Hydraulic characteristics of feedlot manure in an anaerobic leachate bed reactor
Date
2013
Authors
Wasserbach, Kelly, author
Sharvelle, Sybil, advisor
Shackelford, Charles, committee member
Davis, Jessica, committee member
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Abstract
Concentrated animal feeding operations (CAFOs) use the practice of optimizing space for the raising of livestock. By implementing space-saving techniques, these operations end up with large quantities of waste on small parcels of land. One way to utilize the waste is to integrate an anaerobic digester into the waste management approach of a CAFO. Anaerobic digesters efficiently break down waste while creating an energy source. In the Midwestern United States, water is in abundance, and therefore can be added to continuously stirred reactors or other anaerobic digestion technologies. In semi-arid climates, such as in Colorado, water is a treasured commodity. A new technology is being investigated to limit the need for water addition in anaerobic digestion. Water is trickled through a column of manure creating leachate. Leachate is continuously recycled though the leachate bed reactor, and then flows to a compositing tank and ultimately to a high rate anaerobic digester where methane is produced. This method has been used in manure, food and landfill applications. In many cases, clogging occurs either initially or after some digestion has occurred, and the pore space decreases. The objectives of this research were to gain a better understanding of what additives will aid in better flow through manure and to develop a method to characterize hydraulic flow through a column of manure. Intrinsic permeability (k) was measured with respect to compressed air as the permeant fluid on a homogenized sample of feedlot manure. The impact of compression, bulking agents (straw and wood chips), sieving out small fragments, and dispersion media were compared on the basis of the measured k. Applied force, or compression, had the greatest impact on k because the tested manure was greater than 30% air by volume. Straw showed the greatest increase in k of feedlot manure compared to wood chips and particle sieving. After determining which substrate combination would be best suited for liquid flow, experiments based on the mean residence time were set up. No substrate was added in these experiments, because the behavior of the manure was uncertain. Water with an oxidation reduction potential (ORP) of less than -500 mV was used to mimic anaerobic conditions. Three replicate columns were constructed and operated for six weeks. Three tracer tests, each with a hydraulic loading rate of 0.88 m/d, using sodium bromide (NaBr) as the tracer salt were conducted for three replicate columns. Variability in effluent concentrations and flows was observed in the columns, which was expected due to variability in packing within the column. The average mean residence time was approximately 6 hours. Also, the majority of the tracer (60% of the output) leached from the columns in less than one pore volume. Thus, the columns likely experienced preferential flow in that large pulses of water would exit at random times and the majority of the tracer exited the column in less than one pore volume. Tailing of residence time distribution curves and inability to recover all injected salt indicates the likelihood of dead zones within reactors. A ratio (further referred to as ratio R) of the superficial velocity to the hydraulic loading rate was calculated. The ratio R was greater than one for every tracer test indicating that water flows through the column slowly. This indicates retardation through the column consistent with the observation of tailing in residence time distribution curves, also indicating the presence of dead zones. The free drain volume was a small fraction of the pore volume in the total column. The fact that so few pore volumes exited the column indicates severe retardation within the column that can be attributed to dead zones. Each tracer test also showed that very few pore volumes exited the column. This could possibly indicate a retardation in flow.
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Subject
anaerobic digestion
hydraulic characteristics