Low-cost Anaerobic Digesters for Dairy Manure Treatment and Renewable Energy Production
The long-term goal of the project is to provide low-cost treatment options for small to medium-scale farmers that produce renewable energy and reduce environmental degradation and greenhouse gas emissions.
The stated objectives of the project are to:
- Design and construct modified plug flow digesters and assess the ability of these digesters to quantitatively convert substrate inputs into biogas and liquid effluent;
- Conduct an economic analysis to determine the feasibility of installation and operation of these types of systems in the temperate United States and compare it to other designs using a cash-flow analysis;
- To quantify and compare the transformations of carbon (C) nitrogen (N) and phosphorus (P) as waste is processed in the UMD digestion system and compare these transformations to the large-scale BARC digestion system and their composting system, and
- To conduct an emergy analysis that will investigate the emergetic costs of the system and quantify the contributions of climate, logistics, and design requirements to the total input, and to determine the degree to which environmental sustainability and societal benefits are derived.
In the United States, previous digester research has focused exclusively on industrialized systems that are capital and management-intensive, and with an average cost of $1.0 million, are inaccessible to medium and small-scale farmers (USEPA, 2006). Of the 114 existing digesters in the United States, 88 are located on dairy farms. Due to capital requirements, the U.S. EPA recommends digester installation for herds with more than 500 cows, which puts this beneficial technology out of the hands of the overwhelming majority of Maryland stakeholders (USEPA, 2006). Ninety-four percent of Maryland dairies (774 farms) have less than 200 dairy cows, and only 8 farms in Maryland have more than 500 dairy cows. This trend is observed throughout the Chesapeake Bay watershed, with 94.5 % of the dairy farms having less than 200 cows and only 1.2 % having 500 cows or more (USDA, 2004).
Low-cost anaerobic digestion is a proven technology in developing countries, with over 10 million low-cost digesters in India, China, and Latin America (Abraham et al., 2007; Lansing et al., 2008b). The transfer of this technology to temperature zones in the United States has not been explored previously. This project seeks to fulfill this research gap by investigating modified low-cost digesters in Maryland for technology transfer to dairy farms throughout the Chesapeake Bay watershed.
Digester Design: UMD Digestion System
The overall digester design includes nine tubular digesters located within drainage culvert with a geomembrane bag holding the manure, radiant piping under the digester bag and insulation between the digestion bag and the culvert. The manure is pumped from the storage area into two holding tanks (separated and un-separated manure), and then heated by a heating kettle, released by gravity into the digester, where it moves in a plug-flow fashion through the digester over a period of 25 days. The effluent is held in individual effluent holding tanks to enhance effluent sampling, and then pumped to the BARC digestion lagoon.
Pictured are six of the nine 20” long digesters. Each digester holds 750 gallons of wastewater and has a 25 day retntion time, resulting in a total 6750 gallons of manure being treated by the system. Six digesters receive un-separated manure and three digesters receive separated manure.
Modified plug-flow digester highlighting various components.