2001 Aquacultural Engineering Society Issues Forum, November 11-14, 2001, in Shepherdsown, West Virginia, USA
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Modeling Carbon Dioxide, pH and Un-Ionized Ammonia Relationships in Serial Reuse Systems
In serial reuse systems, excretion of metabolic carbon dioxide has a significant impact on ambient pH, carbon dioxide, and un-ionized ammonia concentrations. This impact depends strongly on alkalinity, water flow rate, feeding rate, and loss of carbon dioxide to the atmosphere. A reduction in pH from metabolic carbon dioxide can significantly reduce the un-ionized ammonia concentration and increase the carbon dioxide concentrations compared to those parameters computed from influent pH. The ability to accurately predict pH in serial reuse systems is critical to their design and effective operation. A trial and error solution to the alkalinity–pH system was used to estimate important water quality parameters in serial reuse systems. Transfer of oxygen and carbon dioxide across the air–water interface, at overflow weirs, and impacts of substrate-attached algae and suspended bacteria were modeled. Gas transfer at the weirs was much greater than transfer across the air–water boundary. This simulation model can rapidly estimate influent and effluent concentrations of dissolved oxygen, carbon dioxide, and un-ionized ammonia as a function of water temperature, elevation, water flow, and weir type. The accuracy of the estimates strongly depends on assumed pollutional loading rates and gas transfer at the weirs. The current simulation model is based on mean daily loading rates; the impacts of daily variation loading rates are discussed. Copies of the source code and executable program are available free of charge.
Aquacultural Engineering↗
Factors influencing the nitrification efficiency of fluidized bed filter with a plastic bead medium
The performance of fluidized bed nitrification filters charged with 2 ?? 4 ABS plastic beads (specific gravity 1.06) was evaluated. Three unique bed-height to diameter ratios were established, in triplicate, using column diameters of 12.7, 15.2 and 17.8 cm. Filters received water spiked with recycled nutrients and ammonia (TAN), from one of the three 500 1 feed tank system. With daily ammonia loading fixed at 8.6 g per system, TAN removal increased with column diameter at each of four tests hydraulic loading rates (6, 8, 10 and 12 Lpm). TAN in recirculated water (influent) rose from 0.5 to 1.0 mg/1 as ammonia loading increased from 180 mg/m2-day to 360 mg/m2-day. When hydraulic loading was fixed at 12 Lpm, TAN removal (%) was maximized with ammonia loadings ranging from 225 to 270 mg/m2-day. Biofilm thickness increased with ammonia loading, but decreased with increased hydraulic loading rates. Fluidized beds of ABS plastic beads were effective in reducing energy costs (head loss) of water treatment. ?? 2002 Elsevier Science B.V. All rights reserved.
Aquacultural Engineering↗
Effective flow-through vacuum degasser for fish hatcheries
Since the fry of certain species of fish cannot tolerate even slight amounts of supersaturation, their water supplies must be degassed. Gas content can be reduced to subsaturated, sublethal levels by passing it through the vacuum degasser described here. The system includes a low-cost, flow-through unit easily capable of degassing (to less than saturated conditions) water flowing at 190 liters min −1 . In tests completed to date, dissolved oxygen content was never reduced below 7 ppm.
Aquacultural Engineering↗
Total gas pressure and oxygen and nitrogen saturation in warmwater ponds aerated with airlift pumps
Gas bubble disease and improper inflation of swim bladders in larval striped bass Morone saxatilis have been recently related in laboratory studies to very low levels of gas supersaturation. In other species, kills resulting from gas supersaturation have been reported in natural waters as large as Galveston Bay. We monitored warmwater ponds during spring to determine the extent of naturally occurring gas supersaturation and compared these levels with levels of gas saturation in ponds equipped with airlift pumps. Total gas pressure averaged 110% in the morning at the surface of non-aerated ponds and in the afternoon in ponds with airlift pumps. At other times of the day total gas pressure averaged 106–107% at the surface and bottom, morning and afternoon, in both aerated and non-aerated ponds. No evidence of gas bubble disease was found in 15-day-old striped bass fry cultured for 42 days in either aerated or non-aerated ponds.
Aquacultural Engineering↗
Feasibility of saltwater gradient ponds as a heat supply for hatchery rearing water
Salt gradient solar collector ponds are economically feasible as a source of heat energy for hatchery rearing water. A pond 3 m deep and covering an area of 5000 m 2 will supply approximately 2800 GJ of energy over a three-month period — enough for a one-year smolt program in an Atlantic salmon hatchery. The cost to construct such a pond is approximately $173,000, or the equivalent of seven years' fuel bills using conventional heating equipment. Although the technology is new and not entirely understood, existing ponds have proven to be relatively problem free and require only minimum maintenance.
Aquacultural Engineering↗
Design of airlift pumps for water circulation and aeration in aquaculture
Water flow rates were measured in airlift pumps 3·75-30 cm in diameter to develop performance data that might be useful to aquaculturists. Flows were determined when submergence of airlifts was 100% and when the center-line of the discharge was between 12·5 cm above and 5 cm below the water surface. Air was injected at 15-cm intervals from 15 cm to 120 cm below the discharge of the airlift and air flow was varied from 28 to 1416 liters min −1 . An increase in the vertical lift reduced flow rates greatly in large-diameter pipes, but only slightly affected flow rates in small-diameter pipes. Water flow increased linearly as air flow increased logarithmically.
Aquacultural Engineering↗
A bioassay for production capacity assessment
Given sufficient oxygen, fish production can be limited by a multiplicity of factors. Accurate determination of production capacity requires something other than an estimate of the biomass at which ambient un-ionized ammonia will reach a recommended, non-site-specific, maximum safe concentration. A chronic (1- to 2-month) bioassay can be used to determine the point at which metabolite buildup reduces growth by some predetermined proportion, or causes unacceptable tissue damage. The bioassay is conducted as a series reuse, single-pass system, in which incoming water to each rearing unit is aerated to 90% dissolved oxygen saturation. The assessment of production capacity is based on the relation between cumulative oxygen consumption and growth reduction.
Aquacultural Engineering↗
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/m 2 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.
Aquacultural Engineering↗
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 oxygenwater 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.
Aquacultural Engineering↗
Modeling the effects of serial off-gas reuse on the performance of a hooded surface oxygen obsorption system
A numerical model was used to evaluate the performance of a surface agitation system designed to contact commercial oxygen with water. The modeled system was unique in that oxygen-rich off-gas, normally discharged to the atmosphere, was directed in serial reuse through additional contact stages receiving untreated water. A correlation between the agitator mass-transfer coefficient and power demand, needed to calibrate the model, was established using a single-stage (37 W) contactor of 1·18 m 3 capacity. Additional tests, conducted with both single and three-stage equipment, verified model assumptions and performance predictions. Simulation runs indicated oxygen flow or power input required to meet a given effluent dissolved gas criterion can be substantially reduced by the off-gas reuse step; for example, to achieve an effluent dissolved oxygen of 24·1 mg/litre with a single stage agitator the oxygen feed rate needed was 61·5% greater than that required by a six-stage system receiving the same total power input (standard aeration efficiency, 0·5 kg/kW h; water flow rate, 100 litre/min; influent dissolved oxygen, 9·08 mg/litre at 15°C). The savings achieved increased with (1) greater target effluent dissolved oxygen concentrations, (2) lower oxygen feed rates, (3) higher input power levels, and (4) number of contact stages.
Aquacultural Engineering↗
Design of packed columns for commercial oxygen addition and dissolved nitrogen removal based on effluent criteria
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 Henry's 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.
Aquacultural Engineering↗
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 (N 2 ) 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, N 2 and Ar supersaturation. The N 2 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 "> X = 3·4°C; range 1·1 to 7·9°C) without sufficient gas release.
Aquacultural Engineering↗
Effect of subatmospheric pressure on the performance of an automated packed-column nitrogen desorption system
A portable vacuum degasser was developed to satisfy seasonal hatchery pretreatment needs. Dissolved-gas pressures in water exiting a packed column were regulated automatically with a unique feedback control loop incorporating a gasometer, pressure transducer, electronic (PID) controller and an electrically actuated pump discharge valve. The flow capacity of the system was 160 l/min with a total power requirement of 0·81 kW. Following controller tuning, field tests demonstrated the ability of the feedback loop to set appropriate column vacuum levels quickly in response to varying inlet dissolved-gas pressures or changes in selected controller set points. The degasser's ability to reduce dissolved nitrogen (DN) and increase dissolved oxygen (DO) concentrations was also assessed at four or five pure-oxygen feed rates under each of four column vacuum levels (−4·5, −9·0, −14·6 and −20·0 cm Hg). Performance was then compared with that predicted with a multicomponent gas transfer model. Relative error of model projections averaged just 7·7% for DO and 2·3% for DN (n = 19). Effluent DN ranged between 60·6 and 96·4% of saturation concentrations at 8·8°C with the influent DN at 135% of saturation. The high DN desorption rates achieved allow side-stream pretreatment with blending.
Aquacultural Engineering↗
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/m 3 (3 lb/ft 3 ) 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.
Aquacultural Engineering↗
Gas-phase axial dispersion in a spray tower
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/m 2 s; 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 (% O 2 ) 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) on gas composition were TH, HL, G/L and BG/L. Sample location as a percentage of TH did not have a significant effect on gas composition and accordingly profile slopes were not different from zero ( 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.
Aquacultural Engineering↗
Modeling gas transfer and biological respiration in a recirculating aquaculture system
Recirculating aquaculture system applications of oxygen absorption equipment require consideration of the combined effects of the system's physical, chemical and biological components. Interactions of this type were modeled within a recirculating system incorporating a mixed-flow type rearing vessel, a multi-tube clarifier, a rotating biological contactor (RBC) and a U-tube oxygenator. Finite difference mass transfer calculations, based on reactor theory, were used to predict steady-state dissolved gas levels in component effluents given system operating conditions. The model was calibrated and its predictions verified with data obtained from a pilot scale system of 14 m 3 capacity: errors in calibrated model predictions ( N = 45) average −1·2 mg l −1 (range −4·0 to 0·1 mg l −1 ). Model use indicated oxygen transfer costs are reduced 48% through recycle of U-tube off-gas. Further savings are provided by increasing the water recirculation rate from 250 to 350 l min −1 with low to moderate fish feed rates and by regulating oxygen injection based on diel variations in fish respiration. Increasing the gas transfer coefficient ( K L a ) of the RBC reduced oxygen transfer costs despite resultant elevations in dissolved nitrogen and argon concentrations. Carbon dioxide stripping across the RBC was substantial, varied with K L a , and increased with water recirculation rates.
Aquacultural Engineering↗
Feasibility of measuring dissolved carbon dioxide based on head space partial pressures
We describe an instrument prototype that measures dissolved carbon dioxide (DC) without need for standard wetted probe membranes or titration. DC is calculated using Henry's Law, water temperature, and the steady-state partial pressure of carbon dioxide that develops within the instrument's vertical gas-liquid contacting chamber. Gas-phase partial pressures were determined with either an infrared detector (ID) or by measuring voltage developed by a pH electrode immersed in an isolated sodium carbonate solution (SC) sparged with recirculated head space gas. Calculated DC concentrations were compared with those obtained by titration over a range of DC (2, 4, 8, 12, 16, 20, 24, and 28mg/l), total alkalinity (35, 120, and 250mg/l as CaCO3), total dissolved gas pressure (-178 to 120 mmHg), and dissolved oxygen concentrations (7, 14, and 18 mg/l). Statistically significant (P < 0.001) correlations were established between head space (ID) and titrimetrically determined DC concentrations (R2 = 0.987-0.999, N = 96). Millivolt and titrimetric values from the SC solution tests were also correlated (P < 0.001, R 2 = 0.997, N = 16). The absolute and relative error associated with the use of the ID and SC solution averaged 0.9mg/l DC and 7.0% and 0.6 mg/l DC and 9.6%, respectively. The precision of DC estimates established in a second test series was good; coefficients of variation (100(SD/mean)) for the head space (ID) and titration analyses were 0.99% and 1.7%. Precision of the SC solution method was 1.3%. In a third test series, a single ID was coupled with four replicate head space units so as to permit sequential monitoring (15 min intervals) of a common water source. Here, appropriate gas samples were secured using a series of solenoid valves (1.6 mm bore) activated by a time-based controller. This system configuration reduced the capital cost per sample site from US$ 2695 to 876. Absolute error averaged 2.9, 3.1, 3.7, and 2.7 mg/ l for replicates 1-4 (N = 36) during a 21-day test period (DC range, 36-40 mg/l). The ID meter was then modified so as to provide for DO as well as DC measurements across components of an intensive fish production system. ?? 2003 Elsevier B.V. All rights reserved.
Aquacultural Engineering↗