Utilization of Organic Wastes: On-farm Composting

DANA O. PORTER, P.E.
WVU Extension Service
Agricultural Engineering Specialist

Benefits of using compost

Compost can provide valuable nutrients and organic matter to soil, depending upon the feedstocks (raw materials used) and upon compost management. A chemical analysis of a representative sample of a compost will indicate its total nitrogen, available nitrogen, phosphorus, and potassium. Most composts contain relatively low concentrations of one or more nutrients and are not necessarily considered good "fertilizers"; however, as soil amendments, they are good sources of organic matter.

Nitrogen and phosphorous in compost are generally found in both plant-available forms (NO3, NH4, and P2O5) and organic forms. Much of the nutrients bound in organic forms will be made "plant-available" as the organic matter decomposes. Therefore, readily available nutrients in compost can be much lower than in raw waste, but a "timed-release" effect occurs in the later, slow-release of nutrients "bound" initially in organic forms.

Why should you consider making compost?

The principal advantages in making your own compost are (1) composting is a good treatment option for many agricultural wastes and can produce a marketable product; and (2) composting stabilizes organic materials so they can be stored safely, transported easily, and applied at a convenient time.

Large-scale composting is practiced by municipal wastewater treatment plants, agricultural producers, industrial waste generators, and commercial composters (who are in the business of composting wastes collected from various sources). The compost product is often marketed to "wholesalers" (nurseries, farms, landscaping companies, etc.) or is packaged for retail sale to the general public. Depending upon compost production and markets, prices of commercially available compost may vary considerably.

Raw waste materials may be applied directly to farmland, but there are limitations to the timing of these applications. In fact, when a producer is ready to apply manure to his fields, so are most of his neighbors. Animal waste generators may find themselves "buried" with manure during the winter, and by spring planting be "sold out" of the commodity. If a farmer could obtain the waste when it is abundant, he may be able to pay a lower price. However, storing large quantities of manure can bring negative effects, including odor problems.

Compost is easier to handle and store than raw waste. It does not have an offensive odor and is less likely to contribute to water contamination. A farmer who composts waste materials will likely have greater flexibility; his soil amendment will be available at the time he is ready to use it. The following information may help you decide whether composting is appropriate for your operation.

What you will need to begin composting

The feasibility of composting in your operation depends upon availability of materials, equipment, and labor. Of course, the compost process cannot occur without "feedstocks," or raw waste materials to be composted. Basically, composting is a biological treatment of organic wastes in which microorganisms (bacteria, fungi, and actinomycetes) consume and "break down" organic material into more stable forms. Simply put, we are feeding these organisms, and to make them work efficiently, we attempt to provide them with a balanced diet and a comfortable environment.

To provide nutrients for composting microorganisms, you will need a nutrient-rich feedstock. Animal manures are good sources of macronutrients and micronutrients. Manure can be composted by itself, but it may be too "compacted" for good aeration, and excess nitrogen can be lost through volatilization of ammonia (thereby producing a strong odor). A carbon-rich, relatively low-nutrient feedstock can be mixed with the nutrient-rich feedstock, providing a better "food" for the microorganisms. A carbon:nitrogen ratio between 20:1 and 30:1 is a good target. Since some feedstocks decompose more slowly, this ratio may be finetuned to account for relative availability of carbon and nitrogen.

Dense feedstock mixtures may not have sufficient aeration and become anaerobic (subsequently producing offensive odors and potentially harmful gases). Bulky material, such as wood chips or straw, can be added to reduce the bulk density and increase air volume in the compost pile. Bulking agents can be left in the compost, or they can be screened out and re-used in future compost piles.

Availability of feedstocks is critical to your decision about whether to compost. If you produce one or more of the ingredients within your farm operation, you may be able to obtain the remaining feedstocks and still operate economically. It is important to take transportation costs into consideration. It is not economical to haul feedstock or compost long distances. Consider other producers and industries in your area. Perhaps you can organize a cooperative composting effort, or perhaps you can charge a tipping fee to accept waste from other sources.

You also will need space to build the compost piles, store feedstocks, and maneuver equipment. If possible, locate the composting operation downwind of residences and away from surface water and wells. Be sure to consider your neighbors' feelings toward your composting activity before you begin operations. Odors and flies from stored feedstocks and compost piles, and dust and odors associated with windrow turning operations can be offensive to some neighbors.

The main equipment needed for composting on a moderate to large scale is machinery--to construct, mix, and move material in the compost pile or windrow. A front-end loader and a truck may be all you need. Other equipment to consider may include (1) chipping or shredding equipment (depending upon the feedstock); (2) a windrow turner (available in many sizes and prices); (3) screening equipment (if you wish to recycle bulking agents); (4) aeration equipment ; and (5) composting thermometer or temperature probe (to monitor progress of your compost). You will also need to take labor requirements into consideration. Do you have time to build the windrow and mix it as needed?

How to compost

An excellent practical reference for composting operations is the On-Farm Composting Handbook (Northeast Regional Agricultural Engineering Service[NRAES], 1992). It addresses composting benefits and drawbacks, methods and materials, management, equipment, and marketing of the final product. This inexpensive reference is available from NRAES or from the state extension agricultural engineering specialist.

Composting is a natural process, not an exact science. Therefore, a lot of flexibility exists in developing a composting system. Management of a composting operation may be directed toward one or more of the following objectives.

1. Minimizing time required for composting: If space is limited, an operator will want to maximize the processing rate or minimize the composting area required. Generally, aerobic composting is much faster than anaerobic processes; therefore, more material can be processed on a given area in an aerobic composting system.

2. Maximizing destruction of pathogens and pests: High-temperature aerobic composting destroys weed seeds, insects, and pathogens; therefore, temperature may be a primary concern. Regular mixing of the material will facilitate aerobic conditions and ensure that all feedstock material is exposed to high temperatures.

3. Minimizing nuisance conditions: Aerobic composting is relatively odor free, compared to anaerobic processes. Even in aerobic systems, however, excessive nitrogen in the feedstock mixture can be volatilized, thereby generating odors.

4. Minimizing labor costs: Static piles (not turned after construction) and anaerobic systems require less handling than other systems. Various levels of automation can reduce labor requirements. Of course, automated systems may require high initial capital outlays for equipment and/or high energy costs.

5. Optimizing the final product quality and marketability: Managing nutrient mixtures and feedstocks can result in a compost product tailored for a specific market.

Generally, composting feedstocks are mixed mechanically with front-end loaders or windrow turners. The materials are placed into piles or windrows that should be sufficiently large to generate and store heat to promote high-temperature composting, but not so large that the materials become excessively compacted. Typical windrows range in size from 4 to 8 feet tall and 10 to 20 feet wide at the base. The dimensions should be no larger than available equipment can accommodate.

Windrows may be aerated through passive aeration (natural convection) or forced aeration (by fans or compressed air). They may be static piles or turned regularly (every three days to two weeks). Management method depends upon characteristics of the waste, labor and equipment available, and operator preferences.

A carbon:nitrogen (C:N) ratio of between 20:1 and 30:1 is generally considered appropriate for agricultural wastes. Higher C:N ratios slow material decomposition, because low nitrogen limits microbial activity. Lower C:N ratios may contain excessive nitrogen that may be volatilized as ammonia, thereby producing odors and wasting nitrogen. If odors are a major concern, consider a feedstock mixture with a higher C:N ratio.

Moisture content between 40% and 60% is a good target range. Moisture is needed for microbial activity, but excessive moisture inhibits gas exchange and may result in anaerobic conditions. The compost mixture should feel moist to the touch, but not be soupy. Very wet feedstocks may be dried before mixing, or a dry bulking material can be used to absorb moisture. Consider protecting the compost piles from excessive rainfall or ponded water. Some moisture will be removed from the mixture during the composting process. During dry weather, the mixture may need water added to maintain moisture.

Nutrients are required for microbial activity. Most agricultural wastes and municipal sewage sludges contain sufficient macronutrients and micronutrients to support decomposition. Some industrial wastes may be deficient in one or more nutrients.

Feedstock mixture pH can be important. Microorganisms tend to modify their environment, and products of decomposition may alter pH over time. Near-neutral pH is preferred for most efficient microbial activity. Some food processing wastes and industrial wastes may exhibit levels of alkalinity or acidity that inhibit nutrient availability or microbial activity. Chemical analyses of material samples will indicate whether pH or nutrients need to be adjusted.

Bulk density should be low enough (less than 40 lb/ft3) to allow for good aeration. Dense manures and sludges can be "lightened" by adding of bulking agents, such as wood chips, corn cobs, and straw.

Insulation material can be used if cold weather keeps compost temperatures down. It also can help reduce odor emissions from a pile. Preferred insulation materials include finished (recycled) compost and/or bulking materials.

Odor should not be a problem in a well-managed composting operation. If odors are present, the mixture should be turned to improve aeration. Adding a carbon-rich bulking agent may be helpful to improve the C:N ratio and aeration. Excessive moisture may contribute to odor problems; therefore, management of the process may require shelter and drainage during wet weather. Odors can be generated from stored feedstocks prior to mixing; if odorous materials (e.g., seafood wastes) are to be used as feedstock, they should be mixed immediately upon receipt. Consider having nonodorous bulking agents available before odorous materials are delivered in order to avoid unnecessary delays in mixing.

Some possible feedstock combinations

Almost any organic material can be composted. The main objectives of C:N ratio, moisture content, and bulk density can be achieved with a variety of feedstock combinations. Therefore, gardeners and farmers alike often can easily identify likely "recipes" from materials on-site. Some suggestions include:

I. combinations of poultry litter with bedding material and additional carbon-rich bulking materials, including (1) broiler litter containing wood shavings as bedding material composted with peanut hulls; and (2) broiler litter containing wood shavings as bedding material composted with shredded pine bark (Flynn, et al., 1995).

II. municipal biosolids composted with combinations of sawdust, yard wastes, bark, vegetable trimmings, animal bedding and manures.

III. ground (shredded) yard wastes, dairy manure, and food processing wastes.

To determine an appropriate ratio of feedstocks for your system, determine carbon and nitrogen contents, moisture content, and bulk density for each feedstock. Design your mixture by balancing the C:N ratio with your principal feedstocks; then adjust moisture and bulk density with bulking agents and/or water.

Note that feedstocks can decompose at different rates. For instance, broiler litter may decompose much faster than sawdust in a particular mixture. Therefore, the sawdust may not contribute carbon at a rate sufficient to balance excess nitrogen in the broiler litter. The C:N ratio should be used to generate an initial "recipe," which will be adjusted later to accommodate specific conditions.

For computer users, an EXCEL spreadsheet has been developed at Cornell University and is available at: http://www.cfc.cornell.edu/compost/download.html

For assistance with obtaining poultry litter as a feedstock, you may contact the Poultry Litter Marketing Hotline at 1-888-3litter. This hotline is sponsored by the Potomac Inter-Agency Water Quality Office at Moorefield, W.Va. Or for additional information read: Buy or Sell Poultry Litter the Easy Way: Call 1-888-3LITTER.

Inoculants

Some commercially available inoculants are marketed to improve compost processing. While these materials are unlikely to be harmful, generally they are not necessary. They may improve decomposition of some relatively sterile industrial wastes, and they may even speed the heating cycle of the first compost "batch" for an operation. However, most agricultural wastes contain sufficient microbial populations to initiate composting. Perhaps the best way to inoculate a pile is to mix some compost from the previous batch into feedstock of the new batch. This is especially effective if the feedstocks are similar in the different batches.

What you should expect

Once a windrow is constructed, the microbial activity will generate carbon dioxide, water vapor, and heat. This activity will be evident through increased pile temperature and possibly by visible "steam" coming from the pile. Over time, some of the carbon and water will be removed, thus causing the pile to lose mass (size) and moisture. If water content falls below 40% to 50%, water should be added and mixed into the composting feedstocks. Gradually the C:N ratio will fall as the readily compostable carbon is metabolized by microorganisms, and the nitrogen is converted to nitrate and organic forms.

Pile temperature is a good indicator of the compost process. During the first few days of composting, pile temperature should increase to between 105 oF and 149 oF. The high temperature may be maintained for several days, until the microorganisms begin to deplete their food source or until moisture conditions become less than optimal. Mixing the composting feedstock brings more undecomposed "food" in contact with the microorganisms, replenishing their energy supply. If moisture content in the mixture is too high, insufficient aeration will limit activity of high-temperature aerobic microorganisms. If the moisture content is too low, it will limit microbial activity. Once the optimum moisture level is restored and the feedstocks have been remixed, temperature will increase again. The heating-cooling-remixing cycle is repeated until the readily decomposable material is depleted.

How to know when it is "done"

After the readily decomposable material is depleted, the compost pile will no longer heat upon remixing. The temperature will continue to drop to ambient. Only very slow decomposition will continue. The material should have a pleasant "woodsy" odor and a friable texture similar to a good potting soil. The material will likely feel moist and cool and have a dark brown color. Several tests can be used to determine "doneness" of the compost, including incubation to test for generation of metabolism by-products and respirometer testing to measure oxygen use. Often it is recommended that compost "cure" for several months to allow for continued slow decomposition of more resistant constituents.

Troubleshooting

Some problems encountered in a composting operation may include odors, temperature abnormalities, and fly or mosquito problems. Odor problems, addressed throughout this article, may result from odorous feedstock, excess nitrogen released in ammonia form, and odorous by-products (such as hydrogen sulfide gas) of anaerobic decomposition. Control these with proper storage or timely use of feedstock, sufficient carbon sources and bulking agents to control ammonia release, and appropriate moisture and aeration to ensure aerobic conditions. An acceptable temperature can be maintained by building piles or windrows of appropriate size, maintaining moisture content between 40% and 60%, and mixing properly. Insect problems are generally associated with exposed wet feedstock or standing water on site. Timely mixing of feedstocks and good control of water from drainage, runoff, and leaching will limit opportunity for insect breeding.

Utilizing compost

High-quality compost can provide soil nutrients for improved crop, garden, and/or turf productivity. Even compost relatively low in available nutrients can provide valuable organic matter to soil, thereby improving permeability and water-holding characteristics. Cured compost can be marketed as an amendment to improve soil productivity, as a component in potting soil, or as a mulch for landscaping, gardening, etc. If high temperatures have been maintained in the composting process, the compost will be relatively free of weed seeds, insects, and pathogens compared to untreated organic wastes.

Sources of information

For more information about composting, particularly on a farm or commercial level, the following are recommended.

Flynn, R.P., C.W. Wood, and E.A. Guertal. 1995. "Lettuce Response to Composted Broiler Litter as a Potting Substrate Component." Journal of the American Society for Horticultural Science 120(6):964-970.

Northeast Regional Agricultural Engineering Service. 1992. On-Farm Composting Handbook. NRAES-54. Northeast Regional Agricultural Engineering Service, 152 Riley-Robb Hall, Cooperative Extension, Ithaca, NY, 14853-5701.

BioCycle Journal of Composting and Recycling; 419 State Avenue; Emmaus, PA, 18049; telephone (610) 967-4135.

Useful compost websites

Cornell Composting http://www.cfe.cornell.edu/compost/Composting_homepage.html

Downloadable Excel Compost Mixture Calculation Spreadsheets http://www.cfe.cornell.edu/compost/download.html

The Compost Resource Page http://www.oldgrowth.org/compost/

Backyard Magic: The Composting Handbook http://www.gov.nb.ca/environm/comucate/compost/magic.htm