This article series examines the considerations in forced aeration static pile composting, including the basics of aeration system design and operation, types of aerated static pile systems, and design issues to be evaluated.
Aerated static pile (ASP) composting, using negative aeration and simple timer motor controls, was developed by the U.S. Department of Agriculture’s Beltsville Agricultural Research Center in the 1970s in support of exploring the beneficial uses of sewage sludge. The “Beltsville method” was refined in the 1980s by Dr. Melvin Finstein at Rutgers University, who developed ASPs with positive aeration and a temperature feedback loop to maintain pile temperatures at a constant level. ASP composting has long been used for heavy wet feedstocks like sludges and manures, but it is getting a fresh look as composting facilities are starting to accept feedstocks like food scraps.
This series of articles will examine both process and mechanical design considerations in ASP composting and the newer containerized and covered ASP technologies on the market today. Part I covers process design, including recipe development to maintain free air space, and aeration fundamentals.

Process Design

Process design, whether for ASP, windrow or in-vessel composting, starts with the same procedure: Develop a good recipe for all the anticipated feedstocks, then size all the processing steps in the composting process. A good recipe should be based on valid characterization data on total carbon, total nitrogen, moisture content, volatile solids content and bulk density (mass per unit volume of material) for each feedstock. The recipe should not only balance the carbon to nitrogen (C:N) ratio (>25:1) and moisture content (50-55%), but should also verify content of at least 80 percent volatile solids and has a predicted free air space of between 40 and 60 percent.
Volatile solids (VS) can be thought of as the “biologically combustible portion” of a feedstock, representing the amount of easily biodegradable content. A compost pile with a high content of brushy and woody material (which has a VS content of around 70-75%) will not reach as high a composting temperature as a pile with a high content of food and paper scraps (which would have a VS content more in the 90-95% range).
Free air space (FAS) is a measure of the structural porosity of a pile, which is important in ensuring good air movement through the pile, either by the natural chimney effect of a windrow or by the persistent effect of fan blades. FAS can be predicted by a correlation to bulk density using a formula developed by Albuquerque (2008):
FAS = 100 –
(0.09 x Bulk Density (in kg/m3))
Table 1 provides a sample calculation of FAS. While it calculates the FAS slightly above the target levels, this is a less important process design criteria than C:N or moisture, and could be adjusted by adding more “compost recycle” (finished and typically screened compost, although some facilities use unscreened compost to increase pile porosity). Recycling 10 to 15 percent compost into a mix serves three functions: Provides microbial life already adapted to the facility (serving as an inoculant); absorbs moisture from high-moisture feedstocks like fruits and vegetables; and can provide a small measure of in-situ odor control.
Recipes can also allow for reuse of screened oversized woody particles. Calculating the amount of “overs” available can be done by creating a process flow diagram (PFD) — using the bulk density data to figure out volumes of each ingredient in the recipe, then combining them for a daily, weekly, monthly or annual recipe. If feedstocks are expected to change each day, make a daily recipe and PFD. If feedstocks are seasonal, like a yard trimmings facility, create a weekly recipe/PFD for each of the seasons. Based on assumptions made about volume changes due to grinding, mixing, composting, curing and screening, and about residence times in each process, volumes in each step of the composting process can be estimated. In turn, the physical footprint of each process step, with the blowers and aeration piping sized for the ASP system being designed, can be calculated (more on that in Part II).
Keep in mind that ASP systems are not as forgiving of process design mistakes as are windrow systems, therefore getting C:N, moisture, VS and FAS correct is more important. Traditional ASP systems do not require turning or moving the materials during the active composting process, so grinding, mixing, and feedstock preparation are very important. There is an emerging school of thought in the industry that ASPs should be broken down, remixed and rebuilt at the midpoint of the active composting phase, which allows for midcourse corrections if something in the process design is off-kilter.

Aeration Fundamentals

The purpose of aeration in composting is three-fold: Satisfy the oxygen demand from aerobic decomposition (known as stoichiometric demand); remove excess moisture; and remove excess heat. Of these, the aeration rate to keep a constant temperature by removing excess heat normally governs the aeration requirements of a composting system (Keener, 1997).
Stoichiometric demand refers to the quantity, or weight, of oxygen needed to decompose organic molecules, nitrify (oxidize) ammonia released during decomposition, and oxidize the cellulosic matter in carbonaceous amendments. This oxygen demand refers only to the volatile solids in the feedstocks. Consider the decomposition reaction for food scraps:
4 C18H26O10N + 75 O2 —›
72 CO2 + 46 H2O + 4 NH3
The organic decay stoichiometric demand calculates out to 1.44 grams of oxygen per gram of biodegradable volatile solids (g O2 /g BVS), and the nitrification demand is 0.16 g O2 /g BVS. Similar calculations are done for each feedstock in the compost mix and the results summed.
The stoichiometric demand for oxygen can vary from about 1.0 g O2 /g BVS for highly oxygenated feedstocks like cellulose or starch to 4.0 g O2 /g BVS for some hydrocarbons (Haug, 1993). Because air contains 23.2 percent oxygen by weight, the air demand is the oxygen demand divided by 0.232. This demand concept is different from the air rate required.
The quantity of air needed to remove moisture varies with the moisture content of the feedstocks, the desired moisture content of the final compost product, and the moisture-carrying capacity of the air stream (known as the specific humidity). Assuming a mix moisture content of 60 percent and a desired compost moisture content of 45 percent, with inlet air temperature of 68°F at 75 percent relative humidity and the outlet air at 130°F at 100 percent relative humidity, the oxygen demand for moisture removal in the food scraps decomposition example above is 9.2 g O2 /g dry solids, which is significantly greater than the demand for biological oxidation.
Rates of biochemical reactions generally increase exponentially with temperature, but elevated process temperatures in composting quickly inactivate the microorganisms, so temperature becomes rate-limiting. Removing that heat is an important part of aeration. Some heat will be removed from a compost pile in the final solids, and some lost to the environment, but the majority of the heat loss is in the exhaust gases leaving the pile. Oxygen demand for heat removal is several times greater than that needed for biological oxidation or for moisture removal. In the food scraps example above, the oxygen demand to maintain temperature at 131°F is 38.4 g O2 /g dry solids. If the total aeration needed is 100 percent, then, in the example above, 4.3 percent of that air is needed for biological decomposition, 18.2 percent is needed for moisture removal, and 77.5 percent is needed for heat removal.
The aeration rates needed for composting are determined by converting oxygen demands to aeration demands, then by considering the duration over which aeration is needed. This establishes the average rate of aeration. Aeration rates are usually expressed in cubic feet of air per hour per dry ton (cfh/dt) of mix. Aeration rates will vary depending on where the pile is in the decomposition process, with peak demands exceeding average demands by a factor of 2 or more. In the early and late stages of active composting, aeration rates will be in the range of 200 to 500 cfh/dt, while during times of high microbial activity the peak rate might exceed 2,000 cfh/dt.
Insufficient aeration rates cause pile temperatures to increase due to the inability to provide enough oxygen for heat removal. This creates a tradeoff between the need for larger blowers (which cost more) versus pile temperatures that exceed a desired setpoint (like 131°F). It may be more cost-effective to size the system for less than peak demand and accept process temperatures above the setpoint for a while. Given composting does not completely stop until pile temperatures exceed about 165°F, there is some flexibility in system design.
The type of aeration control system used also affects the ability to meet peak aeration demands, as air is only supplied when the fans are on. While it is possible to simply leave the fans on for the entire duration of active composting, this is not very cost-effective. Some form of control is needed to reduce operating costs and wear-and-tear on the aeration system. The simplest control system is simply a manual on/off switch. Early ASP systems used clock timers to regulate airflow on or off, usually on the basis of 20 minutes on, 40 minutes off per hour. This is still a valid control strategy.
ASP systems can also operate on a feedback control strategy, where some external measurable factor is used to control aeration rates. With a feedback control strategy, the externally measured factor is the controlled variable and the aeration rate is the manipulated variable. The two main controlled variables in ASP feedback control systems are temperature and oxygen content. These systems are often linked to the motors controlling the fans by a variable frequency drive unit, which adjusts the electrical voltage going to the motor, which, in turn, controls fan speed and thus air flow rates. So a temperature feedback control strategy would seek to hold pile temperature at a constant level, adjusting fan speed up as pile temperatures increase (to remove excess heat) and adjusting fan speeds down as piles cool off, whereas an oxygen content control strategy would increase fan speed if O2 content drops below a setpoint like 10 percent and lower fan speed if O2 content rises above 18 percent.
Good process design is an important first step in aerated static pile composting. The next article in this series will examine types of forced aeration systems, including supply fans and aeration piping.
Craig Coker is a Contributing Editor to BioCycle and a Principal in the firm Coker Composting & Consulting (www.cokercompost.com), near Roanoke VA. He can be reached at cscoker@verizon.net. Based in Milton, PA, Tom Gibson, P.E. is a consulting mechanical engineer specializing in green building (www.ProgressiveEngineer.com/consulting.html) and can be reached at tom@progressiveengineer.com.
 
References
Albuquerque, J.A., et. al., “Air Space in Composting Research: A Literature Review”, Compost Science and Utilization, Vol. 16, No. 3, 2008, p. 159-170.
Haug, R.T., “Aeration Requirements”, The Practical Handbook of Compost Engineering, Lewis Publishers, 1993, p. 263.
Keener, H.M., et.al., “Airflow Through Compost: Design and Cost Implications”, Applied Engineering in Agriculture, Vol. 13, No. 3, March 1997, p. 377-384.