the center of prolonging or preventing channeling.
For biofilter applications, the aeration system should be designed to postpone the development of macroscopic channeling for as long a time as possible. For compost applications, channeling will occur in a material bed if it is not reconstituted by mixing or turning. For compost applications, the aeration system can minimize the early formation of pockets that are outside of the desired oxygen, moisture, and temperature conditions of the process thereby extending the interval between reconstitution.
While the truly ideal aerated bed is only a target, the desired features can be approached by careful consideration to the fundamental elements of air flow through the bed as outlined above. The critical design objectives are:
1) Uniform air distribution at the air inlet to the bed – Velocity vectors entering and leaving the bed should be uniform across the inlet and exit faces of the bed.
2) Lowest possible “plenum pressure drop” – To achieve uniform velocity vectors, the pressure across each segment of the bed should be equal. This requires that any pressure gradient induced by the transport of the air through all plenum conduits leading to, or exiting from, the surface of the bed must be as low as possible.
Four other design considerations, not directly related to air flow, are also important: 1) Structural integrity for material handling – While the distributed weight of the bulk material in an aerated bed is usually well under 1,000 lb/ft2, the tire pressure of wheel loaders and other material handling equipment is considerably higher and becomes the loading condition, wherever the materials is placed or reconstituted by wheel loaders; 2) Maintenance and cleaning – Periodic cleaning of surfaces is inherent to biofiltration and forced aeration composting. Ease of maintenance and cleaning is important to the efficiency and cost effectiveness of the aeration system; 3) Corrosion resistance – Use of corrosion resistant materials is critical given the moist, low-pH nature of high-rate biofilter or compost systems, plus the microbial attack against many structural materials. 4) Leachate collection – Leachate produced by the metabolic process occurring in biofilters and compost systems, as well as the adding of water to maintain optimum conditions, means leachate collection and treatment/ disposal/reuse is a critical component of the ideal aeration system.
Fundamental to proper air distribution is the uniform introduction of air being fed into, or withdrawn from, the material bed. This aeration floor or porous support structure must be capable of handling the dead weight of the bed material as well as the pressure exerted on the floor by any material handling equipment.
The perimeter of the aeration floor, however, requires special consideration. For compost and biofilter media not contained on the sides by structural walls, sufficient bed material must extend beyond the perimeter of the aeration surface to prevent short-circuiting of air due to a lower pressure drop at the sides of the bed compared to the center. For compost or biofilter media contained within structural walls, the walls themselves can influence the formation of channels due to the “Coanda Effect.” This is the tendency for a moving fluid – in this case, water-laden odorous air – to attach itself to a surface – in this example, the structural wall – and flow along it, creating uneven drying and collapse of the bed structure in that area. This accelerates the natural decomposition already occurring in the bed of organic material, which causes a differential material breakdown rate between the slow collapse of the organic structure within the bed and the more rapidly decomposing material along the wall, leading to the material falling away from the surface of the wall. In order to address these two compounding effects, design features must be incorporated that artificially increase the resistance to air flow around the perimeter of the bed.
Creating this increased resistance takes two forms. One is the use of an artificial barrier (i.e. curtain or baffle) incorporated at the perimeter of the bed extending from the wall into the bed of material. This curtain creates a longer and, thus, higher resistance path at the wall. The other means is to incorporate a curb, or “dead zone,” between the active aeration surface of the floor and the wall. This also creates a longer, higher-resistance path for the air flow in the region of the wall surface.
While the curtain or “barrier method” provides positive blockage of air flow along the wall, the barrier itself can have long-term adverse effects on the bed material by significantly altering the uniform air flow in the region impacted by the barrier. It is the author’s experience that use of a 12-inch to 24-inch dead zone provides good security against wall channeling if the bed material is initially placed uniformly within the structure. The best means of addressing wall channeling depends on the overall configuration of the compost or biofilter bay. The configuration is often determined by available space for the bay footprint; therefore, no single method is necessarily optimal for all cases.
Closely tied to the need for uniform air distribution at the air inlet to the bed is the need to provide the lowest possible pressure drop through all air pathways leading to or from the bed. A good rule-of-thumb is to have a total static pressure drop in the vertical direction (through the bed) that is ten times the pressure drop within the air manifold piping leading to the material bed itself. Any pressure differential greater than a ten (bed) to one (manifold) ratio further enhances the uniformity of air distribution through the bed. For the sake of energy efficiency, the pressure drop through the bed should be minimized and the aeration floor or manifold should, likewise, be designed with a very low pressure drop within the constraints of the 10:1 ratio.
Fundamental to the design of an aeration supply or suction manifold or floor, is the prevention of abrupt velocity changes in the flow stream as the air goes through changes in piping, trenches, or ductwork of varying cross-sections. Velocity changes in the air plenum should be gradual and, if abrupt changes are required, such changes should be consistent for all similar points in the air handling system such as repetitious branches off a common manifold.
An aeration system can be evaluated on seven factors. These include the six characteristics outlined above: air distribution, plenum pressure drop, structural integrity, maintenance, corrosion resistance, and leachate control. The seventh factor is cost or value. A few of the more common aeration methods are evaluated below.
Manifold-pipe leg method: The manifold-pipe leg method is perhaps the oldest, and still a common, practice of aerating compost and biofilter beds. Air exits the blower into a central manifold, or header. Perforated pipe runs extend laterally from the header, usually into the base of the biofilter or compost bed. A large number of biofilters using a variant of this method were designed with the pipe runs embedded in a subfloor of washed and sized gravel.
Pipe run spacing is typically centered in the range of 5- to 8-feet intervals. Guidelines for perforation spacing and total perforated area (cited in The Practical Handbook of Compost Engineering by Roger T. Haug, 1993) have kept the total perforated area slightly less than the total manifold cross-sectional area and specify that average pressure drop across the perforated opening, or orifice, should be greater than the head loss along the manifold. Corrosion resistant piping materials such as high-density polyethylene (HDPE) are typically specified.
The air distribution pattern is determined by the number of pipes (i.e. pipe interval) and the distance between perforations along the pipes. While plenum pressure drop can be minimized and uniform flow can be readily achieved from pipe-to-pipe along the bed with minimal velocity changes within the manifold-pipe distribution system, the velocity change that most readily impacts microbial performance in the bed occurs at the discharge of each perforation or orifice. The manifold pipe-leg method is weak in this area since the spacing of the orifices is limited by the number of pipe runs, leaving relatively long distances within the bed over which the changes in average interstitial velocity will occur.
This method has been popular because of the low capital cost and the local availability of materials. Perforated HDPE drain tile available at the local home improvement store has been used in many cases and the pipe runs can be placed directly on whatever base material – either a hard-surface slab or bare ground – that is acceptable at the site. In addition, the HDPE provides good corrosion resistance. The major operational disadvantage is the difficulty experienced (and time consumed) in removal of the spent media and damage to the pipe unless all pipe runs are pulled and reset during replacement or turning of the bed materials. Most using this method today in biofilters place the pipe runs in a gravel bed. However, the structural integrity of the gravel bed and piping are often disrupted during media placement. Finally, the primary way to control leachate is to put the pipe runs entirely below the surface of the bed material and in a liquid containment zone.
Manifold-channel method: Several variations of the manifold-pipe leg method have been used throughout the compost and biofilter industry. Distribution channels or trenches covered by spaced blocks or grates, but flush with a hard-surface floor, have been used in place of the pipe runs extending from a common manifold. “Pipe-spigots” floors constructed of pipe runs embedded within a structural floor with equally-spaced “spigots” (orifices) opening at the surface of the floor are another variation. All of these variations address principally the operational inconvenience and cost of placing, removing, and replacing pipe runs into the bed. All allow heavy equipment to operate freely on the surface of the entire aeration floor.
The channels have a higher initial capital cost than the manifold-pipe leg approach, which is partially offset by reduced operating and maintenance costs and convenience. These systems have the ability to enhance the air distribution pattern by increasing the number of ports or slots along the length of the trench. Leachate can be controlled with use of floors that slope toward the trench manifold. However, the structural spacing and construction cost of large numbers of trenches is still a limiting factor in achieving optimal air distribution between the air plenum and the bed material.
Distributed Plenum Floors: A more recent concept is the “distributed plenum floor” that creates an air plenum under nearly the entire bed from which air is exchanged directly with the biologically-active material of a compost or biofilter system. Conventional ductwork carries air externally to or from the bed; there is no ductwork or piping within or under the bed itself. Plates made of polyethylene, polypropylene, or other corrosion-resistant polymeric materials with air distribution slots molded into the structural surface support the bed material (and can withstand the weight of material handling equipment). Typically, these slots have a maximum spacing of 3-inches to 5-inches between each other to provide uniform air flow to the material bed.
Depending on the size of the floor and the air flow rates for the application, the subfloor plenum is either connected directly to the external ductwork via a transition piece or through a subfloor manifold trench. Low flow applications under 1,000 cfm typically do not require a trench; however, this is not a hard number as floor designs vary with floor and component designs.
With distributed plenum floors, the velocity changes associated with air passing from the plenum to the interstitial spaces of the porous bed are lower in magnitude and have smaller critical impact zones compared to other methods. This allows lower air velocities within the floor plenum with corresponding reductions in plenum pressure drop. Leachate is captured in the subfloor plenum area by slightly sloping the plenum and trench floors and installing a drain at the low point of the floor.
Distributed plenum floor systems will typically have a somewhat higher overall capital cost than the other methods but can provide more uniform aeration. In addition, lower energy costs can be realized due to lower overall pressure drop through the floor.
The fundamental principles behind air flow through a porous media are major drivers in determining the efficiency of compost and biofilter processes. All other things being equal, the closer an aeration system facilitates the ‘ideal aerated bed,’ the lower the operating cost of either a biofilter or a compost process. In a biofilter, the functional interval between “restructuring” or bed replacement will be greater. In a composting facility, the rate and uniformity of degradation of the bulk material will increase resulting in more production capacity per unit area and a more consistent finished product. The design of the aeration system that delivers air on the basis of those principles becomes critical to overall process performance. Furthermore, energy for moving air and the time required for placing, reconstituting or replacing bed material are the largest operating costs in both biofilter and aerated compost facilities.
Therefore, the selection of the air distribution system is at the heart of any biofilter or compost system. The financial impact of the chosen system on processing times (compost) or media life (biofilter), material handling costs, and energy consumption must be weighed against initial installation cost to determine the value or cost of a system.
Don Mathsen is chief engineer of BacTee Systems, Inc., based in Grand Forks, North Dakota.

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