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The Application of Pure Oxygen in Recirculating SystemsHarry
Westers IntroductionPure oxygen (LOX) or high purity oxygen (PSA) can oxygenate fish culture water in excess of 100% saturation DO, something not possible with atmospheric air (21% O2) without exceeding total gas pressure (TGP) over 100 percent In aquaculture, especially intensive aquaculture, oxygen availability to the fish has been recognized as the first limiting factor. In intensive aquaculture, under conditions of high loadings (weight of fish per unit flow) and high densities (weight of fish per unit of space), only pure oxygen can meet the demands. This is even more critical in recycling systems where organic matter and nitrification exert significant additional oxygen demands on the system. Oxygen RequiredThe aeration system must be designed to deliver the amount of oxygen required. In a recycling system it is difficult to determine the oxygen required, yet such information is important (Losordo, 1991). The major consumers of oxygen can be identified as the biomass, organic matter (BOD), and the biofilter. Each of these are functions of many physical and biological variables. In this paper fish, specifically the rainbow trout, will represent the cultured organism The oxygen consumption rate (metabolic rate) is primarily a function of water temperature and fish size. These same parameters are used to determine optimum feeding levels. Consequently oxygen consumption is a constant relative to feed fed. For salmonids, as a group, this value is from 200 to 250 g of oxygen per kg of feed (Westers, 1979). For esocids (tiger muskies), a coolwater, non-active fish, it is 110 g (Pecor, 1978), and for the common carp, a warmwater species, it is 230 g (Huisman, 1974). Although probably only 30% is actually utilized as "feeding metabolism" (Machiels, 1987) the feeding level, for practical application, can be related directly to the metabolic rate of the fish (Westers, 1979). Nevertheless, these values, as well as others used in this paper, are not absolute. This, of course, is characteristic of biological functions in general in contrast to those of the science of physics. For rainbow trout, 250 g of oxygen is required per kg of feed. Equation 1 determines the daily oxygen (g) requirement per kg biomass, equation 2 expresses it as mg/L DO per 1.0 % BW feeding level.
gO2/kg fish = % BW x 250 = 2.5 g
---------- (1)
100
mg/L/kg fish = % BW x 250 = 1.74 mg/L
----------- (2)
1.44 x 100
The oxygen demand placed on the unconsumed feed and the feces is a function of the system design. The objective is to rapidly remove the suspended and settleable solids. Accordingly it will be assumed that the BOD5 loading is reduced by 70 percent. Wimberly (1990) determined that the solid waste trapped in the biological filter requires a substantial amount of oxygen. For channel catfish he determined this rate at approximately 2.3 times the BOD5. The author furthermore indicated that the average BOD,sub>5 excretion rate for channel catfish, fed 1.0 % BW, was 2.3 g O2/kg fish. If the same values are applied to rainbow trout and a 70 percent removal is assumed, the daily oxygen requirement (g) for solid waste per kg fish is:
gO2/kg fish = (2.3) 2.3 x 0.30 = 1.59 g (3)
The oxygen required to oxidize one g of ammonia to nitrate is 4.18 g. If it is assumed that 1.0 kg feed generates 30 g of total ammonia nitrogen (TAN), then 1.0 kg feed requires 125.4 g of oxygen for the nitrification process in the biological filter. Equation 4 determines the daily oxygen (g) demand for nitrification per kg biomass per 1.0 % BW feeding level.
g02/kg fish = % BW x 30 x 4.18 = 1.25 g
--------- (4)
100
Understandably, the entire processes are more complicated than presented here. Simple, direct mathematical expressions (stoichiometric values) have been applied to very complicated processes which are influenced by a great many factors and interactions. In the example above, the total daily oxygen demand for the system is 5.34 g per kg biomass fed at 1.0 % BW per day. For a feeding level of 2.0 percent the demand would be twice that or 10.68 g O2/kg/day. It has been suggested by Rosati (1990) to assume a one to one ratio of oxygen to feed. This appears to be a reasonable approach for design purposes. Oxygen DynamicsThe advantage of pure oxygen lies in the fact that DO levels exceeding 100 percent saturation can be achieved without causing TGP to exceed 100 percent (Westers1 1990). Today's trend in aquaculture is towards high density rearing. In salmonids, densities in excess of 100 kg/m3 have been obtained. At such levels, pure oxygen must be used unless very large volumes of water are introduced into the rearing unit. Equations 5 and 6 show the relationship of density (D) or space with loading (Ld) or flow.
D = Ld x R
------- (5)
.06
Ld = D x .06
--------- (6)
R
D is density expressed in kg/m3 Ld is loading expressed in kg/L/min, R is the rearing unit water turnover rate in number per hour and .06 represents 1.0 L/min x 60 min or 60 L which is 0.06 m3. At a density of 90 kg/m3 (5.6 lbs/ft3) and an R of 1.5, the loading is 3.6 kg/L/min (30 lbs/gal/min). The required incoming DO level (DOin) can be determined with equation 7.
DOin = (Ld x gO2/Kg feed x % BW) + DOout
--------------------------- (7)
100 x 1.44
The value 1.44 represents 1 .44g (1.0 L/min @ 1.0 mg/L x 60 min x 24 hr - 1,440 mg/day). At the 1.0 % BW feeding level, and a minimum DOout of 5.0 mg/L, the DOin is:
DOin = (3.6 x 250 x 1.0) + 5.0 = 11.25 mg/L
---------------- (8)
100 x 1.44
Per cubic meter rearing volume with an R of 1.5, the intake flow is 25 L/min. With DOout at 5.0 mg/L, 180 g 02 (25 x 5 x 1.44) is available for BOD and biofilter. The BOD requirement is 1.59 g/kg fish (eq. 3) and the nitrification process requires 1.25 g/kg (eq. 4). For 90 kg fish this adds up to a demand of 225g O2. With 180g O2 available, the deficit is 75g O2. But to maintain a biofilter effluent level of 2.0 mg/L, an additional 72 g O2 must be made available (25 x 2 x 1.44), for a total of 147 g or 4.1 mg/L DO, in addition to the 5.0 mg/L. In other words, the DOin to the filter should be at 9.1 mg/L to satisfy BOD and nitrification. To accomplish this the DOin to the system as a whole should be at 11.25 + 4.1 or 15.35 mg/L. Since 2.0 mg/L should be in the recycled water, 13.35 mg/L must be added to the system. This requires the application of pure oxygen. Table 1. Supplemental DOin mg/L required for rainbow trout maintained at five rearing densities (D) during a doubling of the metabolic rate (from 200 to 400 mg/Kg/hr) for brief periods of 5, 10 and 15 min. DOout = 5.0 mg/L Act. DOout = 3.0 mg/L R is 1.5, flow is 25 L/min/m3 volume
1 2 3 4 5 6 7
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mgO2 req. at activity mg 02 diff. with mg/L DO suppressed mg/L DO suppl. req.
O2 over time (from res. reserve during time during act. period during act. period
D H20 Vol.reserve -------------------------------------------------------------------------------------------
kg/m3 L mg 5 min 10 min 15 min 5 10 15 5 10 15 5 10 15
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30 970 4850 500 1000 1500 4350 3850 3350 4.50 4 3.50 0 0 0
60 940 4700 1000 2000 3000 3700 2700 1700 4 2.90 1.80 0 .40 2.90
90 910 4550 1500 3000 4500 3050 1550 50 3.40 1.70 .05 0 4.70 7.10
120 880 4400 2000 4000 6000 2400 400 -1600 2.70 .50 0 1.90 9 11.30
150 850 4250 2500 5000 7500 1750 -750 -3250 2.10 0 0 6.40 13.20 15.50
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Col. 3 is 5 x Col. 2
Co. 4 is {(200 x D ÷ (60 min)} x min. (5; 10; 15)
Col. 5 is Col. 3 - Col. 4
Col. 6 is Col. 5 ÷ Col. 2
Col. 7 is {(3 x Col. 2) - (Col. 5)} ÷ (25 x min)
Note: Higher exchange rates (R) result in proportionally less suppl.
DO required. For R = 4, for instance, multiply values of
Column 7 by 0.375 (1.5 ÷ 4)
It is well known that fish, when they become excited, by feeding or fright, can double their metabolic rate for short periods of time. This greatly increased demand on oxygen can result in depletion and death. This problem can be especially acute under conditions of high rearing densities since there is little reserve oxygen available. Table 1 illustrates this for five different rearing densities. For the low density of 30 kg/m3, even 15 minutes of hyperactivity will not depress the DO below 3.5 mg/L, while at 90 kg/m3, 10 minutes of hyperactivity will depress the DO down to 1.7 mg/L and 1 5 min of such sustained activity would, theoretically, result in 0.05 mg/I DO, Higher densities cannot even support 5 minutes without depressing DO below 3.0 mg/L, a level considered safe for short term exposure for rainbow trout, the species considered here. For high rearing densities, the oxygen capacity of the aeration system must be designed above that required for the routine needs. The biofilter, too, must not go anoxic since this will kill the bacterial culture. However, since the DOout of the rearing unit, in this example, exceeds by 4.1 mg/L the minimum of 5.0 mg/L required, the situation is not as critical as it appears. Nevertheless, it is advisable to continuously monitor the DOout and select a minimum safe (for fish as well as biofilter) level that will trigger delivery of supplemental oxygen. Oxygen ApplicationPure oxygen can be added to the water in many different ways, such as U-tubes, packed columns, aeration cones and various diffusers (Colt and Watten, 1988). A recent technique is the low head oxygenator (LHO) consisting of a series of chambers in a box. The water enters the top of the box through holes while, at the same time, forming a water seal. Sealing is important to prevent oxygen gas from escaping. For this reason the packed column has been converted to a sealed column, a technique described in detail by Westers (1990). This approach requires a head of five feet, while the LHO was specifically developed to avoid this. It can operate rather efficiently with a head of one foot or even less. Two sources of oxygen are commonly used, LOX (liquid oxygen) and PSA (pressure swing adsorption) generated oxygen. Their advantages and disadvantages are described in some detail by Colt and Watten (1988). Since the papers of Westers and of Colt and Watten referenced above, as well as others, can be found in the publication "Oxygen supplementation, a new technology in fish culture" which is available at no cost from the U.S. Fish and Wildlife Service as Information Bulletin #2 from P.O. Box 25486, Denver Federal Center, Denver, CO 80225, I will not repeat the information here, but instead recommend the reader obtains this document which contains 17 papers related to the technology of oxygen injection, water quality aspects, biological aspects, and monitoring technology. It is a valuable resource for anyone considering the use of pure oxygen. Information on the LHO, a technique of more recent origin, can be obtained from Zeigler Brothers Inc., P.O. Box 95, Gardners, PA 17324-0095, phone 717-677-6181. Oxygen ConsequencesIt has been pointed out that the oxygen requirement can be related, directly, to the feed. Since feed is the source of all "pollutants", oxygen can be used to predict those components. More oxygen means more pollutants that require removal from the system. In the example, with rainbow trout, 250 g of oxygen are required to metabolize one kg of feed. Therefore, 250 g of oxygen which represents 1.0 kg of feed, results in the generation of 30 g TAN, 300 g solids and 325 g of CO2. Carbon dioxide is very soluble in water and difficult to remove. The toxicity of CO2 is aggravated by low DO and low pH. These conditions must be avoided through oxygenation and buffering, either natural (high alkalinity water) or artificially by adding lime or sodium bicarbonate. Lack of CO2 control can be a weak link in recycling systems. An open packed column works well but requires lifting the water an additional five to six feet above the existing design. It could be combined with a trickling biofilter. Rotating biodrums or discs also accomplish some CO2 degassing in contrast to fluidized filter beds and other types of upflow filters. Since the air contains only 0.03% CO2 (300 mg/L), excessive CO2 released from the water can increase the atmospheric CO2 level in a closed environment (building) to manyfold this level, making it increasingly more difficult to "absorb" additional CO2, thus reducing the efficiency of the degasser. Since recycling systems are, generally, indoors, the building should be outfitted with exhaust fans, either directly exhausting the degasser itself or exchanging the building air at the proper rate. What level the carbon dioxide must be reduced to in the culture water depends on a great many factors, such as species, dissolved oxygen level, pH and most likely, other water chemistry characteristics. The literature is not clear on this issue (Westers, 1991) and thus it represents an important topic for research. Oxygen EconomicsOxygen requirement has been expressed in terms of food fed and, consequently, it can be expressed in terms of fish produced. Table 2 provides cost figures for O2 in cents per kg of fish produced. These costs are based on 50 percent and 75 percent absorption efficiencies, as well as three feed conversions (FC), three oxygen costs, and three ratios of oxygen to feed. The cost of oxygen, whether LOX or PSA, should fall somewhere between 30 and 50 cents per 100 cubic feet. This includes the energy requirement, maintenance, equipment depreciation and the rental cost of LOX storage. Table 2. Cost of oxygen in cents per kg fish produced based on two absorption efficiencies, three O2 costs, three feed conversion ratios (FC), and three 02 consumption ratios (kg O2 to kg feed).
Ratio O2 cost in cents Oxygen Cost (cents) per kg of fish produced
O2 Feed per kg per 100 ft3 --------------------------------------------
.75% Absorption .50% Absorption
--------------- ---------------
FC 1.0 1.5 2.0 FC 1.0 1.5 2.0
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7.5 30 4.9 7.5 10.0 7.4 11.2 15.0
.50 10.0 40 6.7 10.0 13.3 10.0 15.0 20.0
12.5 50 8.4 12.5 16.8 12.6 18.8 25.2
7.5 30 7.5 11.1 15.0 11.2 16.8 22.4
.75 10.0 40 10.0 15.0 20.0 15.0 22.6 30.0
12.5 50 12.5 18.7 25.0 18.8 28.2 37.6
1.00 7.5 30 10.0 15.0 20.0 15.0 22.6 30.0
10.0 40 13.3 20.0 26.6 20.0 30.0 40.0
12.5 50 16.6 24.9 33.3 25.0 37.4 50.0
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An 800 cubic feet per hour (95 percent purity) PSA oxygen generating system at one of Michigan's state fish hatcheries delivers oxygen at a cost of 49 cents per 100 cubic feet (Hall, personal communication). This includes the following: 1. Two 40 H.P. rotary screw air compressors $22,400
a. Depreciation (20 years) $ 1 ,600
b. Maintenance $ 3,530
c. Energy @ 7 cents/kwh $16,188
Total $21,318
2. Four 200 ft3 PSA oxygen generators $47,200
a. Depreciation (30 years) $ 1,573
b. Maintenance $ 8,261
c. Energy $ 1,417
Total $11,251
3. 800 ft3 O2 per hour @ 95 percent purity x 8,760 hours equals 6,657,600 cubic feet of oxygen per year. At a total cost of $32,569/yr the cost per 100 cubic feet is $.49 It was pointed out that the application of pure oxygen allows for high density rearing on moderate flows of water. Since more fish are reared per unit space and generating oxygen is cheaper than pumping water, pure oxygen can be an economically sound approach to intensive fish culture, despite the apparent high cost per fish produced. One must carefully consider all aspects. Literature CitedColt, J. and B. Watten. 1988. Application of pure oxygen in fish culture. Aquacult. Eng. 7 pp 397-441. Huisman, E. A. 1 974. Optimalisering van de groli by de Karper (Cyprinus carpio L.). OVB Utrecht, The Netherlands. pp 95. Losordo, T. M. 1990. An introduction to recirculating production systems design. In: Engineering aspects of intensive aquaculture. Cornell Univ., Ithaca, NY. pp 32-47. Machiels, M. A. M. 1987. A dynamic simulation model for growth of the African catfish Clarias gariepinus Burchell 1822) PhD dissertation, Univ. of Wageningen, The Netherlands. Pecor, C. H. 1978. Intensive culture of tiger muskellunge in Michigan during 1976 and 1977. Selected coolwater fishes of North American, ed. R. L. Kendall. Spec. Publ. No. 11. pp 202-209 Rosati, R. 1990. Commercial fish culture using water recirculating systems. In: Intensive Aquaculture. Ill. State Univ., Normal, IL. pp 13-27. Westers, H. 1979. Principles of intensive fish culture. Mich. Dept. of Nat. Res. Lansing, MI (unpublished). pp 108. Westers, H. 1990. The role and application of high purity oxygen in intensive fish (salmonid) culture. Mich. Dept. of Nat. Res. Lansing, MI (unpublished). pp 48. Westers, H. 1991. A comparison of the design, operation and carrying capacity of plug-flow (raceway) versus circulating (round tank) fish rearing units. Mich. Dept. of Nat. Res., Lansing, MI (unpublished). pp 51. Wimberly, D. M. 1990. Development and evaluation of a low density media biofiltration unit for use in recirculating finfish culture systems. Louisiana State Univ., Baton Rouge. pp 162.
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