BioCycle June 2006, Vol. 47, No. 6, p. 48
Dairy lagoons can be a storage facility and manure treatment option when properly sized and not used as a dumping ground for disinfectants and antimicrobial products.
Many dairy farms use earthen lagoons or a “slurry store” type of nonearthen structure for manure storage and treatment. Due to economies of scale, concentrated animal feeding operations (CAFO) have dramatically increased total manure amounts. Inevitably, the huge amounts of manure that accompany CAFOs create enormous problems that need to be addressed – especially the offensive odors.
While farmers can recall the “good old days” when there were fewer regulations and urbanite neighbors complaining about odor, things have changed. While we cannot do much with regulations but to comply with them, we can do something with manure treatment and odor generation.
There are two biological ways to treat wastes: aerobic or anaerobic. Aerobic ones such as activated sludge process are often used in municipal waste treatment where oxygen is introduced to the waste holding tank for the aerobic bacteria to grow and convert organic compounds to carbon dioxide and water. An anaerobic digester is able to take care of wastes with high organic loading and convert them to volatile fatty acids and eventually to methane gas that in turn can be captured for energy. Anaerobic processes tend to be smellier than the aerobic process.
With the exception of the surface layer, a manure lagoon is anaerobic and is not that different from the rumen of a cow. Rations containing grain, silage, proteins and minerals are fed to the cow and digested in the rumen. Proteins and easily digested fibers such as starch from the grain are broken down by a group of bacteria to smaller fragments first and eventually to single units of amino acid and glucose sugar. The amino acids and glucose are then fermented by acid forming bacteria to volatile fatty acids (such as acetic acid, propionic acid and others), hydrogen gas, carbon dioxide gas and other. Most of the volatile fatty acids are absorbed by the cow for energy. Some of the acetic acid is used by yet another group of bacteria to make methane. Methane bacteria can also grow on hydrogen and carbon dioxide. (Methane production represents a waste of energy to the cows and measures have been taken to try to repress methane production).
When too much grain is fed to the cow, fermentation goes too fast and leads to acidosis. Plant fibers such as cellulose also provide energy to the cow but many of these fibers remain undigested and pass out in the manure. In contrast to the rumen, there are few readily digestible fibers left in the manure. Rather, hard to digest plant fibers are present in the manure and these fibers require longer time to break down properly.
The breakdown products from the plant fibers are the same as the easier digested grains to volatile fatty acids, hydrogen, carbon dioxide and others that eventually will lead to methane production. In the manure lagoon, methane production is desired as it will lead to both solid reduction and odor reduction. If the manure lagoon is properly sized to include enough storage space (about 10,000 gallons/cow/year plus allowance for runoff and rain) and proper treatment volume (depending on the location of the farm, less in the warmer climate, more in the colder states), it will fulfill its dual storage and waste stabilization functions with the help of the aerobic surface layer. Through its open surface, oxygen from the air is dissolved in the surface layer to create an odor cap for the lagoon. Volatile fatty acids coming up from the anaerobic lagoon will be consumed by the aerobic bacteria living in the surface layer.
Many CAFO farmers do not have enough storage space or proper treatment volume in the lagoon. Although the amount of manure going into the lagoon per animal remains unchanged, manure from more animals is going into the lagoon. A large amount of volatile fatty acids are produced by fermentation and retained in the lagoon – which leads to a lower pH. Methane bacteria, unfortunately, are sensitive to low pH and are easily inhibited. The robust fermentative bacteria continue to do their work that leads to more volatile fatty acids, causing the whole anaerobic manure system to shut down.
As more manure goes to the lagoon, the lagoon tends to be anaerobic all the way to the top. The aerobic surface layer is minimized or nonexistent and thus cannot function as an odor cap anymore. The accumulation of high concentrations of volatile fatty acids leads to offensive odor. In addition, copper sulfate is commonly used in the animals’ foot bath. Copper sulfate (and other naturally occurring sulfate compounds from the manure) can provide energy to a different group of bacteria known as sulfate reducers, which results in the production of hydrogen sulfide. Smell from the combination of high concentrations of volatile fatty acids and hydrogen sulfite is beyond offensive.
Enhancing Aerobic Conditions
Pro-Act Microbial specializes in manure lagoon treatments. Its patent-pending treatment system simulates the working lagoon by providing air to the surface layer and augmenting the lagoon with aerobic and facultative microbes. These microbes use the excess volatile fatty acids produced in CAFO lagoons and convert them to carbon dioxide and water in the presence of air – essentially recreating the odor cap normally present in a working lagoon to reduce odor. With the volatile fatty acids siphoned off by the aerobic/facultative microbes, the methane bacteria are relieved from inhibition and start to produce methane again. The microbial populations in the lagoon are gradually returned back to normal: hydrolytic bacteria break down the manure solids, fermentation bacteria convert the breakdown products to volatile fatty acids and others, methane bacteria produce methane. Sulfate reducing bacteria are no longer dominating the lagoon and much less hydrogen sulfide will be produced.
Studies on ammonia and hydrogen sulfide concentration in the lagoon water confirmed both compounds were greatly reduced. It is interesting to note that the siphoning off of volatile fatty acids by the aerobic/facultative bacteria resembles the rumen system. In the rumen, the cow absorbs the volatile fatty acids for energy so the feed can continuously be digested by the anaerobic microbes through degradation, fermentation and methane production as mentioned above. In a lagoon, the volatile fatty acids need to be removed. In this case, they are removed by the aerobic and the facultative microbes added as a part of the treatment system to simulate a working lagoon and the rumen system.
When a solids separator is used in conjunction with the surface air and microbes, results are even better. This is because the solids in the lagoon tend to be harder to digest compared to the animal feed. Solids removed by the separator are more than likely the hardest material to digest. Thus, if a separator is used prior to the manure slurry entering the lagoon, microbes (the aerobic/facultative ones added with the treatment system and the anaerobic microbes indigenous to the manure) in the lagoon will be able to liquefy the solids while keeping the odor down. It is important to know that the aerobic/facultative bacteria added to the lagoon are naturally occurring microbes but not indigenous to the manure lagoon so they have to be added periodically to keep the system working.
As a manure lagoon is a living microbial system, anything that upsets the microbes should be avoided. For instance, copper sulfate is used to kill or inhibit the agents of hairy heel warts. It will also kill the bacteria present in the lagoon. The anaerobic microbial system will break down. The lagoon may not smell but solids will become a big problem. Likewise, the use of other disinfecting products such as formaldehyde should be minimized. On the other hand, when a lagoon smells, it is likely that the microbes in the lagoon are out of balance. Restoring the proper microbial balance in the lagoon by taking away excess volatile fatty acids produced by microbial fermentation will ensure that methane production resumes and solids are liquefied.
Chuzhao Lin, Ph.D., is director of research at Pro-Act Microbial in Warren, Rhode Island.
June 26, 2006 | General
Odor Control Methods At Manure Lagoons
BioCycle June 2006, Vol. 47, No. 6, p. 48