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Feasibility and impact of manure storage covers

Tamara Scully for Progressive Dairyman Published on 29 February 2016
liquid manure storage systems

Plenty of things have changed in the world of dairy farming. Advancements in milking systems mean that robots, not people, can perform the several-times-per-day task of milking cows.

Precision technology allows precise field mapping, and GPS systems guide equipment so the proper amount of nutrients can be applied exactly where they are needed.

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Manure, which once was commonly spread on a daily basis, is now stored for long periods of time. As dairies have increased in size, manure handling has advanced into manure storage. Liquid manure storage systems, such as anaerobic lagoons or slurry systems, have become the system of choice for many, particularly larger, dairy farms.

While manure storage systems have advantages, one disadvantage is the emission of greenhouse gases (GHG). As manure is held under anaerobic conditions, the volatile solids found in manure are readily converted to methane, an extremely potent GHG.

“There’s a transition in how manure is being managed. If you move to manure storage, it becomes less aerobic,” said Jenifer Wightman, research specialist at Cornell University.

Approximately 80 percent of the GHG emissions from manure are in the form of methane. Nitrous oxide, another GHG produced from manure, can also be directly emitted from storage systems. Nitrogen emissions come both from storage and from land application of manure.

“Nitrogen emissions are relatively small compared to methane,” she said, and while storage system type can impact nitrous oxide emissions, their impact on climate change is nominal in comparison to the increase in methane production from anaerobic storage systems favoured today as dairies grow larger.

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Policies to protect water quality impact GHG emissions. As environmental standards to protect water quality are implemented, manure storage is becoming more common, especially on dairy farms covered under confined animal feeding operation (CAFO) regulations. Mitigating the increase in methane emissions, stemming from an increasing prevalence of liquid manure storage systems, can be as simple as a manure storage cover.

Mitigating methane emissions

Methane gas production from manure storage is affected by several factors: the mass of volatile solids, the time spent in storage and the temperature. But methane gas from liquid manure storage systems does not have to be lost to the environment. The potential to mitigate the methane created under anaerobic liquid manure storage conditions exists, and the solution seems simple.

“We can just flare it,” Wightman said of the methane. If liquid manure storage systems are covered, conditions become “very anaerobic,” and the resulting methane can then be flared into carbon dioxide.

Methane is combustible and converts to carbon dioxide. Methane is 34 times more potent than carbon dioxide on a 100-year scale and 86 times more potent on a 20-year scale, she said. Combusting methane to create carbon dioxide is therefore a GHG mitigation strategy.

“Putting a cap on it and putting on that flare and destroying it” will “combust the methane and turn it back into one carbon dioxide equivalent,” Wightman said.

Manure storage covers

Covering manure storage systems allows for the capture of GHG emissions. The cost of a manure storage system cover depends upon its surface area. While the size of the dairy will dictate the storage volume needed, the distance to bedrock determines the depth available for storage and thus impacts the surface area and the amount of material needed to cover the system.

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Aside from its impact on GHG emissions, a covered system also decreases the amount of water that is ultimately hauled to the fields along with the manure. Because a cover will keep precipitation from adding to the volume of the catchment, there will be a reduction in the volume of waste product hauled and a reduction in frequency and cost associated with hauling. The reduction of odour is another benefit to storage covers.

When solid-liquid separation occurs along with the covering of liquid manure systems, the volatile solids in the covered system are reduced. This decreases the frequency of solids removal from the storage system. But doing so reduces the overall volatile solids in the covered manure anaerobic manure storage system, thus reducing the amount of methane generated in storage.

When methane is produced and captured in the storage system, the next step is to eliminate it via flaring. But there is not always enough methane produced to do so. In New York, for example, “there is not enough methane to burn” during the cold winter months, and “overall, the flare is working about 80 percent of the time,” Wightman said.

Economic feasibility

The amount of methane emissions from manure storage systems can be calculated using methodology from the EPA. A methane conversion factor (MCF) for each type of manure storage, in a given climate, can be calculated.

The MCF for the Northeast, using solid-liquid separation and covered liquid manure storage, has been calculated at 0.61, using data from three New York farms equipped with retrofitted covers for manure storage systems. These farms were equipped with metered flares and operated for 12 months. This data also indicates that 81 percent of the methane created during storage was flared.

“MCF is very important in terms of your GHG accounting,” Wightman said, as price and policy planning depends on this, and data shows that the capture and flare of methane emissions via manure storage cover usage “is a cost-effective way of addressing GHG emissions on dairy farms.”

The cost of a large manure storage cover has been calculated to be approximately $380,000, based on the six months’ storage needs of a 1,000-cow dairy in New York. This figure includes the cost of the cover, a solid-liquid separator, cover disposal at the termination of its useful life, interest at 4.5 percent on financing and the costs of supplies, equipment, personnel and other incidental expenses.

It also includes calculated savings, over 10 years, on rainwater hauling. A medium cover, based on a 550-cow dairy’s needs for six months of manure storage, is about $270,000.

If grant money were made available for manure storage covers instead of for anaerobic digesters, which also mitigate methane emissions, converting biogas to electricity, data indicates that the same amount of money could mitigate more methane-using storage covers.

About 96 percent of GHG mitigation from anaerobic digesters actually comes from the combustion, or flaring, of biogas. Only 4 percent comes from the offset of fossil fuels, Wightman said.

But the study of the use of manure storage covers and the potential financial feasibility and mitigation of GHG emissions is all conceptual, based on historic data and data from other research studies. There are potential concerns regarding the use of manure storage covers that may make them less viable as a GHG mitigation strategy.

“I’m presenting a concept,” she said.  PD

PHOTO: Mitigating the increase in methane emissions, stemming from an increasing prevalence of liquid manure storage systems, can be as simple as a manure storage cover. Photo by Mike Dixon.

Tamara Scully, a freelance writer based in northwestern New Jersey, specializes in agricultural and food system topics. Wightman spoke at the Dairy Environmental Systems and Climate Adaptation Conference, held in July at Cornell University.

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