0507 ANM: Composting manure and other organic residues


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0507 ANM: Composting manure and other organic residues

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Wednesday, 16 May 2007 03:47

The composting process
Composting is the aerobic decomposition of manure or other organic materials in the thermophilic temperature range (104º to 149ºF). The composted material is odorless, fine-textured and low-moisture. It can be bagged and sold for use in gardens or nurseries or used as fertilizer on cropland with little odor or fly breeding potential. Composting improves the handling characteristics of any organic residue by reducing its volume and weight. Composting can kill pathogens and weed seeds.

Disadvantages of composting organic residues include loss of nitrogen and other nutrients, time for processing, cost for handling equipment, available land for composting, odors, marketing, diversion of manure or residue from cropland, risk of losing farm classification and slow release of available nutrients. During a three-year Nebraska study as much as 40 percent of total beef feedlot manure nitrogen and 60 percent of total carbon was lost to the atmosphere during composting. Runoff and leaching losses of sodium (Na) and potassium (K) were also high (above 6.5 percent each) during composting periods with high rainfall.

Increasing the carbon-to-nitrogen ratio by incorporating high carbon materials (leaves, plant residue, paper, sawdust, etc.) can reduce nitrogen loss. In another study, a 30 percent reduction in nitrogen loss was found during composting of poultry manure in 55-gallon reactors when the C-to-N ratio increased from 15 to 20. Because of nitrogen, carbon, and potassium losses from manure during composting, it may be more desirable to apply the manure directly as a nutrient source unless there are concerns about improving manure characteristics, killing weed seeds and pathogens or reducing odor problems.

Temperature, water content, C-to-N ratio, pH level, aeration rate and the physical structure of organic materials are important factors influencing the rate and efficiency of composting. Homogeneous manure solids can be composted alone without mixing with bulk materials. Bulking agents are needed to provide structural support when manure solids or other organic residues are too wet to maintain air spaces within the composting pile and to reduce water content or to change the C-to-N ratio. Dry and fibrous materials, such as sawdust, leaves, finely-chopped straw or peat moss, are good bulking agents for composting wet manure or organic residues.

Temperature is the most common indicator of how composting is progressing. Elevated temperature is necessary to destroy pathogens and weed seeds in manure or other organic materials. Environmental Protection Agency (EPA) regulations for composting municipal waste require the temperature be maintained at 131ºF or above for at least three days to destroy pathogens. A temperature of 145ºF within the compost pile is needed to destroy weed seeds. Depending on the ambient temperature, a complete composting process may take two to six months. The water content of mature compost should be less than 50 percent and preferably in the range of 30 to 35 percent. The C-to-N ratio should be less than 20.

Composting methods
There are many methods of composting organic materials. These include active windrow (with turning), passive composting piles, passively aerated windrow (supplying air through perforated pipes embedded in the windrow), active aerated windrow (forced air), bins, rectangular agitated beds, silos, rotating drums, containers, anaerobic digestion, and vermicompost (using earthworms). Because of differences in manure characteristics and handling systems for different livestock species, the composting process for each livestock species will be analyzed separately. Other sections will describe composting of other organic residues.

Cattle feedlots
Typically in the central and southern Great Plains, manure scraped from beef feedlots is often mixed with as much as 50 percent soil. About 14.2 pounds of dry manure and soil mixture is collected per head per day when the feedlot is scraped.

Manure collected from feedlots can be composted as is (with no bulking agents) or with high carbon material to increase the C-to-N ratio and reduce nitrogen loss. Feedlot manure can be composted in 60 to 120 days depending on the ambient temperature. Windrow composting is the most common method used for beef cattle feedlot manure. The windrows should be 3 to 6 feet high and 8 to 12 feet wide. Turn with a windrow-turning machine or with a front-end loader.

Swine
Swine manure collected from confinement buildings consists of feces, urine, waste water and feed. Manure collected by deep pits and shallow pits with mechanical scrapers or gravity drainage gutters has 2 to 8 percent solid content, while manure collected by flushing systems is more dilute and has less than 2 percent solids.

For composting, manure solids need to be separated from the liquid. Presses and centrifuges have higher separation efficiencies and produce drier solids than screens. Adding flocculants, such as polyelectrolytes and organic polymers, to manure slurries prior to separation can significantly improve the separation efficiencies. Higher water content manure also can be composted if high carbon bulking materials are added to form a composting mound.

After the bulking materials have been added to swine manure, it can be composted in windrows 3 feet high and 10 feet wide. Frequent turning may be needed to dry the material, increase the temperature in the composting pile, and reduce odor.

Dairy
Manure collected from feeding, lounging and milking barns can be composted. Manure collected in dairy operations contains 75 to 95 percent water and needs dewatering or addition of bulking material for proper composting. The techniques used for dewatering swine manure also can be used for dairy manure. Straw or sawdust bedding are good sources of carbon and drying materials for composting.

Dairy manure contains 3 to 4 percent nitrogen and subsequently benefits from adding high carbon bulking materials to reduce nitrogen loss during composting. Similar to swine manure, dairy manure can be composted after adding bulking material to form a composting windrow.

Animal mortality
Carcass composting can be used for all types of animals. Mortality composting can be accomplished in backyard type bins, indicator composter bins, temporary open bins fashioned from large bales of hay or straw and in windrows or piles on a paved or well-drained soil surface.

Carcasses can be composted in bins using layers of sawdust or chopped straw and dead animals. About 1.5 feet of straw or sawdust is placed under the carcass and 2 feet is needed above the carcass. The biological process of composting animal carcasses is identical to composting any other organic material. The parameters of air, water, nutrient, carbon and temperature need to be regulated. Water content is an important factor to consider when composting dead animals and should be maintained at about 40 to 50 percent. Lower water content promotes dehydration which preserves the carcass while too much water (more than 60 percent) will cause foul odor and may cause runoff from composting piles.

Dead animals are composted in static piles which may be turned once or twice in the active composting period. The pile is then allowed to cure until the pile’s internal temperature is close to the ambient temperature.

Manure from other organic residues
Other organic residues that can be composted include yard wastes, grass clippings, peat moss, sea weeds, fruit and vegetable wastes, food processing wastes, municipal garbage, sewage sludge, etc. When composting any of these materials, consider factors such as air, water content, particle size, C-to-N ratio, pH and temperature.

Management of composting

Adjusting C-to-N ratio and water content
Carbon-to-nitrogen ratio is the weight of carbon divided by the weight of nitrogen in the composting material. When adjusting C-to-N ratio of a mixture for composting, the C-to-N ratio of each added material needs to be considered.

Turning
The composting material should be turned whenever the temperature rises above 145ºF to prevent overheating, which kills the composting organisms. A temperature below 104ºF may indicate lack of adequate oxygen and a need for turning. If the temperature does not rise above 104ºF after turning, the compost should no longer be turned and left for at least one month of curing to complete the composting process.

If the composting material is dry (water content is less than 40 percent), add water to activate the composting process. In some cases, water content lower than 40 percent may result in overheating and a need for watering. If adding water is not an option, regulate the temperature by turning. The composting period may take longer if water content is not maintained at a proper level.

Land application of compost
The composted material is an odorless, fine-textured, low-moisture material that can be used in gardens, potting and nurseries, or used as fertilizer on cropland with little odor or fly breeding potential. Compost can be an excellent source of organic matter, nitrogen and other nutrients. However, nitrogen in compost is stabilized and not as easily available to the crop as nitrogen from the raw material.

Availability of phosphorus, potassium, and micronutrients from compost should be similar or higher than manure or other organic residues used for composting. Since compost is fine-textured and has less water than the raw material, it can be applied more uniformly and with better control. The composted material also can be stored and applied when convenient. Weed seeds or pathogens that can create problems with application of manure or other organic residues should not be a concern when properly made compost is used.

Issues and options
When composting an organic residue, advantages, disadvantages and feasibility of the composting process need to be considered. Composting has benefits and drawbacks and producers need to decide if it is a good option for their operation. Advantages of composting include killing pathogens, fly larva and weed seeds and improving the handling characteristics of manure and other residues by reducing their volume and weight.

Most manure and other organic residues usually contain high nitrogen content and therefore are subject to nitrogen loss during composting. Weed seed destruction, although theoretically possible, needs to be evaluated under field conditions. Manure and other residues may contain significant amounts of water and need to be either dewatered or high carbon bulking agents need to be included for proper composting.

Other considerations for composting include having enough land area, composting odors, economic considerations and environmental consequences. The market for the composted material needs to be developed so composting is a successful venture. ANM

—Excerpts from University of Nebraska Extension website

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