Browsing by Author "Timmons, M. B."
Now showing 1 - 2 of 2
Results Per Page
Sort Options
- An Engineering Analysis of the Stoichiometry of Autotrophic, Heterotrophic Bacterial Control of Ammonia-Nitrogen in Zero-Exchange Marine Shrimp Production SystemsEbeling, 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.
- Using Oxygen Gas Transfer Coefficients to Predict Carbon Dioxide RemovalAitchison, T. F.; Timmons, M. B.; Bisogni, J. J. Jr.; Piedrahita, R. H.; Vinci, B. J. (Commercial Fish and Shellfish Technologies Program, Virginia Tech, 2007-06-01)The purpose of this research was to determine if oxygen gas transfer coefficients, as reflected by overall mass transfer coefficient (KLa) values, could be used to predict carbon dioxide (CO2) removal by degassing in aquaculture production systems. The motivation for this approach was that while there is ample literature related to oxygen gas transfer, there is limited information on CO2 removal. A series of tests was conducted to determine the ratio (φE) of KLa for CO2 to that of oxygen for two commonly used surface aerators and then compare φE to the theoretical 22 International Journal of Recirculating Aquaculture, Volume 8, June 2007 ratio, φT, which is 0.90 based upon gas molecular diameters. Experiments were conducted in a 10,000 L circular tank aerated by means of two different surface agitators. The two aerators were selected to represent aeration patterns with high and moderate water to gas interface exposures or breakup patterns (photos supplied, Figures 2 and 3). The results showed that φE/φT ratios were 96% (for high air exposure) and 74% (for moderate air exposure) for water with an alkalinity of ~130 mg/L as CaCO3. The φE/φT ratio decreased to 0.84 and 0.51 for the high and moderate air exposures, respectively, when higher alkalinity waters (~1,000 mg/L as CaCO3) were used.