Aquacultural Engineering 19 (1999) 99 – 103
Paddlewheel aerator oxygen transfer efficiencies at three salinities Arlo W. Fast a,*, Edmundo C. Tan b, Desmond F. Stevens b, Jeffrey C. Olson b, Jianguang Qin c, David K. Barclay d a
Hawaii Institute of Marine Biology, Uni6ersity of Hawaii at Manoa, P.O. Box 1056, Kaneohe, HI 96744, USA b STO Design Group, Inc., 2500 Redhill A6e., Santa Ana, CA 92705, USA c School of Biological Sciences, The Flinders Uni6ersity of South Australia, GPO Box 2100, Adelaide 5001, SA, Australia d Aquatic Culture and Design, P.O. Box 911, Kapaau, HI 96755, USA Received 4 May 1998; accepted 21 September 1998
Abstract Oxygen transfer rates, or standard aeration efficiencies (SAE) were measured using seven paddlewheel impellers at three salinities (0, 11 and 22‰) and with two aeration devices (0.37 and 0.75 kW) in clean water tests. Oxygen transfer rates increased significantly at higher salinities. With the 0.37-kW aerator, mean SAE values increased 67% at 11‰ compared with freshwater (0‰), while SAE increased 46% at 11‰ with the 0.75-kW aerator. SAE values increased further at 22‰, but the increases were much less. These findings clearly demonstrated a significant salinity affect on oxygen transfer efficiencies with paddlewheel aerators. © 1999 Elsevier Science B.V. All rights reserved. Keywords: Paddlewheel aerator; Water aeration; Oxygen transfer; Re-aeration
1. Introduction Paddlewheel aerators are the most common mechanical aeration devices used in pond aquaculture today. Large paddlewheel aerators of 15 kW capacity or larger are typically used with freshwater catfish culture in the US. (Tucker, 1985), while 1.5 kW or smaller paddlewheel aerators of Taiwanese design are typically used with * Corresponding author. Tel.: +1 808 2367426; fax: +1 808 2367443; e-mail:
[email protected] 0144-8609/99/$ - see front matter © 1999 Elsevier Science B.V. All rights reserved. PII: S0144-8609(98)00044-2
100
A.W. Fast et al. / Aquacultural Engineering 19 (1999) 99–103
marine shrimp culture (Fast et al., 1989, 1990; Fast and Boyd, 1992). Large paddlewheels typically have a single impeller shaft with blades (paddles) spirally arranged along the shaft (Boyd and Watten, 1989), while smaller Taiwanese style paddlewheels typically have two or four impellers each with six or eight inline blades on each impeller. Prior investigations demonstrated much greater aeration efficiencies for the spiral blade design compared with other designs (Ahmad and Boyd, 1988). As part of our present study, we adapted the spiral blade design to the smaller, Taiwan-style paddlewheel aerators, and evaluated aeration efficiencies of seven impeller designs at three salinities.
2. Materials and Methods All oxygen transfer measurements were conducted in the same circular test tank of 6.4 m diameter with 1.04 m water depth. Potable drinking water taken from the tap was used as our freshwater source (0‰), while clean seawater at 35‰ was transported to the test site from the ocean in tanker trucks. Salinities of 11‰ and 22‰ were achieved by blending the two source waters. Standard aeration efficiencies (SAE) of seven paddlewheel impellers were evaluated at three salinities (0, 11 and 22‰), using both a 0.37-kW (0.5 hp) gear-motor test bed and a 0.75-kW commercial paddlewheel aerator of Taiwanese design purchased from Fritz Aquaculture. For simplicity, SAE is used here to designate aerator efficiencies at all salinities, whereas this designation is normally used for freshwater test only. SAE values were based on oxygen saturation values at each salinity (Colt, 1984). The 0.37-kW gear-motor was used to evaluate one impeller at a time and rotated at 103 rpm. Impellers could be raised or lowered in the water. Immersion depths were thus controlled such that all tests were conducted at 3.7 A and 100% continuous motor load. The 0.75-kW paddlewheel aerator used two impellers which rotated at 117 rpm. Amp draws were not controllable, but averaged 6.59, 6.54 and 6.61 A, respectively, at 0, 11 and 22‰ salinities. Four replicate trials were performed with each impeller with each motor at each salinity. Electric service to both motors was 248 V, 60 cycle, single phase. Power to the impellers (brake horsepower) was calculated from efficiency and power factor values provided by the motor manufacturers as described by Boyd and Ahmad (1987). Impellers designated here as 1, 2 and 3 were manufactured commercially by Taiwan companies using injection molds, while impellers 4 – 7 were spiral blade configurations of original design. These impellers were similar to those described by Moore and Boyd (1992), with 8–20 blades each, and blade widths ranging from 5.1 to 10.2 cm. Oxygen transfer rates were calculated using protocol described in detail elsewhere (American Society of Civil Engineers, 1983; Boyd and Ahmad, 1987; Ruttanagosrigit et al., 1991). Briefly, this included de-oxygenating test tank waters using sodium sulfite with cobalt chloride catalyst. The respective impellers and aerator combinations were then operated individually until dissolved oxygen (DO) levels increased to near saturation. DO was measured using a YSI DO meter and strip chart recorder. Standard aeration efficiency (SAE) values at 20°C were calculated
A.W. Fast et al. / Aquacultural Engineering 19 (1999) 99–103
101
for each test based on time for DO to increase from 10 to 70% of saturation. Salinity effects on mean SAE values for each salinity were evaluated using one-way ANOVA tests and general linear models procedures for each aerator. Differences between mean SAE values at each salinity were compared using Duncan’s multiple range test (SAS, 1988).
3. Results Mean SAE values using single impellers with the 0.37-kW aerator in freshwater (0‰ salinity) ranged from 1.6 to 2.1 kg O2 kW · h − 1, with a 1.93 kg O2 kW · h − 1 overall average (Fig. 1). At 11‰ salinity, SAE values ranged from 3.1 to 3.4 with a 3.22 kg O2 kW · h − 1 overall average. At 22‰ salinity, SAE values ranged from 3.3 to 3.7 with a 3.46 kg O2 kW · h − 1 overall average. Overall means were significantly different (P B0.0001), while the multiple range test indicated significant differences (PB 0.0001) between overall SAE means at each salinity. Mean SAE values using two impellers with the 0.75-kW aerator in freshwater ranged from 1.8 to 2.3 kg O2 kW · h − 1, with a 2.07 kg O2 kW · h − 1 overall average (Fig. 1). At 11‰ salinity, SAE values ranged from 2.7 to 3.3 with a 3.03 kg O2 kW · h − 1 overall average. At 22‰ salinity, SAE values ranged from 2.8 to 3.5 with a 3.07 kg O2 kW · h − 1 overall average. Overall means were significantly different (PB0.0001), but only the overall SAE mean for freshwater was significantly
Fig. 1. Mean standard aeration efficiencies (SAE in kg O2 kW · h − 1) for seven paddlewheel impellers evaluated at three salinities (0, 11 and 22‰) with two motor devices (0.37 and 0.75 kW). Confidence intervals (95%) are shown about each mean.
102
A.W. Fast et al. / Aquacultural Engineering 19 (1999) 99–103
different from the other means at this significance level. Overall SAE means at 11 and 22‰ were significantly different from each other (PB0.05). 4. Discussion Prior studies with ‘bubbler’ aeration systems that injected air into water, beginning with Kils (1977) have consistently shown that SAE values increase with increasing salinities (Boyd and Watten, 1989; Ruttanagosrigit et al., 1991; Fast and Boyd, 1992). The underlying reason for this salinity effect on SAE with air injection systems relates to bubble sizes and numbers. Perhaps due to differences in surface tension caused by dissolved salts, injected bubbles are more numerous and smaller in saline water compared with freshwater (Kumar and Kuloor, 1970). This results in greater bubble surface areas, and thus potentially greater gas transfer across the gas–water interface. With aeration systems that splash water droplets into air, the observed effects of salinity on SAE have been equivocal. Rogers and Fast (1986) found that SAE values increased 80% at 34‰ salinity compared with freshwater using a 0.75-kW paddlewheel aerator similar to the one we used in our present study. Boyd and Daniels (1987) observed no difference in oxygen transfer rates using a surface agitator aerator with salinities ranging from 0 to 40‰. Ruttanagosrigit et al. (1991) found that oxygen transfer rates at salinities ranging from 0 to 30‰ were unaffected by salinity when using paddlewheel aerators similar to the one we used here. Our current oxygen transfer trials with seven different paddlewheel impellers and two types of aeration devices clearly demonstrate increased SAE values with increasing salinities (Fig. 1). The greatest SAE increase occurred when salinity increased from 0 to 11‰. SAE values increased 67 and 46%, respectively, with the 0.37 and 0.75-kW aerators. Slight increases in SAE values of 7.5 and 1.3%, respectively, occurred as salinity increased from 11 to 22‰. Increased SAE values with increased salinities in our study are attributed to more numerous and smaller water droplets which were created at greater salinities. This would increase gas–water surface areas and facilitate oxygen transfer into oxygen deficient waters. Conceptually, this is similar to observed decreased bubble size and increased bubble numbers when air is injected into higher salinity waters, as discussed above. We cannot, however, explain discrepancies between oxygen transfer rates reported by different investigators using splasher-type aerators. Although the absolute amount of DO in air saturated water is reduced by increased salinity (Colt, 1984), the rate of oxygen transfer is increased in saline waters compared with freshwater, at least with all air injection systems studied and with splasher aerators described herein. In commercial pond settings, other factors such as pond depth and geometry (Rogers et al., 1991), and dissolved substances other than salts (Shelton and Boyd, 1983) can greatly effect oxygen transfer rates. While it is impractical to measure all these factors, it is important to not undersize aeration capacity. Our findings show that salinity is one of the most important factors affecting aerator oxygen transfer performance, and therefore aeration system sizing.
A.W. Fast et al. / Aquacultural Engineering 19 (1999) 99–103
103
Acknowledgements This study was supported by the US Department of Agriculture through their Small Business Innovative Research Program, grant number 94-33610-0888 to STO Design Group. Hawaii Institute of Marine Biology Contribution Number: 1042.
References American Society of Civil Engineers, 1983. Development of standard procedures for oxygen transfer devices. US Environmental Protection Agency, Washington, DC, EPA-600/2-83-102, 292 pp. Ahmad, T., Boyd, C.E., 1988. Design and performance of paddle wheel aerators. Aquacult. Eng. 7, 39 – 62. Boyd, C.E., Ahmad, T., 1987. Evaluation of aerators for channel catfish farming. Alabama Agricul. Exp. Station, Auburn University, Auburn, AL, Bull. no. 584, 52 pp. Boyd, C.E., Daniels, H.V., 1987. Performance of surface aerators in saline waters. Prog. Fish Cult. 49, 306 – 308. Boyd, C.E., Watten, B.J., 1989. Aeration systems in aquaculture. Rev. Aquat. Sci. 1, 425 – 472. Colt, J., 1984. Computation of dissolved oxygen concentrations in water as functions of temperature, salinity, and pressure. Special Publ. no. 14, Am. Fish. Soc., Bethesda, MD, 155 pp. Fast, A.W., Boyd, C.E., 1992. Artificial aeration, circulation and other pond management practices. In: Fast, A.W., Lester, L.J. (Eds.), Marine Shrimp Culture: Principles and Practices. Elsevier, Amsterdam, pp. 461–500. Fast, A.W., Shang, Y.C., Liao, I.C., Rogers, G.L., 1989. Description of Taiwan intensive shrimp culture farms, and simulated transfer to Hawaii. Univ. Hawaii Sea Grant College Program Rep. Ser. UNIHI-SEAGRANT-ME 89-02, Honolulu, HI, 94 pp. Fast, A.W., Shang, Y.C., Liao, I.C., Rogers, G.L., 1990. Comparative economics of Taiwan-style intensive shrimp culture in Taiwan and Hawaii. J. Aquacult. Trop. 5, 89 – 99. Kils, U., 1977. The salinity effect on aeration in mariculture. Meeresforsch 25, 201 – 206. Kumar, R., Kuloor, N.R., 1970. The formation of bubbles and drops. In: Drew, T.B., Cokelet, G.R., Hoopes, J.W. (Eds.), Advances in Chemical Engineering, vol. 8. McGraw-Hill, New York, pp. 255 – 368. Moore, J.M., Boyd, C.E., 1992. Design of small paddle wheel aerators. Aquacult. Eng. 11, 55 – 69. Rogers, G.L., Fast, A.W., 1986. Surface aerator design for aquaculture and mariculture. Am. Soc. Agricult. Eng., 4, 19 pp. Rogers, G.L., Madenjian, C.P., Fast, A.W. 1991. Estimation of oxygen transfer rates for mechanical aerators in brackishwater aquaculture ponds. J. Appl. Aquacult. 1, 63 – 78. Ruttanagosrigit, W., Musig, Y., Boyd, C.E., Sukchareon, L., 1991. Effects of salinity on oxygen transfer by propeller-aspirator and paddle wheel aerators used in shrimp farming. Aquacult. Eng. 10, 121 – 131. SAS, 1988. SAS user’s guide: statistics, ed. 6.03. SAS Institute, Inc., Cary, NC. Shelton, J.L. Jr., Boyd, C.E., 1983. Correction factors for calculating oxygen-transfer rates of pond aerators. Trans. Am. Fish. Soc. 112, 120–122. Tucker, C.S., 1985. Channel Catfish Culture. Elsevier, Amsterdam, 657 pp.
