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The most common use for biogas is cogeneration, in which an engine produces electricity from the methane while recovering heat. The recovered heat is used to maintain digester operating temperatures, and excess heat can be diffused through a radiator if it is not needed. The carbon dioxide simply passes through the engine. During combustion, the hydrogen sulfide and ammonia in the biogas are converted into sulfur and nitrogen oxides, which are regulated air pollutants. The reduction of hydrogen sulfide (a corrosive compound) is generally desirable, both to meet air permitting requirements and to help preserve engine components. When more biogas is produced than can be burned in the engine, the excess is flared (burned), which converts the methane to carbon dioxide and reduces the impact of releasing the biogas to the atmosphere.
If the goal is to produce gas for pipeline injection or CNG for use as vehicle fuel, then carbon dioxide, hydrogen sulfide, and ammonia in the biogas must all be reduced to very low levels.
Currently, all of Washington’s digesters are using cogeneration systems to produce electricity and heat from the biogas. The DariTech digester design accomplishes hydrogen sulfide reduction through a standalone two-step treatment: a microbial process followed by carbon treatment. The Andgar digester design includes a similar biological process, which occurs inside the digester, that reduces the accumulation of hydrogen sulfide in the biogas. None of Washington’s digesters are currently converting biogas to CNG, although Western Washington University is conducting research on CNG production at FPE Renewables. Another digester, Qualco Energy, is currently producing more biogas than can be used in its cogeneration system, and is considering whether to install additional cogeneration capacity or to develop CNG production capability.
Washington State Department of Agriculture Page 7 Washington Dairies and Digesters, October 2011 Nationally, EPA AgSTAR reports that 130 of the 142 dairy digesters are using the biogas for electricity or heat production, or both. One is treating the biogas for pipeline injection, nine are flaring all of the biogas produced, and the biogas use at one digester was not reported. One digester that uses biogas for electricity generation also uses it to generate CNG for vehicle fuel.
Effluent Processing and Use During digestion, microbes biodegrade a small proportion of feedstock solids into biogas, but due to the high water content of the manure and other feedstocks, the total volume of effluent produced is similar to the total volume of influent.
After digestion, the effluent is separated into liquid and solid components using one or more mechanical processes. Washington’s solid waste permit exemption limits how these components can
• Liquid effluent must be returned to a dairy and managed under a dairy nutrient management plan (DNMP) that has been updated to address management and use of the digested materials.
• Digested solids can be returned to a dairy and managed under its DNMP, delivered to a compost facility, or distributed directly to others if they pass compost quality standards.
For the most part, all six digesters return the liquid effluent to the contributing dairies. The liquid effluent is generally stored in lagoons and applied to crop land as needed to supply nutrients for crop production. Most of the digested solids are used by contributing dairies, or sold to other dairies, for bedding. One digester currently sells all of the digested solids to a company that produces a peat moss substitute for wholesale. Three other digesters include an aerobic treatment process that provides additional reductions in pathogens, odor, and microbial activity. Of these three, two are using and selling solids as bedding and one is land applying solids.
Nutrient Recovery One Washington digester, FPE Renewables, recently added a full-scale nutrient recovery process to its AD system. Nutrient recovery has taken on more importance as adding pre-consumer waste to a dairy digester results in significantly higher nitrogen and more phosphorus than dairy manure alone.
Nutrient recovery processes extract some of the nitrogen or phosphorus, or both, from the liquid effluent. There has been great interest in developing nutrient recovery processes, both as a way to better manage nutrients and to develop marketable products that can contribute to the financial feasibility of a digester.
Dairy Digester Profiles The following profiles of Washington’s six dairy digesters were prepared from information compiled primarily during June and July 2011 using a variety of sources, including interviews with digester owners and operators, site visits and information from regulatory agencies and Washington State University (WSU) researchers. Every effort was made to collect and present accurate information, recognizing that there are seasonal variations in operations. Each profile contains a diagram of the dairy AD system in place in July 2011. The profiles also include technical and operational information about the digesters and the contributing dairies that may be of interest.
• This digester receives manure piped from four lagoons at the Vander Haak Dairy, and periodically imports manure by truck from nearby dairy farms.
• Digested solids are used as bedding at the Vander Haak dairy and on a number of local dairy farms.
• The dairy has a scrape manure management system.
Profile: George DeRuyter & Sons Dairy With more than 5,000 mature dairy cows contributing manure, the DeRuyter digester is by far the largest digester currently operating in Washington, and the only one in eastern Washington. It is farm-owned and operated and was developed in 2006. The digester receives manure from two dairies, George DeRuyter & Sons Dairy and D & A Farms. The only non-manure feedstock currently accepted at this digester is grease trap waste, in contrast to the other five digesters that receive a range of preconsumer waste. Initially, the separated solids were used as bedding or composted on site, but now all solids from this digester are sold to Organix, a Walla Walla company that uses them to produce a peat moss substitute called RePeetTM.
AD System Diagram
• The digester is located on the George DeRuyter & Sons Dairy, and manure is piped to the digester from the farm’s four lagoons and four lagoons at neighboring D & A Farms.
• Both dairies have flush manure management systems.
Profile: Qualco Energy The Qualco digester, developed in 2008, is a public-private partnership between Northwest Chinook Recovery, the Tulalip Tribe, and the Sno/Sky Agricultural Alliance. Although the digester currently receives manure from only one dairy, the digester was designed with the capacity to partner with several nearby dairies. Biogas production currently exceeds the installed generator capacity and Qualco is considering options to make use of the excess biogas. One possibility is adding a second generator. Another option is installing a system that would clean the biogas and produce CNG for transportation fuel. Research collaborations with WSU and the private company Multiform Harvest have investigated the effects of anaerobic digestion on air quality, pathogens, manure nutrient content, and farm economics, as well as phosphorus recovery in the form of struvite. Solids produced are currently being land applied to improve soil conditions.
• The Werkhoven Dairy is about 1 mile away from the digester. Manure and digestate is piped to and from the dairy’s two lagoons.
• The dairy has a flush manure management system.
• The dairy beds with sand and has a system to remove sand from the manure before it is sent to the digester.
Profile: Farm Power Rexville Farm Power Northwest is an independent development company that operates and maintains digesters in partnership with nearby farms. This allows a dairy to participate with minimal risk and expense. So far, Farm Power has developed two digesters in Washington and is in the process of developing a third. Farm Power Rexville was the first of the Farm Power digesters; it started operating in August 2009. The digester is located on property at Beaver Marsh Farms, and takes manure from Beaver Marsh Farms and neighboring Harmony Dairy.
AD System Diagram Sale to other dairies
• Manure is piped from two lagoons at Beaver Marsh Farms and one lagoon at Harmony Dairy. Liquid effluent from the digester is returned to both farms.
• Digested solids are used as bedding at both Beaver Marsh Farms and Harmony Dairy, as well as a number of other local farms.
• Both Beaver Marsh Farms and Harmony Dairy have scrape manure management systems.
• Manure is pumped to and from six lagoons at MJD Farms (about 1 mile away).
• The dairy has a flush manure management system.
• All of the solids separation and storage occurs at the dairy.
• The dairy uses a combination of flushing and scraping to send manure to three lagoons.
• Currently, all of the separated solids produced are used as bedding on site.
• The digester is located on the Van Dyk-S Holsteins property, and manure is piped a short distance from the dairy to the digester. Liquid effluent is returned to the dairy for storage and use.
Page 14 Washington State Department of Agriculture Washington Dairies and Digesters, October 2011 The Importance of Nutrient Management and Recovery Dairies in Washington are required to operate under a dairy nutrient management plan and are inspected regularly by WSDA for compliance. One requirement of the DNMP is that the nutrients in the manure (both nitrogen and phosphorus) are applied to crop fields in the right time, place, and form for good crop production as well as for protection of both ground and surface waters. Dairies also must have sufficient capacity to properly handle and store manure and wastewater on site.
Inspections include a full review of records for nutrient applications and crop production as well as a review of storage and facility conditions and management.
Dairy AD systems in Washington have created new challenges for managing dairy nutrients. Adding pre-consumer waste to dairy digesters not only increases biogas production but also can significantly increase the amount of nitrogen when compared to manure only. The addition of pre-consumer waste can also increase the phosphorus content in the effluent. Under the solid waste permit exemption, these increased nutrient levels need to be quantified and incorporated in the receiving dairies’ DNMP so that their field applications of nutrients are balanced with crop production. This can be a challenge as the type and volume of organic feedstocks can vary considerably during the year as well as from one year to the next. This variability requires the dairies to stay in close touch with the digester operator and to increase nutrient testing of the digestate in order to stay within agronomic rates. So far, Washington dairies receiving liquid effluent from digesters have had to obtain additional land and adjust cropping to make use of the increased nutrients. In some cases where additional land is not available, dairies are exporting excess nutrients to other crop growers.
The anaerobic digestion process converts much of the nitrogen in the feedstocks from organic forms to inorganic forms (ammonia). Ammonia is more readily taken up by plants, which is a benefit.
However, ammonia is also more susceptible to volatilization during storage or application. This also requires additional attention by the dairies to minimize potential water and air quality concerns.
Researchers in Washington have partnered with digester owners and private companies to develop ways to recover nitrogen and phosphorus from manure and liquid effluent. Nutrient recovery can significantly reduce the amount of nutrients a dairy needs to manage. It can also create a range of new opportunities for marketing those nutrients to other growers.
Nutrient Recovery in Washington Nitrogen and Phosphorus Recovery: A full-scale commercial installation of a system that recovers both nitrogen (in the form of ammonium sulfate) and phosphorus (in the form of phosphorous solids) from dairy AD effluent is now operating at FPE Renewables. The system can be operated to produce ammonium sulfate, phosphorous solids, or both. In pilot-scale tests, recoveries of both phosphorus and nitrogen were as high as 80%. A second nutrient recovery system based on this design is planned at Rainier Biogas, a digester under development near Enumclaw. The system was developed based on WSU research; the full-scale system resulted from collaboration between researchers and Andgar.
Phosphorus Recovery as Struvite: Phosphorus recovery from livestock wastewater in the form of struvite has been demonstrated in other parts of the country. A pilot-scale test at the Qualco Energy digester has demonstrated successful struvite recovery from dairy digester effluent, reducing total phosphorus in the effluent by 60-80%. Current collaborative research efforts by WSU and Multiform Harvest include further process development to reduce costs, improve the struvite particle size, and further tailor the process for effective performance on dairy manure.