BioCycle December 2011, Vol. 52, No. 12, p. 22
The four main process control variables are C:N ratio, moisture content, volatile solids content and predicted free air space.
Craig Coker

Like baking, composting benefits from use of a well-crafted feedstock recipe to ensure good process control – resulting in less malodors, shorter processing times and better product quality. The four main process control variables to be considered in developing a recipe are carbon-to-nitrogen ratio (C:N), moisture content (%), volatile solids content (%) and predicted free air space (%). C:N is the ratio of the mass of total carbon in a pile to the mass of total nitrogen, and should be between 25 and 30 to 1. Moisture content should be between 50 and 55 percent (a little higher in the initial mix for aerated static pile composting). Volatile solids (VS) are the non-ash solids in a material. Total VS should be higher than 90 percent, although only about 70 to 75 percent of the VS in an organic material will be biodegradable. Free air space is the ratio of gas (air) volume to total volume and should be between 40 and 60 percent. Not following these guidelines can have a significant effect on process performance in composting, as shown in Table 1.
The first step in building a recipe is to understand the physical and chemical nature of the feedstocks. This may require asking a waste generator for Material Safety Data Sheets and laboratory data on their waste. For example, any industrial residual to be composted should be analyzed for hazardous characteristics using a Toxicity Characteristic Leaching Procedure (TCLP) test. It also should also be tested for the heavy metals listed in the EPA federal 40 CFR Part 503 regulation. Commercial residuals like food scraps or municipal residuals like green wastes don’t need that level of background testing.
Assuming the feedstock is acceptable from a metals and hazardous waste perspective, composters should obtain a representative sample of the material and have it analyzed by a commercial laboratory. Ideally, the sample should be obtained from the point of generation, as this develops an understanding of the source of the waste, how it is generated and handled, and what potential exists for physical or chemical contamination. The analytical lab should be certified under the U.S. Composting Council’s Compost Analysis Proficiency Program for methodologies based on Test Methods for the Examination of Compost and Composting (TMECC). Samples should be tested for total nitrogen, total carbon, moisture content, volatile solids, pH, bulk density and soluble salts.
The bulk density (BD) measurements are used to estimate the free air space (FAS) in the mixture. Agnew and Leonard (2002) reported a linear relationship between bulk density and FAS:
FAScalc = 100 – (0.09 x BD)
where FAScalc is a percentage and BD is the wet bulk density (in kg/m3). Bulk density measurements also offer an easy way for loader operators to know how many buckets of each feedstock to add to the mix. Measuring soluble salts is advised in order to select a bulking agent amendment that does not increase salts to unacceptable levels (e.g., poultry litter).
BULKING AGENTS
Factoring in the right amount and type of bulking agent is important in recipe development. Bulking agents serve two purposes: they provide structure to enhance free air space and pile porosity (important for maintaining aerobic conditions) and they often add biodegradable carbon to help offset the nitrogen-heavy nature of most compostable feedstocks. Bulking agents can be anything from shredded paper and cardboard to agricultural residuals (like corn stover) to sawdust to wood chips. Often, a composting recipe will use more than one kind of bulking agent, with one chosen for its contribution to the C:N ratio and one chosen for its contribution to structural porosity. “We started out doing general recipes and monitoring moisture and bulk densities,” says Geoff Kuter of Agresource in Ipswich, Massachusetts, “but our operators have been doing it for so long they do it by experience now. Get your starting mix right, and everything will roll downhill.” Patrick Geraty of St. Louis Composting has been taking in food waste at one of his yard trimmings facilities in Illinois. “We’re using wax coated cardboard, zoo bedding and leaves as bulking agent, and will soon pilot-test using cross-cut shredded money from the Federal Reserve Bank in St. Louis,” he says. “I always knew there was money in composting!”
Availability of bulking agents vary considerably around the country. In states with bans on landfilling or incinerating yard trimmings (see “State of Garbage In America,” October 2010), bulking agents can be readily available. In other areas, competition for wood chips can require innovative solutions. “We’re surrounded by an 80 MW biomass power plant, three paper mills and two plywood plants,” says Ken Newman of Royal Oak Farm in Evington, Virginia, “We were paying over $20/ton for wood chips but have purchased a Komptech slow-speed shredder to go out and grind up stumps and debris that these other wood chip markets won’t touch.” Others echoed Newman’s refrain. “We’ve had to be opportunistic,” says Nelson Widell of Peninsula Compost in Wilmington, Delaware. “We developed a partnership with the construction company that built our facility. They have their construction crews source-separate clean construction lumber which we grind up with a Doppstadt slow-speed shredder for some of our bulking agent.” There are also locally available materials, like peanut hulls in the southeastern U.S., which several composters use as bulking agents.
DEVELOPING THE RECIPE
How much time and effort to put into recipe development and management depends on how many feedstocks come to the composting facility. For a yard trimmings site, the recipes should be seasonal, so site operators can adjust the mix to account for high amounts of grass clippings in the spring and early summer, high amounts of leaves in the fall and early winter and mostly brush coming in at other times. Nitrogenous amendments that can be used to offset the high C:N ratio of green waste feedstocks in some seasons include poultry litter, food wastes and inorganic fertilizers like urea.
Small composting facilities can use their senses of observation to know if the mix has a good recipe. Visually, the operator can look to see if the feedstocks are evenly mixed; moisture content can be assessed with a squeeze test, and, after composting has begun, detecting foul odors is a signal that something is wrong with the mix. Notes Kuter: “Use observations of process performance to make adjustments to recipes. If you smell a bad odor or your temperatures don’t rise, you know you’ve got a problem in your recipe.”
Large composting facilities that take in more than three to four different feedstocks should do the math (for an explanation of the math behind balancing C:N ratio and moisture, visit the Cornell University composting website at http://compost.css.cornell. edu/calc/simultaneous.html). There are various Excel spreadsheet models available that are very helpful in developing recipes. An example of one developed for a small on-site university composting program is shown in Table 2. In this model, the blue-tinted cells are input data, the orange-tinted cells are output data.
No recipe will work well without proper proportioning and good mixing. Site operators don’t necessarily need to understand the math of recipes but they do need to understand the volumes of the recipe (e.g., three loader buckets of sawdust for every loader bucket of manure). Mixing can be done by bucket-blending, building a “layer cake” of feedstocks and turning them in together with a windrow turner, or with one of the many mechanical mixers on the market.
Summary
If the ingredients in a baked cake are not proportioned and mixed correctly, the cake will not rise and it will not taste very good. Similarly, if the feedstocks in a composting pile are not proportioned and mixed properly, not only will it not compost well, there is a significant risk of process problems ranging from malodors to lack of pathogen inactivation. Recipes are easy ways for site operators to proportion the blends of feedstocks correctly, especially when using one of the computer-based tools available for facility managers.
Craig Coker is a Contributing Editor to BioCycle and a Principal in the firm Coker Composting & Consulting (www.cokercompost.com), near Roanoke Virginia. He can be reached at cscoker@verizon.net.
REFERENCE
Agnew, J.M. and Leonard, J.J., 2002, “Using a modified pyncometer to determine free air space and bulk density of compost mixtures while simulating compressive loadings”, Proceedings of the 2002 International Sypmposium: Composting and Compost Utilization, Columbus, Ohio.
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What Are Alternative Bulking Agents?
What else besides wood chips can be used as a bulking agent? Other materials that have been tried include:
Shredded or Chipped Tires
Long-lasting, but they cost about $25/ton and there are safety issues (protruding metal from steel cords) and chemical issues (heavy metals, volatile organics leaching).
Old Corrugated Cardboard (OCC)
Provides both structure and carbon amendment but mills are paying $165 to $175/ton for OCC.
Shredded Paper
Nonrecyclable soiled paper absorbs water and provides carbon, but not much structural porosity.
High Density Polyethylene (HDPE)
Hard plastics, like the media found in air pollution control scrubbers, has been used, but is subject to breakage from materials handling equipment, resulting in contamination of the finished product.
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How Do Compostable Plastics Affect Recipes?
Increasingly, compostable plastics are finding their way into composting facilities. Biodegradable polyesters like polylactic acid (PLA) and polyhyrdroxyalkanoate (PHA) are complex carbon- hydrogen-oxygen molecules. “They are almost pure carbon, with a C:N ratio above 200,” explains Bruno De Wilde, Laboratory Manager for OWS-Dranco in Ghent, Belgium, who has analyzed compostable plastics. “This is bioavailable carbon, unlike woody materials that contain lignin, which cannot be decomposed by bacteria. Bioplastics also have a very high VS content, greater than 99 percent.” One important potential benefit is also their role as a bulking agent, e.g. cups, cutlery, rigid packaging and also plates and trays will decrease the density and increase the free air space.