This Washington State University (WSU) Extension publication is an abbreviated review of information contained in the Renewable Natural Gas and Nutrient Recovery Feasibility for DeRuyter Dairy (Coppedge et al. 2012). It is part of the Anaerobic Digestion Systems Series, which aims to provide information that improves decision-making for anaerobic digestion systems. Readers interested in further details are encouraged to read the full WSU report. Furthermore, individuals interested in a specific digester installation should seek expert guidance from an Anaerobic Digestion System technology provider. The US EPAAgSTAR project database contains a list of technology providers across the country.
This publication introduces key concerns regarding nutrient management. It discusses and compares the profitability of three nutrient recovery technologies that work in tandem with anaerobic digester systems. While one of the nutrient technologies was developed in collaboration with WSU, this publication does not intend to promote the WSUpatented technology. Rather, the emphases are on the technology’s process of removing nutrients from dairy waste and an examination of its contribution to the profitability of an anaerobic digestion project compared to current alternatives.
Nutrient management is one of the most important concerns identified by dairy owners that can potentially have negative economic impacts (Bishop and Shumway 2009). Every year, a dairy cow generates 164 kg total nitrogen, 80 kg total ammonia nitrogen, and 28 kg total phosphorus (ASAE 2005). The microbial breakdown of manure can emit ammonia and other compounds that cause concerns in air quality and respiratory health (Archibeque et al. 2007; Erisman and Schaap 2004; McCubbin et al. 2002), as well as methane, which is a potent greenhouse gas (Shih et al. 2006). Meanwhile, long-term manure application on agricultural lands may result in excess nitrogen and phosphorus accumulation, which has contributed to problems with nitrate leaching, eutrophication, ammonia toxicity, and nitrate-induced blue baby syndrome (EPA 1996).
As a result of these concerns, a number of efforts to develop and commercialize nutrient recovery technologies appropriate for dairies and other livestock operations are underway. This publication focuses on the economic feasibility of nutrient recovery technologies that work in tandem with anaerobic
Anaerobic Digestion and Nutrient Recovery Technologies
Anaerobic digestion (AD) creates an anaerobic environment (without oxygen) in which naturally occurring microorganisms convert complex organic materials in manure and other wet organic wastes, such as food processing wastes, to biogas. Biogas can be used in a combined heat and power (CHP) model to generate renewable energy, or it can be further refined to renewable natural gas (RNG) which can be used as a transportation fuel. In addition to biogas, AD creates many cobenefits, including reductions in greenhouse gas emissions, decreased odors, waste stabilization, and lower pathogen counts (EPA 2004; EPA 2005; EPA 2008; Martin and Roos 2007).
Coppedge et al. (2012) examined an anaerobic digester in the Yakima Valley that processes feedstock from 5,000 dairy cows, generating 165,000 gallons of concentrated manure per day. Little to no offfarm materials are fed into the digester. The study looks at different configurations of AD projects. The current baseline is an ADCHP system and has been operational since 2008. This system produces biogas that fuels a modified diesel engine and generator, which then produces electricity and recovered heat. The electrical power is sold to the grid. The initial capital cost for an ADCHP system is approximately $4.4 million.
The second AD project is an investment scenario where biogas produced by the anaerobic digester is upgraded to RNG. The ADRNG system is assumed to start producing renewable fuel two years after development of the infrastructure. The ADRNG system is evaluated here under the scenario wherein the current ADCHP is converted to this particular system (i.e., in 2012). The estimated total capital cost is $10.1 million, comprised of the initial investment for the ADCHP ($4.4 million) plus a $5.7 million investment to convert the existing system and construct the infrastructure needed to produce renewable fuel. The RNG infrastructure includes a gas cleaning unit, digester modification for accepting nonmanure inputs such as food processing substrates, transportation of gas off site via pipeline to an injection point, and a fueling station.