Direct land application of livestock manure is often the preferred method of utilization, but it is not always feasible. If the land to be used for application is distant or the location is sensitive to odor, some type of manure treatment may be desirable. Unfortunately, treatment will not necessarily reduce the land area needed for application and may result in increased loss of ammonia nitrogen from manure.
Livestock manure is treated for several reasons:
Treatment processes fall into three categories—physical, chemical, and biological. Physical treatment systems involve such simple processes as settling, filtering, and drying to change the characteristics of the manure. Chemical treatments add something to help condition it. Biological treatments take advantage of naturally occurring microorganisms in the manure to change its properties.
It is sometimes desirable to separate the solid and liquid portions of livestock manure. This can be accomplished through physical treatment for the following purposes:
Settling takes advantage of gravity to separate the solids from the liquids. Livestock manure is placed in a stilling basin to allow solids to settle to the bottom. A detention time as short as 30 minutes can be used to settle out solids from dilute wastewater such as open-lot runoff. Septic tanks installed ahead of leach fields and settling basins used with vegetative infiltration areas are examples of settling systems. Refer to Chapter 5, Farmstead Runoff Control, for more information on settling basins. Solids must be removed regularly to maintain treatment efficiency of settling systems and to recoup the storage capacity.
Centrifuge separators function similarly to a centrifuge to dewater manure and rely on the differences of density between solid and liquid material. Since solid material is denser, it will settle out in an applied sedimentation field. Centrifuge separators are in general more efficient in dewatering manure than other mechanical separators.
Filtering and screening systems use a medium to hold solids as the liquid moves through. Gravity, vacuum, or pressure can be used to move the liquids through the media. Liquid-solid separators that use stationary and vibrating screens remove solids from flushing water. Sand drying beds are a simple application of filtering where gravity carries the liquid down through the sand and the solids form a cake on top. Vacuum filters often use cloth or a wire screen to hold the solids as the liquid is drawn through. Presses also use cloth or wire screens to hold the solids as the liquid is pushed through.
Drying is used primarily for volume reduction by encouraging the water to evaporate, concentrating the solids. In Ohio, drying systems must be covered to protect them from rainfall, and supplemental heat or forced air is needed to encourage rapid evaporation.
Coagulating agents such as ferric chloride, lime, alum, and organic polymers can greatly improve the dewatering characteristics of livestock manure. These chemicals bring the solids in manure together so they settle more rapidly. Bringing the smaller particles together also improves solids removal by filtration. Care must be taken when handling coagulants. Some are corrosive and others are very slippery if accidentally spilled.
Raising the pH of livestock manure to pH 12 for 30 minutes kills many of the microorganisms that live in manure. The result is to eliminate odor production and limit the spread of disease. Quick lime (CaO) or hydrated lime (CaOH) is usually used to raise the pH level of livestock waste. One limitation to this treatment is the immediate loss of ammonia from the manure. Both quick lime and hydrated lime are highly reactive and need to be handled with extreme care. Consult manufacturer guidelines for proper safety procedures.
Various manure-storage pit additives are marketed to reduce manure odor. The additives are made of chemicals, microbes, bacteria, enzymes, or plant derivatives used individually or in combination. The effectiveness of additives varies, depending upon the manure characteristics and storage configuration. The National Pork Producers Council Odor Solutions Initiative Committee published the booklet Odor Solutions Initiative Testing Results, Manure Pit Additives in 2001. The booklet summarizes the testing of 35 pit additive products. Most additives tested did little to control ammonia or odor emissions.
Anaerobic lagoons stabilize livestock manure by taking advantage of natural processes. In the absence of oxygen, high-strength organic wastes, such as livestock manure, is digested by anaerobic bacteria. Anaerobic lagoons are commonly used in Ohio for treatment of swine manure from pull plug gutter systems and treatment of washwater from egg packaging facilities.
Anaerobic lagoons for livestock manure have several advantages:
Anaerobic lagoons for livestock manure also have several limitations:
In an anaerobic lagoon, bacteria break down the manure in a two-step process (Figure 17). One group of bacteria converts the manure to organic acids. The second group converts the organic acids to methane gas and carbon dioxide.
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| Figure 17. Anaerobic digestion process. (Source: Ohio State University Extension Bulletin 604, 1992 Edition.) |
The management requirements of an anaerobic lagoon are primarily concerned with creating the right environment for the methane-forming bacteria. These bacteria are upset by sudden changes in temperature, a drop in pH, “slug loads” of organic waste, or toxic substances. Anaerobic lagoons are usually constructed as deep as allowable by soil conditions or pumping equipment limitations. A cross section of an anaerobic lagoon is shown in Figure 18. Anaerobic lagoons can be constructed as single- or two-stage lagoons. Single-stage lagoons are more typical in Ohio; however, two-staged lagoons are used where high-quality water is desired for pit recharge.
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| Figure 18. Anaerobic lagoon cross section. (From Natural Resources Conservation Service (NRCS), Animal Waste Management Field Handbook, Figure 10-21.) Used by permission. |
Anaerobic lagoons work best in warm weather. Even in warm weather, several months are required to fully stabilize manure in an anaerobic lagoon. As the temperature drops, more time is needed. In areas like Ohio, where winter water temperatures can drop near or below freezing, lagoons can experience “turnover” in the spring and fall. Turnover occurs as the lagoon is heating up in the spring or cooling down in the fall. During turnover, water from the bottom (which is high in odor) comes to the top and water from the top moves to the bottom. Agitating the lagoon can help shorten the turnover period.
A well-functioning lagoon will have a neutral pH (7.0 to 8.0). If the first group of bacteria, the organic-acid formers, grows and multiplies faster than the methane formers, the pH of the lagoon can drop. If the lagoon is left untreated, it will go “sour,” methane production then ceases, and strong odors are released. If the lagoon pH drops below 6.7, it is important to add hydrated lime or caustic soda—use extreme caution as these are highly reactive chemicals; consult the manufacturer’s guidelines for safety procedures—daily at a rate of 1 pound per 1,000 cubic feet of lagoon volume until the pH is raised above 7.
A well-functioning anaerobic lagoon requires continuous loading of manure and wastewaters. When starting up a lagoon, fill it one-third to one-half full with clean water to dilute the manure and reduce shock on the system. Failure to do so will result in high odor production. Constant amounts of manure should be added each day. Slug loads of manure can cause an increase in organic-acid production, a drop in pH, and strong odors. Slug loading is especially discouraged in the winter, when biological activity is lowest. Store the excess manure until it can be slowly added to the lagoon.
Certain compounds are toxic to the organisms in an anaerobic lagoon. Keep chemicals such as arsenic, copper, and antibiotics out of the lagoon.
An alternative design to a single- or two-stage lagoon is the anaerobic lagoon/settling basin. The anaerobic lagoon/settling basin is designed to hold the majority of total and volatile solids in the settling basin which significantly reduces the lagoon loading and resulting size. The system will typically be desired by a producer who wants to utilize the nutrient value of the manure and have recycled water available for flushing. This system should not be considered when odors are an issue because the settling basin behaves like a holding pond.
Additional information on the planning and design of anaerobic lagoons can be found in Ohio Natural Resources Conservation Service (NRCS) Conservation Practice Standard 359, Anaerobic Waste Treatment Lagoon, at: www.oh.nrcs.usda.gov/technical/ohio_eFOTG.html and Chapter 10 of the NRCS AWMFH available at: www.wcc.nrcs.usda.gov/awm/awmfh.html.
Aerobic lagoons stabilize livestock manure through the addition of oxygen. By adding large amounts of oxygen to the manure, naturally occurring bacteria will begin to break down the manure and reduce its odor in one to six months. Aerobic digestion is a one-step process. Bacteria use oxygen to convert manure to carbon dioxide and water. Aerobic lagoons can be either naturally or mechanically aerated.
Naturally aerated lagoons operate within a depth range of two to five feet to allow oxygen entrainment necessary for the aerobic bacteria to digest the manure. They are designed for an allowable loading, ranging from 27 to 37 pounds (depending upon location in Ohio) of BOD5 (biological oxygen demand) per acre per day. These lagoons will have a large surface area in comparison to an anaerobic lagoon or mechanically aerated aerobic lagoon. A cross section of a naturally aerated lagoon is shown in Figure 19.
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| Figure 19. Cross section of naturally aerated lagoon. (From Natural Resources Conservation Service (NRCS), Animal Waste Management Field Handbook, Figure 10-24.) Used by permission. |
Mechanically aerated lagoons use mechanical aeration to supply the oxygen needed to treat manure and minimize odors. Two kinds of mechanical aerators are used—the surface pump and the diffused-air system. The surface pump floats on the surface of the lagoon, lifting water into the air, thus assuring an air-water mixture. The diffused-air system pumps air through water, but is generally less economical to operate than the surface pump.
Aerators are designed primarily on their ability to transfer oxygen (O2) to the lagoon liquid. Of secondary importance is the ability of the aerator to mix or disperse the O2 throughout the lagoon. Poor mixing or shutting off the aerator will result in strong odors.
Aerobic bacteria need oxygen, so the lagoon must be managed carefully to make sure that adequate oxygen is always present. Dilution water is needed from the start-up of the lagoon, and a steady daily supply of manure is required. Slug loads will quickly use up the oxygen and result in a strong odor.
Aerobic lagoons used for livestock manure have several advantages:
Aerobic lagoons also have limitations:
Anaerobic digesters are used to more fully control the anaerobic processes taking place in an anaerobic lagoon. Digesters are covered, heated, and stirred to shorten the time needed to stabilize the manure, to control odors, and to capture the methane produced. Due to the shortened treatment time, the treatment volume required for anaerobic digesters is almost 100 times smaller than the treatment volume required for anaerobic lagoons; however, the total system volume must also include storage of treated manure between periods of land application (Figure 20).
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| Figure 20. Total treatment volume and level of management for three treatment systems. The same volume of raw manure is treated in each system. (Source: Ohio State University Extension Bulletin 604, 1992 Edition.) |
Anaerobic digesters work under the same two-step biological process as anaerobic lagoons. One group of bacteria converts manure to organic acids, and another group converts the organic acids to methane and carbon dioxide. The same factors that upset an anaerobic lagoon will upset a digester, such as sudden temperature changes, a drop in pH, slug loading of manure, and toxic substances.
Anaerobic digesters are operated at relatively high temperatures to stabilize manure as quickly as possible. They are heated to maintain a temperature of 70ºF to 140ºF, with 100ºF being optimum. Maintaining the right temperature is the single most important management factor in operating an anaerobic digester.
Mixing within the digester helps keep the bacteria in contact with the manure and keeps solids from settling out in the digester. Several mixing systems can be used, such as mechanical mixers, pumps, or bubbling with digester gas. To eliminate the need for mixing, plug-flow digesters have been developed to slowly move the manure through a tube-shaped vessel. In plug-flow digesters, the manure added today will leave the other end of the digester in about a month (Figure 21).
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| Figure 21. Basic digester types. (Source: Ohio State University Extension Bulletin 604, 1992 Edition.) |
Mixed digesters are usually used with liquid manure, and plug-flow digesters are best loaded with semi-solid manure (about 13% solids). Large amounts of bedding and soil should not be added to a digester. A carefully controlled mixed digester can stabilize manure in 20 to 30 days. As with an anaerobic lagoon, the digester should be loaded with steady amounts of manure daily. Slug loads will upset the digester. The first sign of upset is a drop in pH. If the pH drops below 6.7, first check the temperature and then check and possibly reduce the feed rate. Adjust the pH if needed.
The biogas produced by anaerobic digesters is about 50 to 60% methane (natural gas), 40 to 50% carbon dioxide, and less than 1% other gases such as hydrogen sulfide. The digester gas is often burned to heat the digester. Although digester gas can be burned to generate heat and electricity, the trace gases and water vapor in digester gas are corrosive to equipment and must either be removed before burning or equipment must be more extensively maintained.
For every 100 pounds of raw manure added to a digester, four pounds are converted to biogas. The remaining 96 pounds still contain all the potassium and phosphorus present in the manure. Some of the organic nitrogen in the manure is converted to ammonia during digestion, increasing the possibility of nitrogen loss during land application.
Daily attention is required to measure the anaerobic digester’s temperature and pH and check the manure-loading and gas-collection systems. Daily management takes about 15 to 30 minutes. The digester should be emptied every one to two years to remove solids from the floor, clean heating pipes, and make necessary repairs. Major repairs and preventive maintenance will also be needed.
Safety considerations are important with anaerobic digesters. The methane produced is flammable, and the greatest threat for explosion is in a confined space. Because methane is lighter than air, a continuous ridge vent in the building housing the digester equipment is necessary. The hydrogen sulfide in biogas is also hazardous. Hydrogen sulfide levels become life-threatening in 30 minutes at 300 ppm. Because hydrogen sulfide is heavier than air, sensors in buildings housing digester equipment should be placed near the floor.
Anaerobic digesters used for livestock manure have several advantages:
Anaerobic digesters also have limitations:
Composting is a natural biological process requiring air, moisture, and the right proportion of carbon to nitrogen to stabilize organic material. It is predominantly an aerobic process and is used to stabilize all types of organic wastes. The process consumes oxygen and releases heat, water, and carbon dioxide (CO2). The microorganisms use the most readily biodegradable substances as their food source. The compost that remains resembles humus and can be used as a soil conditioner, organic fertilizer, or as a food base for organisms that suppress plant diseases. Composting reduces the volume and mass of the parent materials by 40 to 80% and destroys pathogens if the process is controlled properly.
In conventional composting, ingredients are brought together, mixed, then put into a pile to compost. Rynk (1992) describes materials used as being primary (material of interest to be composted), amendment (material added to adjust C/N or water content), and bulking agent (material added to “open” up the compost mix, giving it porosity so air can move through the pile to provide oxygen and cooling). Generally, the mix is turned every three or four days, but sometimes every day or only weekly or monthly.
In some systems air is forced through the compost to control temperature and keep the pile supplied with oxygen. When little or no heat output is observed, the material is removed, remixed, and put into a curing pile for several months. The rate of composting can be controlled by adjusting the air, moisture, and carbon and nitrogen contents. Manure mixes typically take several months to a year to compost and cure.
While composting occurs naturally, the process requires proper conditions to occur rapidly, minimize odor generation, and prevent nuisance problems. More than 20 controllable factors affect composting (Keener et al., 1993). Of these factors, nutrient balance, water content, porosity, and temperature will be discussed.
The ratio of carbon to nitrogen (C/N ratio) is critical to the composting process. The recommended range is 25:1 to 40:1, with the ideal ratio about 30 parts carbon to 1 part nitrogen. This C/N ratio in the compost meets the needs of microorganisms for high rates of decomposition while minimizing the loss of nitrogen as ammonia. Many manures have a C/N ratio that is too low to compost efficiently, so amendments that contain a high C/N ratio must be added. Plant materials such as wood chips, sawdust, chopped corn stover, or straw are ideal amendments. Phosphorus and other principal and trace elements are generally available in satisfactory amounts for microorganisms when manures are blended with amendments to achieve a proper C/N ratio.
Composting is a biological process requiring air (oxygen). If the material to be composted is too wet, as with most manures, the limited amount of air available in the pile will hinder the process. Adding a bulking agent to manure, such as wood chips, corn stover, or leaves, allows air to get into the pile. Turning the pile over is another way to ensure that air gets into the composting material. Fans can also be used to draw air through the compost pile to ensure proper aeration (Figure 22).
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| Figure 22. Aerated static pile. (Source: On-Farm Composting Handbook, NRAES-54, 1992. Natural Resource, Agriculture, and Engineering Service, Ithaca, N.Y.) Used by permission. |
Moisture is also an important factor for microorganisms to function during composting. The ideal moisture content is about 60% moisture with an acceptable range of 50 to 70%. It is important to avoid excess water because of the potential for odor and leaching conditions. If the mixture feels moist, but no water drips from it when a handful is squeezed, the mixture probably has adequate water content. Moisture is lost from the pile throughout the composting process. It is important to add water if the compost gets too dry and to stir or add more bulking agent if it gets too wet. Composting in the open air is affected by rainfall, and in some regions, rainfall saturates compost piles, causing leachate and odor problems.
The optimum temperature range for composting is 110 to 145ºF. Temperatures above 131ºF kill most animal pathogens, plant pathogens, and weed seeds if sustained for three days or longer. At temperatures above 145ºF, microbial activity declines, with activity approaching low values as compost temperatures exceed 160ºF.
Optimum composting temperatures are achieved by regulating airflow and/or pile size and allowing heat generated through microbial activity to leave the pile. A compost mass stored in piles more than five-feet high by 10 feet across usually allows the temperature to reach 140ºF in less than two days. Maximum practical depth ranges from 5 to 11 feet, depending on the material to be composted. Deep piles (depths >5 feet) sometimes lead to spontaneous combustion.
Composting systems can be classified as static piles, aerated static piles, turned windrows, and aerated turned windrows. The terms windrow and pile can be used interchangeably in these descriptions. The terms in-vessel and tunnel indicate that the pile or windrow is contained within a structure. Which system represents the best technology depends on the material to be composted and takes into account not only environmental issues (health, safety, public nuisance, etc.) but also the economics. A brief description of various systems follows.
Composting material is placed in a pile (or windrow) and left to compost with minimal turning and without using forced ventilation. This approach is generally used with materials unlikely to generate offensive odors if anaerobic, materials such as leaves and ground yardwaste which have high C/N ratios (>50). However, dead animal composting also uses the static pile approach to composting.
When escaping odors may be a problem, the pile is capped with a biofilter type material to trap and destroy the odors. These systems are usually turned using a front end loader once at the end of the first phase (high rate) of composting and once between a secondary phase (stabilizing phase) and a curing stage.
Composting material is placed in piles (or windrows) with ventilation ducting underneath the piles. Past studies on aerated pile systems show they work best using forced ventilation as opposed to suction because of higher pressure drops and accumulation of water in the piping under suction. Even so, many systems ventilate by negative-pressure on the compost and exhaust the resulting gases through a biofilter system.
The rate of aeration required to prevent anaerobic metabolism varies with the product being composted. Pile height is generally limited to 8 feet and less than 100 feet in length to balance airflow required to control temperature, static pressure drops, and fan power requirements. Spacing of ducts is usually controlled by pile height since duct spacing is approximately equal to the height of the pile. A block approach, as opposed to windrows with aisles, is often used to maximize use of space. Pipe sizing, both header and aeration duct, and hole placement in the aeration duct are critical for successful operation of this system.
Composting material is placed in a windrow (pile) and is turned at regular intervals. Pile heights are generally limited to five to eight feet because of the ability of equipment to handle material and porosity considerations. Equipment ranges from tractors with buckets to payloaders and from pull type turners handling 5 feet (height) x 8 feet (width) windrows to straddle turners handling 8 feet x 10 feet windrows (or larger). Length of the windrow depends on site location.
Porosity of compost is important to prevent anaerobic conditions in windrows, as with the static piles, since both depend on natural convection (chimney effects) to ventilate the pile. Turning the pile does incorporate oxygen, but if the pile lacks porosity, the center of the windrow becomes anaerobic within minutes. Many turners designed today can introduce water back into the windrow to maintain conditions favorable for a high rate of composting.
Windrow systems often generate some odors early in the process with highly putrescent material. One method to minimize odor is to cap the piles with biofilter type material to trap escaping odors and delay turning the windrows until the temperature of the high-rate phase of composting has declined to <135ºF and oxygen levels have recovered in most regions of the compost pile.
In-vessel composting is used to more fully control the composting process that takes place in a compost pile. In the reactor vessel, the optimum mix of organic waste, moisture, and bulking agent is mixed and aerated. With careful control, composting can be completed in a few weeks. Composting vessels are usually housed in a building to control moisture and reduce odors.
An enclosed reactor version of in-vessel is the tunnel system, which completely controls the path of exhaust air leaving the system. In-vessel systems can be used for all types of prepared (particle size is critical) materials and have the capabilities of controlling emissions. They have high fixed and operating costs and should be selected based on materials to be composted and site requirements.
Composting used for livestock manure has several advantages:
Composting also has limitations:
For more information on composting, refer to Ohio State University Extension Bulletin 792, Modern Composting, and Natural Resource, Agriculture, and Engineering Service NRAES-54, On-Farm Composting Handbook, available through your local county Extension office.
Keener, H. M., C. Marugg, R. C. Hansen, and H. A. J. Hoitink. 1993. Optimizing the Efficiency of the Composting Process. Science and Engineering of Composting. 59-93.
Rynk, R., Editor. On-Farm Composting Handbook. NRAES-54. 1992. Natural Resource, Agriculture, and Engineering Service (NRAES), P. O. Box 4557, Ithaca, NY 14852-4557, www.nraes.org.