Anaerobic digestion is a process that results in the production of biogas that can be used a renewable source of electricity on-farm or sold to the distribution grid. Temperature is a critical parameter for anaerobic digestion since it influences both system heat requirements and methane production. Although anaerobic digestion can take place under psychrophilic (15-25°C), mesophilic (35-40°C), and thermophilic (50-60°C) conditions, temperatures of 35-37°C are typically recommended for methane production from animal manure. However, digesters require significant amount of heat energy to maintain temperatures at these levels. There is limited information about methane production from dairy digesters at temperatures less than 35°C and results in the literature are presented from laboratory-scale rather than field-scale systems.
The objective of this study was to evaluate the effect of two relatively low digestion temperatures (22 and 28°C) on methane production using replicate continuously-fed, field-scale dairy manure digesters at two organic loading rates. The results were compared with those from identical digesters operated at 35°C.
Anaerobic digestion experiments were carried out using six modified Taiwanese-model field-scale (FS) on-site digesters (Fig. 1) at the USDA Beltsville Agricultural Research Center (BARC). Each FS digester has a total capacity of 3 m3 and was operated at a liquid capacity of 67% (2 m3 working volume) with 33% headspace for biogas collection. The FS digesters are plug-flow reactors and operated without mixing. First, duplicate field-scale (FS) anaerobic digesters were maintained at one of three set temperatures (22 ± 2, 28 ± 2 and 35 ± 2°C) and fed with solids-separated manure for 80 days (period 1). The digesters were subsequently operated under the same temperature regime (22 ± 2, 28 ± 2 and 35 ± 2°C) but were fed at a higher organic loading rate (OLR) using solids-separated manure amended with manure solids for 56 days (period 2). The hydraulic retention time (HRT) was 17 days for all digesters throughout the study. Digesters were fed once daily five days a week with 160 L d-1 of separated manure for period 1, and 148 L d-1 of separated manure amended with 16 kg d-1 (wet weight) manure solids (roughly 12 L in volume) for the period 2.
Our results suggest that anaerobic digesters treating dairy manure at lower temperatures can be nearly as effective as digesters operated at 35°C, even with a relatively short 17-day retention time. Methane production from digesters operated at 28°C was about 90% of that from digesters operated at 35°C but the differences were not statistically significant. Digesters operated at 22°C produced about 70% as much methane as digesters operated at 35°C without affecting digester stability. Small farm digester systems that may not have access to waste heat from electrical generation, could efficiently operate at these lower temperatures to produce methane and reduce greenhouse gas emissions and odors. Larger digester systems could also choose to operate at these lower temperatures if reducing digester heating would allow for more valuable uses of their heat energy such as drying solids or treating liquids to remove nutrients.
We are currently investigating the fate and effect of antibiotics and feed additives during the anaerobic digestion of manure.
Osman Arikan, Assoc. Prof., Istanbul Technical Univ., Dept. of Environmental Eng., Istanbul, Turkey. Visiting Scientist, USDA-ARS, BARC, Beltsville, MD, Visiting Assoc. Prof., University of Maryland, Dept. of Environmental Science&Tech., College Park, MD. firstname.lastname@example.org
Walter Mulbry, Research Microbiologist, USDA-ARS, Beltsville Agricultural Research Center, Beltsville, MD. Stephanie Lansing, Assistant Professor, University of Maryland, Department of Environmental Science and Technology, College Park, MD.
Data is to be published.
The authors gratefully acknowledge Jose Colina and Lorianny Rivera for assistance in operating the digesters and Anna Kulow for analyzing biogas and effluent samples.
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