Browsing by Author "Ebeling, J. M."

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    An Engineering Analysis of the Stoichiometry of Autotrophic, Heterotrophic Bacterial Control of Ammonia-Nitrogen in Zero-Exchange Marine Shrimp Production Systems
    Ebeling, J. M.; Timmons, M. B.; Bisogni, J. J. (Commercial Fish and Shellfish Technologies Program, Virginia Tech, 2009-06-01)
    After dissolved oxygen, ammonia-nitrogen buildup from the metabolism of feed is usually the limiting factor to increasing production levels in intensive aquaculture systems. Currently, large fixed-cell bioreactors are the primary strategy used to control inorganic nitrogen in intensive recirculating systems. This option utilizes chemosynthetic autotrophic bacteria, ammonia-oxidizing bacteria (AOB), and nitrite-oxidizing bacteria (NOB). Zero-exchange nitrification management systems have been developed based on heterotrophic bacteria and promoted for the intensive production of marine shrimp and tilapia. In these systems, the heterotrophic bacterial growth is stimulated through the addition of an organic labile carbonaceous substrate. At high organic carbon to nitrogen (C/N) feed ratios, heterotrophic bacteria assimilate ammonia-nitrogen directly from the water, replacing the need for an external fixed film biofilter. As a result, build-up of suspended solids may become the second limiting factor after dissolved oxygen. This paper reviews two nitrogen conversion pathways used for the removal of ammonia-nitrogen in aquaculture systems; autotrophic bacterial conversion of ammonia-nitrogen to nitrate nitrogen, and heterotrophic bacterial conversion of ammonia-nitrogen directly to microbial biomass. The first part of this study reviews these two ammonia removal pathways, presents a set of balanced stoichiometric relationships, and discusses their impact on water quality. In addition, microbial growth energetics are used to characterize production of volatile and total suspended solids for autotrophic and heterotrophic systems. A critical verification of this work was that only a small fraction of the feed's carbon content is readily available to the heterotrophic bacteria. For example, feed containing 35% protein (350 g/kg feed) has only 109 g/kg feed of labile carbon. In the paper's second part, the results of a study on the impact C/N ratio on water quality is presented. In this experimental trial sufficient labile organic carbon in the form of sucrose (sugar) was added daily at 0%, 50%, and 100% of the system feeding rate to three prototype zero-exchange systems. The system was stocked with marine shrimp (Litopenaeus vannamei) at modest density (150 /m2) and water quality was measured daily. Significant differences were seen between the three strategies in the key water quality parameters of ammonia-nitrogen, nitrite-nitrogen, nitrate-nitrogen, pH, and alkalinity. The control (0%) system exhibited water quality characteristics of a mixed autotrophic/heterotrophic system while the other two systems receiving supplemental organic carbon (50% and 100%) showed water quality characteristics of pure heterotrophic systems.
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    In-situ Determination of Nitrification Kinetics and Performance Characteristics for a Bubble-washed Bead Filter
    Ebeling, J. M.; Wheaton, F. W. (Commercial Fish and Shellfish Technologies Program, Virginia Tech, 2006-06-01)
    Intensive recirculating aquaculture systems rely almost exclusively on some form of fixed film biofilter for nitrification.Currently there is no standardized way to determine and report biofilter performance to facilitate user selection among the numerous options. This type of information is critical for the end user, and also important for both the design engineer and the manufacturer. In an attempt to address this issue, a simple procedure for estimating nitrification reaction rate kinetics is described and applied to a bubble-washed bead filter. Reaction rate kinetics were determined through a series of batch reaction rate experiments with a commercially available 0.06-m3 (2.0-ft3) bubble-washed bead filter. Empirical mathematical models for the nitrification of ammonia-nitrogen to nitrate-nitrogen were developed. The kinetics of nitrification were found to fit a simple first-order reaction model, when the ammonia-nitrogen concentration was less than 1 mg NH4-N/L, and a zero-order reaction when the ammonia-nitrogen concentration was greater. The exact breakpoint between first and zero-order reaction kinetics was found to be a function of the flow rate. In addition, the first-order kinetic reaction rate constants were also a function of the flow rate, reflecting the influence of high nutrient gradients and associated higher nutrient gradient across the biofilm. No effect of flow rate was found for the zero-order reaction rate constants. Kinetic reaction rate parameters, maximum reaction rates, and half-saturation constants were determined for the Monod kinetics model as functions of hydraulic loading rate. Based on these results, an evaluation tool was proposed to help characterize bead filter performance based on reaction rate kinetics. A series of performance characteristic curves were developed to show maximum removal rates as a function of ammonia-nitrogen concentration and flow rates through the bubble-washed bead filter.