By: Saqib Mukhtar
Livestock and poultry operations frequently use anaerobic lagoons as liquid waste storage and treatment structures. In a lagoon, organic waste is diluted with water and bacteria decompose the organic matter. During decomposition, solids that cannot be liquified by bacteria settle out as sludge. The treated liquid and dilution water, called effluent, are used by livestock and poultry producers as fertilizer. Effluent also can be recycled for manure handling in a flush system.
To be efficient, a lagoon must be properly designed. The size of a lagoon is based upon several storage volumes-sludge storage volume, minimum design volume for volatile solids loading rate (treatment volume to ensure bacterial decomposition of manure), volume of total waste entering the lagoon, and precipitation and runoff volumes. Designs also include approximately 2 feet of freeboard for embankment protection.
In a system with two-stage lagoons, the first cell (primary lagoon) is designed to hold the sludge and the minimum design volume. The second cell (second stage lagoon) receives overflow from the first cell and has storage volumes for further treatment of volatile solids, total livestock waste, runoff, precipitation and freeboard. Plans for future expansion in the livestock or poultry operation also should be considered in determining the final size of the lagoon. A properly sized two-stage lagoon is easier to manage than a single-stage system and improves treatment efficiency by producing higher quality effluent for reuse (flushing or land application).
In a properly functioning lagoon, two types of bacteria, acid formers and methane formers, are prominent. Acid formers convert biodegradable organic matter to volatile acids. Methane formers convert these volatile acids to methane and carbon dioxide. Under optimum conditions, a balanced rate of biological reactions by the two types of bacteria is achieved. Variations caused by environment (temperature fluctuations, for example), improper design and poor management will disturb this balance and cause excessive odor and sludge accumulation.
When a lagoon is in operation, proper management is extremely important.
Start-up management is the most critical stage. It may take more than 1 year for bacteria populations to reach the level required to treat the wastes. A new primary lagoon should not receive any waste until the water level reaches 50 percent of the minimum design volume. Diverted surface water and roof drainage may be used to add water to the lagoon.
Warm temperatures stimulate bacterial growth. Therefore, manure should be added to a new lagoon in the spring when bacterial populations are increasing.
Remember, not all materials entering the lagoon are biodegradable. Keep sand and supplies such as metal containers, plastic, glass, and other non degradable material from entering the lagoon. These materials not only reduce the functional life of the lagoon, but also damage the pumping and irrigation equipment during effluent removal.
Pumping from the lagoon should be conducted annually, at least, to ensure that the bacterial activity is not inhibited. However, the drawdown should not be below the level of minimum design volume. Install a stake or marking device in the lagoon to indicate this critical level.
Sludge buildup should be monitored periodically. If the lagoon is filling with sludge faster than the designed life of sludge storage, frequent agitation and pumping may be necessary. In addition, solids separation by use of mechanical separators or settling basins may be required to reduce sludge buildup.
Electrical conductivity, a measure of the level of salts, can be monitored to observe lagoon performance. An electrical conductivity range of 2,000 to 8,000 µmho/cm (micro mohs per centimeter) indicates acceptable levels of salt concentration. A level greater than 10,000 µmho/cm can indicate high salt concentrations in the effluent that will inhibit plant growth. At very low levels of 10 to 13µmho/cm, the lagoon’s anaerobic bacteria almost cease to function.
Lagoon pH should be monitored and maintained between 7.0 and 8.0. A pH below 6.5 (acidic conditions) generates excessive odor and sludge production. A low pH may indicate an excessive loading rate; volatile acids form faster than they can be converted into the end products of methane and carbon dioxide. A lagoon that develops crust (e.g., dairy lagoons) usually indicates a high loading rate. To increase pH, add lime and mix it throughout the lagoon.
Any intakes for recycling water or irrigation should be placed below the depth of floating solids or 1 foot below the water level.
Maintain all embankments. Check closely for minor damage that may lead to major structural problems. Berms should be covered with grass and mowed to reduce erosion and improve appearance. Do not plant trees near the embankment. Erosion caused by animal traffic should be checked. Grass should not be allowed to grow out over the water surface because this will interfere with pumping and increase the volatile solids load. Lagoons that may create a safety hazard for animals and humans should be fenced.
Keep records of all the design information, volume of the effluent pumped monthly, and any data (pH, sludge levels, seepage, etc.) collected. Records provide necessary information during emergencies and when trouble-shooting problems.
ASAE. Design of Anaerobic Lagoons for Animal Waste Management. ASAE EP403.2 1998. ASAE Standard 1998. American Society of Agricultural Engineers, St.Joseph, MI 49085.
Fulhage, Charles. 1995. Design and Management of Lagoons to Minimize Odor. Proceedings of the International Livestock Odor Conference ’95. Iowa State University, Ames, Iowa. pp. 196-199.
Loehr, R. C. 1974. Agricultural Waste Management. Problems, Processes, and Approaches. Academic Press Inc., New York, NY 10003.
Sweeten, J. M., and M. L. Wolfe. 1994. Manure and Wastewater Systems for Open Lot Dairy Operations. Transactions of the ASAE. 37(4) pp.1145-1154.
Sweeten, J. M. 1980. Waste Treatment: State-Of-The-Art. Livestock Waste: A Renewable Resource. ASAE Proceedings of the 4th International Symposium on Livestock Wastes-1980. Amarillo Civic Center, Amarillo, TX, pp. 334-338.
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