Barnaby J. Watten

Applications of pure oxygen in fish culture

Abstract The use of pure oxygen can increase the carrying capacity of a fish culture system when dissolved oxygen is the most limiting factor. The actual increase in carrying capacity will depend primarily on temperature, pH, and the efficiency of the solids removal system used. While the primary design parameter is the concentration of dissolved oxygen from the absorber unit, the concentration and partial pressures of other gases are also important. Due to change in the oxygen demand over the day, the ability to adjust the concentration of dissolved oxygen from the absorber unit may significantly reduce oxygen usage, but may reduce the overall system reliability. Important operating characteristics are dissolved oxygen concentration, absorption efficiency, utilization efficiency, and transfer efficiency. These parameters are all interrelated and the selection of the actual operating point may involve significant trade-offs between the various parameters. While the mass-transfer characteristics of the different types of absorption units are relatively well understood, operational procedures and constraints may significantly impact what can be achieved in practice. The ‘best’ type absorber unit for a given application will depend on site conditions, production schedules, and the layout of the rearing units. General design procedures are suggested for the design of pure oxygen systems.

Influence of tank design and hydraulic loading on the behavior, growth, and metabolism of rainbow trout (Oncorhynchus mykiss)

Subadult rainbow trout (Oncorhynchus mykiss) stocked at 48 kg/m3 (3 lb/ft3) were subjected to treatments of tank design (rectangular plug flow, circular, and cylindrical cross flow) and water exchange rate (1·5 and 2·5 exchanges/h) to determine their effects on fish behavior, growth, and metabolism. Ambient light levels and current velocities were also measured in each of three tank sectors (upstream, middle, and downstream) to determine their relative contributions to behavioral effects. Tank design significantly affected fish orientation to current, contact time with tank surfaces, and frequency of agonistic encounters, though aggression levels were relatively low overall. Gradients in fish distribution by sector were greatest in plug-flow tanks. Effects were either modified or eliminated by increasing the water exchange rate from 1·5 to 2·5/h. Multiple-regression analysis showed the following hieararchy of independent-variable effects on fish distribution: tank type > exchange rate > aggression level > current velocity > light level. Significant effects of tank design were also observed on fish growth in terms of biomass gain (cross flow > plug flow > circular). These results were matched in metabolic studies, where both oxygen consumption and ammonia excretion were highest in circular and lowest in cross-flow tanks. Reduction (cross-flow compared with circular tanks) in oxygen consumption averaged 13·6%, ammonia excretion 17·5%. These results were also modified by an increase in water exchange rate. Tank-design effects on fish metabolism and growth may be mediated, at least partly, through changes in fish behavior.

Comparative hydraulics and rearing trial performance of a production scale cross-flow rearing unit

The hydraulic characteristics of a cross-flow rearing unit of 20·3 m3 capacity were identified, based on an analysis of residence time distribution (RTD), and compared with the characteristics of a standard rectangular vessel. A tilapia (Oreochromis aureus) production trial was also conducted, in triplicate, within each of the two tank types. Results of the RTD analyses confirmed that the cross-flow rearing unit provided mixed flow reactor behavior despite its rectangular shape. Hydraulic performance was improved over that of an earlier design by repositioning the influent and effluent manifolds at floor level and by rounding the lower sides of the vessel to streamline flow. Influent jet wake distributed uniformly over the tank floor provided surface velocities of 10·0 to 14·7 cm/s with a manifold pressure of 0.6−1.65 m H2O (gauge). The jet wake and resultant cross-flow circulation pattern continuously purged fecal solids and waste feed at a water exchange rate of 1·4 per hour. Whole body composition, mortality, lengths, and weights of tilapia produced in both tank types did not differ significantly (P > 0·05). The food conversion ratio was higher (P < 0·05) in the cross-flow than the standard rectangular vessel (2·12 versus 1·85 kg feed/kg gain), while net oxygen consumption (2·56 versus 2·85 g O2/kg biomass d) and ammonia excretion (126·3 versus 162·7 mg NH4-N/kg biomass d) were lower. The cross-flow design appears particularly suited for high-density culture applications that require minimal flow.

Gas transfer within a multi-stage packed column oxygen absorber: model development and application

A packed column oxygen obsorber was developed in which oxygen flow is directed, in serial reuse, through parallel packed column stages receiving equal portions of the liquid being treated. The relative performance of the absorber was established using a computer simulation program employing finite difference-mass transfer calculations. The program was calibrated using packing specific mass transfer coefficients derived from pilot scale test data. A separate series of tests served to verify model assumptions and performance predictions. Simulation data indicated multi-stage operation can substantially reduce the column height required to achieve a selected oxygen absorption efficiency (AE); for example, the column height required to achieve an AE of 76·5% with an inlet volumetric oxygenwater ratio of 0·008 (column packing, 3·81 cm plastic ACTIFIL®; water temperature, 20°C; influent dissolved oxygen, 9·08 mg/litre; operating pressure (absolute), 760 mm Hg) was 0·27 m using a 10-stage system versus 1·39 m using a single-stage absorber. Reductions in column height achieved were related to oxygen and water feed rates, number of stages employed, mass transfer characteristics of the column packing used, and concentrations of dissolved gases in the liquid being treated.

Modeling the effects of sequential rearing on the potential production of controlled environment fish-culture systems

The production potential of an aquaculture system can be achieved only if the feed carrying capacity (kilograms per day) of the system is maintained at the maximum acceptable rate throughout the rearing period. Continuous production of fish through successive stocking and harvesting provides a higher mean feeding rate than conventional batch culture methods. Algorithms were developed to optimize sequential fish production given the feed carrying capacity of the system, rearing period (RP), length of fish at stocking (Lt = 0, growth rate (ΔL), feed conversion (FC), and mortality rate (Z). Algorithm use indicates that production per rearing period increases at a diminishing rate with the number (n) of stocking-harvesting intervals. For example, using rainbow trout (Oncorhynchus mykiss), where RP = 270 d, Lt = 0 = 7·6 cm; ΔL = 0·0.086 cm/d, FC = 1·8, and Z = 0·0, the increase in production over than predicted with the batch method (n = 1) for selected values of n was as follows: 44% (n = 2), 65% (n = 3), 87% (n = 5), 98% (n = 7), 104% (n = 9), and 110% (n = 12). Improvements in production with n > 1 diminish with a greater Lt = 0 or Z, buth increase with RP or ΔL. Sequential rearing can substantially reduce the water flow required to achieve a chosen production goal.

Gas phase axial dispersion in a packed column oxygen absorber

Gas phase axial dispersion was characterized within an enclosed packed column receiving oxygen and water under counter-current flow conditions. Steady-state gas phase profiles (longitudinal) were measured during a series of 90 column runs in which, at each of three bed depths (0·362, 0·699, and 1·041 m), all combinations of the following independent variables were tested: influent volumetric oxygen-liquid ratio, 0·8, 1·6, 2·6, 4·0, and 8·0%; hydraulic loading, 32·0 and 61·2 kg/m2 s; and packing type, 2·54 cm Tri-Pack®, 3·81 cm Nor-Pac® and 5·08 cm Nor-Pac®. Over the range of operating conditions tested, gas phase mixing was extensive and for all practical purposes could be considered complete. A homogeneous gas phase within the column, unlike true counter-current flow, results in an exponential decay in dissolved gas deficits as the liquid passes through the packing. Thus, increases in packed bed depth will result in diminishing increases in gas absorption/desorption.

Design of packed columns for commercial oxygen addition and dissolved nitrogen removal based on effluent criteria

Abstract Successful application of the packed column in fish culture requires a design based not only on oxygen supplementation rates (kg/day), or other standard performance indicators such as oxygen absorption efficiency (kg absorbed/kg applied) or transfer efficiency (kg/kWh), bvt also on dissolved nitrogen and total dissolved gas pressure limits and predicted changes across the system. Such a design procedure was developed by applying a mass balance on the gas and liquid phases of the system along with Henrys law and previously published packing-specific mass-transfer correlations. The design approach is unique in that column pressure and oxygen feed rates are calculated for target changes in dissolved oxygen and nitrogen without the use of iterative numerical procedures. An alternative calculation sequence was developed to establish the sensitivity of column performance to changes in oxygen feed rates at a selected column pressure or packed bed depth.

Modeling gas transfer in a spray tower oxygen absorber

A computer model characterizing the performance of a spray tower oxygen absorption system was developed based on finite difference mass transfer calculations. Performance was assessed in terms of oxygen utilization, transfer efficiency, and economy. Pilot scale tests verified model assumptions and performance predictions. Simulation runs indicated spray tower head and oxygen feed requirements for desired changes in dissolved oxygen (DO) exceeded those required for packed column equipment. Spray tower performance was improved by increasing hydraulic loading from 35 to 85 kg m−2 s−1 and by increasing tower height from 1·25 to 2·50 m. The effluent DO concentration that minimized variable costs of oxygen transfer was lower in the spray tower than in the packed tower, indicating clean water use of the spray tower will be limited to moderate effluent DO requirement applications (DO <20 mg l−1).

Gas supersaturation in surface waters of aquaculture ponds

During the 1988 and 1989 growing seasons, dissolved gas pressures were measured in 0·04 ha to 0·06 ha earthen ponds (n = 17) used to produce channel catfish or bluegill sunfish, both with and without supplemental aeration. Surface waters in early morning hours (0630 to 0830 h) were typically saturated with nitrogen (N2) and argon (Ar), undersaturated with dissolved oxygen (DO), and supersaturated with carbon dioxide. Total gas pressures (ΔP) were low, averaging −40 mmHg (range −127 to 62 mmHg). During afternoon hours (1300 to 1500 h) ΔP increased to a mean of 111 mmHg (range −46 to 334 mmHg); 34% of afternoon ΔP values were above that known to cause mortality in channel catfish during continuous exposure bioassays (115 mmHg), yet no excessive mortality was observed. High afternoon ΔP values resulted from DO supersaturation caused by phytoplankton photosynthesis and, to a lesser extent, N2 and Ar supersaturation. The N2 and Ar supersaturation apparently resulted from cooling and saturation of surface waters with air during evening hours, and the subsequent increase in water temperature during daylight hours (daily increase X = 3·4°C; range 1·1 to 7·9°C) without sufficient gas release.

Gas-phase axial dispersion in a spray tower

Abstract Gas-phase axial dispersion (mixing of the composition of the gas phase along the longitudinal axis) was characterized in an enclosed spray tower for purposes of establishing reactor type for the solute-solvent pair oxygen and water. Test condition variables were spray tower height (TH), 1·52, 2·03 and 2·54 m; hydraulic loading (HL), 44·2, 66·3 and 88·4 kg/m2s; the ratio of volumetric oxygen injection to water flow rate (G/L), 1·0, 2·5 and 5·0%; the ratio of volumetric bulk tower gas recirculation flow rate to water flow rate (BG/L), 0, 500 and 700%; and bulk tower gas recirculation direction, counter-current to and co-current to the water flow. Gas composition measurements (% O2) made across the long axis of the tower under steady-state conditions provided 1020 independent observations and 240 gas composition profiles. Factors showing a significant effect (P 0·05). Profile data indicate a completely mixed gas phase within the tower. The dispersion observed was attributed to the lack of a significant pressure drop along the axis of the reaction vessel, forces due to nozzle operation, and to bulk tower gas recirculation.