ROTARY COMPOSTERS

Composting

Composting is the biological decomposition and stabilization of organic material. The process produces heat that, in turn, produces a final product that is stable, free of pathogens and viable plant seeds, and can be beneficially applied to the land. As the product stabilizes, odors are reduced and pathogens eliminated. When composting high moisture materials, bulking materials are necessary for reducing moisture content and maintaining the integrity of the pile. Ideally, composting will enhance the usefulness of organic by-products as fertilizers, privately and commercially.

Composting is receiving increased attention as an alternative manure management practice due to increased pressures from society to reduce the impact on the environment. The producer may see alternative benefits to the reduction in volume of manure due to composting. Land base required to apply manure compost may stay the same but the producer can economically haul compost further than manure.
Benefits

• Reduces mass and volume
  - lower hauling costs
• Reduces odor
• Pathogens are destroyed
• Kills weed seeds
• Improves transportability
• Soil conditioner
• Improves nutrient qualities
  - the nutrients from compost are released slowly and steadily
• Decreases pollutants
  - stabilizes the volatile nitrogen into large protein particles, reducing losses
• Land application when convenient
• Saleable product
• Increases water retention of soil

Compost Uses

Compost has numerous agronomic and horticultural uses. It can be used as a soil amendment, fertilizer supplement, top dressing for pastures and hay crops, mulch for homes and gardens, and a potting mix component. In these examples, the compost increases the water and nutrient retention of the soil, provides a porous medium for roots to grow in, increases the organic matter and decreases the bulk density or penetration resistance.

The Composting Process

Under controlled conditions, composting is accomplished in two main stages: an active stage and a curing stage (Figure 1). In the active composting stage, microorganisms consume oxygen (O₂) while feeding on organic matter in manure and produce heat, carbon dioxide (CO₂) and water vapor. During this stage, most of the degradable organic matter is decomposed. A management plan is needed to maintain proper temperature, oxygen and moisture for the organisms. Testing temperature, moisture content, and oxygen levels can help make decisions on composting activities, such as turning, aerating, or adding moisture. These tests can be performed quite simply on site giving quick feedback - from minutes for temperature or oxygen to overnight for moisture content. In the curing phase, microbial activity slows down and as the process nears completion, the material approaches ambient air temperature. Finished compost takes on many of the characteristics of humus, the organic fraction of soil. The material will have been reduced in volume by 20 to 60%, the moisture content by 40% and the weight by up to 50%. One of the key challenges in composting is to retain as much nitrogen as possible. Composting may contribute to the greenhouse effect because carbon dioxide (CO₂), methane (NH₄) and nitrous oxide (NO₂) will be emitted to the atmosphere during composting.

Factors Affecting the Composting Process
Controlling the process factors can accelerate the natural composting process. Each of these factors has the potential to significantly affect the composting process. Some of the important factors in the composting process are shown in below with their acceptable ranges.

Factors affecting the composting process and acceptable ranges

Temperature
Temperature is a very good indicator of the process occurring within the composting material. The temperature increases due to the microbial activity and is noticeable within a few hours of forming a pile as easily degradable compounds are consumed. The temperature usually increases rapidly to 50 - 60°C (122 - 140°F) where it is maintained for several weeks. This is called the active composting stage. Biochemical reaction rates approximately double with each 10°C (50°F) increase in temperature, yet higher temperatures will increase ammonia loss during the composting process. The temperature gradually drops to 40°C (104°F) as the active composting slows down and the curing stage begins. Eventually, the temperature will become that of the surrounding air.

The highest rates of decomposition occur when temperatures are in the range of 43 - 66°C (110 - 150°F). During the active composting stage, the temperature may start to fall because of a lack of oxygen. Turning the material introduces new oxygen and the active composting stage continues. The temperatures can exceed 70°C (158°F) but many microorganisms begin to die, which stops the active composting stage. Cooling the material by turning helps to keep the temperature from reaching these damaging levels. Heat loss occurs primarily because of water evaporation from the material. Heat loss can also occur if the pile is too small or is exposed to cold weather. If the moisture content falls too low it increases the chance of obtaining damaging high temperatures.

The temperature should be maintained at 55°C (131°F) or higher for a minimum of 14 days to destroy the viability of many pathogens and weed seeds. Remember, the edges of the windrow are cool, therefore they must be turned into the center to kill the weed seeds.

The temperature can be measured with a one meter (three foot) long dial temperature probe.


Temperature measurement probe.

Carbon to Nitrogen Ratio
The carbon to nitrogen ratio (C:N) of manure is a very important factor that affects the whole composting process because microbes need 20 to 25 times more carbon than nitrogen to remain active. The ratio should be between 25:1 and 30:1 at the beginning. The microorganisms digest carbon as an energy source and ingest nitrogen for protein and reproduction. Softwood shavings, sawdust and straw are good sources of carbon. Other inexpensive sources of carbon include municipal waste and shredded newsprint or cardboard. Most manures are a good source of nitrogen but may be low in carbon depending on the amount of bedding used. The table below lists the C:N ratio for materials commonly included in farm compost. The content of materials on your farm can be estimated using the table or a laboratory can perform the analysis.

If the ratio is too high (insufficient nitrogen), the decomposition slows. If the ratio is too low (too much nitrogen), it will likely be lost to the atmosphere in the form of ammonia gas. This can lead to odor problems (refer to the troubleshooting table in the back of the manual for solutions). Most materials available for composting do not fit the ideal ratio so different materials must be blended. Proper blending of carbon and nitrogen helps ensure that composting temperatures will be high enough for the process to work efficiently and ensures other nutrients are available for microbes in adequate supply.

Aeration
The minimum desirable oxygen concentration in the composting material is 5%. Greater than 10% is ideal to avoid anaerobic conditions and high odor potential. Aeration adds fresh air in the center of the composting material. Rapid aerobic decomposition can only occur in the presence of sufficient oxygen. Aeration occurs naturally when air warmed by the compost rises through the material, drawing in fresh air from the surroundings at the base of the windrow. Initial mixing of materials usually introduces enough air to start composting. Porosity and moisture content affect air movement through the composting material. Regular mixing of the material, referred to as turning, enhances aeration in the composting material. Good aeration during composting will encourage complete decomposition of carbon (C) to carbon dioxide (CO₂) rather than releasing carbon as methane (CH₄). Too much aeration, however, can actually reduce the rate of decomposition by cooling the composting material and may cause the release of too much CO₂. Excessive air flow can remove a lot of moisture. Another consequence of excessive aeration is ammonia loss, especially with high nitrogen (low C:N ratio) mixes. As the material dries out, more ammonia volatilizes and consequently, more nitrogen is lost.

The oxygen concentration can be measured with an oxygen probe. However, temperature provides an adequate indication of the process conditions. If the supply of oxygen is limited, the composting process slows and the temperature begins to fall. In this case the composting materials should be turned.

Moisture Content
Moisture plays an essential role in the metabolism of microorganisms and indirectly in the supply of oxygen. Microorganisms can utilize only those organic molecules that are dissolved in water. Moisture content between 50 and 60% (by weight) provides adequate moisture without limiting aeration. If the moisture content falls below 40%, bacterial activity will slow down and will cease entirely below 15%. When the moisture content exceeds 60%, nutrients are leached, porosity is reduced, odors are produced (due to anaerobic conditions) and decomposition slows. The squeeze test can be used to check the moisture content. The material is too wet if water can be squeezed out of a handful and too dry if the material doesn’t form a ball when squeezed.

Caution: Material in the pile will be very hot, use a shovel to remove material.

If the pile becomes too wet, it should be turned. This allows air to circulate back into it and loosens the materials for better draining and drying. Adding dry material, such as straw, sawdust or finished compost can also remedy excess moisture problems.

If the material is too dry, water can be added. An effective practice is to turn the material and rewet materials in the process. Shaping the pile can assist in shedding excess water from the pile. A windrow cover can be used to keep unwanted moisture from the elements out of the windrow and conserve moisture within the windrow. Optimum moisture content of raw materials should be between 50 and 60% (wet basis), depending on particle size, available nutrients and physical characteristics.

Porosity
Porosity refers to the spaces between particles in the compost material. These spaces are partially filled with air that can supply oxygen to the organisms and provide a path for air circulation. As the material becomes water saturated, the space available for air decreases, thus slowing the composting process.

Compacting the composting material reduces the porosity. Excessive shredding can also impede air circulation by creating smaller particles and pores. Turning fluffs up the material and increases its porosity. Adding coarse materials such as straw or woodchips can increase the overall porosity, although some coarse materials will be slow to decompose.

pH of Materials
The optimum pH for microorganisms involved in composting lies between 6.5 and 7.5. The pH of most animal manures is approximately 6.8 to 7.4. Composting alone leads to major changes in materials and their pH as decomposition occurs. For example, release of organic acids may, temporarily, lower the pH (increase acidity), and production of ammonia from nitrogenous compounds may raise the pH (increase alkalinity) during early stages of composting. On-site laboratory tests of pH can be used to maintain process control and product quality at a composting site.

Nutrients
Adequate levels of phosphorus (P), potassium (K), carbon (C), nitrogen (N), etc. are important in the composting process and are normally present in farm organic materials such as manure and livestock mortalities. Nutrient loss can occur through volatilization, losses to the atmosphere and leaching. Composting converts the nutrients in manure to stable forms that have a low ability to be lost by volatilization and leaching when applied to the land. However, during the composting process substantial amounts of nitrogen will be lost through ammonia volatilization. The ammonia emissions during composting reduce the fertilizer value of the finished compost. Nitrogen losses can also occur from emission of nitrous oxides or nitrogen gas.

Toxic Substances: CAUTION
Some organic materials may contain substances that are toxic to composting bacteria or bacteria required for composting. Heavy metals such as manganese, copper, zinc, nickel, chromium and lead fall into this category and may be immobilized chemically prior to composting. A laboratory can analyze samples of raw materials for toxic substances. Weathered fly ash, after equilibrating with atmospheric CO₂, is called lagoon ash, which has an alkaline pH and provides a good fixing agent to suppress the availability of heavy metals in manure compost. Clopyralid is a long-lasting herbicide used to control broadleaf weeds. It does not pose a threat to humans or animals. It passes through animals and the composting process with little breakdown. Compost contaminated with clopyraid may harm certain types of broadleaf or ornamentals and vegetables such as beans, peas, peppers, tomatoes and potatoes. If you suspect that manure or compost is contaminated with clopyralid, it is better to send a manure or compost sample to the lab for testing.

Material Characteristics
It is important to be familiar with the material used in composting. Make sure to have proper C:N ratios. Materials can be blended together to attain the proper ratio. The table below contains characteristics of common on-farm composting materials. In order to blend materials in suitable proportions several factors must be considered. The necessary formulas and a sample calculation are in Appendix A.

Characteristics of common composting materials Material Nitrogen (dry weight) (%) C:N (dry weight) Moisture Content (%) Bulk Density @Moisture Content (kg/m3) Beef - Feedlot with bedding 1.3 1.8 68 710 Dairy - Solid manure handling 1.7 18 79 710 - Liquid slurry 2.40 - 3.60 16 88 – 92 990 - Solids separated from slurry 1.45 23 77 650 Pigs - Liquid slurry 0.15 - 5.00 20 93 - 99 1000 - Solids separated from slurry 0.35 - 5.00 1.9 75 – 80 270 - 860 Poultry - Broiler breeder layer 3.6 10 46 470 - Broiler litter 4.7 15 25 330 - Turkey litter 4.2 14 33 380 Horse Manure with Bedding 1.40 - 2.30 22 - 50 59 – 79 725 - 960 - with straw 1.5 27 67 - - with shavings 0.9 65 72 - Sheep Manure 1.30 - 3.09 13 – 20 60 – 75 - Straw - general straw 0.30 - 1.10 48 - 150 27-Apr 58 - 357 - oat straw 0.60 - 1.10 48 – 98 14 130 - 192 - wheat straw 0.30 - 0.50 100 - 150 10 135 - barley straw 0.75 - 0.78 - 18-Dec - Legume Grass Hay 1.80 - 3.60 15 – 19 10 – 30 - Cardboard 0.1 563 8 154 Leaves 0.50 1.3 40 – 80 38 60 - 80 Paper 0.20 - 0.25 127 – 178 18 – 20 130 Sawdust 0.06 - 0.80 200 – 750 19 – 65 207 - 267 Woodwaste (Chips) 0.04 - 0.23 212 – 1313 15 – 40 264 - 368

Blending Sample Calculation

Formulas for Only Two Ingredients
Required amount of ingredient a per kg b
To obtain desired C:N ratio:	a = % Nb x (R-Rb) x (1-Mb) .>..% Na (Ra-R) (1-Ma)
.
To obtain desired moisture content:	a = Mb-M .>..M-Ma

Formulas for a Mix of Materials
C:N ratio	= weight of C in ingredient a + weight of C in b + weight of C in c +… .>...>..weight of N in a + weight of N in b + weight of N in c +… = [%Ca x a x (1-Ma)] + [%Cb x b x (1-Mb)] + [%Cc x c x (1-Mc)] > [%Na x a x (1-Ma)] + [%Nb x b x (1-Mb)] + [%Nc x c x (1-Mc)]
.
Moisture content	= weight of water in ingredient a + weight of water in b + weight of water in c + ... .>...>...>....>...>...>..>..total weight of all ingredients = (a x Ma) + (b x Mb) + (c x Mc) … .>...>.>....a + b + c + ...0

Troubleshooting

Table 5. Manure composting troubleshooting

Problem	Possible Causes	Solutions
The inside of the windrow is dry	Not enough water	Add water when turning the windrow
Temperature is too high	1. Low to moderate moisture	1. Add more water and continue turning the windrow
	2. The windrow is too big	2. Try to decrease the size of the windrow
Temperature is too low	1. Insufficient aeration	1. Turn the windrow more frequently to increase the airflow
	2. Wet condition of the windrow	2. Add more dry material
	3. Low pH	3. Add lime or wood ash and remix
Ammonia odor	1. High level of N (C:N ratio less than 20:1)	1. Add high carbon material, such as sawdust, woodchips, or straw
	2. High pH	2.Lower pH by adding acidic ingredients (leaves) or avoid adding more alkaline materials such as lime and wood ash
Hydrogen sulphide odor	Windrow material is too wet and its temperature is too low	Add dry bulk material