Forced aeration composting takes on many forms. In all cases the intent is improved process conditions to speed up composting while producing less odors. But while there is little agreement on how to assess or improve the effectiveness of a specific design, it is understood that airflow capacity and energy consumption are key performance indicators. In this article we discuss how a science and engineering approach can inform the design of an aeration system to help you get the most out of your investment.
Composting is fundamentally the process of oxygen breathing microbes converting degradable organic matter (also called bio-available solids) to into CO2 and water in an exothermic (heat generating) process (Figure 1). The faster this reaction progresses, the sooner the gooey stuff in the mix is oxidized to CO2 and the remaining organic matter is stabilized.
Published research has repeatedly shown that moderated temperatures and high oxygen levels produce stable compost faster while generating less emissions of odors and VOCs (volatile organic compounds) – which are generally the primary constituent of compost odors. But as they say, “composting happens” whether it’s slowly in a passively aerated static pile or more quickly in a highly aerated and tightly controlled, and capital-intensive system. Both have their place. But the current trend of diverting more food waste and building larger facilities in more demanding regulatory environments is driving requirements toward the higher end of process control spectrum, which requires well designed aeration systems.
Every composter knows that fresh piles are the most active piles. We can measure this time-varying rate of bio-oxidation in the lab. Figure 2 shows the CO2 production of a bench-scale vessel composting a fresh mix of digestate and ground wood . Semi-optimized process conditions (oxygen >15%, temperature ~140°F) allowed the aerobes to produce a huge early CO2 spike that then rapidly decreased. CO2 production is proportional to heat generation and thus the “Aeration Demand” required to cool and oxygenate the pile. This data shows how focusing the aeration capacity on a shorter period early in the composting cycle can both accelerate stabilization and reduce fan energy consumption.
To specify compost system’s airflow we need a unit aeration rate; which is commonly expressed as airflow (cubic feet per minute) per cubic yard of compost (cfm/cy). If the airflow is to be controlled in response to heat generation, we see in Figure 2 that an adaptive design would consider both the peak and much lower average aeration rates.
Some might wonder why we are focusing on cooling and not oxygen demand in this discussion. It turns out that a dive into thermodynamics reveals that over 10 times as much air is required to cool a pile as to supply oxygen to those hard-working aerobes; if an aeration system provides even modest cooling, much less strong temperature control, there will be plenty of oxygen supplied
The first step in specifying an aeration system should consider peak and average aeration rates, and the aerated composting cycle retention time. The retention time will be function of the stability requirements and how well the process conditions are controlled, which in turn is a function of the peak and average aeration rates. In practice we see dizzying range of peak aeration rates 0.3 cfm/cy up to 50 cfm/cy and retention times from 5 days to 14 weeks. The CREF’s 2021 Compost Handbook cites typical peak aeration rates on days 1 to 10 of 3 to 10 cfm per cubic yard of pile volume, and on days 10 to 20 of 1.5 to 3 ft3/min per cubic yard of pile volume. Various scientific efforts have explored this including work by Dr. Tom Richards at Cornell, PHD candidate David Notton at Cardiff, and our own biothermal model developed based on equations from R.T. Haug. In our experience, designs based on rules-of-thumb rarely provide a best-value composting process; only bench or pilot scale can determine optimized aeration rates and retention times.
The total fan energy consumed, kWh per ton of compost processed, is a function of aeration rates, retention times and how efficiently the aeration system delivers the air. Figure 3 shows an example fan system. The delivery efficiency depends on how the aeration system is configured and controlled, and on the mechanical design of the fans, ducting and pipes that comprise the aeration system. One simple way to reduce fan power requirement is to follow established air-handling design practices found in design handbooks. In our experience competent system design results in fan energy consumption in the range of 3-14 kWh per ton. Design choices that impact the aeration system efficiency are discussed in the following sections.
Centralized and decentralized fan designs offer different advantages. Below are some of the key considerations:
Compost systems with distributed fans use a single large fan and distribution duct to send air to multiple zones in a system. Commonly, automated control systems modulate airflow using individual zone dampers based on a temperature feedback control sequence.
Compost systems with decentralized fans use individual fans for each compost zone. While these are often timer controlled, they can also be integrated with an automated control system to control VFD speed or Timer based on zone temperature.
For a fixed rate of delivered air, the fan efficiency and system loss determine any energy consumption differences. So what are the differences between positive and negative applications?
Compost systems require pressure to overcome distribution loss (friction in duct, corners, aeration pipe surface) and the pressure of the compost pile material (a function of porosity, density, and velocity). For both positive and negative applications, the pile and distribution systems are mostly similar. However, the negative fan has additional ductwork and biofilter media to overcome, which requires a slightly higher total pressure than a positive only system.
While moving warm saturated air requires a stainless rather than mild steel to avoid corrosion, the impact on mass flow rate is small since their is minimal difference in density. For example: warm, high Relative Humidty (RH) air is only slightly less dense than cooler air at lower RH.
70°F at 50% RH => 0.074 lb/CF
120°F at 100% RH => 0.065 lb/CF
Although the fan efficiency will be slightly different between a positive vs negative application, this is very low relative to the effect of pressure.
Overall, systems with negative aeration tend to require 20-30% more energy to move the equivalent volume of air as a positive system.
Since the composting microbes will require varying rates of air over the process retention time, we recommend matching the fan output to this biological demand. This can be done through cycling the air on a duty cycle (either with a damper, or on/off control of a fan), or by modulating airflow (either using a modulating damper or fan VFD).
Two common factors:
Maintaining consistent process conditions (and oxygen levels) helps the biological process. In general, continuous duty aeration or very short duty cycles, on the order of minutes rather than hours, help to maintain favorable conditions.
Fan efficiencies tend to decrease as they approach zero flow. While modern VFD’s help minimize these system efficiency losses fan performance tends to drop off rapidly below 20% speed. This can tend to favor duty cycles for applications that would result lower fan speed.
Fans play a critical role in forced aeration composting. They deliver the oxygen and cooling for the biological process, impact the system design and capital cost, and have an impact on the operating expenses. Because of this, we believe it is important to engineer a compost fan solution based around best-value. This should include analyzing or testing feedstock aeration requirements followed by a review of potential system designs. At ECS, we find that most applications achieve best efficiencies and project economics by using distributed fans and automated controls. However, there are some large-scale systems with very short retention times where the decentralized fan approach offers greater overall value
We’d love to hear from you and discuss what fan configuration is right for your project!