There are a wide variety of farms. They vary in their resources and their environmental concerns. Some farms have access to more capital, skilled labor, management ability, land resources, water resources and markets than other farms. Different manure treatment and handling methods are needed to match the resources and needs of different farms. Recent studies have shown manure-handling costs on farms can be significant. Figure 1* shows costs collected from western New York dairies in 1996. These do not include storage costs and they do not include additional costs that increased management from the implementation of a Comprehensive Nutrient Management Plan (CNMP) would require.
There is potential cost reduction in better fertilizer management, use of more efficient equipment and the use of alternative handling methods on smaller farms.
Manure has been traditionally applied fresh to the land as a fertilizer and soil amendment. Although this practice will continue, many farms will need to change this relatively simple manure handling to more complex storage and treatment methods to respond to the environmental concerns that are increasingly being raised.
Society has recognized animal agriculture can lead to excess nitrates in groundwater, pathogens and excess nutrients in drinking water, Biological Oxygen Demand (BOD) and sediment in surface water. To avoid these problems, manure will increasingly be spread on dry soils in fields where the chance of runoff and leaching are low. Environmental agencies are prescribing these changes. There are now many state, provincial and federal regulations on the timing and amounts of manure spreading. In order to hold manure until the appropriate time to spread, storage will be a standard practice on most farms. In 1997, only 10 percent of the dairy farms in New York had more than six months of storage.
Manure in storage is generating many complaints about odor. When manure is stored, it starts to decompose anaerobically. The byproducts of incomplete anaerobic decomposition are very smelly. Society objects to bad odors as much as, if not more than, dirty water. Therefore, treatment for odor control will become much more common as farms are forced to convert to storing their manure.
Planning for phosphorous
Phosphorous has been identified as the most common limiting nutrient in freshwater. As higher phosphorous levels are building up in some agricultural soils, soluble phosphorous is being released with water flows leaving the fields. Preferential flow allows phosphorous to readily leave tile-drained fields during wet weather applications. States, provinces and federal governments are responding by requiring phosphorous-based nutrient management plans. Manure generally provides a higher amount of crops’ needs for phosphorous than nitrogen. Phosphorous-based plans will require manure to be spread thinner and hauled longer distances to cover more fields. Therefore, there will be an increased need for treatment that concentrates phosphorous, making it easier to haul long distances. Treatment that reduces the mass of manure would meet this need as well.
Development of byproducts that can be sold off the farm could help maintain profitability while improving the environment. Compost or organic matter that can be used as a soil amendment may develop into a market of which farms can take advantage. Organic farms and landscapers are growing businesses that may be looking for more of this type of material. Prices for compost-like material have been reported ranging from $5 to $30 per cubic yard.
Pathogens from manure can easily enter the environment. Both society and regulators are increasingly trying to reduce the amount of pathogens or indicator organisms in drinking water and contact with recreational water. Detection methods for disease-causing microorganisms have become more sophisticated, such as to be able to trace the source of the pathogens. Soon, treatment for pathogen reduction will be needed. Gas releases that form smog, greenhouse gases or contribute to acid rain may be regulated in North America in the future. Europe is regulating the amount of ammonia farms can release. Controlling these gases will require treatment methods for manure.
Depending on location and management’s personal values, each farm can have different environmental concerns. Those operating in a watershed that supplies drinking water may be more interested in controlling pathogens and phosphorus. Those upstream of a freshwater lake may be more concerned with sediment and phosphorus. Those with close or sensitive neighbors may be more concerned with odors. Those in a porous aquifer may be more concerned with nitrogen leaching and pathogens. Others may only be concerned about BOD loading that kills local fish. Nutrient loading far downstream may be a concern to some farms. Manure treatment methods will be required to deal with each of these issues.
Composting has been proposed for years as one of the best methods to treat dairy manure. It has been used on excessively bedded dairy manure, separated dairy manure solids and on the drier manure produced by other animal species to reduce odors. The costs of composting may be offset by sales of the compost. Most dairy manure is too wet initially (12 percent dry matter [DM]) to compost well, as shown in Figure 2*. It needs to have a moisture content of less than 75 percent to heat up and start composting easily. Smaller farms using more bedding may have manure with a lower moisture content.
There may be niche opportunities on some farms with a source of a high-carbon waste stream. Farms with cheap sources of old hay, waste paper, bark, sawdust or recycled compost may be able to add enough solids to support a composting operation. Charging tipping fees for the material brought in or aggressively marketing the compost produced can add a profitable enterprise to the dairy operation.
Mechanical separation of the manure solids can produce a “solid” portion (15 to 30 percent DM and about 20 percent of the original mass) and a “liquid” portion (4 to 8 percent DM and about 80 percent of the original mass). Liquids are easier to handle than a semisolid. Solids can be recovered for bedding, soil amendment or exported off the farm. High capital and operating costs for the mechanical equipment have caused some farms to quit separating. Maintenance of the equipment is a problem. Marketing of the solids may not be successful on all farms. The real problem with this system is the small volume of manure that is composted and the large volume of liquid waste remaining.
Biodrying of the manure by recycling dry compost as the amendment in the alleys and using the heat generated in the aerobic decomposition to dry the manure-compost mix with forced air has been proposed. Odor, volume, weight and pathogen reduction would occur. Equipment for solids handling is available on most farms, so adoption by many farms should be easy. Storage of solids is safer environmentally than liquid storage because of the lower risk of catastrophic failure. The compost material may be marketed as an income source and to move the nutrients off the farm. The management of the drying process will be critical and the costs of the operation may be high. Additional amendment may be required.
Using the heat of composting to evaporate water and dry compost has been used in municipal systems for quite some time. Biodrying had been shown to work on animal wastes in the past, but the in-vessel machinery was seen as too expensive to run on a dairy farm. Recent catastrophic failures of liquid storages, high costs of liquid handling and the odor problems associated with liquid manure storage, started the search for a solid handling system for dairy manure. Systems that quickly composted and dried the solids under a roof and without turning made the composting shed seem possible. Studies showing composting could remove significant moisture when liquid manure was added reinforced the idea. To explore the idea of biodrying on a farm, a proposal from NYSERDA was developed with the following goals:
Description of biodrying
If managed carefully, the heat generated by aerobic composting can provide the energy to reduce 12 percent DM manure to a 60 percent DM residual. Forced air composting under a roof with a carefully controlled airflow would optimize this process. Composting works best with an initial moisture content below 70 percent.
Recent applications of composting operations have shown the feasibility of this process by using forced air to compost 6-foot high layers of manure in 21 days. Recycled compost or a mix of compost and sawdust, or other amendment, at 40 percent DM could be spread in the cow alleys about 3 inches thick to absorb one day’s production of 12 percent DM manure. The mixture could be scraped into a shed, piled 6 feet deep and aerated to produce 40 percent DM compost in three weeks.
Figure 3* shows a side view, plan view and cross-section of the biodrying shed. The building was designed with a high overshot roof, open walls and 4-foot eaves to provide good ventilation while keeping the process protected from precipitation. Manure and recycled compost can be loaded from either side, although preliminary trials have shown a side delivery manure spreader can build a 6-foot high pile that is 40 feet long. A control system can be developed to run the fans that will optimize the composting operation.
The resulting material would then be used as the dryer material, mixed with an amendment, if needed, and placed in the alley each day. This recycle loop could be continued indefinitely. One-third of the compost produced each day would not need to be recycled and would be stockpiled for sale or land application on the farm.
This process could potentially compost all of the manure produced, with little additional amendment needed. The compost would be reduced one-half in volume and one-sixth in weight from the original manure due to water loss and solid conversion to gasses.
For 85 cows, 100 heifers and 30 calves, it is estimated a shed to compost the material will need to be 40 feet by 120 feet if the material is piled 6 feet deep. About 365 cubic feet of compost will need to be mixed with the 616 cubic feet of manure and bedding produced each day. About 865 cubic feet of compost plus manure will be placed in the compost building each day. This will produce 624 cubic feet of compost at approximately 40 percent moisture in 21 days. Taking away what is needed for recycling, there will be a daily production of 260 cubic feet or 5,170 pounds of compost per day. This is shown in a schematic in Figure 4*. Yearly production would be 950 tons or 3,500 cubic yards.
Pathogen and odor control would be substantial. Heat produced during the compost process has been shown to reduce pathogen viability substantially. The aerobic nature of the composting process produces few odors, if managed correctly.
With odor controlled, spreading the compost during the growing season can occur without offending neighbors. Storing and spreading a high-solids product should reduce the runoff and leaching potential of land spreading and eliminate the potential of a catastrophic failure from the storage system.
If the compost is sold off the farm, all the phosphorous can be exported. This can be a huge advantage for those farms in a phosphorous-excess situation.
The reduced volume from a composting system that minimizes the amount of amendment needed will keep both spreading costs down as well as costs for the amendment. Selling the compost produced can potentially add a cash enterprise to the farm. Selling to a wholesaler may produce less profits, but it will keep the marketing effort low.
If successful, this system would likely be adopted by many small- and medium-sized farms that have yet to adopt to liquid storage systems. Farmers and the community will enjoy the odor reduction. Environmental agencies will enjoy the pathogen reduction and the ability to export phosphorous. The capital cost for this system would consist of a three-sided composting shed with an aeration system installed in the floor. The estimated cost of this is $192,000. Fifteen 1.5-horsepower fans will be needed in this installation. Piping every 32 inches will deliver the air. A control unit on the fans that has a feedback system to the temperature of the compost will be needed for each fan. Additional ventilation, condensing or reversing of the airflow may be needed in the winter to optimize the process. Additional testing and feedback systems will be installed. It is anticipated the dairy farm will have the needed material handling equipment on the farm. The additional material handling, amendment (if needed) and power for the aeration equipment would be the operating costs. These costs may be offset by sales of the product, use of the compost as bedding or reduced storage and spreading costs.
The composting shed would need to be large enough for 21-day storage of the compost-manure mix piled 6 feet high. Additional storage for the excess compost could be provided on a pad with controls for rainwater runoff. The air control-temperature feedback system will need additional controls and testing to optimize moisture removal. The rest of the system is well within the management capabilities of most dairy farm operators. The costs of the present manure-handling system are shown in Table 1*.
It should be noted that the present system is unacceptable environmentally, as the phosphorous is being concentrated on the fields close to the barn and spreading is done on a daily basis, with the risk of loss to the environment. The milkhouse waste cost is included in this table. It would need to be fixed, as it is presently discharging inappropriately and to better compare the manure-handling systems that will deal with the milkhouse waste. The cost of this system is comparable to the costs shown in Figure 1* for farms, without considering milkhouse waste.
Installing a traditional liquid manure-handling system would have the costs shown in Table 2*. The soils at the site are not suitable for an earthen storage. The milkhouse waste could be handled with the liquid manure after pumping to the storage. This system would save on fertilizer costs, but add a huge labor demand during the spring when corn planting and first cutting of hay already demand high labor. This plan would spread out the phosphorous, but would still be providing excess to the land over time. The liquid manure would add odor as a concern to the farm and the community. Catastrophic failure and pathogen pollution are also risks with this method.
Meeting a CNMP by biodrying and spreading the compost on the farm result in the costs shown in Table 3*. This method of manure handling reduces the odor potential and reduces the pathogen concerns. It is the most expensive alternative. The cost of operating a biodrying system can be offset if the compost material is sold off the farm. This exports the phosphorous, so it adds to the environmental benefits as well. Fertilizer would need to be purchased for the farm, but the fertilizer could be obtained at the correct proportions, reducing the problem of phosphorous build-up. The costs for this are shown in Table 4* using $5 per cubic yard for the profits from the sale of the compost.
Selling the compost at a higher price can have a large positive effect on the system’s cash flow. Selling the compost for $20 per cubic yard can actually show a $136 profit per cow per year for manure handling.
Biodrying may be a way to meet environmental, odor and economic considerations for dairy farms. Costs of the system have been estimated to vary from $797 per cow per year if the compost is utilized on the farm, to a cost of $480 per cow per year if the compost is sold for $5 per cubic yard. ANM
References omitted due to space but are available upon request.
Tables and Figures omitted but are available upon request to email@example.com.
—From PRO-DAIRY, Cornell University Extension
Peter Wright, Senior Extension Associate, PRO-DAIRY, Agricultural and Biological Engineering Department, Cornell University