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Why Study High-Solids Anaerobic Digestion?
Colorado is the second highest producer of high solids cattle waste (HSCW) in the United States. Despite the available resources, Colorado currently has only one operational anaerobic digester treating manure (AgSTAR EPA 2011), which is located at a hog farm in Lamar. Arid climate and limited water resources in Colorado render the implementation of high water demanding conventional AD processes. Studies to date have proposed high solids AD systems capable of digesting organic solid waste (OSW) not more than 40% total solids (TS). Lab tests have shown that HSCW produced in Greeley (Colorado) has an average of 89.4% TS. Multi-stage leach bed reactor (MSLBR) system proposed in the current study is capable of handling HSCW of up to 90% TS.
What Did We Do?
Hydrolysis is carried out in a trickle flow leach bed reactor (TFLBR) and methanogenesis can be carried out in a high rate anaerobic digester (HRAD) like an upflow anaerobic sludge blanket reactor or a fixed film reactor. The objective of this research is to evaluate and optimize the performance of the TFLBR. The system was operated as a batch process and the organic leaching potential of a single pass TFLBR configuration was evaluated. The organic leaching potential was measured in terms of chemical oxygen demand (COD).
Three series’ of reactor experiments were carried out in total. Each subsequent experiment was based on the results on the previously conducted experiment. First set of reactor experiments included three TFLBRs (triplicate) loaded with HSCW. The difficulty encountered during the operation of this experiment was that the flow rate of water through the TFLBR slowed down over time and eventually dropped to zero within the first 24 hrs. This caused water build-up on top of the manure bed, resulting in the failure of hydrolysis. Second set of reactor experiments included six TFLBRs (two sets of triplicates). One set of triplicate was loaded with HSCW and the other set of triplicate was loaded with HSCW bulked with straw (5% by mass) to improve the porosity through the reactor. A layer of fine sand was added on top of the manure bed to facilitate water dispersion through the reactor.
The third set of reactor experiments included the comparison between nutrient dosed and non-nutrient dosed reactors (each carried out in triplicates). The idea behind dosing nutrients to an operational TFLBR was to check if the reactors were nutrient limited during the digestion process. Composite sampling technique was adopted so as to capture the exact leaching potential from each of the reactors.
What Have We Learned?
The first set of reactor experiments helped in identifying the clogging issues in operational TFLBRs handling HSCW. The second set of reactor experiments validated the use of fine sand as a better alternative to improve hydraulic flow when compared to the use of bulking agents. The third set of reactor experiments indicated that the addition of nutrient solution to a single-pass TFLBR operation is essential in improving the overall system yield. Leachate collection by composited sampling method instead of the instantaneous sample method improved the system efficiency by approximately 50%. The average TS reductions in the non-nutrient dosed and nutrient dosed TFLBRs were 23.18% and 22.67% respectively. The non-nutrient dosed TFLBRs underwent approximately 66.32% of COD reduction and the nutrient dosed TFLBRs underwent approximately 73.51% of COD reduction due to COD leaching during hydrolysis, over the period of six weeks. Biochemical methane potential (BCMP) test results indicate high biogas yields from the weekly composited leachate from the reactor experiments proving successful system operation. Approximately 0.43 L CH4/g COD is produced from the leachate collected from the non-nutrient dosed TFLBRs and 0.57 L CH4/g COD is produced from the leachate collected from the nutrient dosed TFLBRs.
The proposed MSLBR system recommends TFLBRs operating under leachate recirculation. The addition of nutrient solution in a leachate recirculated TFLBR would not be unnecessary since the nutrients in the system would be conserved. The success of hydraulic conductivity and leaching quality in a leachate recirculated TFLBR is unknown. More research is required to completely understand the operation and success of the MSLBR system treating HSCW. Pilot scale reactor experiments should be conducted to monitor the operation of the TFLBRs under leachate recirculation.
Asma Hanif, Graduate Student in Civil & Environmental Engineering, Colorado State University, firstname.lastname@example.org
Dr. Sybil Sharvelle, Assistant Professor in Civil & Environmental Engineering, Colorado State University, Sybil.Sharvelle@colostate.edu
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