Turning Manure into Clean Energy: The Smart Farmer
February 10, 2021
For centuries, farm animals have been essential for civilizations to survive, but nowadays they are destroying the environment. They have always left behind manure that releases the toxic greenhouse gas, methane gas, which is harmful to carbon levels. However, as the world’s population grows so does the number of animals and methane gases they produce. A single cow can consume about 50-60 pounds of dry food every day. That is a lot of manure they leave behind. And it is not just cows, pigs and chicken are also big contributors. So, farmers are finding ways to turn their waste into energy.
Livestock manure has been used to produce energy for as long as they have been around. Manure would be used in fire when no wood was available and at the end of the 19th century, used as fuel for streetlamps. Now a days, farmers are using it to power and heat their farms and contribute back into the grid.
What are Farms Doing Today?
Usually, the waste of animals on a farm goes to a deep pond where it continues to naturally decompose as if it were still in a cow’s stomach (4 stomachs, actually). Natural bacteria eat the residues and emit biogas, a combination of methane and CO2. A cow can produce 4 to 5 tons of CO2 per year, this is in essence the same greenhouse gas footprint as a car.
One method a farm may use to reduce their gas emissions is by working together with a Dairy Digester Development Operations Company (DDDOC). The DDDOC implements digester systems at the farms. The digester is a device that captures gas emissions, so they do not vent into the air, but instead, redirects to more beneficial uses like biogas. They even help eliminate the smell of manure in groundwater. The process takes a normal manure pond and covers it with a flexible membrane preventing oxygen from getting inside. This causes the bacteria to believe they are in the cow’s stomachs so they keep eating the remaining calories and emitting the methane gas. The biogas produced by the dairy digester usually travels through a pipeline to a production facility that uses it as fuel or turns that renewable natural gas into fuel directly.
The farm can also add products to the waste ponds to help increase the amount of gas energy. To add to the benefit of the farmer’s use of the digesters, grocery stores often compensate them for disposing of wasted vegetable excess as opposed to disposing it at costly landfills. This method requires a reasonable initial investment, but when done correctly, pays off in the end.
The Importance of Manure in a Dairy
In a Dairy farm, the symbiotic relationship between manure and other processes such as growing food is really important. On one hand, they get the nutrients that manure delivers to certain food processes, but the disposal of the remains can also become another source of income for their owners. Once they produce the biogas in the farm’s ponds digester, the rest of the process is done at a plant that takes this raw gas and makes it clean. After that, one of the options today is to transform it into a Compressed Natural Gas, CNG as an end product, no different than the natural gas that reaches homes through the pipes. One of the big uses today for this CNG is vehicle fuel. Reselling this gas will generate profit not only for the process plant but also for the farm that could generate, depending on the operation, a six-figure income.
Other Efforts for the Poultry Manure
In the Chesapeake Bay watershed area, some farms are testing the idea of turning poultry litter, which is rich in nitrogen and phosphorus, into an energy source for heating the poultry houses. Regularly, farms use burners to keep poultry houses warm. On large farms, this new process burns about 80-85% of the poultry litter produced, on average 450 tons. The process is very natural, however, only furnaces with certain combusting configurations for burning a variety of materials can be used in this process. The greatest challenge with this process is the issue of properly disposing of the burner emissions.
The Process of Moving Manure
Manure slurry pumping consists essentially of solids suspended in water. It typically contains bedding materials such as straw and sand and the water used in flushing and washing. As manure is cleaned from an area, other particle-like grits from concrete slabs, large solids such as rocks, concrete chunks, and other debris are washed from the dairy operation. Large industrial pumps are used to move the slurry from the operation area to the digestion systems.
These heavy-duty pumps are capable of processing straw and other large solids that could clog common pumps. The pumps also must be able to withstand the grit and sand which causes abrasion on the impeller and casing of a pump. Diluting the slurry with more water so a pump can process the solids can only be done to a certain extent to maintain the solid consistency that is optimum for the digester process. Also, manure slurry tends to be acidic, which may affect pump seals and other components, so the pumps must be of excellent quality.
For supplying manure slurry to a central digester and returning digested effluent back from the digester to the dairies, both return and supply pumps may be required, depending on the application.
For selecting the right pump for the job, there are some factors to be considered:
- Pump materials and construction are appropriate for the possibly abrasive and acidic nature of the manure slurry to be pumped.
- Consider the largest expected solid or mass that can pass through the impeller and volute.
- Check the other critical variables in the application, like piping, the grade of elevation, etc.
- For good pump performance, consider the application challenges, most commonly centrifugal and positive displacement pumps are used in dairy farms for this purpose. Also, decide if an electrical or diesel pump is more suitable.
Centrifugal pumps are preferable for longer distances because starting torque requirement is lower, and they are less susceptible to clogging. Centrifugal pumps are available for pumping manure slurries having maximum total solids up to 10 to 12%. Manure pumping can use flooded suction or feed pumps to move material to digesters, buried storage tanks, or lagoons, but a feed pump is usually required when going the other way. Following the decomposition process, when all gasses are captured, the excess turns into a solid waste that can be used as fertilizer for the field. This waste will be pumped by centrifugal pumps through irrigation systems for liquid waste or to spreader trucks.