Design of packed columns for degassing

Aquaculmral Engineering 3 (1984) ..31-_ ~- -,-r~ ~

Design of Packed Columns for Degassing

John Colt Teufel and Associates, 1303 Lake Boulevard. Davis. California 95616, USA

and

Gerald Bouck Division of Fish and Wildlife (PJS), Bonneville Power Administration. PO Box 3621, Portland. Oregon 97208. USA

A BSTRA CT The packed cohtmn can be used to degas large JTows o f water. For a given htfluent Ap, temperature, and dissolved oxygen, the e/fluent Ap will depend on the media size arm the height o f the column. Cohtmn heights #t the range 1-3 m are required to meet criteria o f A p = 20 mm Hg and dissolved oxygen concentration (DOC) >I 90% o f saturation. Selection o f media size will depend on consideration o f column height, loading capacity, attd media costs. Gas composition within the column has a secondarv effect on the effluent Ap. Generally, the mole fraction o f oxygen bzside the eolttmn cannot be reduced more than 10% and still satisfy the dissoh, ed oxygen criteria. For waters high in dissolved oxygen, the use o f a vacuum column can reduce the required column height.

NOMENCLATURE

BP C

Ci. Co~t C*

F

Barometric pressure (ram Hg = Torr) Concentration of a gas in water (mg liter -1) lnfluent concentration of a gas to the column (nag liter -1) Effluent concentration o f a gas to the column (rag liter -~) Saturation concentration of a gas in liquid (mg liter -~) Infinite series in terms of nKd + K Z

251 Aquacultural Engineering 0144-8609/84/$03.00 © Elsevier Applied Science Publishers Ltd, England, 1984. Printed in Great Britain

252 G K

PH ,.0 AP &Pi

z2kPin ~ou t

APtn (i) ~keou t(/) T Vac X X' Z (I)

J. Colt, G. Bouck Overall system constant for packed column and distribution system (dimensionless) Aeration coefficient of media (m -~) Aeration coefficient of screen or distribution plate (dimensionless) The number of screens or distribution plates Vapor pressure of water (mm Hg) Difference between total gas pressure and the barometric pressure measured by the membrane diffusion method (ram Hg) Differential partial pressure between the liquid and gas phase (mm Hg) for the ith gas lnfluent ~ to the column (mm Hg) Effluent zSd~ from the column (mm Hg) Influent APi for the ith gas (mm Hg) Effluent z5.¢',- for the ith gas (ram Hg) Water temperature (C) Applied vacuum to column (ram Hg) Mole traction of a gas in atmosphere Mole fraction of a gas inside the column Height of column (m) Bunsen coefficient (liters liter - l a t m -~) (KLa)gaJ(KLa)oxygen; where KL is the mass transfer coefficient

INTRODUCTION Supersaturated dissolved gases (both individually and collectively) are common in water from wells, springs, streams and lakes and may vary considerably between seasons and operations. Chronic exposure of hatchery fish (especially eggs and larvae) to supersaturated gases may result in developmental problems (Peterson, 1971), increased incidence of infectious diseases, and mortality (summarized by Weitkamp and Katz, 1980). One remedy is a 'packed column' which is a fihn-flow aerator that can be used simultaneously both for oxygen transfer and for removal of gas supersaturation. Packed columns are inexpensive and reliable, and are used widely at Pacific salmon hatcheries (Owsley, 1981), but general design procedures are not readily available. Therefore, we developed a multicomponent transfer model in terms of AP based on a single-component gas transfer model described by Hackney

Design of packed columns for degassing

253

and Colt (1982); we now present experimental verification of this model and design characteristics for its use in hatcheries. Special emphasis is placed on the reduction of column height and the use of vacuum applications.

GAS TRANSFER IN THE PACKED COLUMN Transfer of a single gas, such as oxygen, argon, nitrogen or carbon dioxide, in the packed column (Hackney and Colt. 1982) can be written as: 1 C* t = do[nKa + K Z I In C-~--CoutJ

(1)

This equation is based on a two-fihn mass-transfer model. Gas transfer within the packed column takes place due to both the distribution system and the media. The performance of the packed cohmm depends strongly on even distribution of the water over the media. Expanded metal screens, perforated or slotted aluminum screens, and drilled orifice plates have commonly been used as distribution systems in degassing systems (McLean and Boreham, 1980; Hackney and Colt. 1982). At 20°C, values of Kd range from 0.08 to 0.68 (McLean and Boreham, 1980; Hackney and Colt, 1982). For the orifice distribution system, K a increases slightly as the surface loading rate is increased (Hackney and Colt, 1982). Ka values at other temperatures can be computed from : Ka(r) = Ka(2o°c) 1-024 (7-- 20~

(2)

The value of K depends on loading rate and the media size tHackney and Colt, 1982). For pall rings, typical values of K at 20°C, range from 1-0 to 2-5 m -I and can be adjusted to other temperatures by eqn (2). For a particular medium, the value of K is constant until the column starts to tlood. Flooding in a packed c o l u m n is defined as an inversion of phases so that the liquid phase becomes continuous within the void space between the media (Norton Co., 1977). Flooding is caused by excessive hydraulic loading and results in a significant change in the mass transfer characteristics of the column. Flooding produces three conditions that oppose degassing: (1) air flow through the column is

254

J. Colt, G. Bouck

reduced; (2) the pressure within the column is increased; and ( 3 ) t h e air-water surface area is reduced. The elevated pressure inside the column increases the effective C* value above the atmospheric value. Therefore, the water leaving a flooded column will be supersaturated regardless of the column design. Maximum loading rates for packed columns are presented by Hackney and Colt (1982). The transfer of different gases in a given system depends on the diffusivity of the individual gases. The ratio of the mass transfer coefficient of the ith gas divided by the mass transfer coefficient of oxygen is represented by q5 and can be used to adjust the transfer rates of the different gases. For packed columns, a value of 0-85 and 1.00 should be used for qSN~+Ar and ~co~, respectively (Speece and Humenick, 1973; McLean and Boreham, 1980). The Co,,t concentration for an individual gas can be computed from eqn (1). However, the risk to aquatic animals depends primarily on the zXP (Bouck, 1980; D'Aoust et al., 1980). zXP is the differential hyperbaric gas pressure and is equal to the difference between the total dissolved gas pressure and the local barometric pressure. The ,..YPcan be measured directly by instruments described by D'Aoust et al. (1980) and Bouck (1982) and is reported typically in millimeters of mercury (ram Hg). Given this relationship, degassing columns should be designed based on z2ff', rather than on the concentration of a single gas. The criterion for z2xP depends on the tolerance of the species, its development stage and the depth of the culture system. Chronic exposure to values of 30 mm Hg can increase the mortality of tish (Bouck, 1976). In shallow rearing systems, clinical signs of gas bubble disease can be produced in larval striped bassMoronesa.vatilis at ZXP= 22 mm Hg (Cornacchia and Colt, 1984). Growing evidence indicates that extremely sensitive organisms should not be exposed in hatcheries to z3a° >~ 0, i.e. Atlantic salmon S a l m o salar, brown trout S a l m o trutta. Another requirement is that the dissolved oxygen in the effluent of a packed column must be or exceed 90% of air saturation. Higher dissolved oxygen allows support of a greater mass of fish, given the same flow of water. As the dissolved oxygen approaches the saturation level, the transfer efficiency of aerators decreases (Colt and Tchobanoglous, 1981). A criterion of 90% of oxygen saturation is based on trade-offs between carrying capacity and costs. Therefore, the design of packed columns must consider both the z2~Pand the dissolved oxygen criteria.

Design of packed columns for degassing

255

The difference between the pressure of a gas in a liquid and the atmosphere for the ith gas is referred to as z~°,.. For the major gases (Colt, 1983) the values of--APi are equal to:

6/'0:- Co: ;30,. (0.5318) APc°:

--Xo: (BP --PH:o)

Cco.

/3co. (0.3845)--Xco:(BP--PH~o)

Co ~o:

(3)

(4)

Cco~ (0-3845) &o:

Z2U°N,.Ar = BP +~2x_P-- v: (0.5318) . . . . . . -- XN: ÷Ar (BP

--PH20)

(5)

Addition of eqns (3)-(5) shows that: --~ '~LPo:"{- ~kPN~. +At +

~19C0~

(6)

Substitution of eqns (3)-(5) into eqn (1), results in the following equations: Oxygen:

In [ z3aPin(°'-------'--~)] = 1.00(nKa + KZ) ~&Pout(O,)a

Nitrogen + argon: In [ zS,PintN~÷~)] = 0.85(nKa + K Z ) [£~kPou t(N: +Ar) -I Carbon dioxide:

[ z~kPin(CO: ) ] = In [ ~ ) j 1.00(nKa + KZ)

(7) (8) (9)

or: Z~Dout(O:) = g~Din(O2)

e-(nKd+KZ)

z~,Dout(N, ÷Ar) = z~kPin(N:+Ar )

e -°'85(nKd+Kz)

Z]kPout(CO~) = Z~Din(CO:) e-(nKd +KZ)

(10) (11) (1 2)

Since:

~Pout = £xPout(O:) + &Pout(N:+~) + --/~out(coo ~Pout = Z~in(O,) C-(nKd+KZ) + ~in(N~+Ar) e-O'85(nKd+KZ) q- z~kPin(CO: ) e-(nKd +KZ)

(13)

256

J. Colt, G. Bouck

Equation (13) can be used to predict the performance of a packed column used for degassing. In most surface waters, the partial pressure of carbon dioxide is small and typically in analysis its pressure is included with nitrogen + argon. Equations (5) and (6) can be written as:

~ N : +At+CO, = BP -- ziP-- Co; 13o= (0-5318) -- PH=o - -

XN: +~r+ CO: (BP

- -

PH:O)

AP = APo~ + APN, + ~ + c o .

(14) (15)

Since the contribution of carbon dioxide to APN~+Ar+CO:is small, the error due to assuming that CI,co: = CbN,+Ar is also small, and APout is equal to:

Z~/Oout = Z~in(O=) e -("Kd+KZ) + ~kPin(N=+Ar+CO2) e -O'85(nKd+Kz) (16) If APincco=) is above 5 mm Hg, eqn (13) should be used. Some spring or well waters may contain excessive amounts of carbon dioxide (Mrsic, 1933). Removal of carbon dioxide may be slower than predicted by eqn (13) because detention time in the packed column can be less than the time required for all the carbonate species to approach equilibrium. Equation (16) can be rewritten as:

z~Pout = ~ i n e-O'85(nKd+KZ) "q- z~Pin(O,.)F

(17)

If the second tenn is neglected, then: APout = APi, e -~Ss("'rd+Kz)

(18)

Equation (18) should not be used for design, but the essential characteristics of the packed column can be shown by examination of this equation. After passing through the distribution system, AP will decrease exponentially as the water passes through the media. For a given system at constant temperature and flow,

~out - APin

- constant

As APi. changes, APou t will change in a linear manner.

(19)

Design oF packed colum ns ¢br degassing

25 7

The design of a packed column for degassing will require measurement of ~_YP, temperature, barometric pressure, dissolved oxygen, water temperature and salinity. The values of ~ for the c o m p o n e n t gas are then computed from eqns ( 3 ) - ( 5 ) or eqns 13) and (14). Then the --~o.t value can be c o m p u t e d from either eqn (13} or eqn (16). For a given distribution system, media and ~-YPin values, the height of the column is the major engineering parameter. The height required to satisfy a given /-YPout criterion will in general require a trial and error solution. Limited information is available on tile values of Kd and K. The overall system constant G can be defined as: G = nKa + K Z

(20)

For oxygen, the value of G can be computed by substitution of eqn (20) into eqn I 1):

[c':_-<,,]

G =In kC*--Cout ]

t2 )

Therefore, even if the values of Kd and K are unknown, the value of G for a given system can be computed from measurement of Cin and Co~t. Substitution of G into eqns (13) or (16) can be used to estimate the performance o f the system under different zXPi. values. Once a distribution system and media have been selected, the performance of a degassing column will depend primarily on the column height (eqn (118)), Low AP values require high column heights and reduction of &Pout to zero theoretically requires an infinitely high column. A vacuunl system has been suggested to reduce the column height (Fuss, 1983), and a vacuum-siphon system has been suggested to reduce pumping costs (Monk et al., 1980). Vacuum systems reduce the effective barometric pressure, hence increase the 2t/-' and therefore increase the gas transfer rate. The design of vacuum packed columns can be approached from the following. The '.XPi, values are computed from eqns ( 3 ) - ( 5 ) or eqns (3) and (.14) using a value of BP equal to B P - - V a c . The ~-XPo~t is then computed from either eqns (13) or (16). This value is the ~ of the water at the bottom of the column. The final &Po~t after the water leaves the column is equal to /-kPout- Vac. However, some precautions are warranted in the use of vacuunl degassing as described by Marking et al. (1983) (see this paper, 'Design of

Vacuulm Columns').

258

J. Colt, G. Bouck MODEL V E R I F I C A T I O N

Model verification will be based on the work reported by Bouck et al. (1984). The performance of a number of different media was evaluated as a function of influent &P and column height. Since these workers did not use a distribution plate and n = 0, eqn (7) should be rewritten as

hi r ~kPin(°;'] = K Z LZXPout(o:)l

{~) - "

or

log

[ £~r~in(O ~) ] LAPout(o:) ]

-

K ..'OUO "~ -, ,-,.,

(23)

letting K' = K/2.303 and rearranging eqn (23-) results in: log [LkPout(o:)] = log [APin(O:)] -- K ' Z

(24)

This equation has the form: l o g ( y ) = b + mZ

(25)

where b = Iog[LkPin(o:)l and m = --K'. A similar expression can be developed for N2 + Ar + CO:. Using the data presented in table 3 of Bouck et al. (1984), log [ zXPout] was plotted against Z in Fig. l. Except for low values of_.A.Po: and ~tPx:+Ar÷co2, where measurement errors become more serious, the perfomlance of the column can be described by eqn (25). The overall removal of 2xP can be described as a negative exponential decrease with column height (eqn (18)). The zSPout depends in a linear manner on zXPi, t Fig. 2) as is predicted by eqn (19). Using independently collected data, the essential features of the proposed mass transfer model are confirmed. This model will now be used to investigate the effects of various parameters on the performance o f the packed column for degassing applications.

DESIGN OF PACKED COLUMNS F O R DEGASSING Tile performance of packed columns was computed from eqn (16) as a function of temperature, dissolved oxygen and _4.P. The system

Design of packed columns for degassing

259

1000

I00

A

1(

0 ~'

1 0.00

•

- - APN..... CO,

•

--

~,

~Po,

I 0.30

I O.GO

I 0.90

J~ 1."~0

Column Height (m)

Fig. 1.

Performance of the packed column as a function of column height (cf. Table 3 of Bouck etal., 1984).

J. Colt, G. Bouck

260 .XP ~, Sat.'

133

( m r . Pig)

BC o/

A I

!SO

/

I

I I I 126

I

I00

I I I

n-

I

I I

'/ /g /

tLI 1 2 0

O .a

/

/

!

113

00-

106

50 -

100

0

z

i 0

I

i

l

] 25

t

r

I

I

I

I

I

50

t

,

]

I

I

I

75 .1P ( m m H O )

I

[

I

I

0

103

107

110

% Salutation I

at 760 m m H g

O U T - F L O W WATER

Fig. 2.

&Pout

versus ~Pin

for the packed column.

modeled had a single distribution plate (Kd = 0.40 at 20°C). Tile K values tested were 1.05, 1.58, t.71 and 2 . 5 0 m -1, and correspond to 8.89, 5.03, 3.81 and 2.54 cm diameter plastic pall rings (Norton Co., Akron, Ohio). The saturation concentrations for the four gases were computed from Weiss (1970, 1974). The barometric pressure was assumed equal to 760.0 mm Hg. Except when stated to the contrary, the criteria for AP and dissolved oxygen were 20 mm Hg and 0 . 9 0 C * , respectively. The column height includes an additional total of 0.20 m for the distribution system and drain system. If both criteria were met after passing through the distribution plate (no column required), the height o f the system was assumed to be 0.15 m. Gas-to-liquid ratio (G/L) is the volume o f gas that passes through the column per unit volume of liquid and is expressed as a dimensionless

261

Design o/ packed columns ]'br degassing

ratio. The G/L ratio was based on the criterion that the mole fraction of oxygen in the gas leaving the column was equal to 99~ of the normal mole fraction in the atmosphere. Gas flows were computed in terms of standard conditions (20°C, I atm, relative humidity = o6:,c) ' ~ (Yunt. 1979). Effect of K on column height

The required column height depends on the media selected and ._AP (Fig. 3). At 15°C and a dissolved oxygen concentration (DOC) = 5.0 mg liter -~ ira the influent, the column height ranges from 0-9 to 3.4 m. At a given influent DOC, media size and temperature, the column height required to satisfy the dissolved oxygen criterion is a constant and does not depend on z2LP. The column height required to meet the ,.,kP criterion is zero for z2~Pi, = 0, and increases as --APi, is increased. Since the column is required to meet both criteria simultaneously, the required column heights are c o m p u t e d for both criteria and the larger of the two values is used for design. At low ,.X_, ° values, the column height is controlled by the dissoh'ed oxygen criterion. Above

K.IO5

J

J

J

J

10

E

Ko:~,

a=

10

I

/

I

I

'

..~P. (ram Hg)

Fig. 3.

Effect of K on c o l u m n height ( 1 5°C, influent DOC = 5.0 m# li~er -z ),

262

J. Colt, G. Bouck '°i

!

f I J

g

lo.

i 1.G

/

i~

I

I

I

I

aP. (mm Hg)

Fig. 4.

Effect of temperature on column height (K = 1.71 m-1 at _0 C, influent DOC = 5.0 mg liter -z ),

a ~kPin of approximately 60 mm Hg, the column height is controlled by the zXP criterion. Effect of temperature on column height For a given &P, tile required column height decreases at higher temperatures (Fig. 4), This is a direct result of the effect of temperature on Ka and K. An increase in temperature from 5 to 30°C will increase the K value threefold. If the dissolved oxygen criterion was based on an absolute concentration in nag liter -1, a different response would be produced. The increase in the K values will be counteracted by the decrease in the C::" value (eqn (1)). Effect of dissolved oxygen on column height At a given APi, value, increasing DOC in the influent, reduces tile column height slightly {Fig. 5). This results because the mass transfer rate of oxygen is larger than N2 + Ar + CO.,. Increasing DOC at a constant _4J' increases the Afio: value. At ~ = 0, the column height

Des(gn of packed columns for degassing

263

: . P . • lO0

I0

~p. - o

o o~o

*.

. . . . .2

~

,I~,

s,

,

i

~

I

g

~ ~o

I 11 ..) o

Fig. 5.

Effect of dissolved oxygen on column height ( K = 1-71m -1 at .,0 C, T = 15°C).

/ Ir i'

i$o

,co 2O0

o

!

[

!

loo

~sa

, i 200

,

2SO

~ P . {ram Hg!

Fig. 6.

Effect o f dissolved oxygen and AB on column height (K = 1.71 m "1 at 20°C. T = 15°C).

264

3". Colt. G. Bouck

is controlled solely by the oxygen criterion. For influent DOC above 0.90C* or 9-07 mg liter -1 at 15°C, a column is not needed (Fig. 5). Effect of dissolved oxygen and AP on column height At 15°C and the 3.81 cm media, the required column height ranges from 0 to 2.2 m (Fig. 6). At low AP values, the required column height depends strongly on the influent DOC. At higher AP values, dissolved oxygen has a secondary effect on column height.

Effect of AP and dissolved oxygen criteria on column height As the A/' criterion approaches 0, the column height increases in an exponential manner (Fig. 7). Reduction of the AP criterion from 50 to 5 mm Hg increases the required height by a factor of 2-3. At low values of zXPi,, the column height is controlled by the dissolved oxygen criterion and is therefore independent of the APcriterion. At 15°C, DOC = 5-0 mg liter -~ and K = 1-71 m -1 at 20°C, a cokmm

3.O

211

OO o;

Fig. 7.

I

I

i

I

I

J

so

1~

Iso .',P. ( r a m H g }

~oo

2so

3Qo

o

Effect of AP criterion on column height (K = 1.71 m -l at 20 C. T= 15°C, influent DOC = 5-Omgliter-').

Design of'packed columns/or degassing

265

height of 1.03 m is required to satisfy the dissolved oxygen criterion. If the dissolved oxygen criterion was reduced to 85% (rather than 90%) the column height needed to meet the dissolved oxygen criterion would be reduced to 0-76 m. The required column height would only be decreased in the zone bounded by the column heights equal to 1.03 and 0.76 m. Effect of the mole fraction of oxygen on column height Tile composition of gas within the column has little effect on /_kPout but has a critical effect on effluent dissolved oxygen. The C* concentration for oxygen is proportional to the partial pressure of oxygen within the column: C* c~ Xo:(BP--PH:o)

(28)

As the value of Xo. decreases, the required column height significantly increases, especially in the dissolved oxygen limited area (Fig. 8). At an

x,a~o ig~

g

2O

.1=

i

so

I voo

! , 1so

~

t ~so

! Joo

.~P~ (ram Hg)

Fig. 8.

Effect of the mole fraction of mole oxygen on column height (T= 15°C, K = 1.71 m-1 at 20~C).

266

J. Colt, G. Bouck

X~), of 0- 195 O0 or less, the column height is controlled totally by the dissolved oxygen criterion• For influent dissolved oxygen levels less than the criterion, the partial pressure of oxygen in the column must be greater than or equal to 90% of the atmospheric value: X'o.(BP--PH:o) >f 0"90Xo:(BP--PH:o)

(29)

In columns at atmospheric pressure, then X~: > 0.1885

130)

If X ' < 0-1885, then the C* value within the column is less than the criteria, and the dissolved oxygen criterion cannot be satisfied under any conditions. For influent dissolved oxygen greater than the criterion, the critical X~: will depend on the influent DOC and APi~. The maintenance of adequate air flow through the column will generally require installation of low pressure blowers (Hackney and Colt, 1982). Self-ventilating

"°r J f , r

f o

o10C"

J o

•

,o

w o i~

ao

"3

o 2.a

~P (ram HO)

F i g . 9.

Effect of temperature on tile nlaxinmm G/L ratio (intluent DOC = 0 mg l i t e r -x , K = 1.71 m -~ at 2 0 ° C ) .

Design of packed co lure ns for degassing

267

columns can be designed using l a g e media and low surface loading rates. Detailed information on the design o f this type of column is not available at this time. The adequacy of column air flow can be checked by direct measurement of X~: with a dissolved oxygen probe. The cost of maintaining adequate air flow through the column is low in comparison to adding additional column height to compensate for a reduced value of X'o. Effect of temperature and dissolved oxygen on the G/L ratio The air flow required to maintain X~): = 0.99Xo, is developed in terms of the G/L ratio. This ratio depends on the dissolved oxygen level, &P, temperature and criteria for dissolved oxygen and &P. The maximum G/L ratio (DOC = 0.0 mg liter -1 ) ranges from 3 to 6 over the normal temperature and &Pi, values (Fig. 9). The G/L ratio is larger at low temperature because a given L2kPin represents more gas on a mass basis.

~4a

~*°

f J

°I "o ._~

~20

oo.5oJ

k !

oo.r$ -

",P(ram Hg)

Fig. 10.

Effect of dissolved oxygen on the GIL ratio at 15°C (K = 1-71m -1 at 20°C).

268

J. Colt, G. Bouck

Therefore, a larger air flow is needed to achieve the effluent mole fraction criterion. At 15aC. the G/L ratio for typical dissolved oxygen and A_Pin values ranges from 2 to 3 (Fig. I0). For typical design, G/L = 5 should be adequate, These gas flow rates are computed in terms of standard conditions (1 atm, 20"C, 36,,G relative humidity): actual flow rate will depend on local temperature, pressure and relative humidity (Yunt, 1979). Design of vacuum columns Increasing the vacuum applied to the colunm reduces the required column height for the AP criterion, but increases the column height needed for the dissolved oxygen criterion. The required height for both criteria decreases at low vacuums, then starts to increase at intermediate vacuums (Fig. 1 I). At higher vacuums, the required column height is larger than for the atmospheric columns. The dissolved oxygen criterion is the limiting parameter under most conditions.

/ 3.C

2.0

g

! to

o,o

I

lo

I

2o

I

3a

[

~

I

so

I

so

I

7o

ao

Vacuum {ram Hg)

Fig. 11.

Effect of vacuum and dissolved oxygen on column height IT= 15°C, APin = 150 mm Hg, K = 1.71 m-~ at 20°C).

Design of packed columns ]or degassing

269

Uio

oo

o

,io

--

to

~

i,'l

s~o

~o

,~o

a¢l

Vacuum (ram Hgl

Fig. 12. EtTect of vacuum and mole fraction on column height (T= 15°C, ~'Pin = 150 mm Hg. dissolved oxygen = 5.0 mg liter -1, K = 1.71 m-1 at 20°C).

For inlluent dissolved oxygen levels less than the criterion, the partial pressure of oxygen inside the column must be equal to or exceed 90% of the atmospheric value: X~):(BP--Vac--PH:o) ~> 0.90Xo~(BP--PH:o)

(31)

If the mole fraction of oxygen in tile column is equal to the atmospheric value (X~). = Xo,), then the maximum vacuum inust be ~<76.0 mm Hg (Fig. 1 I). If the mole fraction o f oxygen within the column is less than the atmospheric value, the maximum vacuum that can be used is reduced (Fig, 12). The maximum allowable vactmm can be computed from eqn (31). Vacuum degassing systems must be designed to vent air into the column. The required G/L ratios for the vacuum column are identical to those for the atmospheric column (Figs 9 and 10). For inlluent DOCs above the criterion, vacuums above 76.0 mm Hg can be used, but system failure can result from small increases in column height or reduction in influent dissolved oxygen levels. For an influent D O C = l O m g l i t e r -1 and a V a c = 8 0 m m Hg (Fig. 11), the

270

J. Colt. G. B o u c k

required column heiglat is 0.60 m. If a 0.80 m column is used, DOC will be reduced below the criterion of 9.07 mgliter -~ and the system fails. A single design value of the G/L ratio = 5.0 is recommended for the atmospheric column because the cost of the tow pressure fans is low. For waters containing high dissolved oxygen levels, this design value of the G/L ratio may be overly expensive to maintain in vacuum systems. Continuous monitoring of the partial pressure of oxygen within the column can allow control of the air flow rate to maintain the required value of Xo=. The use of vacuum columns is seriously limited by the necessity of meeting both the dissolved oxygen and zXP criteria (Marking e t al., 1983). The use of a vacuum column for waters low in dissolved oxygen is not recommended without pretreatment by a packed column. Air entrainment, heating and photosynthesis can produce oxygen supersaturation as welt as total dissolved gas supersaturation (Weitkamp and Katz, 1980). Under these conditions, the use of a vacuum column can result in a significant reduction in colunm height and pumping costs. s

GENERAL DESIGN CONSIDERATIONS Optimum design of a degassing system will depend on a knowledge of the seasonal variation in zkPi,(S:+Ar+CO=), Z~kein(O2) and temperature. Gas supersaturation may typically peak in the spring and summer (Lindroth, 1957; Harvey, 1967; Bouck, 1976). In rivers, dissolved gas concentrations may depend strongly on the water flow, hydroelectric or other operation conditions (Harvey and Cooper, 1962; D'Aoust and Clark, 1980) and therefore may vary significantly on a daily basis. Under these conditions, continuous dissolved gas monitoring (D'Aoust and Clark, 1980; Bouck, 1982) may be required. The chronic effects of gas supersaturation may require prolonged exposure before mortality results. This delay in response coupled with the seasonal variation in zkP require that dissolved gas data be collected in a systematic manner. Degassing columns should be designed with at least 1 year's data (collected once or twice a week). Since column height depends on both zXP and temperature, the maximum column height may not correspond to the maximum ZM°. Therefore, the design of the column should be based on the maximum required column height plus a safety factor. Collection of adequate data for design is expensive, but can prevent overdesign or system failure. Continuous monitoring of zkP

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(D'Aoust and Clark. 1980: Bouck, 1982) and dissolved oxygen may allow automatic control of the degassing system and reduce power costs. The use of vacuttm systems requires special design considerations. Conservative design of column height (excessive column height) in the atmospheric columns only increases the pumping costs. In tile vacuum column, this procedure can result in system failure if the excess height reduces the dissolved oxygen below the criterion. Poor distribution of the liquid over the media and flow down the sidewall degrade tile performance of the packed column. These problems may be critical to the production of ,'_kP values less than 2 0 r a m Hg. For these applications, the minimum number of points at which tile water should be applied to the surface is unknown. The minimum number in the chemical industr3 is 45 points m -2, while Hackney and Colt (1982) used 800 points m-:. The effects o f wall flow can be reduced by the use of large cotunms and redistribution plates. The work reported by Hackney and Colt /1982) is based on a I m high column. The placement of a redistribution plate every I m may be advisable. While several types of manufactured redistribution plates are available (Norton Co., 1978), a simple annulus formed out of sheet plastic may be adequate. The width of the annulus should be ~-~ of the column diameter. A safe &P is not a well-defined criterion for chronic exposure of most fish, crustaceans and molluscs. Typical work on gas bubble disease has been conducted on Pacific salmonid fishes, usually exposed to acutely lethal ~ values (Weitkamp and Katz, 1980). Eggs and larval fish appear more sensitive to dissolved gas supersaturation than juvenile or adult fish but this varies between species. Packed columns can produce gas levels which have proved safe for rainbow trout Salmo gairdneri. However, preliminary but strong evidence suggests that rainbow trout are considerably more tolerant than brown trout Salmo trutta and Atlantic salmon to A~P levels in the range 0-25 mm Hg. Therefore, additional research is needed to assure maximum safety for the fish at minimum costs to operators.

ACKNOWLEDGEMENTS The authors would like to thank Kris Orwicz for help in the computer modeling and for a critical review of this article.

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J. Colt, G. Bouck REFERENCES

Bouck, G. R. (1976). Supersaturation and fishery observations in selected alpine Oregon streams. In: Gas Bubble Disease, eds D. H. Fickeisen and M. H. Scheider, CONF-741033. Technical Information Center, Energy Research and Development Administration, Oak Ridge, Tennessee, pp. 37-40. Bouck. G. R. (1980). Etiology of gas bubble disease, Trans. Am. Fish. Soc.. 190, 703-7. Bouck, G. R. (1982). Gasometer: an inexpensive device for continuous monitoring of dissolved gasses and supersaturation. Trans. Am. Fish. Soc., 11 I, 505-16. Bouck. G. R., King, R. E. & Bouck-sehmidt, G. L. (1984). Comparative rentoval of gas supersaturation by plunges, screens, and packed columns. Aquaeultural Eizgbleerbzg, 3, 159-76. Colt, J. E. {1983). The computation and reporting of dissolved gas revels, lear. Res., 17, 84l-9. Colt, J. & Tchobanoglous. G. (1981). Design of aeration systems for aquaculture. lu: Proceedhzgs of the Bio-Engineering Symposium for Fish Culture. eds. L. J. Allen and E. C. Kinney, Fish Culture Sections, American Fisheries Society, Bethesda, Maryland, pp. 138-48. Cornaccttia. J. W. & Colt, J. E. (1984). The effects of dissolved gas supersaturation of larval striped bass Morone saxatilis (Walbaum). J. Fish Dis., 7, 15-27. D'Aoust. B. G. & Clark, M. J. R. (1980). Analysis of supersaturated air in natural waters and reservoirs. Trans. Am. Fish. Soc., 109,708-24. D'Aoust, B. G.. Stayton. L. & Smith, S. S. (1980). Separation of basic parameters of dcco.npression using fingerling sahnon. Undersea Biomed. Res., 7, 199-209. Fuss, J. T. (1983). Efl'ective flow-through vacuum degasser for fish hatcheries. Aquaculmral Engineerilzg, 2, 301-7. Hackney, G. E. & Colt, J. E. (1982). The performance and design of packed column aerators for aquaculture. Aquaculutral Enghleering, 1,275-95. Harvey, H. H. (1967). Supersaturation of lake water with a precaution to hatchery usage. Trans. Am. Fish. Soc., 96, 194-201. Harvey, H. H. & Cooper, A. C. (1962). Origin and treatment of a supersaturated river water. Progress Report No. 9, International Pacific Salmon Fisheries Commission, New Westminster, British Columbia. Lindroth, A. (1957). Abiogenic gas supersaturation of river water. Arch. Hydrobiol., 53,589-97. Marking. g. L., Dawson, V. K. & Crowther, ,I. R. (1983). Comparison of column aerators and a vacuum degasser for treating supersaturated culture v.'ater. Prog. Fish-Cull., 45, 81-3. McLean, W. E. & Boreham, A. L. (1980). The design and assessment of aeration towers. Fisheries and Oceans, Vancouver, British Columbia, Canada (unpublished).

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Monk. B. E., Long, C. W. & Dawiey, E. M. (1980). Feasibility of siphons for degassing water. Trans. Am. Fish. Soc., 109,765-8. Mrsic. W. (1933). Die Gasblasendrankheit der Fische. Ursachen, Begleiterscheinumgen und Abhilfe. Zeitschri/t ffir Fisherei. 31.29-67. (Quoted in Weitkamp & Katz, 1980.) Norton Co, (1977). Design information for packed towers. Bullet#z DC-ll, Akron, Ohio. Norton Co. (1978). Packed tower internals. Bulletin TA-80. Akron, Ohio. Owsley. D. E, (1981). Nitrogen gas removal using packed columns. In: Proceedings of the Bio-engbteering Symposium for Fish Culntre, eds L. J. Allen and E. C. Kinney, Fish Culture Section. American Fisheries Society. Bethesda, Maryland. pp. 71-82. Peterson. H. (1971). Smolt rearing methods, equipment, and techniques used successfuUy in Sweden. In: Atlantic Salmon Workshop, Unipress Co., Fredericton, New Brunswick, pp. 32-62. Speece, R. E. & Humenick, M. J. (1973). Carbon dioxide stripping from oxygen activated sludge systems. J. War. Poll Cont. Fed.. 45, 412-23. Weiss, R. F. (1970). The sohtbility of nitrogen, oxygen, and argon in water and seawater. Deep-Sea Res., 17,721-35. Weiss, R. F. (1974). Carbon dioxide in water and seawater: the solubility of a nonideal gas. Mar. Chem.. 2,203-15. Weitkamp, D. E. & Katz, M. (1980). A review of dissolved gas supersaturation literature. Trans. Ant. Fish. Soc., 109,659-702. Yunt, F. W. (1979). Gas flow and power measurement. In: Proceedings: Workshop Toward an Oxygen Transfer Standard, EPA-600/9-78-021, ed. W, C. Boyle, US Environmental Protection Agency. Cincinnati. Ohio, pp. 105-27.