Summerfelt, S. T. 1996. Engineering design of a water reuse system. Pages 277-309 in R. C. 

Summerfelt, editor. Walleye culture manual. NCRAC Culture Series 101. North Central Regional 

Aquaculture Center Publications Office, Iowa State University, Ames. 

Engineering Design of 

Water Reuse System 

a 

Steven T: Summerfelt, The Conservation Fund’s Freshwater Institute, PO Box 1746, Shepherdstown, WV 

25443 

Introduction 

Recirculating aquaculture systems are a type of 

flowing water (intensive) fish culture technology in 

which a high percentage of the water is reused after 

treatment. The recirculating system consists of interacting unit processes that form a complex whole. Such a 

system implies an organized set of complementary 

parts. A deficiency in one component, e.g., the clarifier, 

will affect performance of the other components and the 

entire system. 

Water quality criteria required to maintain a healthy and 

fast growing fish (Table 1) are the basis for designing 

water reuse processes for closed-systems. The parameters of primary concern are dissolved ammonia, nitrite, 

oxygen, carbon &oxide, nitrogen and solids. These 

parameters are important because their production or 

reduction (Table 2) can lead to concentrations that 

affect fish growth and health. Further, it is not only the 

individual components, but the aggregate of all the 

water quality components, which affect fish growth and 

health (Meade 1989). 

The reused water must be sufficient in both quantity 

and quality to maintain acceptable growth and health of 

the cultured organism. Most often, treatment systems 

utilized to prepare water for reuse are designed to 

reduce fish metabolites, such as suspended and settleable solids, dssolved nitrogen compounds (ammonia 

and ammonium), and BOD, as well as processes for 

controlling dissolved gases, pH, and pathogens. 

Recirculating aquaculture systems typically use 

clarifiers or filters to remove particulate solids, biological filters to reduce dissolved wastes, strippers/aerators 

to add oxygen and decrease carbon &oxide levels, and 

oxygenation units to increase oxygen concentrations 

above saturation (Table 3). Processes to provide 

advanced oxidation and pH control may also be 

required. 

The purpose of this chapter is to present engineering 

criteria to construct a recirculating system to culture 

walleye. The system design presented here is not unique 

and consists of components common to many commercial and research recirculating systems. All components 

have been researched as individual unit processes, but 

not in combination as a functional system. 

It should be noted that a variety of commercially viable 

designs and technologies exist, and that a randomly 

selected group of experts will have different preferences 

on what constitutes the most effective combination of 

components. Different technologies used in recirculating aquaculture systems will not be reviewed because of 

the availability of several reviews (Liao and Mayo 

1972, 1974; Lucchetti and Gray 1988; Mayo 1991 ; 

Rosenthal and Black 1993; Losordo 1993; Timmons 

and Losordo 1994). 

Design guidelines 

Generally, a fish production system is designed to 

meet a predetermined production capacity, expressed as 

pounds (kg) fish produced per year. To maximize its 

efficiency, the production system must be operated at or 

near its maximum carrying capacity defined by the 

maximum pounds that can be supported by the system. 

Therefore, the starting point for a rigorous design of a 

recirculating aquaculture system begins by determining 

the system’s carrying capacity. Procedures for calculating carrying capacity based upon limiting water quality 

NCRAC Culture Series 101 - Walleye Culture Manual 277 
criteria have been reviewed by Meade (1988), Colt 

(1 991), Losordo and Westers (1 994), Soderberg (1 995), 

and others. Determination of yearly production capacity, on the other hand, is much more complex because it 

depends upon both the characteristics of the fish 

(health, growth, etc.) and on the production strategies 

(i.e., frequencies and rates of stocking and harvesting 

and fish size when stocked and harvested) used to 

maintain the culture facility near maximum carrying 

capacity. Rigorous determination of a system’s yearly 

production capacity has been reviewed elsewhere 

(Summerfelt et al. 1993). 

Table 1. Water quality standards for fish culture. Unless otherwise stated, units 

are in mg/L (Meade 1989). 

Parameter Desirable range Maximum 

Alkalinity (as calcium carbonate) 

Aluminum 

Ammonia 

Arsenic 

Barium 

Cadmium 

Alkalinity < 100 mg/L 

Alkalinity > 100 mg/L 

Calcium 

Carbon dioxide 

Copper 

Alkalinity < 100 mg/L 

Alkalinity > 100 mg/L 

Dissolved oxygen 

Hardness, (as calcium carbonate) 

Iron 

Lead 

Magnesium 

Manganese 

Mercury 

Nitrogen 

Nitrate 

Nitrite 

PCB (polychlorinated biphenyls) 

Pot assi u m 

Salinity 

Selenium 

Sodium 

Sulfate 

Sulfur 

Total dissolved solids 

Total suspended solids 

Zinc 

PH 

10-400 

0.01 

0.02 

0.05 

5.0 

0.0005 

0.005 

4-1 60 

0-1 0 

0.006 

0.03 

5 mg/L to saturation 

10-400 

0-3.0 

0.01 

0.02 

0.01 

0.2 

15.0 

110% (total gas pressure) 

103% (as nitrogen gas) 

0.1 in soft water 

0.002 

6.5-8.0 

5.0 

5 yo 

0.01 

75 

50.0 

1 .o 

400.0 

80.0 

0.005 

278 NCRAC Culture Series 101- Walleye Culture Manual 
It is difficult to define the optimum size of a recirculating system used to produce food-size fish. To protect 

against a ruinous loss from system malfunction or 

disease, a large scale production operation should use a 

number of independent recirculating system modules 

rather than one gigantic system with common treatment 

units. Adding more recirculating system modules is also 

a simple method to increase the overall facility production capacity. Decsions on the size of the recirculating 

system modules must consider benefits to be obtained 

from economies of scale (i.e.¶ lower cost per unit flow 

treated for larger systems) against hfficulties that could 

arise from distributing flow and removing solids within 

the culture tank, gradmg and harvesting fish, removing 

mortalities, or isolating the biofilter while treating the 

fish with a chemotherapeutant; and other criteria. There 

is a definite trend, however, towards large recirculating 

system modules for food-fish production. Further 

commercial experience is needed to help define the 

optimal size of these systems 

The principle components of the recirculating system 

design described here include: a microscreen filter for 

solids removal; a fluidized-sand reactor for biofiltration; 

a cascade column for both aeration and carbon dioxide 

stripping; a unit for purified oxygen injection (e.g., 

multi-stage low head oxygenator or U-tube); and a 

circular tank for fish culture. The recirculating aquaculture system design also requires methods to add ozone 

and manage pH. Each of these components is discussed 

Table 2. Metabolite production and consumption. 

Metabolite 

Ib metabolite per 

Species Ib feed fed Reference 

Oxygen demand 

Total ammonia 

nitrogen produced 

Nitrate produced 

Walleye 0.2 

Trout 0.22 

Salmon 0.54 

Walleye 0.027 

Nonspecific 0.032 

Trout 0.031 

Salmon 0.029 

Nonspecific 0.087 

Salmon 0.024 

Phosphate produced Nonspecific 0.005 

Salmon 0.01 6 

Settleable solids 

produced 

Nonspecific 0.3 

Salmon 0.52 

Total solids produced Catfish 0.4 

cBOD produced Catfish 0.4 

(5-day, 20°C) Salmon 0.60 

COD produced Salmon 1.60 

Carbon dioxide 

produced 

Coho salmon 0.285 

Steel head 0.43 

Yager and Summerfelt (1 994) 

Willoughby (1968) 

Liao and Mayo (1974) 

Forsberg and Summerfelt (1994) 

Piper et al. (1 982) 

Speece (1 973) 

Liao and Mayo (1974) 

Piper et al. (1 982) 

Liao and Mayo (1974) 

Piper et al. (1 982) 

Liao and Mayo (1974) 

Piper et al. (1 982) 

Liao and Mayo (1974) 

Speece (1 973) 

Speece (1 973) 

Liao and Mayo (1974) 

Liao and Mayo (1974) 

Liao and Mayo (1974) 

Liao and Mayo (1974) 

NCRAC Culture Series 101 - Walleye Culture Manual 279 
separately below. In addition, each section below Solids removal by microscreen filtration 

includes an example illustrating the design or sizing of 

the component described when used within a moderately-large system with a design recirculating flow rate 

of 1,000 gpm (3,800 Lprn). 

Solids removal is the most critical process to manage 

in recirculating aquaculture systems, because solids 

influence the efficiency of all the other component 

functions as well as the potential for disease (Chen et 

al. 1994). Solids production is proportional to feed 

input (Seymour and Bergheim 1991; Chen et al. 1994). 

Table 3. Summary of the unit processes typically used within recirculating 

aq u acu It u re systems. 

Unit process Effect Example types 

Clarification removes 

settleable and/or 

suspended solids 

Biofiltration removes 

dissolved 

organics and 

ammonia 

Stripping/aeration shifts 

concentrations 

of dissolved 

carbon dioxide, 

nitrogen, and 

oxygen towards 

equilibrium 

values 

Oxygenation adds dissolved 

oxygen to levels 

generally greater 

than equilibrium 

microscreen filters (drum, TriangelTM, and disk) 

settling basins 

tube/plate settlers 

roughing filters (packed with random rock or 

swirl separators 

pressurized filters (sand and plastic bead) 

gravity filters (high rate sand and slow sand) 

flotation/foam fractionation 

plastic, and with structured plastic) 

fluidized-media reactors (sand and plastic bead) 

rotating biological contactors 

trickling filters 

submerged large media reactors 

pressurized bead filters 

mechanical-surface mixers 

diffusers 

columns (open to atmosphere, and enclosed with 

forced ventillation) 

a. packed or tray 

b. spray 

shallow air-lifts 

corrugated inclined plane 

stair-type drops 

U-tubes 

columns (atmospheric pressure and pressurized) 

a. multi-staged (e.g., low head oxygenators) 

b. packed or tray 

c. spray 

oxygenation cones 

oxygen aspirators 

diffusers 

enclosed mechanical-surface mixers 

280 NCRAC Culture Series 101- Walleye Culture Manual 
Particulate matter (feces, feed fines, uneaten feed and 

sloughed biofilm) is the major source of carbonaceous 

oxygen demand and nutrient input into the water, 

especially if it degrades within the system. The best 

solids removal practice, therefore, removes solids from 

the system as soon as possible and it exposes the solids 

to the least turbulence and mechanical shear. Conventional solids removal processes generally remove solids 

larger than 100 pm but, with few exceptions, they do 

not remove colloidal solids smaller than 20 p or 

dissolved solids (Chen et al. 1994). 

Microscreen filters are commonly used withn commercial recirculating aquaculture systems for removing 

solids. They are commonly used by producers with 

large recirculating systems, because they are modular, 

not permanently fixed to one spot, and relatively easy to 

install. They also have a high hydraulic capacity, a low 

space requirement, and an acceptable headloss. 

Microscreen filters also perform well withn recirculating systems because they remove a large proportion of 

the solids produced during each pass and because they 

do not store the solids they have removed. 

Microscreen filters are sieves. They strain water-bound 

particles larger than the filter screen (mesh) openings. A 

cleaning mechanism is necessary to remove solids from 

the filter. Depending on the type of microscreen filter, 

the filter is cleaned by either hydraulic flushing, 

pneumatic suction, mechanical vibration, or ralung. 

Cleaning of the sieve can occur continuously, periohcally, or on demand. The drum, TriangelTM, and 

disk microscreen filters are the three main 

variations used in the United States. Drum filters 

are now used by many commercial facilities 

because they cost less than TriangelTM filters per 

unit flow treated. Drum filters are also desirable 

because they can be plumbed so that if ever the 

filter screen wash mechanism breaks down 

(allowing solids to accumulate and plug the 

microscreen panels), water overflows to the 

pumping sump and not out of the system 

through the solids collection trough. 

inside to the outside of the drum during solids removal. 

When the drum rotates, solids trapped on the filter 

screen in the cells of the supporting grid are gently 

lifted out of the water. When the drum filter is operated 

without rinsing, however, the particulates begin to clog 

the screens, which causes the water level within the 

drum to increase. To maintain flow, solids are washed 

from the filter with a high pressure spray during drum 

rotation. Some drum filters have used air suction to 

remove solids from the surface of the screens. These 

vacuum cleaned drum filters generally operate with the 

flow passing from the outside to the inside of the drum. 

Drum rotation can be either continuous or intermittent, 

when automatically controlled with a level switch 

located within the drum. During automatic control, the 

drum is not rotated until the difference in water level 

between the inside and outside of the drum reaches a 

predetermined upper level. The drum rotates 180" or 

more each wash cycle. In either mode, pressure sprays 

rinse the solids off the screens into collection troughs 

where they are piped away for disposal. Because 

microscreen filters are washed frequently (Libey 1993; 

Summerfelt et al. 1994), trapped solids are not stored 

for a significant time within the recirculating aquaculture system. 

Microscreen filters have been shown to effectively 

remove solids from recirculating aquaculture systems 

(Libey 1993; Summerfelt et al. 1994), and from the 

influents (Liltvedt and Hansen 1990) and effluents from 

Drum filters (Figure 1) use fine mesh screens to 

remove solids from high volume flows. The 

filter screen is supported on a grid, which in turn 

is attached to the outer circumference of the 

drum. Each drum contains several grid-supported screens. Flow is usually passed from the 

Figure 1. Working mechanism of a drum filter (Courtesy of 

HydroTech). Arrows indicate the paths taken by the treated flow 

and the filterable solids as they pass through the unit. 

NCRAC Culture Series 101 - Walleye Culture Manual 281 
single-pass systems (MBkinen et al. 1988; and 

Bergheim et al. 1993). Stuhes withm a recirculating 

system at the Freshwater Institute indicated that 

microscreen filters, with sieve panels containing 80 pn 

openings, removed a large portion of the net solids 

produced each pass (Summerfelt et al. 1994), particularly when ozone was added to the system. The fine 

particles that were not removed accumulated within the 

recirculating system. Particles that accumulate within a 

recirculating system using microscreen filters are 

smaller than 20-40 p and constitute 50% (by mass) or 

more of the particles approaching the filter in the 

recirculating flow (Heinen et al. 1996). 

Size of the microscreen mesh also affects the filter’s 

hydraulic capacity, total solids removal, sludge water 

production rate and concentration, and filter wash 

frequency. Summerfelt et al. (1 994) demonstrated that 

microscreen filters with panels of small openings 

removed more TSS but also required much more 

frequent washings than filters with panels having larger 

openings. More frequent washings resulted in more 

sludge effluent and less concentrated sludge. Conversely, microscreen filters installed with panels 

containing larger sieve openings Qd not capture as large 

a proportion of the suspended solids from the recirculating flow each pass, but required fewer washes and 

produced less volume, but more concentrated sludge 

water effluent. 

Microscreen filters require relatively large capital 

investment and operating expenses. Operating costs 

included electricity for the pumps, labor to periodically 

clean the sieve panels, and labor and parts to restore 

worn-out pressure pumps. Pressure pumps cycle on and 

off several hundred to several thousand times per day 

(Libey 1993; Summerfelt et al. 1994). As a result, 

pressure pumps can fail and are often the most likely 

items within the entire recirculating system to cause 

problems, When pressure pumps fail, sieve panels plug 

and water overflows past the filter unit. When selecting 

and installing microscreen filters, attention should be 

given to planning for water overflow when plugged 

sieve panels cause flow to bypasses the filter unit. 

Microscreen filters are available from distributors in 

capacities ranging from under 80 to over 13,000 gpm 

(under 300 to over 50,000 Lpm). Larger models can 

generally be purchased at a large economy of scale. 

Fully equipped drum filters using 60 pm microscreen 

panels, for example, range from $1,000 to 4,000 for 

every 100 gpm (380 Lpm) of flow capacity. Commercial distributors of microscreen filters may often be of 

assistance when selecting a microscreen filter based on 

the system flow rate, fish loading, and desired 

microscreen opening size. 

Example design: drum filter 

Microscreen drum filters are usually purchased from 

manufacturers because they are difficult to fabricate 

locally (unlike fluidized-sand biofilters, aeration/ 

stripping towers, or U-tubes). Several companies 

manufacture microscreen drum filters. In late 1995, the 

price for a drum filter with panels containing 60 pn 

sized openings capable of treating more than 1,000 gpm 

(3,800 Lpm), and equipped with pressure pumps and 

solids-flushing control mechanism costs about $17,000. 

Biofiltration by fluidized-sand biofilters 

Fluidized-sand biofilters are used to treat dissolved 

wastes within recirculating aquaculture systems 

(Cooley 1979; Burden 1988; Owsley et al. 1988; Paller 

and Lewis 1988; Wimberly 1990; Thomasson 1991; 

Weaver 1991; Bullock et al. 1993; Heinen et al. 1996; 

Summerfelt and Cleasby 1996). Microbes attached to 

the surface of the sand oxidize ammonia to nitrate and 

oxidize or metabolize and incorporate organic compounds. Attachment of the microbe population to the 

sand, which has a density 2.65 times that of water, 

keeps the microbes from being flushed out of the filter 

and provides the biosolids retention time required for 

biological oxidation of ammonia to nitrates, The fine 

sands used in fluidized beds make a good support for 

microbial attachment because they have very high 

specific surface areas (i.e., available surface area per 

unit volume). Adhtionally, filter sands are relatively 

inexpensive, inert, non-compressible, non-biologically 

degradable, and environmentally friendly (compared 

with plastic beads). 

Fluidization requires relatively high water velocities to 

tumble the sand in the water and transport ammonia, 

nitrite, organic compounds, and oxygen in close contact 

with the biofilm-coated sand. The high velocities 

improve transfer of dissolved compounds into the 

biofilm by exposing all portion of the biofilm surface to 

the solution and by replacing and decreasing the 

thickness of the stagnant boundary layer surrounding 

the biofilm. The high velocities also produce forces 

282 NCRAC Culture Series 101- Walleye Culture Manual 
(both hydraulic and the physical 

particle-particle or particle-wall 

interactions that occur in the region 

of water distribution under the 

fluidized bed) that continuously 

shear the growing biofilm. Some 

control over the biofilm thickness 

can be maintained by selecting a 

given sand size, by manipulating 

shear forces, and by replacing sand. 

becoming fluidized, when the velocity of water through 

the bed is sufficiently large to result in a pressure loss 

greater than the apparent weight (actual weight less +Static 

Fluidized-sand biological filters 

have been used by commercial 

aquaculture producers with large 

recirculating systems. Relative to 

other large scale biofilters, fluidized-sand biofilters are compact, 

reliable, efficient at removing 

ammonia, and are cost competitive 

($0.002-0.0001/ft2 sand surface 

area, depending upon characteristics 

of the sand). 

bed+-Fluidized bedDesign considerations 

Fluidized-sand beds operate by 

-A4- 

4- 

A 

Interface between . 

clear fluid and expanded bed 

Interface between 

clear fluid and static bed 

Water distribution and 

media sumort mechanisms II 

Expanded bed 

(vo ’ Vmf) 

Figure 2. Mechanics of flow through a granular bed at velocities (v,) above 

and below the minimum fluidization velocity (V,,,~). 

injecting an equal distribution of water across the 

biofilter’s cross-section at the bottom of the sand 

(Figure 2). Water flows up through void spaces between 

sand grains in the bed. Viscous and inertial forces resist 

the water passing through the static bed, causing the 

superficial velocity’ (Figure 3). The bed expands, 

3 ‘rl 

A 

2 

m pressure loss (static bed) to increase with increasing 

buoyancy) per unit cross-sectional area of the bed. The 

relative amount of bed expansion is dependent upon the 

shape and hameter of the sand and the velocity and 

temperature of the water. Once the bed has been 

constant at all bed expansions (Figure 3). 

p1 

a 

3 

E 

2 

2 

fluidized, the pressure drop across the bed remains 

m 

LE 

If oxygen is not limiting, design of most nitrifying pc 

biofilters is based on the amount of surface area 

Ay~ -‘ Real bed (channeling) 

Superficial velocity is a term used for calculating hydraulics 

within granular media. The superficial velocity is the velocity 

of the flow that would be measured if no media were present. 

‘mf 

Superficial velocity 

It can be calculated by dividing the average volumetric flow 

rate by the cross-sectional area of the reactor that is 

perpendicular to the flow. In the remainder of the chapter, 

superficial velocity is referred to simply as velocity. 

Figure 3. Hydraulics of flow through a bed of granular 

media at velocities above and below the minimum 

fluidization velocity (vmf) (after Fan [1978]). 

NCRAC Culture Series 101 - W a l l e y e C u l t u r e Manual 283 
required to daily convert a given amount ammonia to 

nitrate. For most types of biofilters, capital costs 

increase roughly in proportion to increased requirements for surface area. In fluidized-sand biofilters, 

however, the total available surface area can be increased relatively inexpensively due to the super high 

specific bed surface areas (ranging from 1,000-14,000 

ft2/ft3 [4,000-45,000 m2/m3]) and relative low cost of 

the media (around $1-2/ft3 [40-70/m3] of graded sand). 

Although the cost per unit surface area is low compared 

to other biofilter types, the total head pressure required 

across the fluihzed-sand biofilter is moderate (generally c 6 psi [4 m of water head]). The head pressure 

required to fluidize a given sand depth does not change 

with sand diameter; only the water velocity required 

changes with the sand size selected and the desired bed 

expansion (Table 4). Therefore, the design of a fluidized-sand biofilter is based on both hydraulic and 

nitrification considerations. The first step in designing a 

fluidzed-sand biofilter is to select a sand and a bed 

expansion. Assuming that the flow rate to be treated by 

the fluidized-sand biofilter withm the recirculating 

aquaculture system was previously set, selection of 

these two criteria sets the velocity needed to fluidize the 

sand to the desired expansion. The velocity, flow rate, 

sand size, and depth of sand controls the size (e.g., 

diameter) and oxidation capacity of the biofilter. When 

selecting sand size, bed expansion, and bed depth, you 

must ensure that the biofilter has excess capacity for 

nitrification and sufficient oxygen loading to maintain 

an aerobic effluent. Completing the fluidized bed 

requires design of a distribution mechanism for 

injecting flow at the bottom of the sand bed and 

uniformly fluidizing the sand. 

Sand selection & bed expansion 

Summerfelt and Cleasby (1 996) reviewed the hydraulic 

design of fluidized-beds. Fluidization hydraulics 

depends on characteristics of the sand. Density, 

sphericity, and porosity can be approximated for the 

Table 4. Velocity (v,) required to fluidize a uniformly sized sand of equivalent diameter 

(De,) to a given bed expansion, assuming a temperature of 25°C and characteristics 

of typical sands* (Summerfelt and Cleasby 1996). Bed specific surface area (Sb) and 

minimum fluidization velocity (vmt) are also shown for each diameter. 

at 50% at 100% at 150% 

De, (mm) Sb (ft2/ft3)$ vmf (gpm/ft2)t Expansion Expansion Expansion 

0.05 

0.1 

0.15 

0.2 

0.25 

0.3 

0.35 

0.4 

0.45 

0.5 

0.6 

0.7 

0.8 

0.9 

1 .o 

26,800 

13,400 

8,900 

6,700 

5,400 

4,500 

3,800 

3,400 

3,000 

2,700 

2,200 

1,900 

1,700 

1,500 

1,300 

0.04 

0.1 6 

0.36 

0.64 

1 .oo 

1.44 

1.95 

2.54 

3.1 9 

3.92 

5.56 

7.42 

9.46 

11 -62 

13.88 

0.2 

1 .o 

2.2 

3.4 

5.1 

7.6 

10.1 

12.8 

15.4 

18.1 

23.2 

28.4 

33.2 

38.1 

42.6 

0.5 

1.9 

3.8 

6.9 

10.6 

14.4 

18.2 

22.1 

25.7 

29.4 

36.6 

43.4 

50.0 

56.2 

62.2 

0.8 

2.9 

6.0 

10.6 

15.4 

20.0 

25.0 

29.5 

34.1 

38.4 

46.9 

54.8 

62.5 

69.7 

76.6 

*i.e., a sand density of 2.65 g/cm3, a loose-bed porosity of 0.45, a sphericity of 0.45, and a water temperature of 25°C. Variations 

in sand characteristics from different quarries can result in significantly different expansion velocities requirements than those 

reported here, particularly as the diameter of the sand increases. Therefore, the numbers in Table 4 should only be used for 

preliminary design estimates; a hydraulic test on a sample of the sand selected should be completed to determine the actual 

expansion velocities on a case by case basis; * 1 cm-l= 100 m2/m3 = 30.48 ft2/ft3; t 1 cm/s = 14.7 gpm/ft2. 

284 NCRAC Culture Series 101- Walleye Culture Manual 
Table 5. Example particle size analysis provided by a sand distributor (data for FilterSilTM 

filtration sand from Unimin Corporation’s facility in Oregon, Illinois). 

Mesh 

(ASTM E-11) 

Sieve analysis 

Typical mean % retained on individual sieves 

20 

30 

40 

50 

70 

100 

140 

200 

270 

Pan 

1.5 

45.0 

45.0 

5.0 

2.0 

1 .o 

0.4 

- 

- 

15.0 

60.0 

20.0 

3.0 

1 .o 

0.4 

0.1 

- 

- 

3.0 

36.0 

40.0 

13.0 

6.0 

1.4 

0.1 

trace 

- 

1 .o 

20.0 

34.0 

24.0 

15.0 

5.0 

1 .o 

0.1 

1 .o 

15.0 

47.5 

30.0 

6.0 

0.5 

trace 

Size designation 

Effective size (mm) 

U n ifo r m i ty Coefficient 

0.45 0.35 0.25 0.1 5 0.1 0 

0.45 0.34 0.23 0.1 6 0.1 3 

1.4 1.4 1.8 2.0 1.7 

purpose of design (Summerfelt and Cleasby 1996). 

Diameter and uniformity of the sand can be determined 

from a sieve analysis provided by the supplier. The 

characteristics for a given sand are used to estimate the 

velocity required to achieve a given bed expansion. 

Equations to determine the hydraulics of flow through 

expanded beds of sand and the typical values used in 

these calculations are summarized by Summerfelt and 

Cleasby (1996). Estimates of the velocities required to 

fluidize a given diameter sand to a given expansion are 

presented in Table 4. 

Suppliers of silica filter sand are listed in the AWWA 

(1 994) Buyer’s Guide. Fluidized-sand biofilters in 

aquaculture typically use an extremely hard, whole 

grain crystalline silica sand, which is finely graded and 

has a mean effective diameter of 0.1-1 .O mm, and a 

uniformity coefficient between 1.3 to 1.8. Average bed 

expansions are often between 20 and 100%. Because 

sands are not perfectly uniform, larger sands move to 

the bottom of the fluidized beds where they expand less 

than the smaller sands that have migrated to the top of 

the bed. The average expansion of a bed at a given 

velocity depends upon the size gradation within the bed 

(i.e., uniformity coefficient), which makes it important 

to predict expansion of both the largest and smallest 

fractions of sand (Cleasby and Fan 1981). In particular, 

the largest fraction of sand must expand at the velocity 

selected. 

The following example illustrates why expansion of the 

entire bed must be considered when selecting a sand. A 

supplier provides a particle size analysis (Table 5) for 

several sizes of sand. Assume the sand selected is the 

one with an effective diameter of 0.34 mm and a 

uniformity coefficient of 1.4 (Table 5). To design a 

fluidized bed using this sand, the first step is to determine how much expansion would be obtained withn 

the largest and smallest size fractions. Examination of 

the sieve analysis (Table 5) indicates that the largest 

15% of the sand was retained on the 30 mesh sieve, but 

passed through the 20 mesh sieve, and that the smallest 

4.5% of the sand passed through a 50 mesh screen. 

Table 6 can be used to convert sieve size to approximate 

sand diameter. Roughly, the largest 15% of the sand has 

a diameter > 0.595 mm and < 0.841 mm, and the 

smallest 4.5% of the sand has a diameter e 0.297 mm. 

The relationship between sand size, velocity, and bed 

expansion can be estimated from Table 4. Sand with a 

mean effective size of 0.34 mm requires a velocity of 

around 17-18 gpm/ft2 (1 -20 cm/s) to expand 100%. The 

largest 15% of the sand has a diameter > 0.60 mm (but 

< 0.841 mm). It would fluidize to some extent at a 

velocity of 17.6 gpm/ft2, but expansion would be less 

than 50%. Most of the smallest 4.5% of the sand would 

expand slightly less than 150% at a velocity of 17.6 

gpm/ft2. Accordingly, the 0.34 mm sand would comNCRAC Culture Series 101 - Walleye Culture Manual 285 
Table 6. Size of opening corresponding to the U.S. sieve series 

designation number (Perry and Chilton 1973). 

Sieve designation Size of opening Sieve designation Size of opening 

nu mbert (mm) numbert (mm) 

16 

18 

20 

25 

30 

35 

40 

45 

50 

1.19 

1 .oo 

0.841 

0.707 

0.595 

0.500 

0.420 

0.354 

0.297 

t Number of meshes per inch. 

pletely fluidze at a velocity of 17.6 gpm/ft2 and would 

have a mean expansion of about 100%. 

Biofilm growth on the sand increases expansion as it 

decreases the effective density of the sand. Increased 

expansion due to biofilm growth can be of special 

significance with fine sands (< 0.5 mm), as the biofilm 

thickness may become greater than the Qameter of the 

sand. 

Biofilter geomefry 

The design of the recirculating aquaculture system 

described in this chapter assumes that the biofilter will 

treat the full flow of reused water. With this flow rate 

(Q) and with a velocity (v,) that was set when the sand 

and overall bed expansion were selected, the crosssectional area (Ab) requirements for the biofilter can be 

calculated: 

Due to practical considerations based on biofilter 

geometry, pressure drop, and reactor oxygen demand, 

the depth of sand in aerobic fluidized-sand biofilters is 

generally designed to be 3-6 ft (I -2 m), unexpanded. 

After setting the depth of sand within the bed, the total 

available surface area (SAavail) should be checked to 

ensure that there is more surface area available than 

surface area required (SAreqd) to remove the total 

ammonia nitrogen (TAN) produced by the fish: 

60 

70 

80 

100 

120 

140 

170 

200 

230 

0.250 

0.21 0 

0.177 

0.149 

0.125 

0.105 

0.088 

0.074 

0.063 

PTAN 

RTAN SAreqd = ~ (3) 

where: 

SAavail = surface area available in biofilter for 

SAreqd = surface area required for nitrifying all of the 

S, = specific surface area of the bed of sand (ft2/ft3); 

V,, = volume of sand within the biofilter (ft3); 

P,= TAN generation rate (lb TAN produced per 

RTAN = area specific nitrification rate (lb TAN 

removed per day per ft2 of surface area). 

nitrification (ft’); 

TAN produced (ft2); 

day); 

The rate that TAN is produced (PTAN) within the system 

is proportional to the product of the culture biomass and 

the feeding rate: 

where: 

&ish = density of fish in the culture tank (lb fish per 

Vct = volume of water contained within culture 

‘feed = feedmg rate (lb feed per lb fish per day); 

aTAN = TAN produced as a proportion of feed fed 

ft3 culture volume); 

unit (ft3 culture volume); 

(lb TAN per lb feed). 

286 NCRAC Culture Series 101- Walleye Culture Manual 
The fluidized-sand biofilter design should have excess 

available surface area. Generally spealung, the expense 

of the excess sand and slightly larger biofilter tank is 

relatively low, and the excess is necessary to ensure 

stable operation. Excess capacity is important to ensure 

complete nitrification (prevent accumulation of nitrite) 

and to prevent gelling of the biofilm. The latter problem 

may result in channelization and meda wash-out, 

which has been reported to occur when sands of less 

than approximately 1 .O mm diameter were used within 

fluidized beds operated under high substrate loading 

rates (approximately 6.0 x lb TAN removed per 

day per ft2 of surface area, as calculated from data 

presented by Burden [1988]). Burden (1988) did not 

report gelling in fluidized beds of sand larger than 

approximately 1.5 mm. Research on nitrification in 

fluidized-sand biofilters at the Freshwater Institute 

indicated that area specific removal rates ranged from 

0.45 x to 2.0 x lb TAN removed per day per 

square foot of surface area (20-100 mg/d/m2), depending upon the amount of excess sand present (unpublished data). However, nitrification rates on sands are 

highly dependent upon the sand size, water temperature, and ammonia and organic loading rates. In 

addition, because sands have such high specific surface 

areas, it can be argued that the nitrification capacity of 

the filters would be more accurately represented by the 

total sand volume, rather than the total sand surface 

area. More applied research is needed in this area. 

Large fluidized beds can be circular or 

rectangular, constructed withn plastic, 

fiberglass, concrete, or enamel-coated 

steel tanks, and can generally be 

constructed on site. If necessary, the 

design can be modified by adjusting 

tank dameter, sand diameter, or 

recirculating sytem flow rate to 

provide a tank of convenient size, or to 

allow the use of a graded sand which 

is available locally. 

F/o w distribution 

A reliable flow distribution mechanism is critical to effectively operate a 

fluidized-sand biofilter. The flow 

distribution system must deliver an 

equal amount of flow across the base 

of the bed, prevent loss of media, 

operate without detrimental fouling 

(or have a fouling prevention system) and, in some 

cases, support the bed. There are a wide variety of 

distribution mechanisms used to inject water into the 

bottom of large fluidized-sand biological filters. Each 

mechanism differs in how flow is transported and 

distributed. However, most mechanisms used in 

recirculating aquaculture systems transport the flow 

through a manifold, starting at the top of the biofilter, 

that runs down the inside of the reactor to the base of 

the sand. Bringing the flow into the biofilters from 

above avoids distribution pipes piercing the biofilter 

wall. A check valve to stop siphoning, installed after the 

pump and before the dstribution manifold above the 

biofilter, prevents the hydraulic head of water in the 

tank from producing back flow into the distribution 

system when normal flow is lost. Backflow can carry 

sand into the distribution pipes and plug them, which in 

turn can cause uneven fluidzation of the bed. 

A flow dstribution mechanism is described below that 

is a simple modification of the pipe-manifold system 

used in wastewater treatment filters. This distribution 

mechanism introduces the flow through a manifold 

structure located at the top of the biofilter (Figures 4 

and 5). The overhead manifold branches to equally 

spaced pipe laterals that transport the flow down an 

inside wall to the reactor base. At the base, each lateral 

elbows 90" and runs across the floor to the far wall 

where they elbow 90" up from the reactor bottom and 

Figure 4. A modified pipe-lateral distribution system before sand was 

added. 

NCRAC Culture Series 101 - Walleye Culture Manual 287 
thrcaded / “l 

inlet pipe 

reducing manifold pipe 

abrasion resistant base and lower sidcwalls 

1 

side perspective 

elbow 

/ 

reducing 

tee 

lateral pipes with equally spaced orifices 

reducing 

elbow 

f tcc and 

thrcadcd caps 

threaded caps 

top perspective 

reducing 

elbow 

f tcc and 

thrcadcd caps 

threaded caps 

top perspective 

valve 

Figure 5. A modified pipe-lateral distribution system for fluidized-sand biofilters (side and top perspectives). 

run the length of the wall to the top of the reactor 

(Figures 4 and 5). Flow distribution orifices are located 

on each section of pipe running along the vessel floor 

(Figures 4 and 5). Tees fitted with screw caps are 

located at the end of each manifold pipe. This configuration allows for installation of control valves on 

indvidual pipe laterals, and provides a mechanism for 

unplugging individual pipes by flushing water, sand, or 

other debris by temporarily removing the threaded cap 

on the side-tee at the top and end of the plugged pipelateral (Figure 6). 

Figure 6. A plugged lateral was cleared by removing 

the screw cap from the overhead tee at the lateral 

end and flushing with water. 

Criteria for spacing of the pipe-laterals, and the spacing 

and dameter of orifices, are provided in several 

engineering texts (Weber 1972; AWWA 1990). Water is 

distributed from the pipe-laterals along the floor of the 

vessel through two rows of 0.25 to 0.5 in (6.4 to 12.7 

rnm) diameter orifices that are “directed downward (on 

each side of a lateral pipe) so as to dissipate the energy 

of the water jets” (Weber 1972). Laterals and orifices 

are normally spaced at roughly the same interval, 

between 3 to 12 in (7.5 to 30 cm) apart. Additional 

guidelines given for lateral design includes the following ratios: 

Total area of orifices: cross-sectional area of bed 

Cross-sectional area of pipe-lateral: total area of 

Cross-sectional area of manifold: total area of pipe0.0015 to 0.005:l (5) 

orifices served 2 to 4: 1 (6) 

laterals served 1.5 to 3:l (7) 

Conformation to these ratios will assist with the basic 

approach used to obtain uniform flow distribution 

(Montgomery 1985): to size the orifices small enough 

to introduce a controlling headloss, and to scale the 

distribution pipes so that the flow velocity within the 

pipes are reasonably low and uniform throughout the 

entire filter area. Orifices of pipe-lateral systems 

generally create a headloss of at least 2 ft (0.6 m) of 

water (Montgomery 1985). It is rationale to size the 

orifices to create a headloss greater than or equal to the 

head required to fluidize the bed. Much larger orifice 

headloss would produce much stronger jetting actions 

288 NCRAC Culture Series 101- Walleye Culture Manual 
that are more likely to damage the vessel or distribution 

pipes and are more likely to scour the biofilm from the 

sand in the jetting zone. Much smaller orifice headloss 

would not maintain proper distribution of flow under 

the sand bed. 

The following equation can be used to estimate the 

headloss for a given orifice diameter and flow rate: 

1 .- ‘In A I n 

‘orif 

where: 

HLorif = headloss due to flow through orifice (ft of 

Qorif = flow rate of water through orifice (ft3/s); 

Aorif = area of orifice (ft ); 

g = gravity constant (32.2 ft/s2); 

C = orifice discharge coefficient for sharp-edged, 

submerged orifices (0.6). 

The pressure loss across a fluidized bed is constant at 

all bed expansions (Figure 3) (i.e., velocities > than the 

minimum fluihzation velocity), and is equal to the 

buoyant weight of the media per cross sectional area of 

the bed. The head required to fluidize sand is between 

0.9-1 .0 ft of water for every 1 .O ft of sand in the loosely 

packed bed. 

water); 

2 

When the modified pipe-lateral distribution mechanism 

was evaluated within a cubical 6 x 6 x 6 ft (1.8 x 1.8 x 

1.8 m) tank at the Freshwater Institute (Figure 4). The 

orifices were oriented at an angle 45” below horizontal. 

Water jets emitting from the downward facing orifices 

sand blasted holes through the 0.25 in (6 mm) thick 

fiberglass floor of the biofilter vessel in 7 d (Figure 7). 

The AWWA (1971) has also reported on jet action 

producing problems at the base of filter beds. We 

modified the design to prevent the jet action from 

“sand-blasting” through the tank wall by pouring a 

concrete pad on the biofilter floor (4 in thick pad with 

side walls 3 in thick by 4 in hgh), after first removing 

the sand and distribution pipe-laterals. After about one 

year of operation the sand was removed and the 

concrete pad showed little erosion from the water jets. 

During evaluation, this distribution system maintained 

uniform sand expansion (30% expansion) throughout 

the bed (3 ft deep, unexpanded) with no maintenance, 

even though flow was interrupted several hundred 

cycles (30 min flow followed by 30 min no flow). Flow 

Figure 7. Water jets emitting from distribution orifices 

sand-blasted holes through the bottom of an unshielded 

fiberglass vessel. The vessel floor was later shielded 

with 4 in (10 cm) of concrete to prevent tank damage. 

interruption did not plug the pipe laterals. However, 

when sand was initially loaded into the bed with a front 

end loader, plant stalks, leaves, and roots that had 

grown on the sand pile entered the pipe manifold with 

the recirculated, unfiltered flow and blocked flow 

through the orifices at the end of two pipe-laterals. 

Pipe-laterals were unplugged by flushing water down 

the pipe after the screw caps were removed from the 

overhead tees at the end of each plugged lateral (Figure 

6). If more vigorous action had been required to unplug 

a lateral, a garden hose or wire-rooter could have been 

run down through the top of the tee fittings (after 

temporarily removing the threaded cap). A similar 

distribution manifold has been successfully used to treat 

200 gprn in a fluidized-sand biofilter withn a recirculating system at the Freshwater Institute. 

Example design: fluidized-sand biofilfer 

Fluidized-sand biofilters are complex devices, but they 

are easily fabricated locally using a vessel and PVC 

pipehittings and concrete. The most critical design 

factor for a fluidized-sand biofilter is the water dmtribution manifold. Assuming that the 0.34 mm hameter 

filter sand (Table 5) described earlier was selected, a 

velocity of 17.6 gpm/ft2 would be required to obtain an 

average bed expansion of 100%. Equation 1 can be used 

to calculate the biofilter’s cross-sectional area based 

upon the desired velocity (17.6 gpm/ft2) and system 

flow (1,000 gprn): 

NCRAC Culture Series 101 - Walleye Culture Manual 289 
Either a circular or rectangular vessel can be selected to 

provide the biofilter's cross-sectional area requirement. 

A circular vessel would be 8.5 ft (2.6 m) in diameter or 

a rectangular vessel could be 6.5 ft (2 m) wide by 8.7 ft 

(2.65 m) long. For this example, a rectangular vessel 

was selected because it could be fabricated out of 

concrete. Nine distribution pipe-laterals, spaced on 0.75 

ft (0.23 m) centers, would fit within the vessel parallel 

to the long walls. Their would be 23 orifices through 

the underside of each lateral (placed 15-45" below 

lateral center). Twelve of the orifices would be located 

on one side of each lateral and 11 orifices located on the 

opposite side. The distribution mechanism would 

contain a total of 207 orifices and each orifice would 

distribute roughly 4.83 gpm (18.4 Lpm). The orifice 

diameter can be selected using equation 8 to provide a 

controlling headloss greater than the head required to 

expand the sand. If the biofilter had 6.6 ft (2 m) of sand 

(static), it would require about 6.6 ft (2 m) of water 

head to expand. An orifice of 6/16 in (9.53 mm) 

diameter would create a controlling head loss of 8.4 ft 

(2.6 m). The laterals and manifold should be scaled to 

minimize pressure losses within the distribution system. 

Each distribution lateral should have an inside diameter 

from 3 to 4 in (7.6 to 10.2 cm), accordmg to the criteria 

in equation 6. The hstribution manifold at the top of the 

vessel should have an inside diameter anywhere from 

8 - 12 in (20 - 30 cm), according to the criteria in 

equation 7. 

The 0.34 mm sand in the fluidized bed would have a 

specific surface area of about 4,000 ft2/ft3 (13,000 m2/ 

m3)(from Table 4). The total volume of sand in the 

biofilter, 376 ft3 (10.6 m3), would likely cost about 

$800, excluding shipping. This fluidized bed would 

have about 1,500,000 ft2 (140,000 m2) of surface area 

available for microbial growth (using Equation 2). If we 

assume a conservative area specific nitrification rate 

(I .0 x 1 0-5 lb TAN removed per day per ft2 of surface 

area), then the biofilter could be expected to remove 15 

lb (6.8 kg) TAN per day. Assuming that 0.03 lb TAN are 

produced for every lb of feed eaten (Table 2), then at 

maximum loading this biofilter could treat the ammonia 

produced from feeding about 500 lb (230 kg) per day. 

Aerationktripping by cascade column 

Commercial oxygen is widely used to supplement 

oxygen levels and boost fish production in intensive 

aquaculture systems. However, accumulation of hgh 

levels of carbon dioxide can become a limiting toxicity 

factor with high fish densities and inadequate water 

exchange; i.e., high fish loadings (Colt and 

Tchobanoglous 1981; and Colt et al. 1991). Carbon 

dioxide toxicity is more likely to occur in intensive 

aquaculture systems whch inject pure oxygen because 

oxygen injection unit processes use insufficient gas 

exchange to strip much carbon dioxide (Watten et al. 

1991). Additionally, more carbon dioxide is usually 

produced in systems which inject pure oxygen because 

these systems have the oxygen to support higher fish 

loadmg rates. When aeration is used to supply oxygen 

to aquaculture systems, however, fish loadmg levels are 

lower than can be obtained with pure oxygen and 

enough air-water contact is generally provided to keep 

carbon dioxide from accumulating to toxic levels 

(Speece 1973). 

Air stripping and aeration are mass transfer processes 

that occur together when water is contacted with air to 

bring its concentration of dissolved gases (such as 

nitrogen, carbon dioxide and oxygen) into equilibrium 

with the partial pressures of these gases in the surrounding atmosphere. The rate of mass transfer (J), as defined 

by Fick's Law, is equal to the product of the overall 

mass transfer coefficient (KL), the total interfacial 

contact area (A) per unit system volume (V) and the 

concentration gradient ( Xes - X) (Treybal 1980): 

A 

J = K~ . -(xeq V - X) (9) 

The concentration gradient is the driving force for mass 

transfer. In the expression for the concentration gradient, Xes is the limiting molar concentration of Qssolved 

gas defined by Henry's Law and X is the actual molar 

concentration of dissolved gas in the water. The term A, 

represents the specific interfacial area. 

V 

Carbon dioxide can be transferred from water with any 

of the non-closed aeration systems that Boyd and 

Watten (1989), Colt and Orwicz (1991) and others have 

described. However, because carbon dioxide has a 

Henry's Law constant 200 to 300 times that of oxygen, 

it is more difficult to strip carbon dioxide than to add 

oxygen to water; consequently, bubbles formed from 

diffused aeration readly become saturated with carbon 

dioxide, requiring enormous quantities of air to achieve 

substantial carbon dioxide transfer rates when compared with the air-flow rates required for oxygen 

transfer alone. Passing water through air provides the 

290 NCRAC Culture Series 101- Walleye Culture Manual 
larger ratio of air to water volume needed for carbon 

&oxide exchange. This makes it more effective to strip 

carbon dioxide by moving water through air, as is done 

with surface aerators and air strippers, than by moving 

air through water, as is done with subsurface aerators 

(Colt and Orwicz 1991). Also, when water droplets are 

formed while passing through air, the shortened 

dffusion distance (a function of droplet diameter) 

enhances mass transfer out of the liquid phase. 

The obvious way to let water fall through air is by a 

gravity drop, which can be: over a weir, onto a splashboard, through plastic meda or stacked splash screens, 

down an inclined corrugated sheet (with or without 

holes), or down a stair-stepped surface. Because carbon 

dioxide stripping requires such a large volume of air per 

unit volume of water, it is most effective when large 

volumes of air can be forced through cascadng water 

within enclosed columns. Increasing the air-water 

contact area by packing the columns with high voidage 

plastic media or stacked screens improves both carbon 

dioxide stripping and aeration (Figure 8). Air stripping 

in forced ventilation columns has been described by 

Onda et al. (1968) and reviewed extensively in the 

waste water treatment field by Kavanaugh and Trussell 

(1980), Cornwell (1990), Haarhoff and Cleasby (1990), 

and Thom and Byers (1 993). Design of air-stripping 

columns requires selection of the following parameters: 

liquid loading rate; 

air to water volumetric loading ratio (ths selection 

paclung depth; 

packing material (size and type); 

determines the air loading rate); 

Shenvood and Holloway (1940) and Piedrahita and 

Grace (1989) have measured KLa values for packed 

C0,-stripping chambers. Summerfelt (1 993) reviewed 

and reported criteria for carbon &oxide stripper design 

within aquaculture systems, they are: 

a hydraulic fall of 3-5 ft (1-1.5 m), (10) 

a hydraulic loading of 25-35 gpm/ft2 (1 -0-1.4 rn3/ 

min/m2>, (1 1) 

a volumetric air:water ratio of 6: 1 to 10: 1. (12) 

High porosity pachng or splash screens are needed to 

avoid flooding or gas hold-up. If high solids loadings 

are expected, a stripping tower with screens or trays is 

easier to maintain than a packed tower. Also, the air 

blown through the air stripper should be vented out of 

Distribution Plate 

/ 

Air 1- 

Tray Tower 

.Air Out 

'ay or Scr 

Water Ou 

.e e 

t 

Water In- Distribution Plate 

MediaMedia Suppor 

Screen \ 

Air In---) 

4 /I 

-b Air Out 

-b Water Out 

Packed Tower 

Figure 8. Two common types of air stripping 

columns. 

the building to prevent carbon dioxide from accumulating inside the building that contains the recirculating 

aquaculture system. In a cold climate, heat can be 

conserved by venting the air through an air-air heat 

exchanger. 

As an alternative to air stripping, carbon dioxide can be 

treated by chemical addition as described in the pH 

Control section. Chemical addition is already required 

to maintain alkalinity in closed recirculating aquaculture systems. 

NCRAC Culture Series 101 - Walleye Culture Manual 291 
Example design: aeratiods tripping 

column 

Aeratiodstripping columns 

(Figure 8) are relatively simple 

devices that can be locally 

fabricated in a sheet metal shop. 

For a flow of 1,000 gpm (3,800 

Wmin) the air stripper should 

provide a drop of 4.0 ft (1.2 m, 

Equation lo), across a crosssectional area of 33 ft2 (3.2 m2, 

Equation 1 l), and force 1,300 

scfm (38 m3/min, Equation 12) of 

air flow counter-current to the 

water. Water is distributed across 

the cross-section of the column 

with an orifice plate located at the 

top (similar to that used on a 

LHOTM, as shown in Figure 9). 

Flow will be equally dstributed 

multi-stage luw head uxygenarur (LHO rv) U-tube 

Figure 9. An LHOTM or U-tube can be used for tansferring pure oxygen 

into water. 

to all of the orifices in the distribution plate and a water 

seal will be formed above the distribution plate by 

selecting the number and size of orifices to create a 4-6 

in (10-15 cm) head loss (Equation 8). The column can 

operate with or without splash media, but more carbon 

&oxide will be stripped using media. Several expanded 

metal panels (0.75-1 .0 in openings) that are placed 

perpendicular to the flow work well to break the falling 

water and also resist fouling better than some plastic 

meQa (Figure 8). Air exhausted from the stripper 

should be vented outside of the building and through an 

air-to-air heat exchanger in cold climates. 

Oxygen injection with low head oxygenators or 

U-t u bes 

Aeration and oxygenation are processes used to 

maintain adequate levels of oxygen within recirculating 

fish culture systems. Fortunately several outstanding 

reviews on aeration and oxygenation have been written 

by Speece (1981), Visscher and Godby (1987), Colt and 

Watten (1988), Speece et al. (1988), Boyd and Watten 

(1989), Visscher and Dwyer (1990), and Watten (1994). 

In the aeration process described above, air is brought 

into contact with water so oxygen transfers from the air 

into the water. The aeration unit also serves as the 

carbon dioxide stripping unit. In the oxygenation unit 

process, pure oxygen gas is used (instead of air) to 

achieve oxygen levels in the water flow that are above 

standard atmospheric saturation levels. Increasing 

oxygen in the flow increases the system’s carrying 

capacity, resulting in an economical means of boosting 

system production (Collins et al. 1984). 

Oxygen can be produced on site using pressure swing 

adsorption (PS A) equipment or purchased from 

commercial sources as a bulk liquid or gas (Speece 

1981; Boyd and Watten 1989; Watten 1994). Producing 

oxygen on site requires a source of dry air at pressures 

of 90 to 150 psi as well as the PSA unit, which amounts 

to a considerable capital investment. The cost of 

generating oxygen using PSA units, which varies with 

electric rates, is about 0.5 kwh of electricity per pound 

of oxygen produced. The cost of liquid oxygen varies 

with location. At the Freshwater Institute, the operating 

and capital costs (using net present value over a ten year 

period) of using liquid oxygen versus on site generation 

were compared; we found that liquid oxygen costs 

about three times more annually than would oxygen 

generated on site. However, the capital investment and 

risk of system failure would be lower if liquid oxygen 

were used (Boyd and Watten 1989). 

Oxygen is not an insignificant cost, it may represent 

about 15% of feed costs if generated on site (E. Wade, 

Freshwater Institute, unpublished data), and thus the 

transfer of oxygen into the fish culture water must be 

efficient. Gas absorption equipment is designed to take 

advantage of the factors governing the rate of mass 

transfer. Gas absorption is dependent on the area of the 

292 NCRAC Culture Series 101- Walleye Culture Manual 
gas-liquid interface and the thickness and rate of 

surface film renewal (both dependent upon the energy 

transfer in the gas-liquid contactor) and also on the 

gradient between the saturation and existing concentrations of the gas in the water. When pure oxygen rather 

than air is transferred into water, the water's saturation 

concentration for oxygen is increased nearly 5-fold over 

the saturation concentration obtained when air is used. 

The saturation concentration can also be increased by 

increasing the total pressure in which the transfer 

occurs (e.g., via a pump or hydrostatic head). Increasing 

the pressure during oxygen transfer from 1 to 2 atmospheres nearly doubles the saturation concentration of 

oxygen in water. Increasing the absorption pressure to 

increase the oxygen transfer, however, may also 

increase the operating costs, and this must be considered when selecting an oxygenation unit. Additionally, 

mechanisms for strippinghenting nitrogen and argon 

gas released during oxygen absorption are important 

both to reduce the total gas pressure of the water and to 

increase the eficiency of oxygen transfer. 

Many types of oxygen transfer equipment have been 

described: U-tubes, multi-staged low head oxygenation 

units, packed columns, spray columns, pressurized 

columns, oxygenation cones, oxygen aspirators, bubble 

diffusers, and enclosed mechanical-surface mixers. Two 

methods are particularly well suited to transferring 

oxygen to water within large recirculating systems: Utubes and multi-staged low head oxygenation units 

(Figure 9). Both methods can be readily scaled-up, are 

easy to control, and require only a modest hydraulic 

head. Performance and design criteria for each are 

given below. 

Multi-stage low head oxygenators 

A multi-stage low head oxygenation unit (LHOTMj was 

patented by Watten (1989) and described by Watten and 

Boyd (1990). They maximize oxygen transfer efficiency 

by reusing the oxygen feed gas though a series of 

contact chambers. Water flow is typically distributed 

equally to each chamber at the top of the LHOTM via 

head-controlling orifices. Water flowing through the 

orifices typically drops anywhere from 1 to 4 ft (0.3 to 

1.2 m) into a plunge pool which seals the lower portion 

of each LHOTM chamber (Figure 9). The chambers 

typically do not contain packing, but can be packed 

with plastic media to improve mass transfer (Weber et 

al. 1995). 

Most evaluations of LHOTM units (Dwyer and Peterson 

1993; Wagner et al. 1995; Weber et al. 1995), have been 

in cold water (12-17"C), where the saturation of oxygen 

is higher than at the temperature used to raise walleye 

(20-25°C). These studies demonstrated that LHOTM 

units provide excellent (60 to 90%) oxygen transfer 

efficiencies, when operated at gas to liquid ratios of less 

than 1 volume of oxygen feed gas for every 100 

volumes of water flow (G:L = 0.01:l). However, the 

low oxygen loading rates (G:L e 0.01:1 j, which are 

good for maximizing oxygen transfer, limit the increase 

in dssolved oxygen concentrations within the flow to 

generally < 8 mg/L above saturation. Weber et al. 

(1995 j found that to produce dmolved oxygen concentrations of approximately 25 mgL in 12°C water 

required increasing the G:L to around 0.02: 1. The 

oxygen transfer efficiency dropped to 50 to 60% at the 

increased G:L, even though plastic pachng was used 

within the LHOTM chambers. Results from these studies 

make it clear that oxygen off-gas recycling is necessary 

to conserve oxygen when trying to double (or more) the 

saturation concentration of dissolved oxygen within the 

flow leaving an LHOTM unit. The studies also demonstrated that LHOTM units remove nitrogen gas when 

they add oxygen, which helps prevent large total gas 

supersaturations, even when large oxygen supersaturations are obtained. 

U-tube 

A U-tube maximizes oxygen transfer by using hydrostatic pressure to temporarily increase the saturation 

concentration of oxygen as the flow passes through the 

depth of the vertical, U-shaped conduit (Figure 9j. 

U-tubes operate by continuously diffusing pure oxygen 

into the water entering the top of the conduit and by 

then entraining the oxygen bubbles in the flow passing 

down one side of the conduit and up through the other. 

Bubbles are entrained by designing the cross-sectional 

area of the first conduit to create a velocity greater than 

the buoyant velocity of the bubbles. Watten and Beck 

(1985) developed an equation to predict the dissolved 

oxygen concentration in the effluent of a U-tube (DOo) 

for a given conduit depth (D in m), water temperature 

(T in "Cj, influent dissolved oxygen saturation (S in %), 

and volumetric gas to liquid ratio (G:L in %), assuming 

that the diffuser is placed at the beginning of the 

downward conduit: 

NCRAC Culture Series 101 - Walleye Culture Manual 293 
-3.77 + 12.198. In I 

DOCF = exp< 

(13) 

DO0 = +0.9069 - D - 0.1405. T 

+0.0575 - %S r 

+0.00279 - D 

-0.00143 - T 

-0.00078 - %S 

+0.00393 - %R 

L 

Boyd and Watten (1989) show data indicating that Utube systems without off-gas recycle provided oxygen 

transfer efficiencies of only 30-50%, but that when offgas recycling was used, U-tubes could increase their 

oxygen transfer efficiencies to 55-80%. Watten and 

Beck (1985) provide an equation to correct for the 

effects of off-gas recycling: 

(DOo) recycling = DOCF - (DOo) no recycling (1 

where 

0.1 6347 - 0.0922 - In I 

and 

,100 (16) off gas recycled 

off gas recycled + new gas 

%R = 

The pump pressure required for a given U-tube, 

generally from 4- 15 ft of water head, can be estimated 

by adding the head losses resulting from pipe friction, 

velocity, and two-phase flow. The head loss resulting 

from two-phase flow can be estimated with an equation 

provided by Watten and Beck (1985): 

10.888 + 0.0758 - D] 

DHL = expi +1.021-ln (;) - 1 (17) 

At a constant pipe velocity, large hameter U-tubes used 

to oxygenate high volume flows have less total head 

loss than do correspondingly small U-tubes that 

oxygenate low volume flows. 

Because U-tubes operate at higher pressures than 

LHOTM units, it is possible to add more oxygen to a 

flow with a U-tube than could be achieved by using a 

LHOTM. However, U-tubes do not strip nitrogen as 

readily as LHOTM units, and a U-tube would produce 

higher total gas pressures per unit of oxygen transferred 

than an LHOTM unit, 

Example design: LHOTM 

LHOTM units are relatively simple devices that are 

patented and licensed to Ziegler Brothers (Gardners, 

PA). For this example, the oxygen transfer unit should 

be expected to raise oxygen concentrations to at least 

17 mgL within the recirculating flow, A LHOTM unit 

can easily add 7- 10 mg/L to the flow at a transfer 

efficiency of 50-70%. Ziegler Brothers will manufacture 

and sell LHOTM units to oxygenate 1,000 gpm (3,800 

Lpm). They will also manufacture an aerationhtripping 

column that can stack on top of an LHOTM unit. This 

requires that both units use about the same hydraulic 

loadmg rate, whch presents no problem because a 

hydraulic loadmg rate of 25-35 gpm per ft2 cross 

sectional area works well for both LHOTM units (Weber 

et al. 1995) and cascade columns (Summerfelt 1993). 

Stacking the two gas transfer processes saves on space, 

tanks, and plumbing. (A U-tube would also have 

worked in this example). 

Culture within circular tanks 

Tanks for the intensive culture of fish are of varied 

shape and flow pattern (Piper et al. 1982). They are 

designed with considerations for production cost, space 

utilization, water quality maintenance, and fish management. Geometry, water velocity, and flow patterns are 

particularly important design considerations. The 

rationales for several common tank designs are reviewed by Piper et al. (1982), Watten and Beck (1987), 

Young and Timmons (1 99 l), and Timmons and Young 

(1991). 

Circular tanks are good for culturing walleye, because 

the tanks are relatively easy to maintain and provide a 

healthy and uniform culture environment. The main 

reason circular tanks are advantageous is because they 

operate with a rotating flow about the center drain. 

Rotational velocity can be controlled with properly 

294 NCRAC Culture Series 101- Walleye Culture Manual 
designed water inlet and outlet structures (Klapsis and 

Burley 1984; Tvinnereim and Skybakmoen 1989). 

Rotational velocity should be swift enough to carry 

solids and make the tank self cleaning, yet not faster 

than required to avoid over-exercising the fish. Water 

velocities of 0.5-2.0 times fish body length per second 

were reported in a recent review (Losordo and Westers 

1994) to be optimal to maintain fish health, muscle 

tone, and respiration. To generate centrifugal forces 

capable of driving settleable solids to the tank’s center 

drain, velocities should be greater than approximately 

15 to 30 cm/s (Burrows and Chenoweth 1970; MBlunen 

et al. 1988). 

Circular fish culture tanks can be managed as “swirl 

settlers” because the rotational flow concentrates solids 

at their bottom and center (Goldsmith and Wang 1993). 

Concentrated solids can be removed in a small flow 

stream (as low as 510% of the total flow leaving the 

tank) by using a bottom-drawing center drain as part of 

a double-drain system (Makinen et al. 1988; Lunde and 

Skybakmoen 1993; Losordo et al. 1995). The remainder 

of the flow leaving the tank (roughly 90-95% of the 

total) is withdrawn through a fish-excluding port 

located above the bottom-drawing drain or part-way up 

the tank’s side wall (M. Timmons, Cornel1 University, 

personal communication; S. Summerfelt, unpublished 

data). According to Losordo et al. (1 995), removing 

solids with a double-drain system has the potential to 

improve solids removal withn recirculating aquaculture 

systems. 

Circular tanks provide about complete mixing, which 

maintains uniform water quality throughout the tank. 

Complete mixing means that the concentration of a 

constituent in the water flowing into the tank changes 

instantaneously to the concentration that exists throughout the tank. Complete mixing also means that the 

concentration of the constituent in the tank will be the 

same as in the water leaving the tank through the center 

drain. Thus, if good mixing can be achieved, all fish 

within the tank are exposed to the same water quality. 

The tank water exchange rate can be set to maintain the 

water quality throughout the tank. 

The importance of homogeneous mixing can be 

illustrated by looking at how the concentrations of 

dssolved oxygen and carbon dioxide, and the total gas 

pressure change within circular tanks with high 

densities of fish. It is not uncommon for tanks with high 

densities of fish to receive influents with, for example, 

dissolved oxygen and carbon &oxide concentrations of 

22 mgL and 10 mg/l, respectively, and a total gas 

saturation of 108%. Within a well mixed tank, however, 

fish respiration (oxygen uptake and carbon dioxide 

excretion) can bring the concentration of oxygen and 

carbon dioxide throughout the tank to 8 mg/L and 30 

mgL, respectively, and bring the total gas pressure to 

less than 100% saturation. As this example illustrates, 

homogeneous mixing and hydraulic exchange within 

circular tanks can be used to maintain water quality. 

Example design: culture tank 

Circular culture tanks can be manufactured from many 

materials: fiberglass, concrete, enamel coated steel, and 

supported plastic liners. The circular tank is best sized 

based upon its exchange rate. An exchange rate of at 

least l.O/hr is a good rule of thumb. Exchange rates of > 

2.0hr are not that uncommon in ultra-intensive systems 

using complete mixed tanks. Accordingly, a circular 

tank scaled for 1-2 exchangefir with a flow of 1,000 

gpm (3,800 Lpm) would range in size from roughly 

60,000 to 30,000 gal (230 to 11 5 m3), respectively. 

Tank geometry is set by considering the flow distribution necessary to achieving homogeneous mixing and 

solids flushing, the head required to gravity flow 

through the drum filter, and the strategies designed to 

manage the fish. A 40 ft (1 2.2 m) diameter tank with 5 

ft (1.52 m) of water (6 ft side wall) is one option for 

providmg 47,000 gal (180 m3). At 1,000 gpm flow the 

water volume in this tank would be exchange 1.3 times 

every hour. Price of this tank would depend upon 

construction material. Fiberglass tanks of these dimensions cost about $1 6,000. 

An oxygen balance on the flow indicates that 120 lb (55 

kg) of dissolved oxygen would be available for fish 

consumption, assuming that the oxygen entering the 

tank was I7 mg/L and that a minimum of 7 mg/L were 

maintained within the tank. Assuming that fish require 

0.25 lb of oxygen for every lb of feed fed (Table 3), 

then there would be enough oxygen available to support 

fish consuming 480 lb (220 kg) of feed per day. The 

example fluidized-bed biofilter can support this feed 

rate. If a daily feeding rate of 1.5% of body rate were 

fed to grow out food-size walleye, then the tank could 

support a maximum biomass of roughly 36,700 lb 

(14,700 kg), equating to a maximum density of 0.78 lb/ 

gal (82 kg/m3) of tank volume. 

NCRAC Culture Series 101 - Walleye Culture Manual 295 
Advanced oxidation with ozone 

Ozone is a powerful oxidizing agent that can be put to 

numerous beneficial uses within aquaculture (Rosenthal 

198 1 ; Hochheimer and Wheaton 1995). Because many 

contaminants in water used for aquaculture are oxidizable, ozone can be used in applications ranging from 

disinfection to general water quality control. Ozone is 

particularly well suited to aquaculture because it is a 

strong disinfectant, capable of a wide range of oxidizing uses, with a rapid reaction rate, few harmful 

reaction by-products, and oxygen is produced as a 

reaction end product. Ozone has been used within 

recirculating aquaculture systems to reduce fish &sease 

(Owsley 1991; G. L. Bullock, Freshwater Institute, 

personal communication); but most often ozone has 

been added to recirculating systems to oxidize nitrite, 

dissolved non-biodegradable organic material, and/or 

organic particulate matter (Otte et al. 1977; Otte and 

Rosenthal 1979; Rosenthal and Otte 1980; Rosenthal 

1981; Williams et al. 1982; Sutterlin et al. 1984; 

Rosenthal and Kruner 1985; Paller and Lewis 1988; 

Poston and Williams 1988; Reid and Arnold 1992). 

Oxidation of organic material can produce 

microflocculation (Maier 1984; Chang and Singer 

1991) and improve solids removal via sedmentation 

and foam fractionation (Sander and Rosenthal 1975; 

Otte and Rosenthal 1979; Williams et al. 1982), 

granular filtration (Wilczak et al. 1992; Rueter and 

Johnson 1995), or microscreen filtration (S.T. Summerfelt, unpublished data). Ammonia, unfortunately, is not 

readly oxidzed by ozone except at pH values > 9 (Rice 

et al. 1981). 

Proper application of ozone requires consideration of 

four unit processes: ozone gas generation, gas to liquid 

absorption, contact time for reaction, and ozone 

residual removal. 

Generation 

Ozone is most commonly generated in large quantities 

by passing relatively pure oxygen gas through a corona 

discharge where a portion of the oxygen (0,) molecules 

are excited to form ozone (0,) molecules (Bablon et al. 

1991). 

0, + energy + 0 + 0 

0 + 0, + 0, (19) 

The corona &scharge is produced when electric current 

is made to jump between two parallel electrode 

surfaces. In situations were only small amounts of 

ozone are required, ozone can also be generated by 

exposing oxygen gas to ultraviolet light at wavelengths 

less than 200 nm (Dohan and Masschelein 1983). 

However, producing ozone with ultraviolet light 

requires 6-30 times more energy than corona discharge 

systems (Bablon et al. 1991). 

Ozone can be produced using a feed gas of air or of 

purified oxygen. In corona-type systems, 2-3 times less 

energy is required to generate ozone using concentrated 

oxygen gas rather than air (Bablon et al. 1991; 

Dimitriou 1990). Using pure oxygen instead of air also 

increases the amount of ozone produced from about 1- 

3% to 24% by weight (Dimitriou 1990). Because 

oxygen is also typically being used to maximize 

carrying capacity in intensive aquaculture systems 

where ozone would be a candidate, purified oxygen is 

the logical choice for generating ozone. 

Ozone generation requires a low moisture feed gas (dew 

point temperature < -65"C), without particulates and 

coalescible oil mists (Dimitriou 1990). Moisture, 

particulates and oil mists can all greatly reduce ozone 

production by fouling the dielectrics within the corona 

discharge cells. Efficient ozone production also requires 

adequate cooling of the feed gas while it passes through 

the corona discharge to prevent elevated temperatures 

that would decompose a large portion of the ozone 

produced (Carlins 1982). Typically, heat is transferred 

to water or air flowing outside the &electric cell as a 

single-pass, straight, shell and tube heat exchanger. 

Ozone generation capacity can also be greatly reduced 

if the dielectrics within the ozone generator's corona 

discharge cells are fouled by hydrocarbon contamination of the oxygen feed gas. Oxygen feed gas used to 

generate ozone should have hydrocarbon concentrations 

less than 5 ppm (Dirnitriou 1990) or they can combust 

within the corona discharge, depositing a layer of 

carbon upon the dielectric surfaces that reduces both 

ozone production efficiency and concentration of ozone 

output. At the Freshwater Institute, hydrocarbon 

contamination was blamed for fouling the &electric cell 

in the ozone generator about every 3-6 months. According to engineers at PCI Corporation (West Caldwell, 

New Jersey), hydrocarbon concentrations > 20 ppm 

within the oxygen feed gas can occasionally occur in 

systems using industrial grade liquid oxygen. This 

results because hydrocarbons with vapor pressures 

similar to oxygen are retained when liquid oxygen is 

distilled. Hospital grade liquid oxygen, though more 

296 NCRAC Culture Series 101- Walleye Culture Manual 
expensive, must meet strict limits on contaminants and 

will not produced fouled &electric panels. Oxygen 

produced with pressure swing absorption systems is 

also low in hydrocarbons, because hydrocarbons are 

largely absorbed by the molecular sieves used to strip 

nitrogen out of air (Dimitriou 1990). 

Rice and Netzer (1 982) recommend that ozone generators be operated at 50-75% of their rated production 

capacity (kg/d). They claim that operating at below 

rated ozone production capacity reduces dielectric wear 

and also provides excess capacity that can be used 

during surges in ozone demand or when another 

generator (plumbed in parallel) is serviced. Ad&tionally, when feed gas rates through the ozone generator 

are reduced, higher concentrations of ozone can be 

generated within the feed gas. Energetically it is less 

efficient to produce these higher ozone concentrations 

(Rice and Netzer 1982). However, when higher concentrations of ozone are generated, the same daily amount 

of ozone can be met with less oxygen requirements. 

Thus, the ozone concentration produced can be adjusted 

so that the aquaculture system’s ozone requirements can 

be met at the same time that the system’s oxygen 

requirements are met. 

Ozone has a half-life in the ambient atmosphere of 

about 12 hours (Rice et al. 1981). Ozone’s relative 

stability in air is such that it cannot be stored and must 

be generated on site. However, ozone can be piped 

considerable &stances with little decomposition (Rice 

et al. 1981). 

A bsorpfion 

The relatively high costs of ozone and oxygen make 

their efficient transfer into water important. Because 

ozone can be co-transferred with oxygen, similar 

transfer units can be used (see earlier section on oxygen 

transfer for more details). Adding ozone to a recirculating system that is already using purified oxygen only 

requires installation of an ozone generator and the 

accompanying ozone distribution, monitoring, and 

control mechanisms. All of the other necessary equipment (oxygen supply and dstribution system, gas 

transfer units, and control mechanisms) are already in 

place. 

When ozone is transferred to water, the overall rate of 

ozone &sappearance from the gas phase depends upon 

the rate it reacts with constituents within the water and 

the type of contacting system used. The type and 

quantity of constituents withn the water sets the rate 

that ozone reacts. Rapid reaction with oxidizable 

inorganics and organics will maintain a low apparent 

equilibrium concentration of ozone within the liquid 

film and increase the rate of ozone transfer. As mass 

transfer and reaction occur in series, either one can 

become rate limiting. 

There are a wide range of devices that can be used for 

transferring ozone within air or purified oxygen into 

water (AWWA Research Foundation 1991 ; Rosenthal 

1981). Units that have a continuous gas-phase (i.e., 

units that &sperse liquid drops and films within a gas) - 

such as spray columns, packed columns, and multistage low head oxygenators (LHOTM units) - provide 

transfer but very little time for reaction. Units that have 

a continuous liquid phase (i.e., units that disperse gas 

bubbles within a liquid) - such as U-tubes, Speece 

cones, aspirators, bubble dffusers, and enclosed 

mechanical surface or subsurface mixers - provide both 

ozone transfer and some reaction time. Because they do 

not provide contact time for reaction, transfer units 

where the gas phase is continuous are generally smaller 

and are sometimes less costly than units where the 

liquid phase is continuous. Adhtionally, systems that 

pass water through air can be designed for much higher 

transfer efficiencies than systems designed to pass air 

through water. Higher transfer efficiencies are achieved 

by the high interfacial area provided by the packed 

systems and the more efficient counter-current plug 

flow contacting in both phases in the continuous gas 

phase systems (Montgomery 1985). When using units 

based upon absorption within a continuous gas phase, 

however, a separate contact chamber may be required 

for reaction (as described in the reaction section next). 

Ozone transfer within continuous gas phase units, 

though not well published, can be quite good. Ozone 

transfer efficiency was 100% in the LHOTM units 

evaluated in the recirculating system at the Freshwater 

Institute (S.T. Summerfelt, unpublished data). In this 

system, complete ozone transfer occurred because: 

ozone is 13 times more soluble than oxygen in water 

according to Henry’s law; gas residence times within 

the LHOTM chambers were approximately 45 minutes; 

and, there were nitrite and dissolved and suspended 

organic material in the water that rapidly reacted away 

the ozone. 

NCRAC Culture Series 101 - Walleye Culture Manual 297 
Contacting and reaction 

Ozone is relatively unstable in water. In a solution of 

pure water, the half-life of ozone is -165 min at 20°C 

(Rice et al. 1981). In real systems containing organic 

carbon (TOC), the half life of ozone may be less than a 

few minutes (Glaze 1990). The levels of TOC in a high 

density recirculating aquaculture systems produced 

typical ozone half-lives that were I 15 s (G. L. Bullock, 

Freshwater Institute, personal communication). 

Although ozone is reactive, it is fairly selective in what 

it reacts with (Pankow and Morgan 1981). Ozone is 

capable of oxidizing a great many organic and inorganic 

substances. When ozone reacts with organic carbon, the 

reaction often takes place at many of the bonds which 

can not be readily oxidized through biological metabolism and makes the partially oxidized organic compounds biologically degradable at a faster rate (Rice et 

al. 1981). Rice et al. (1981) explained that the increased 

biodegradation rate is partially due to formation of 

smaller molecules and partially due to fewer higher 

order covalent bonds. Additionally, ozone oxidation can 

cause dissolved organic molecules to precipitate and 

colloidal organic solids to microflocculate (Maier 

1984). These reactions enhance the removal of organic 

matter from process flow streams. 

The desired end results produced by adding ozone (e.g., 

microflocculation, nitrite and/or color oxidation, 

dsinfection, etc.) can be controlled by either mass 

transfer or lunetic limitations. Microbial reductions, in 

particular, are largely controlled by lunetic limitations. 

This means that for microbial reductions to occur, a 

certain dissolved ozone concentration must be maintained for a given amount of time. For disinfection, the 

required residual ozone concentration is usually 

between 0.1-1 .O mg/L and the hydraulic retention times 

are any where from 0.5 to 20 minutes (AWWA Research Foundation 1991). 

Waters within recirculating aquaculture system contain 

high levels of organic matter and nitrite, and these 

compounds (particularly nitrite) will react with ozone in 

approximately stoichiometric amounts (Rosenthal 198 1 ; 

Sutterlin et al., 1984; Rosenthal and Kruner, 1985; 

Bablon et al. 1991). However, under most conditions 

ozone does not convert organic molecules to carbon 

doxide, but only fragments organic molecules into 

smaller pieces. When ozone is used only to remove 

color, nitrite, and microflocculate organic matter, it is 

not as important to maintain a measurable ozone 

residual as when microbial reductions are desired. To 

improve water quality it is often satisfactory to just add 

sufficient ozone. Microflocculation, however, will 

require time after the ozone has reacted away. 

The hgh ozone demand of water in a recirculating 

aquaculture system makes maintaining an ozone 

residual difficult, particularly when gas-phase ozone 

transfer units are used. Therefore, achieving large 

microbial reductions in recirculating systems requires 

much more ozone than would be needed to dxinfect the 

influent of typical single-pas s aquaculture systems (G. 

L. Bullock, Freshwater Institute, personal communication). Because liquid-phase ozone contact units allow 

ozone gas to be transferred into water for longer periods 

than gas-phase units, liquid-phase units are more often 

used to achieve microbial reduction than gas-phase 

units (AWWA Research Foundation 1991). 

Until recently, criteria on how much ozone should be 

added to recirculating aquaculture systems to produce 

perceptible benefits were not available. Recent research 

at the Freshwater Institute has shown that ozone 

addtion at a rate of 0.025 lbs ozone per lb feed improved water quality and microscreen filtration (S.T. 

Summerfelt, unpublished data), and reduced BGD 

associated mortalities and chemical treatments required 

to control BGD epizootics (G.L. Bullock, Freshwater 

Institute, personal communication), in a recirculating 

system used to culture rainbow trout. Adding ozone at a 

higher rate (0.036-0.039 lb ozone per lb feed) produced 

similar results but was much more likely to produce fish 

mortality when on occasion ozone accumulated to toxic 

levels. Although ozonation reduced BGD mortality, it 

failed in nearly all cases to produce even a 1 log,, 

reduction (i.e., 90% reduction) in numbers of heterotrophic bacteria in the system water or on gill tissue 

(G. L. Bullock, Freshwater Institute, personnel communication). Failure of the ozone to lower numbers of 

heterotrophic bacteria or to prevent the causative BGD 

bacterium from occurring on gills was attributed to the 

short exposure time to ozone residual (35 s contact 

chamber) and rapid loss of oxidation (ozone half-life 

I1 -1 5 s) caused by levels of nitrite and dissolved and 

particulate organic material. Since ozonation equipment 

is expensive, it is rational to add ozone at the lowest 

effective rate necessary to achieve the desired results. 

Adding ozone at the lower rate is also justified to 

reduce potential for fish to be exposed to ozone, 

298 NCRAC Culture Series 101- Walleye Culture Manual 
particularly when little hydraulic retention time is 

available between the fish culture tank and the ozone 

transfer point. 

The economy of using ozone within recirculating 

aquaculture systems is sometimes questioned, because 

it requires large capital and operating costs. However, if 

the oxygen feed gas required for increasing fish loadmg 

rates is used to generate ozone (assuming that 4.5 kwh/ 

Ib ozone are required to produce 6% ozone in the feed 

gas) and if only electric costs are considered, then it 

would cost less to generate ozone in the oxygen feed 

gas (about 0.27 kwMb oxygen used) than it would cost 

to generate the oxygen feed gas (about 0,5 kwh/lb 

oxygen generated). We should also note that although 

ozone improves recirculating aquaculture system 

performance, these systems can function at a somewhat 

lower carrying capacity without ozone. 

Residual removal 

Ozone may be present in the water after the contact 

chamber, depending upon the applied ozone dose, the 

ozone demand of the water, and the contact time. There 

are four methods to eliminate dxsolved ozone: use 

greatly extended contact times; pass the flow through a 

biofilter or bed of activated carbon; strip the ozone into 

air with either a bubble chamber or a packed bed 

aeration column; or destroy the dissolved ozone with 

high intensity ultraviolet light (AWWA Research 

Foundation 199 1). 

No ozone gas should escape. All residual gases should 

be collected and vented to an ozone destruction process 

that will destroy the ozone before releasing the gas to 

Table 7. Toxicity of dissolved ozone to fish. 

the atmosphere. Ozone gas destruction can be catalyzed 

by heat, me&a (such as granular activated carbon or a 

manganese dioxide or other coated media), or a 

combination of both (AWWA Research Foundation 

1991). It is possible that ozone gas can escape from 

ozone transfer units, particularly those that are characterized by a continuous gas phase, because often these 

transfer units use an orifice plate to dntribute the water 

flow across the top of the transfer unit. The layer of 

water covering the distribution plate and the water 

surface at the bottom of the unit are the barriers that 

prevent ozone containing gas from escaping the unit. If 

water flow to ths type of unit is interrupted, however, 

the water barrier above the orifice &stribution plate 

disappears and ozone gas within the unit escapes 

directly into the surroundings. To safely use a unit with 

a continuous gas phase for ozone addition, an automatic 

method must be provided to shut off the ozone when 

flow to the unit ceases. It is possible that a similar 

problem could result within units that pass air through 

water if, after water flow stopped, gas pressure overcame static water head and forced an exit out either the 

water influent of effluent paths. 

Ozone toxicity 

Ozone can be harmful to humans and lvghly toxic to 

aquatic organisms at low concentration levels. Ozone is 

capable of oxidizing many biochemical compounds 

present in living organisms, including amino acids, 

pyrimidme nucleotides, fatty acids, flavins, and proteins 

containing sulfhydryl groups (Camichael et al. 1982). 

The most severe oxidation of living tissue is the 

production of short-lived free radicals that are highly 

reactive and damage membrane-bound enzymes and 

lipids. 

Effect 

Ozone 

concentration 

(mg/L) Species Reference 

96-hr LC50 0.0093 rain bow trout Wedemeyer et al. (1 979) 

lethal 0.01 -0.06 rain bow trout Roselund (1 975) 

60% mortality after 4 wks 0.01 bluegill Paller and Heidenger (1 979) 

lethal 0.2-0.3 fat head minnows Arthur and Mount (1 975) 

24-h LC50 0.38 white perch Richardson et al. (1 983) 

24-h LC50 0.06 bluegill Paller and Heidenger (1 979) 

96-h LC50 0.08 striped bass larvae Hall et al. (1981) 

NCRAC Culture Series 101 - Walleye Culture Manual 299 
Fish 

Ozone destroys epithelium covering the gill lamella 

which results in a rapid drop in serum osmolality 

(Paller and Heidinger 1979: Wedemeyer et al. 1979) 

and, if mortality does not occur immediately, can leave 

the fish highly susceptible to microbial infections 

(Paller and Heidinger 1979). The minimum ozone 

concentration lethal to fish is about 0.01 mg/l, but it 

depends upon species and life stage (Table 7). 

Technology does not exist to continuously monitor the 

concentrations of dissolved ozone that can be lethal to 

fish. Oxidation reduction potential (ORP) has been used 

by many, with varying degrees of success, as an indirect 

means to monitor and control ozone addition. 

Humans 

Exposure to ozone in air is hazardous to humans as 

ozone attacks the respiratory tract, producing inflammation, pulmonary edema accompanied by capillary 

hemorrhage and, with prolonged exposure, ozone has 

been shown to cross over into the bloodstream and 

attack blood cells and serum proteins (Carmichael et al. 

1982). OSHA standard allows for a permissible 

exposure level of less than 0.1 ppm on a time weighted 

average for an 8-hour work period and a maximum 

single exposure level of 0.3 ppm for less than 10 

minutes duration. 

Example design: ozonafiun 

Based on current research, the optimum effects of 

ozonation within recirculating aquaculture systems are 

obtained at doses of about 0.025 lb ozone per lb of feed 

fed. It is recommended that the ozone generation 

capacity be around 1.5-2.0 times the ozone demand. In 

this example, a single modular recirculating aquaculture 

system would require an ozone generator with the 

production capacity of roughly 25 lb (1 1 kg) of ozone 

per day. An industrial grade ozone generator of this 

capacity would cost around $25,000, making ozonation 

the most expensive unit process within the recirculating 

system. 

As discussed above, ozone is most efficiently generated 

within an oxygen feed gas. Because oxygen is already 

required within the recirculating aquaculture system, it 

is most efficient to co-transfer ozone with the oxygen 

feed going into the system’s oxygen transfer unit (i.e., 

LHOTM in thls example). A draw-back of addmg ozone 

within the LHOTM unit is the potential to accumulate 

residual ozone within the fish culture tank (placed just 

downstream of the LHOTM unit). An ozone reaction 

chamber below the LHOTM unit (Le., LHOTM sump 

tank) that provided several minutes of water retention 

time would provide additional time to expend the 

ozone’s oxidative power prior to the fish culture tank. A 

5,000 gal (19 m3) tank would provide 5 rnin retention 

time for 1,000 gpm (3,800 Lpm) flow. 

Dissolved ozone cannot be measured continuously at 

the levels required to protect fish (~0.01 mg/l). An 

inhrect measure of ozone residual is the water’s 

oxidation reduction potential (ORP), which is a 

measure of a water’s potential to oxidize and thus a 

measure of the waters toxicity to fish. The ORP can be 

monitored within the fish culture tank and can be used 

to control ozone addition such that ozone residual is not 

present within the fish culture tank. A safe ORP for 

freshwater is around 300 mv, dependmg upon pH. 

pH control 

Alkalinity and p~ play an important role in aquaculture. The pH is a measure of the hydrogen ion concentration and controls acidhase chemistry. Alkalinity, a 

measure of the acid neutralizing capacity of a solution, 

depends on the concentrations of the bicarbonate, 

carbonate, hydroxide, and hydrogen ions. An alkalinity 

of at least 50 mg/L as calcium carbonate should be 

maintained during nitrification to prevent pH instability 

(Gujer and Boller 1984). Malone et al. (1993) recommends an alkalinity of at least 100 mg/L as calcium 

carbonate. 

The equilibrium of many of the chemical species 

important in aquaculture is controlled by pH. Of great 

importance is the influence of pH on the equilibrium of 

the ammonia and carbonic acid systems. Taking the 

equilibrium of ammonia and ammonium for example: 

NH, + H,O eNHi + OHBecause the ammonia (NH3) associates with water 

(H20) to form hydroxide (OH-) and ammonium 

(NH4+), the resulting equilibrium is a function of pH: 

r 1 

L .I 

and temperature (at 25OC, pK, = 9.245): 

300 NCRAC Culture Series 101- Walleye Culture Manual 
pK, = 0.09018 + 2729.92/T,"K (22) 

such that increasing pH or temperature shifts the 

equilibrium to form more ammonia. The location of 

equilibrium is important because ammonia (unionized) 

is much more toxic to aquatic organisms than ammonium (Meade 1985). 

The pH-equilibrium relation between carbon dioxide 

and the species in the carbonic acid system is equally 

important, though more complex. Dissolved carbon 

dioxide can combine with water in a hydrolysis reaction 

to form carbonic acid: 

C02 + H20 H H2C03 (23) 

However, there is about 633 times as much carbon 

&oxide in water as carbonic acid under equilibrium 

conditions. Depending upon pH, carbonic acid dissociates, releasing hydrogen ions and bicarbonate ions. 

Similarly, bicarbonate ions dissociate, releasing 

additional hydrogen ions and carbonate ions: 

H,C03 @ H+ + HCOS 

HCOT w H+ + COT 

(24) 

When pH is 6.5-9.5, the equilibrium carbon dioxide 

concentration can be approximated using pH and 

alkalinity (Summerfelt 1993): 

1000 I 

3 4 5 6 7 8 9101112 

PH 

0.lI ' I ' I ' I ' I ' I J-I 

Figure 11. Percentage of carbon dioxide and ammonia 

as a function of pH (at 25°C). 

(6.3-pH) Ccoz = Alk * 10 

where CCO 

mg/L (Figure 10). 

is the concentration of carbon dioxide in 2 

Comparing the range of pH values where ammonia and 

carbon dioxide coexist indicates that the smallest 

fractions of both carbon &oxide and ammonia coexist 

at a pH of 7.5-8.2 (Figure 11). Changing the system pH 

only 1 unit changes the corresponding equilibrium 

carbon dioxide or ammonia concentration 

10 fold. 

Chemical treatment can be used to maintain 

a pH that will minimize the potentially toxic 

effects of ammonia and carbon dioxide in 

recirculating aquaculture systems. The 

treatment process consists of adding a 

supplemental source of alkalinity such as 

lime, caustic soda, soda ash, or sodium 

bicarbonate to the water (Bisogni and 

Timmons 1991). Lime, caustic soda, and 

soda ash react with carbon dioxide to 

Alkalinity 

400 

300 mg/L 

200 mg/L 

mE/L 

produce bicarbonate alkalinity. Adding 

sodmm bicarbonate is simply a source of 

~ 50 bicarbonate alkalinity and a means to - increase pH. An alternate way to maintain 

pH with lower chemical additive costs 

would be to increase make-ur, water 

6.0 6.5 7.0 7.5 8 .O 8.5 9.0 

PH I 

Figure 10. Carbon dioxide equilibrium with the carbonic acid system 

as a function of pH and alkalinity (at 25°C). exchange rate if the make-up water conNCRAC Culture Series 101 - Walleye Culture Manual 301 
tained medium to high levels of alkalinity. Make-up 

water may require heating and the costs saved in 

chemical additives may not be justified due to increased 

heating costs. 

Lime may be purchased as calcium oxide (CaO), which 

is also known as “burned lime,” “quick lime,” or 

“unslaked lime.” When calcium oxide is added to water 

it hydrates, forming calcium hydroxide (Ca(OH)2) 

which is also known as “slaked lime.” Although 

calcium hydroxide is very soluble in water, the dosages 

that are normally used produce a slurry that is called 

“milk of lime” because of its chalky appearance. When 

calcium hydroxide dissolves in water, it produces 

hydroxide ions that react with carbon dioxide forming 

bicarbonate ions: 

OH- + CO, w HC0.I L J 

Caustic soda. Sodium hydroxide (NaOH), is more 

expensive than lime. It is very soluble in water. When 

sodium hydroxide dissolves in water, it dissociates and 

releases hydroxide ions into solution that react with 

carbon dioxide to form bicarbonate ions. 

Soda ash. Sodium carbonate (N%C03), is also a more 

expensive source of alkalinity than lime. It, too, is very 

soluble in water. When soda ash dssolves in water it 

dissociates, releasing carbonate ions into solution that 

react with carbon dioxide to form bicarbonate ions: 

COT +C02w 2HC03 

Sodium bicarbonate (NaHC03) is a popular chemical 

additive for maintaining alkalinity in a recirculating 

aquaculture system (Bisogni and Timmons 1991 j. It is 

very soluble and hssociates in water to release bicarbonate ions. Bicarbonate ions do not react with carbon 

dioxide, but they can shift the equilibrium of the 

carbonic acid system through a shift in pH. 

The rate of interconversion of various carbonate forms 

affects the buildup of carbon &oxide in water. The rate 

of ionization of carbonic acid into bicarbonate and 

carbonate ions is nearly instantaneous, but the rate of 

hydration of carbon dioxide into carbonic acid is 

relatively slow (Kern 1960). 

CO, + H20 e H2C0, (29) 

Accumulation of carbon dioxide occurs when the rate 

of production of carbon dioxide by the fish exceeds the 

rate at which carbon dioxide is lost to water replacement and to the atmosphere. Carbon dioxide will then 

build up the carbonic acid concentration. As it does, the 

concentration of bicarbonate will increase and the 

concentration of carbonate will go down. These 

changes in the relative amounts of carbonic acid and its 

ion products will lower pH. If carbon dioxide is 

removed from solution, carbonic acid will slowly 

dehydrate to reestablish the equilibrium. As this 

happens, bicarbonate will dnproportionately release 

carbonic acid and carbonate ions into solution. 

Conclusions 

The technology described here consists of some of the 

most cost and technologically effective unit processes 

available to recirculating systems. The descriptions 

provided here can be used by an engineer to design a 

recirculating aquaculture system capable of treating 

from as little as 100 gpm to as much as several thousand gpm (or more). The system scale is primarily 

limited to the amount of risk one is willing to place in a 

single recirculating production module and the ability 

to manage the fish and the hydraulics in the fish culture 

tank. 

Although recycle aquaculture systems have many 

advantages, the systems have large capital investments 

and high operating expenses relative to other production 

technologies. To date, few U.S. fish producers have 

found it economically viable to culture fish where large 

amounts of energy must be expended for pumping or 

heating water. As a result, many recirculating systems 

used to culture fish commercially have not been 

technically or economically effective and have failed. 

Recirculating systems that have attained profitability 

rely on technologies that minimize capital and operating costs for water treatment. Marketing and production 

management, however, may play an even larger role in 

the economic success of the recirculating system 

(Hanluns et al. 1995). 

The economics of fish production are the controlling 

criteria for determining whether-or-not to construct the 

facility. However, the economics of walleye production 

within a recirculating system were not presented in this 

chapter. Wade et al. (1996) reported the economics of 

302 NCRAC Culture Series 101- Walleye Culture Manual 
fish production at several scales are currently being 

evaluated within a system as described here. 

Example design: system integration and layout 

General guidelines for integrating and laying out a 

recirculating aquaculture system follow: 

Solids should be removed from the recirculating 

flow immedately after they leave the culture 

tank, before any pumping occurs and before 

biofiltration. 

Solids should not be stored in the recirculating 

system or they will begin to degrade and leach 

nutrients. 

Pumping should occur only once each pass. 

Carbon dioxide should be air stripped after reaching 

its highest concentration before oxygen supersaturations are produced, and before returning to 

the fish culture tank (i.e., just after the biofilter 

and just before the oxygenation unit). 

Alkalinity is best added for pH control after carbon 

external 

s tand-pipe 

dioxide has been stripped and just before the fish 

culture tank (i.e., within the sump tank of the air 

stripper). 

transfer unit, because the aeration it produces 

brings the oxygen levels in the flow up to near 

saturation. Bringing an oxygen saturated flow to 

the oxygenation unit allows all of the oxygen 

added to go towards supersaturating the water. 

Oxygen should be added to supersaturate the flow 

just before the fish culture tank. The water should 

be kept from significant contact with the atmosphere after the oxygen supersaturation has been 

obtained. 

used to supersaturate the flow. 

Air stripping should occur before the oxygen 

Ozone should be generated in the oxygen feed gas 

The recirculating system designer must also consider 

the hydraulics that are encountered when transporting 

water from unit to unit. In this example, water is 

bac 

r 

:k to culture tank + 3 

17 ft 17 ft 

16.5 

Figure 12. Layout of example recirculating aquaculture system. 

NCRAC Culture Series 101 - Walleye Culture Manual 303 
pumped only once. Gravity must be used to move water 

in all other places. Elevations suggested to integrate and 

create gravity flow through the example recirculating 

system are presented in Figure 12. 

Acknowledgments 

I would like to thank Barnaby Watten (National 

Biological Service, Wellsboro, Pennsylvania), Charles 

Oulman (Iowa State University, Ames, Iowa), Joseph 

Hankins (Freshwater Institute, Shepherdstown, WV), 

Edward Wade (Freshwater Institute, Shepherdstown, 

WV), and Glenn Snapp (Water Management Technologies, Mechanicsburg, Pennsylvania) for technical 

assistance. Parts of this material were based upon 

research conducted at the Freshwater Institute using 

support provided by the United States Department of 

Agriculture, Agricultural Research Service. 

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