The key to a successful composting operation is matching and maintaining the level of operational and structural BMPs to the specific facility conditions. This requires well-trained operators who understand the basics of compost process biology and are given the tools they need to efficiently monitor and manage the process.
By Tim O’Neill
Composting is like a low temperature fire. However, whereas a fire indiscriminately oxidizes the carbon-based fuel, compost only oxidizes the bio-available volatile solids (BVS) portion of the feedstocks leaving behind the more resilient and fibrous carbon-based materials, ie the compost. Figure 1 offers a simplified diagram of the process. When fire is characterized by efficient combustion fuel rapidly consumed, very little smoke is emitted. Similarly, when the bio-oxidation process that is composting is efficient, bio-available solids are rapidly converted to CO2, water and heat, and very little volatile organic compounds (VOCs) and odors are emitted. The application of both structural (design-based) and operational Best Management Practices (BMPs) are the keys to sustaining efficient bio-oxidation during composting. This article will cover some of the more impactful BMPs one should consider when designing and operating a composting facility.
Operational BMPs
Operations often determine the success of a composting facility; said another way, buying highly engineered composting technology will not protect you from poor operations. A good starting point is to make sure operators get formal training through either the U.S. Composting Council Research and Education Foundation or Composting Council of Canada sponsored classes.1 In these classes they will learn the ABCs of making a BMP mix as well as how to monitor and adjust the process to keep it efficient.
Starting with A BMP Mix
A BMP initial mix is critical to the entire compost process. There are three key parameters in a mix: moisture, density and C/N (carbon to nitrogen) ratio. These parameters set up proper conditions on the biofilm on decomposing waste particles (Figure 2), which is where all the “magic” happens. If all three are within BMP ranges, the subsequent composting process will have the greatest chance of maintaining the conditions for efficient bio-oxidation. If one or more parameters fall outside the BMP range, the process will become inhibited, and is likely to move into anoxic or high ammonia conditions where substantial concentrations of odorous VOCs are produced and the process slows dramatically.
Table 1 lists the three key mix parameters and their BMP target ranges. Of these parameters, the C/N ratio is generally fairly consistent throughout a season; infrequent testing is adequate. If the feedstocks include dense, wet, nitrogenous materials such as food waste, digestate, biosolids, etc., then the target ranges can only be achieved with the addition of a lower density, drier and high carbon amendment. This amendment is most commonly some form of ground wood. In this case, the weights of each the feedstock components, along with the three mix parameters in the Table 1, should be measured and logged regularly to provide assurance that a BMP mix is being made in each and every batch. If an odor event happens or if stability goals are not achieved, chances are that one of these metrics will be out of range. A well-kept mix QC (quality control) data log will provide direction on how to address the issue.
Monitoring the Process
The first weeks of composting is the most critical period. There tends to be lots of degradable carbon available so the bio-oxidation process will either be efficiently consuming lots of oxygen and making lots of CO2 and heat, or if the process is inhibited, it will be making lots of VOCs and odors. To manage and maintain process efficiency, operators will need to monitor the key process indicators and be ready to make adjustments during this period.
Assuming the mix parameters are within BMP guidelines given above, the key driver of bio-oxidation efficiency is the availability of oxygen level in the biofilm. This is where the mesophilic and thermophilic bacteria combine oxygen with the BVS to accomplish their work. Much like blowing on a fire, the more oxygen that is available, the more efficiently they bio-oxidize the feedstocks. The concentration of oxygen dissolved in this water film layer is determined by both the temperature and the oxygen levels in the pile. Higher temperatures drive gasses out of a solution, and higher oxygen levels over the biofilm cause more oxygen to be absorbed. This relationship is shown in Figure 3, published by the compost researchers at UK Environment who recommend a minimum oxygen concentration above 3 ppm.2 This author has, through field testing, found that concentrations above 4 ppm further reduce air emissions.
Assuming the forced aeration system is designed to provide at least modest cooling (discussed in the Structural BMPs section), only temperature needs to be measured regularly. This is because the airflow required for cooling is five to 10 times higher than airflow required to supply oxygen.3, 4 So as long as the aeration system runs consistently, the oxygen levels should generally be above 16 percent. This is handy because oxygen measurements tend to be either labor or technology intensive, whereas temperature measurements are relatively simple and easily integrated into an automatic control system, which makes managing the process easier for operators.
Assuming the oxygen concentrations in the biofilm are sufficient, and that no other significant process inhibitions are present (such as low pH or moisture levels, or densification), temperature plays a direct role in the rate of bio-oxidation. As the pile heats up, the rate of bio-oxidation increases by roughly doubling every 10°C (18°F) up to about 65°C. At this temperature, even the heat-loving bacteria (thermophiles) begin to faulter. As temperatures surpass 70°C, the process slows down markedly and the odor and VOC emissions rise sharply.4
One of those other inhibitors mentioned, low pH, is also generally managed through temperature control. Facilities accepting food waste pretty much always start with a feedstock mix with low pH in the range of 4.5 to 5.5 (7.0 is neutral). This is okay so long as the pH comes up near neutral early in the process. To do this, the temperatures should be kept below 45°C for the first couple of days so that the mesophilic (medium temperature) bacteria can break down the organic acids that cause the low pH. Once the pH rises above about 6.5, the temperatures can be allowed to increase into the thermophilic regime without inhibiting the process. If, however, the temperatures rise too quickly, the mesophiles will not have the opportunity to raise the pH and the thermophiles, which are very sensitive to low pH, will be inhibited. In this case the bio-oxidation will be inefficient and significantly higher concentrations of VOCs and odors will be produced. Once again, the key process indicator is the pile temperature, and most expedient tool most operators have to control temperature, is to adjust aeration rate (airflow per volume of compost) to balance the heat generated by bio-oxidation.
But how about the other process inhibitors of low moisture levels and densification? As discussed above, it is fairly straightforward to measure and manage moisture content (%MC) and density when making the initial mix. However, once material is sitting in an aerated static pile, it becomes a lot less convenient to measure or change these parameters. The goal should be to start with a BMP mix and achieve efficient composting for 10 to 18 days, then accept that to re-establish BMP %MC and densities the pile will have to be wetted and mixed by adding water, breaking it down and rebuilding it (rewetting a pile from the surface without mixing is ineffective). It is good practice to measure the %MC when piles are being broken down in order to understand how fast the heat generation is drying out the feedstocks and allow operators to optimize the primary composting retention time. When the bio-oxidation during primary composting is efficient, the material going to curing will have far less BVS remaining. This compost will be far less prone to producing VOCs and odors and will require less management on the part of the operators or composting infrastructure. A summary of the process BMPs for primary composting is given in Table 2.
Structural BMPs in Primary Composting
If operators are asked to provide BMP compliant operating conditions, they will need a composting system with the structural BMP’s to meet their process control objectives. There are, however, many facilities that manage their processes adequately while not complying with all of the operational BMP’s in Table 2, but these facilities have some combination of the following attributes: a remote location, low BVS feedstocks, small volumes and a relaxed regulatory environment. For the rest of the industry, a more significant investment in the structural BMPs will likely be required to succeed. From a process quality perspective, the heart of this investment should be in the aeration and control system that determines the efficiency of the bio-oxidation.
Forced aeration removes heat from piles primarily by evaporative cooling; the air enters the pile at ambient conditions and leaves hot (often at 50° to 70°C) and at 100 percent RH. The higher the airflow, the more heat is removed. Per Figure 1, when a pile is strongly aerated and the exit temperature remains elevated, this can only mean that the BVS is being rapidly converted to CO2, water and heat. On the other hand, if the aeration rates are low relative to the BVS in the mix, then the combination of excessive temperatures and low oxygen will inhibit the process. Table 2 gives guidelines for peak and average BMP aeration rates used in design.
To deliver the air uniformly to the pile requires a knowledge of general air handling design and some specialized knowhow as it applies to composting. The design of the above grade fans and ducts should comply with the practices taught by ASHRAE and SMACNA.5 This will minimize power use and provide even and dependable distribution of air to each of the aeration floors under the composting piles. Large and/or complicated systems should be modelled to optimize their performance characteristics (see Figure 4). The design of the aeration floors requires more specialized engineering skills given the harsh and unusual conditions.
Aeration floors are to compost facility what the water film layer is to the composting particle; they determine the bio-oxidation efficiency more than any other single element in an ASP.6, 7 In many cases, they have the highest initial cost of any element. Their job is to uniformly deliver airflows up to the “Peak Aeration Rate” without excessive back-pressure (which reduces fan power efficiency) after 30-ton loaders have piled them 9 foot deep with wet sticky material. Two of the most common types of aeration floors are high pressure spargers and low-pressure trench systems (see Figure 5). Spargers consist of long pipes running under the slab with risers every 2 to 3 feet that locate an orifice plate just below the surface. The orifices diameters are calculated to create enough back-pressure to cause semi-uniform airflow across the floor. They work best in positive (pressure) aeration since they tend to pull particles into the orifices when in negative (suction) aeration. Trench type aeration floors operate at lower pressures and rely on low friction losses in the distribution of the air to many smaller orifices to achieve semi-uniform distribution. They require more complicated form work during construction, but have similar parts cost as sparger floors and lower power consumption for a given airflow rate. Trench type aeration floors are also more effective at collecting surface water and operating in negative aeration. Both floor types require a gravity drainage system and a straightforward method of removing solids from all below-grade components.
As mentioned above, the rate of heat generation constantly changes as the BVS is consumed and as various process-inhibiting factors come and go. If BMP temperatures are to be maintained most of the time, the airflow rate must vary in response. Using an automated system with temperature feedback to modulate airflow, by controlling fan speeds and damper positions, is a fairly straightforward way to achieve this. By following the aeration demand, these systems also reduce fan power consumption. At a minimum, a smart system should provide operators an easy way to visualize and record the key process indicators of pile temperature and aeration system status (so they know the oxygen concentrations in the biofilm). Full featured compost control systems offer many other labor-saving and risk-reducing tools. A summary of the principle structural BMPs for primary composting is given in Table 3.
The Key to Success
The key to a successful composting operation is matching and maintaining the level of operational and structural BMPs to the specific facility conditions. This requires well-trained operators who understand the basics of compost process biology and are given the tools they need to efficiently monitor and manage the process. | WA
Tim O’Neill is President of Engineered Compost Systems (ECS) (Seattle, WA). He learned to compost from his dad while growing up in Seattle. He earned his BS and MS in Mechanical Engineering at the University of Washington. Since 1999, ECS has provided engineering and process technology to hundreds of large-scale composting facilities in North America, Europe and New Zealand. Tim serves as a Trustee for the USCC’s (United States Composting Council) Research and Education Foundation and he teaches classes in facility design and odor management for both the USCC and the Washington Organic Recycling Council (WORC). His professional passion is combining science-based inquiry, professional engineering and operational experience to help the industry improve. For more information, e-mail i[email protected] or visit www.compostsystems.com.
Notes
1. Sauer N., “Odour Technical Guide 3, Oxygen Solubility in Compost” UK Environment Agency (2012)
2. Sundberg C., “Effects of pH and Microbial Composition on Odour in Foodwaste Composting”, Waste Management 33 (2013) 204-211
3. Sundberg C., “Higher pH and Faster Decomposition of Biowaste Composting by Increased Aeration”, Waste Management 28 (2008) 518-526
4. Haug, R.T. (1993) The Practical Handbook of Compost Engineering. Lewis Publishers
5. www.ashrae.org, www.smacna.org
6. Coker, O’Neill, BioCycle, June 2017, Vol. 58, No. 5, p. 30, Aeration Floor Fundamentals, Part I
7. Coker, O’Neill, BioCycle, July 2017, Vol. 58, No. 6, p. 28, Composting Aeration Floor Functions and Design, Part II