The need to safely dispose of manure from concentrated animal feeding
operations is resulting in some imaginative approaches. One North
Carolina hog producer thinks one approach might be to attach a
commercial greenhouse operation to the hog business, where a portion of
the liquid manure is used to generate another cash crop – in this case,
The need to safely dispose of manure from concentrated animal feeding operations is resulting in some imaginative approaches. One North Carolina hog producer thinks one approach might be to attach a commercial greenhouse operation to the hog business, where a portion of the liquid manure is used to generate another cash crop – in this case, tomatoes.
|Average tomato production from the greenhouse is 920 lbs. per day of large-fruited cultivars and 92 pints per day of grape tomatoes. Submitted photo|
The Julian Barham Farm in Zebulon, North Carolina, has created two new income streams from its liquid manure. Firstly, it has installed an ambient-temperature anaerobic digester to capture biogas from the manure to generate electricity. Secondly, some of the stabilized liquid manure discharged from the digester is used to fertilize tomatoes in a commercial greenhouse operation.
Why tomatoes? Barham chose tomatoes because he felt they would absorb a higher volume of treated wastewater than other vegetable crops.
Some of the water is also used to recharge hog barn manure pits, because by the time it has been treated in the digester, 92 percent of the harmful organics have been destroyed. For odor control, wastewater diverted to the farm’s greenhouses and hog barn manure pits undergoes a biological nitrification process. The remaining liquid waste stream is stored in a lagoon and eventually applied to cropland.
The Barham Farm is actively participating in research to develop manure management systems to help North Carolina cope with a major increase in the generation of hog manure. Swine production increased in the state from 2 million hogs in 1987 to 10.1 million in 2001. Because of the huge increase in hog production, this raised environmental concerns regarding animal waste management practices. So North Carolina State University (NCSU) is working with Julian Barham to evaluate an integrated manure management system involving anaerobic digestion for waste treatment, biofilter nitrification for odor control, and greenhouse tomato production as a possible method to divert some of the wastewater away from cropland irrigation.
|The Julian Barham Farm in Zebulon, North Carolina, has installed an ambient-temperature anaerobic digester to capture biogas from hog manure to generate electricity. |
|The operation has also built a commercial greenhouse operation that uses a portion of the stabilized liquid hog manure discharged from the digester to fertilize hothouse tomatoes. |
|The farm installed a Caterpillar 3406 engine with a 120-kilowatt generator to burn the biogas coming off the digester to generate electricity. The engine is capable of handling gas containing significant amounts of hydrogen sulfide, so nothing is done to clean the biogas before it is burned. |
|For odor control, wastewater diverted to the farm’s greenhouses and hog barn manure pits undergoes a biological nitrification process. |
The Barham farm is a farrow-to-wean swine operation with approximately 4000 sows, generating 36,720 gallons per day of wastewater flow. The manure pits are drained to release the wastewater and recharged with nitrified water from the storage lagoon every eight days.
The farm’s manure management system consists of an ambient-temperature anaerobic digester, cogeneration of electricity and heat through combustion of biogas produced from the digester, a storage pond, nitrification biofilters for ammonia emission control, and tomato production for nutrient recovery.
Installation of the Barham Farm manure management system started in 1996 and has evolved over the years.
“We wanted to try to get out in front of the environmental and odor issues,” says Barham.
Until recently, that area of North Carolina was experiencing substantial residential growth, so Barham wanted to keep good relations with his neighbors. When raw liquid manure is applied to cropland, this often leads to complaints and Barham wanted to avoid that. Once he discovered the amount of biogas that could be generated from the liquid manure, he worked through the Environmental Protection Agency’s AgStar program, which hired California consultant, RCM Digesters, to build the ambient-temperature anaerobic digester. It is 20 feet deep, has a surface area of about 265-feet by 265-feet, has a three-foot clay liner at the bottom, and holds about 865,500 cubic feet of wastewater. The swine wastewater flows by gravity to the digester where bacteria interact with the raw liquid manure and generate biogas. About 1383 cubic feet of biogas is generated per hour, consisting of about 63.7 percent methane and 20 percent carbon dioxide. The hog farm installed a Caterpillar 3406 engine with a 120-kilowatt generator to burn the biogas coming off the digester to generate electricity. Barham says the Cat engine is capable of handling gas containing significant amounts of hydrogen sulfide, so nothing is done to clean the biogas before it is burned.
Once the farm had harnessed the biogas, started producing power, and selling it to a local utility, the next issue was how to manage the treated wastewater stream coming off the digester. Its disposal on cropland in that part of North Carolina is challenging because of the area’s clay soil, rolling terrain, and wet winter environment. So, Barham came up with the idea of building a greenhouse to grow tomatoes and to use some of the liquid by-product from the digester as organic fertilizer in tomato production. At present, the farm has two, 28,000 square foot greenhouses.
“We added the greenhouses really just to make the water go away,” says Barham. “Originally, we thought we could get rid of about 8000 to 10,000 gallons of water per day. That was overly optimistic. We are moving about 2000 gallons per day, which helps, but is not anywhere near what I need to be moving.”
The farm also installed a 400,000 BTU boiler and a 10,000 gallon hot water tank, with the intention of capturing and using waste heat from the Cat engine to heat the water. NCSU estimates that using the waste heat from the engine is one way the farm could cut down on its greenhouse heating bills, particularly in winter. Right now, they and the hog barns are heated using liquid propane.
The greenhouse facility is easy to build, Barham says. The management requirement to produce the tomato crop is a whole different matter. He says it is important for hog producers to evaluate whether they want the extra challenge of operating a commercial greenhouse business in addition to a hog farm before taking the plunge. It was an entirely new venture for the Barham’s that required that they learn a lot about growing and marketing a vegetable cash crop. The liquid manure is not simply applied randomly to the tomato plants. The farm has installed a nutrient control system to ensure that the tomato plants are fed the right nutrient mix to optimize production.
Average tomato production from the greenhouse is 920 lbs. per day of large-fruited cultivars and 92 pints per day of grape tomatoes. Recently, the large tomatoes have been selling for about $1.99 per lb. and $1.50 per pint of the grape tomatoes, meaning that daily income is about $2000. From that, the hog farm needs to pay its expenses. The greenhouse has six dedicated employees.
Like other cash crops, production is also weather-dependent. Barham Farm aims for most of its production in winter when local field tomatoes aren’t available.
Barham has found it difficult to compete with tomatoes grown elsewhere, and being dependent on a single product has left him somewhat dependent on what buyers are prepared to pay.
“Unfortunately, somewhere on this earth, there are field tomatoes being grown and they bring them in,” says Barham. “The only advantage we have is that we are local, and it depends on how much the customer feels that they are worth.” He’s investigated the possibility of being certified as an organic producer, but is so far not convinced that the time and effort is worthwhile, given the mixed reviews he has received.
He is looking at possibly diversifying his line of vegetables and marketing the operation directly to local consumers more as a market garden, because he feels customers might be more willing to visit the operation to purchase several vegetables rather than just tomatoes, especially if he can make a variety of fresh garden vegetables available in the off season.
It’s obvious that Barham and NSCU have made a considerable investment in time and money to put a number of novel ideas for making better use of the farm’s manure resource, and are even studying the use of the carbon dioxide stream from the biogas to improve tomato production.
However, what Barham has discovered recently is that theory is not always in step with reality.
At the present time, the biogas is being flared because he is trying to negotiate a new power purchase agreement with the local utility. Unlike many other states, North Carolina does not have a Renewable Portfolio Standard (RPS) requiring local utilities to purchase a portion of their power from renewable sources, although state legislators are moving in that direction. So this has made negotiations with local utilities more challenging.
“Everybody knows that renewable energy makes sense and there are a lot of lawmakers behind it,” says Barham. “Eventually, it’s going to work.”
Right now, if the system was operational, the power produced from the biogas would provide about 75 percent of the hog farm’s power needs, which is why Barham is still dependent on the local utility for electrical services.
In an attempt to possibly circumvent that dependency and also to generate more biogas in winter when the greenhouse operation really needs it for heat, the hog farm is thinking about installing a smaller, separate, thermophilic digester. This type of digester accelerates the digestion process by operating at a higher temperature, but it requires a greater amount of monitoring and control to ensure that the internal bacteria culture remains healthy.
Despite some of the hog farm’s immediate challenges, “I still like our approach,” Barham says.