What do farms, manure, and a developing technology for creating fertilizer have to do with the Chesapeake Bay? Well, almost one-quarter of the Chesapeake Bay’s 64,000 square mile watershed is agricultural land. Runoff from farmland inevitably drains into the local streams, creeks and rivers that flow to the Chesapeake Bay.
When best management practices are not implemented on agricultural lands, runoff can carry animal waste and excess fertilizer into these waterways, overloading them with nutrients, bacteria and pathogens.
A developing technology called anaerobic digestion has been proposed to reduce phosphorus runoff from many farms. Pilot studies have been conducted in several locations around the world, including at least three Chesapeake Bay watershed states.
Anaerobic digesters, or biodigesters, have become an increasingly popular tool for managing manure on farms. Biodigesters are thought to have several benefits, including reducing farm animal waste runoff, producing nitrogen-rich liquid that can be used as fertilizer, and producing phosphorus-rich solids that can be processed into mulch and other products that would reduce runoff.
Biodigesters are increasing in popularity for use with dairy farms and manure handled as a liquid, slurry or semisolid. However, a Bay Program website visitor wanted to know about the effectiveness of using biodigesters on poultry farms with litter feedstock to improve water quality in the Bay and its tributaries.
One study conducted in the Bay watershed for the Propane Education Research Council tried to determine if this method could decrease the phosphorus in the liquid effluent from the digester exit point. Unfortunately, the study concluded that this was not the case. Phosphorus was only decreased by approximately 5 percent – the same rate of reduction without the anaerobic digestion process. The council concluded that significant phosphorus reduction could be possible if a separate post-digester step was added.
According to that study, the use of biodigesters would not be an effective way for farmers to help improve water quality.
John Ignosh is a scientist with the Virginia Cooperative Extension at Virginia Tech, working on agricultural byproduct utilization. “As far as digesters [used for] litter,” he said, “there have been a few pilot projects looking at this. The main challenge is that digestion is better suited for slurry type feedstocks.”
Most discussion of anaerobic digesters is in reference to digesters using a slurry type feedstock, but Ignosh said there have been pilot projects with litter feed conducted in Maryland, Virginia and West Virginia, among other locations.
An important note is that regardless of the type of feedstock used for the biodigesters, there is not a significant reduction in nutrients from the waste. Nitrogen enters the digester as ammonium and organic nitrogen, and the ammonium is not destroyed in the digester. Instead, the organic nitrogen is converted to ammonium. So the nitrogen in the effluent from the digester typically ends up being higher than when it went in. Similarly, the microorganisms used in the digester do not consume phosphorus. Although some of the phosphorus can be converted to a soluble form, the total mass of phosphorus remains constant.
Therefore, while anaerobic digesters may be useful for producing biogas to create energy and manage waste, they do not reduce the amount of nutrients in the fertilizer or other products it might result in. So fertilizer that is made from a biodigester and is used on farmland would not decrease the amount of nitrogen and phosphorus that would run off the land. These devices also tend to be prohibitively expensive for many farms and do not provide the best benefit for the investment.
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