BioCycle November 2004, Vol. 45, No. 11, p. 62
Researchers develop a layered mesophilic compost system to provide fish farmers – as well as others using liquid systems – with an alternative to expensive solids management technology.
Paul R. Adler and Lawrence J. Sikora

ANNUAL estimates of manure solids from trout production are about 26.8 million pounds; From dairy cattle and swine, the amounts each year are roughly 54 and 17 billion pounds respectively. Liquid manure is typically generated since many of these facilities are confined feeding operations with wash down systems to handle manure.
In aquaculture and animal production, the goals of solids treatment systems are volume reduction (e.g. thickening and dewatering) as well as stabilization. Stabilization of solids reduces pathogens (both human and animal) and eliminates offensive odors and the potential for putrefaction. Municipal biosolids treatment and management systems are typically capital intensive, whereas on farms, they are more land extensive, relying more on natural ecological processes to achieve the desired goals.
There are a number of natural biosolids treatment and management systems currently in use; some are anaerobic (anaerobic digesters, lagoons, and wetlands) and others are aerobic (vermicomposting, composting, and land application) processing systems. Anaerobic storage of manure in settling basins has been the standard practice for liquid animal manure. However, the anaerobic storage of manure produces offensive odors and air pollutants and reduces the organic matter and nutrient content of manure. Land application and composting are the most common aerobic organic solids treatment systems. The value of land-applied solids is greatly reduced by anaerobic storage prior to land application. Although land application of manure has a long history of being an effective and practical technology, since it cannot occur year-round, anaerobic storage when it is liquid manure, is a necessary component of the whole system. Composting can be a practical system alternative to storage in a lagoon and gives the farmer more flexibility in time of application. The compost systems most likely to be used on farms are windrows and aerated piles.
MESOPHILIC TEMPERATURES
High temperatures and anaerobic conditions can be reached in pile compost systems and require turning or air injection to keep them aerobic. Mesophilic temperatures (50-104°F) and aerobic conditions can be maintained during decomposition without mechanically mixing or injecting air, as is necessary with thermophilic composting. These conditions can be maintained by either ensuring that the surface area to mass ratio is large enough so that heat can be dissipated rapidly or the highly reactive material is close enough to the surface so passive gas exchange can meet the high oxygen requirements of the decomposing material. As the material decomposes and becomes more stable, it has a lower requirement for oxygen and can be buried because the reduced passive gas exchange with depth is sufficient to meet the lower needs of the more stable material. The layered mesophilic compost system described below has both a high surface area to mass ratio and keeps the highly reactive material close to the surface, only slowly being buried as new layers are added on top over time.
With increased capacity to maintain aerobic conditions, the layered mesophilic compost system can also operate at higher water content. Composting liquid manure typically requires use of excess carbon to keep the system aerobic. Fish manure (similar to other liquid animal manures) typically has a solids content of about two to ten percent (water content of 90 to 98 percent) after initial primary treatment and for thermophilic composting in pile systems, the water content of the compost mixture needs to be reduced to around 60 percent. Often the adjustment of moisture is combined with the desire to increase the C:N ratio to about 25:1 thereby using a carbonaceous bulking agent. At a solids content of two percent, achieving the target of 60 percent moisture content would require almost 10 times more carbonaceous bulking agent than required for a C:N ratio of 25.
Manure in cold water aquaculture production systems is typically settled in quiescent zones created within raceways to concentrate it prior to being pumped to storage in off-line settling basins. As the water passes through the basins, the solids settle out and the excess water is discharged to surface water. Since the basins are not frequently emptied, the organic solids mineralize and nutrients flow out of the basins with the surface water discharge. Although most P is initially in the solids, significant amounts can be lost to the surrounding water during storage in the settling basins and then discharged to surface water. Reducing the loss of P from solids can significantly reduce the amount of P lost to the water environment. Decreasing the loss of P from the solids could be achieved by reducing the amount of time they are stored in the settling basins.
Our objective was to develop the protocol for a land-based aquaculture solids management system that minimizes leaching of P to the surface water environment by storing and treating solids on land rather than in water in off-line settling basins. In this system, solids would continue to be settled in the off-line settling basins but pumped onto straw nearby for storage and stabilization. A layered mesophilic compost system was developed to provide fish farmers with an alternative to expensive solids management technology; it uses resources and equipment that are readily available and fit within current management operations.
DESIGN CRITERIA IN FARM-SCALE STUDY
A farm-scale study was conducted at The Conservation Fund’s Freshwater Institute in Shepherdstown, West Virginia to evaluate design criteria for a layered mesophilic composting system and the effect of carbon source on the operating parameters. Field plots (about 144 ft2) were established with two carbon sources (wheat straw or oak sawdust) and three blocks. A six-inch layer of course wood chips was spread out as a base layer to facilitate passive oxygen transfer from the base and drainage followed by a layer of either wheat straw or oak sawdust.
Arctic char (Salvelinus alpinus) manure was applied at a loading rate of about 0.4 inches (8 percent solids) every 10 days using a vacuum tank spreader modified for side discharge over the plots. The rate of application was determined by dividing the total volume of manure applied by the total area covered during application. Carbon was added after each application at an initial rate to maintain a C:N ratio of about 20. After the first application, it was determined that the application rate for wheat straw could be cut in half due to its very low density nature compared to the oak sawdust, leading to a C:N ratio of about 16. The oak sawdust was applied at a rate to maintain a C:N ratio of about 20 throughout the study.
At the completion of the study, the surface layer represented “time zero” with age of compost increasing with depth. Samples were taken from layers that represented three stages of compost age. The top layer extended from zero to 63 days, the middle layer from 63 to 118 days, and the bottom layer from 118 to 167 days. Compost maturity was determined using the Dewar self-heating test. The Dewar self-heating test was performed in 2-L insulated Dewar flasks. Temperatures of mixtures were recorded using minimum-maximum thermometers.
The loading rates for the experiment were about (pounds/acre/day): 58.3 N, 18.8 P, 902 Arctic char manure, and 1,089 wheat straw or 1,545 oak sawdust. At these loading rates, the land requirements were about one acre to compost the manure for each 446 tons of Arctic char produced annually, but could be significantly lower for the wheat straw since experimental loading rates were limited by the reduced capacity of the oak sawdust compost. The application rate of wheat straw was about 30 percent lower than for oak sawdust. As described earlier, the loading rate of wheat straw was reduced in half from the initial rate needed to give a C:N ratio of about 20. The C:N ratio ended up being about 16 for the wheat straw. To maintain a moisture content of about 60 percent, about six times more carbon would have been required than at 85 percent moisture. Depending on the initial solids content of the manure, it would have taken from four to over 28 times more carbon to compost the manure with a solids content from two to ten percent using thermophilic composting when moisture content is reduced to 60 percent.
COMPOST STABILITY EVALUATED
As organic matter decomposes, it becomes, more stable and less susceptible to change. The Dewar self-heating test was used to determine compost stability. The oak sawdust compost mixture did not reach this stable stage until some time during the 118 to 167 day period of composting. However, the wheat straw compost mixture reached stability during the 63 to 118 day period in about 50 d less time than the oak sawdust. At the end of the experiment (167 days and 17 applications), the depth of the compost pile was 16.5 inches (oak sawdust) and 33.0 inches (wheat straw).
The structure of carbon source affected both the potential for runoff and the oxygen content. The wheat straw’s open structure made it possible for it to absorb liquid manure without runoff during both the summer and winter season. The particle size of the oak sawdust, however, was too fine and the surface layer would crust over between applications thereby inhibiting infiltration, and runoff often resulted during application. If sawdusts were used, some mechanical breakup of the surface would be required so that the liquid would be readily absorbed. Higher rates of carbon would not have prevented the sealing of the surface from occurring. During the winter, the sawdust was frozen solid and even less permeable to infiltration, whereas the structure of wheat straw continued to be open enough to accept application even during freezing temperatures.
Higher oxygen content was maintained in the wheat straw compost throughout the study. Oxygen levels recovered from low levels after less than five days with both carbon sources; however oxygen dropped to lower levels in oak sawdust and did not recover to levels seen in wheat straw. Since there were only small decreases in oxygen levels in wheat straw after manure application, loading rates could have been significantly higher compared to oak sawdust.
Heat generated during composting can be harnessed to kill human, plant, or animal path-ogens by making piles large enough to self-insulate and raise the temperature to thermophilic levels. However, large compost piles need to be aerated either by turning the material like in windrow systems or injecting air as used in aerated piles to prevent them from becoming anaerobic. Aerobic decomposition under ambient temperature can be a less resource intense management alternative to thermophilic composting.
Standard practices have been defined to achieve pathogen reduction for thermophilic composting systems. Pathogen survival in raw manure and the soil environment has been studied. Until more information is available on pathogen survival in mesophilic compost, standards used for application of raw manure at organic farms should be followed, where it is not applied within 90 to 120 days before harvest, depending on the type of crop.
Mineralization and nitrification rates were higher with wheat straw as indicated by the higher levels of both ammonium and nitrate probably due to the higher rates of decomposition. Mineralization and nitrification generate acidity thereby decreasing compost pH as observed in this study.
Phosphorus content and form varied with C source. Although the amount of P added per plot was similar with both carbon sources, the P concentration in the wheat straw compost was about two times higher because about half the amount of carbon or bulking agent was added compared with the oak sawdust. Some P in the oak sawdust compost was also lost due to runoff during application. Although the inorganic fraction of total P is consistently high in composts, about 70 to 95+ percent, the fraction of total inorganic P which is water-extractable range from being relatively low one to 12 percent to high 15 to 40 percent. In contrast to inorganic N, water-extractable P decreased as the organic matter decomposed with either carbon source. The percent of total P that was water-extractable decreased over time in this study from 16.6 to 4.8 percent in the oak sawdust and 9.1 to 5.6 percent in the wheat straw compost. There were also higher amounts of water-extractable P in the wheat straw (1.34 to 2.90 pounds/ton) than in the oak sawdust compost (0.70 to 1.28 pounds/ton). These values were similar to those measured in other studies. Others have also observed a decrease in water-extractable P over time with different types of composts. When P was fractionated, it was found that other forms of inorganic P such as HCl-soluble P increased and water-extractable P decreased. This trend suggests the possible transformation of water-extractable P to more stable forms which would reduce the potential for runoff losses.
Water soluble P can be sequestered by having a higher C:P ratio. Net immobilization of P occurs when the C:P ratio is > 300 and net mineralization when < 200. The average C:P ratio of the initial compost mixture ranged about 63 for wheat straw to 98 for oak sawdust compost, so would not be expected to biologically immobilize P. Although organic matter decomposition has a significant impact on P cycling in soils, inconsistency in use of the C:P ratio as an index of immobilization or mineralization has been suggested to possibly be due to variable amount of inorganic P in organic materials, presence of limiting nutrients (e.g. N, S), and quality of the carbon.
VERSATILITY OF SYSTEM
Although storage and land application of liquid manure has a long history of being an effective and practical technology, since it cannot occur year-round, anaerobic storage is a necessary component of the whole system. Composting increases the value of manure and can be a practical system alternative to storage in a lagoon, giving the farmer more flexibility in time of application. Currently manure from fish grown in raceway systems is settled and stored in basins. During storage, phosphorus and other nutrients are continually released from the manure into the surrounding water in the basin. Because of how manure is managed on farms, water flows through these basins daily carrying these released nutrients to surface water.
The layered mesophilic compost system described in this study allows the manure to be stored and treated on land after it has been settled in the basins, greatly reducing the potential discharge of phosphorus to surface waters. Since building infrastructure, such as a pad, is not necessary to the success of this system, it can be located on cropland and periodically moved so crops can utilize any leached nutrients for crop growth rather than allow them to build up to high levels in the soil. Proper site selection is important to reduce the chance for runoff losses of nutrients.
Land requirements were about one acre to compost the manure for each 446 tons of Arctic char produced annually, but could be significantly lower for the wheat straw since experimental loading rates were limited by the reduced water absorption capacity of the oak sawdust compost. Mesophilic composting of high liquid content manures does not require excess carbon to reduce its water content as is needed with thermophilic composting because of high oxygen transfer rates associated with the high surface area to mass ratio and the layer that has the highest requirement for oxygen is near the surface.
Depending on the initial solids content of the manure, it would have taken from four to over 28 times more carbon to compost the manure with a solids content from two to 10 percent using thermophilic composting when moisture content is reduced to 60 percent. This mesophilic composting system is a less resource intense system than standard composting technology and is a practical technology that could be readily adopted on fish farms, and other farms with liquid manure systems, using current equipment.
Paul Adler is with the USDA-Agricultural Research Service (ARS) Pasture Systems and Watershed Management Research Unit located at University Park, Pennsylvania (paul.adler@ ars.usda.gov). Larry Sikora is with the USDA-ARS Animal Manure and By-products Laboratory based at the Beltsville Agricultural Research Center in Beltsville, Maryland.