As I noted in my last post, Withers & Ravenel’s Swine Farm Biogas Renewable Energy Project, located in Bladensboro, North Carolina, is one of this year’s ACEC Excellence in Engineering Award winners.
I asked the folks at Withers & Ravenel to give share more about their project, how it was conceived, and how it was designed.
Why Swine Farms? What was the genesis of this Project?
North Carolina is “pork proud”, with approximately 9 million hogs on farms scattered throughout the eastern part of the State. These farms rely on open pit lagoons and land application for the treatment and disposal of animal waste. However, open-air treatment lagoons have a poor reputation among some lawmakers, residents and environmentalists. They are accused of creating sickening odors, allowing methane to escape into the atmosphere, and contaminating groundwater and streams.
Because of environmental concerns, the State temporarily suspended permitting the construction and operation on any new swine farms utilizing lagoon treatment systems in 1997, and required new farms to meet “environmentally superior technology” (EST) standards. Since the enactment of the suspension, there have been no new hog farms introduced in North Carolina.
Tax Credits, Legislation, and Funding
In 2007, the State passed Senate Bill 3, which pushes the use and development of renewable energy standards, the State took a big step toward encouraging innovative treatment technologies for swine waste by mandating utilities to purchase Renewable Energy Credits (REC) generated from swine waste. The Bill also provides a 35% State tax credit in addition to the Federal 30% tax credit.
Spurred by the availability of the NC Green Business Fund grants from the 2009 American Resource Recovery Act, Withers & Ravenel conceived the 600kW renewable energy project and assembled the project team, which included developer AgPower Partners LLC, Withers & Ravenel engineers, and Barnhill General Contractors.
The project was able to receive over $2 million dollars in grants and tax credits, including a $500,000 grant from the NC Department of Energy and $1.5 million grant from the US Treasury. The Owner, Billy Storms, was able to finance the balance of the project cost through a loan with the Cape Fear Farm Credit Association.
Engineering a Plan for the System
In North Carolina, swine farms flush the houses with water in a closed system with a lagoon providing storage and treatment of waste which is then applied to crops. With this process, the waste is diluted to around 1-2% solids. However, in order to reduce the water content for more efficient temperature control of the anaerobic digestion, the waste needs to be between 5-10% solids.
With the implementation of scraper technology, swine waste volume was significantly reduced by eliminating the added liquid from the flushing system. This made the Storms Farm much less reliant on the volume required in the existing lagoons, reduced the required size of the anaerobic digester, providing benefits to both the waste handling concerns and to energy production. The scraper system also reduced the amount of ammonia gas in the barns, which is beneficial to animal and human worker health.
Most digester systems for swine manure in North Carolina have relied on ambient covered lagoons. However, at Storms Farm, with the scale of a 600 acre farm and swine houses separated by as much as a mile, it was not cost-effective to build and cover a new lagoon to treat the waste using anaerobic digestion. The distance between the 23 barn complex made it problematic to pump waste because of build-up in the pipe known to cause maintenance and failure problems.
After review of viable technologies, Withers & Ravenel recommended DVO Anaerobic Digesters to supply the digester technology for Storms Farm. DVO has an extensive tract record using their patented mixed plug-flow digester technology on dairy farms, but there was no comparable swine waste facilities using mesophilic digestion in the US. Europe has a substantial number of facilities that use swine manure mixed with other substrates, but there was no reliable source of data for the gas yield using solely swine waste substrate.
As a result, Withers & Ravenel took multiple manure samples from local farms and had them tested to help estimate the biogas yield. Even with this data, there was very little information to collaborate the projected biogas yield. After all alternatives were evaluated, the most cost-effective, efficient option was to construct a heated mesophilic digester system with cogeneration and to convert the barns to scraper manure removal systems.
Geotechnical borings were done and revealed the need to raise the digester above grade due to a high ground water table. This required sloping the backfill around the tank as insulation to maintain the needed 95 degree temperature in the mesophilic process. The report also revealed the need to pre-load the site to avoid potential settling of the digester and cogeneration building. This additional grading and site work was necessary from the original conceptual site plan.
The Digester Operation
The scraper system scraps the waste to a gravity collection system and storage tanks behind each barn. Two vacuum trucks and drivers empty each of the 23 tanks daily to collect the manure collected from each of the barns, drive to the digester facility and empty the manure into the influent pump station.
Manure is pumped into the 1.1 million gallon in-ground concrete digester where the temperatures are maintained above 95 degrees (mesophilic) and the natural occurring anaerobic bacteria destroy the volatile solids, produce the bio-gas containing 65% methane, kill pathogens, and produce a high quality inorganic waste product virtually pathogen and odor free for storage and, eventually, land application as fertilizer. At Storms Farms, about 60,000 gallons of swine waste is processed each day. The biogas is “scrubbed” of corrosive components and combusted in an 845 HP gas driven engine/generator integrated system provided by Martin Machinery, from Latham, Missouri.
The digester produces wastewater that is free of pathogens and odors and removes 90% of the phosphorus and 75% of ammonia nitrogen. The electricity – enough to power over 300 homes – is sold to North Carolina Electric Membership Corp.’s grid network.
Through this design, Withers & Ravenel was able to develop Storms Farm into the largest swine biogas renewable facility in North Carolina, generating 600kW of power with an operating capacity of 95%.
The volume reduction due to the implementation of the scraper system and digestion process has allowed the treated effluent to be returned to one of the existing lagoons, reducing the dependence on the original six lagoons on the site.
Future projects to remove the inorganic solids remaining in the effluent by dewatering and to treat for additional phosphorous and ammonia nitrogen removal are in the planning stages in order to meet the additional requirements of an EST standard farm.
What was the Owner’s involvement?
Billy Storms, owner of Storms Farms was instrumental in getting the Project off the ground, through financing the project, through the willingness to change the method of manure management and by accepting the technological challenge of running what is, essentially, a small wastewater treatment and power plant. He was a true partner in the project and was directly responsible for the selection and installation of the manure scraper system in all of the barns, implementing the operations of the truck collection system, the digester and generator systems as part of the farm operations.
What can we learn from this project?
This project demonstrates a method to economically build new swine farms without total dependence on the historical open air lagoon treatment system, and flushing system collection methods, a conversion that is necessary to meet the State EST standards. The system also develops renewable energy meeting the goals in Senate Bill 3 requiring swine waste to be utilized in a percent of the production of renewable energy.
However, because of the cost, risk, and complexity of the project, its applicability may be limited to a handful of existing farms in North Carolina. Larger farms, preferably more than 50,000 animals, are required in order to have the scale to produce the amount of energy to have an economical rate of return on investment.
Thanks, Withers & Ravenel, for the detailed project description. Your turn: Thoughts? Comments? Questions for the team? Shoot me an email or post in the comments below.
Photos (c) Withers & Ravenel; Pig outline courtesy Pixabay.
Please note that the Anaerobic digestor does not remove ANY nutrients; 75% of nitrogen and 10% Phosphorus which came out of the hog barn is still in the digestate (end of the process residuals) liquid portion, the other 25% of N is in the digestate solids with the 90% of Phosphorus.
Digestate will be used on fields as fertilizer, and the mineralized nutrients (nitrogen and phosphorus) will in fact be much more effective than straight out of the hog.
Thanks Robert for your information.