Progressive Dairy Editor Karen Lee
Covering manure storage is growing in popularity for a number of reasons, from odor control to minimizing rainwater accumulation to capturing biogas. “A cover keeps oxygen and ambient air out,” explai
ns Craig Hartman, civil engineer with Hartman Engineering. Anaerobic conditions, without the presence of oxygen, allow the microbes that produce biogas to thrive. Open-air storage basins can be influenced by sunlight and wind.
The introduction of atmospheric oxygen is non-conducive for anaerobic microbial activity in the top 3 to 4 feet of the lagoon. “Adding a cover converts the full pond to an anaerobic digester,” says Hartman, who has worked on covering 54 units in the last 13 years, most of which are lined earthen lagoons covered with a high-density polyethylene (HDPE) material for biogas production. “HDPE is thick and pretty strong,” he says, noting it works with a very small amount of gas pressure, has a very low permeability for water and gas, and can hold up against the sun’s ultraviolet (UV) rays.
Dave Anderson, technical sales manager at IEC Covers, also uses this material for covering large dairy and hog ponds and holding tanks. “Polyethylene is a great product. It is impervious to biogas and most wastewate
r constituents, while providing the end-user with a very long lifespan,” Anderson says. Over the past 31 years, he has worked on over 1,300 covers for a wide range of clients across North America and 20 different countries. The material is flexible and in a positive-pressure system will bag up to hold the gas produced. In areas that are subject to higher wind velocities, the negative pressure cover design is typically preferred because they lay flat on the water and are less susceptible to movement caused by wind gusts and storms. In the negative-pressure system, a vacuum removes the gas, which prevents the cover from ballooning and improves the longevity of the material. Different mil thicknesses, design and type of polyethylene is selected depending upon the type of basin and the engineer’s and farmer’s operational parameters.
Colder climates typically need insulated covers to retain water temperature during winter months to promote and maintain biological activity. As an engineer, Hartman says it is important the covers adhere to building and fire protection codes. Since the cover will hold dangerous gases, such as hydrogen sulfide, it needs to be safe in instances of high winds. Earthen lagoons make up 90% to 95% of the cover installations Anderson has worked on. In this scenario, the edges of the cover are buried in an anchor trench around the lagoon and are backfilled and compacted. He said the cover could be attached to concrete and metal using stainless steel batten bar and hardware. Plumbing is installed to collect the biogas so it can be cleaned and sold as renewable natural gas, used to fuel an engine or flared. If the amount of biogas can be quantified as it is drawn from the storage and analyzed for content, the project may qualify for grants and credits for reducing carbon emissions, Anderson explains. “It is a way of offsetting the capital cost,” he says. “Depending on the number of head and enough [manure] to generate biogas, the cover will pay for itself and generate money for the farm. ”Capturing and processing the biogas also reduces odor on the farm.
The covered anaerobic digester is positioned within a farm’s manure management system. On the flush dairies Hartman primarily works with, the manure leaves the barn and is processed through a screen separator and sand lane before entering the lagoon digester. The effluent entering the digester contains 1% to 2% total solids. The microbes will aid in processing any remaining organic material, and a sludge removal system can be installed prior to the cover to take out any inorganic material that settles in the storage. From the covered digester, the effluent typically flows into an open storage pond. The volume of the digester remains fairly stable, whereas the storage pond will fluctuate as it is used for the farm’s agronomic needs. The goal is to never have to remove the cover until it needs to be replaced. “With really good solids separation up front, you can get 20 years life out of it,” Anderson says.
For maintenance, the cover must be monitored for any defects from UV or stress, as well as puddling from rainfall. Anderson explains their covers have sand tubes that create a spine and rib pattern. Rainwater falls into these depressions and flows to a collection site. “A properly designed cover will not have any issues,” he says. Each cover is custom-designed and built to the project specifications. The most economical method of installing covers is on dry, empty ponds. Covers can be installed with liquid; however, it takes slightly longer and requires different methods for deployment. Reducing odor with activated carbon Another cover option that has been implemented in Latin America and is starting to gain traction in the U.S. is focused on odor reduction without the use of chemical additives .Its primary use has been in municipal, industrial, and food and beverage applications, but could also be used in agriculture.
“The systems consist of geomembrane covers with housings for activated carbon filters. Airflow has to go through the filter, and they capture and treat odor from the tank or pit,” says Bob Negley, sales manager for Anue Water Technologies. The membrane is supported by a cable grid and batten bars above the surface, making it unaffected by aeration, changing effluent levels, foaming, bacteria and other common issues. The lifetime of the carbon filters is nine to 18 months, depending on emission levels.The filter is engineered to allow gases and water to flow freely through while the activated carbon in the filter traps the odor-causing contaminants. Gas-specific filters for hydrogen sulfide and ammonia can be combined in the system to control the various odors from manure.
“Each system is custom-designed to the specification,” says Greg Bock, vice president general manager of Anue Water Technologies. They can be made to fit any shape and size and are expected to last over 10 years, even in harsh conditions. The edges are fastened to the perimeter of the metal or concrete storage structure and can be engineered for an earthen basin as well. Each cover can also contain viewing ports to see what is happening inside the structure. There are no pumps or motors involved, and the system has been shown to remove hydrogen sulfide by as much as 95%.
Negley says activated carbon is currently used to remove carbon dioxide in other applications. This system may qualify for carbon credits if the amount of gases present both before and after the filters can be quantified for the individual application. “I think this is possible but not something we’ve done for [agricultural] application,” he explains. Whether controlling odor or collecting biogas, cover options available today are custom-built for many applications and require minimal, if any, maintenance.
Industrial and Environmental Concepts (IEC) recommends proper subgrade preparation. The pond subgrade is critical in draining liquid and venting gas for membrane liner systems. The long-term performance of a pond liner system depends heavily on the condition of the prepared subgrade.
Most soils can be used for a subgrade as long as they don’t contain rock or materials that can puncture the liner. Fine grained soils such as sand, loam and clay can be used for the pond subgrade.
The prepared surface should be uniform, well compacted, and free of sharp rock fragments, stones, tree roots etc. should be removed or covered with a protective material or geotextile. The surface should not have any natural or foreign object that protrudes above the surface of the subgrade.
Rock and aggregate in photo to be removed prior to liner installation.
The protective soil should be a minimum of (6″) thick and should be compacted. This bedding thickness may have to be increased depending on local site conditions. Another alternative for wastewater ponds is the use of a non-woven geotextile.
Smooth, rounded stones less than (2″) can remain within the prepared subgrade, however, these should compacted into the subgrade so they don’t protrude above the finished surface. The general rule of thumb is that all stones and rocks, regardless of shape and size, and clay lumps that lie above the subgrade surface should be removed.
The prepared subgrade should be compacted in accordance with design specifications and standard engineering practice. Generally, this means that the subgrade should be compacted to a minimum 95% of maximum dry density according to the standard Proctor test (ASTM D698). At a minimum, the subgrade should be firm and unyielding, and should be compacted to a level that permits the movement of construction equipment, liner deployment equipment, and other related traffic without causing rutting and/or deformation of the surface. IEC recommends wastewater lined basin floors be slightly sloped to promote gas venting and drainage.
Compaction is especially important around pipe penetrations and concrete structures. In most projects, IEC can offer liner options for redundant sealing and recommendations around piping and structures.
Standing water, windrows and rock to be corrected prior to liner installation.
All ponds or lagoons to be lined should not include pockets or voids of any kind and should not be rutted or have high areas on the floor. In addition, the surface should be free of frost lumps and ice.
The prepared subgrade should also be shaped and graded to facilitate surface drainage both prior to, and during the installation of the lining system.
Care must be taken to maintain the prepared subgrade following completion. Vehicular traffic on the completed subgrade should be limited. Marks or ruts left in the subgrade by vehicular traffic should be smooth prior to pond liner installation. The subgrade should be protected from desiccation, flooding and freezing. Standing water should be removed so that the earthwork does not become saturated).
On projects that involve IEC, the subgrade will be inspected upon arrival at site. Our project supervisors will inspect the condition of the subgrade and will issue a “Certificate of Acceptance if requested . Corrective actions and activities to maintain the subgrade in a suitable condition for lining (including dewatering) are the responsibility of the owner or the general contractor.
For questions on lining or covering a basin, you can contact Dave Anderson at IEC’s office, 952-829-0731.
A novel approach was taken to cost-effectively close a 4.86 ha unlined industrial wastewater sludge lagoon in North Carolina and reduce leachate production. A floating geosynthetic cover system gave crews and equipment access to the lagoon surface.
Geomembranes are permeable fabrics which when used together with soil have the capacity to isolate, filter, support, protect and drain. Geomembranes were previously used as canal and pond liners. However the current and largest application is controlling dangerious wastes. In several of these applications, geomembarnes are concurrently used with geotextiles or mesh which offer reinforcement or protection to the flexible geomembrane while at the same act as outlets for gases and leachates generated in certain wastes.
Rosendale Dairy – Home of new digester
Located at the Rosendale Dairy, owned by the company Milk Source, the biodigester will process 240 tons of manure per day from the dairy’s 8,500 cows, generating biogas burned as fuel in power production and a pathogen-free and nutrient-rich waste stream that is land applied as organic fertilizer. Some will be used on the dairy’s own farmland and the rest marketed at a discounted rate compared to commercial fertilizer to the dairy’s feed providers for use on their land.
Installation of Roeslein Alternative Energy’s biogas technology creates renewable
natural gas from Smithfield Foods Hog Production’s two million pigs in northern Missouri and takes sustainability to the next level.
In 2013, the Blue Mountain Biogas Power Generation plant came on line in Beaver County, Utah. Owned and operated by Alpental Energy Partners of Provo, Utah – developers of alternative energy power plants – the $17 million, 3.2 MV plant is generating electricity from biogas provided by the anaerobic digestion of swine manure. A portion of the capital costs was eligible to be covered by a U.S. Treasury 1603 Grant.
