Bouck 1984 Aquacultural-Engineering

7/28/2019 Bouck 1984 Aquacultural-Engineering

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Aquacu lrura l Eng ineer ing 3 ~1984) [ 59 - [ 76

C o m p a r a t iv e R e m o v a l o f G a s S u p e r sa t u r a ti o n b yP l u n g e s , S c r ee n s an d P a c k e d C o l u m n sG e r a l d R . B o u c k

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D e p a r t m e n t o f C h e m i c a l E n g i ne e r in g , W a s h i n gt o n S t a t e U n i v e rs it y .P u l lm a n , W a s h i n g to n 9 9 1 6 3 , U S A

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Degass ing t e s t s were c on du c ted by us ing p lunges , s creens and co lum nspacked wi th G l i t ch Ba l las t r ings (1 .58 , 2 .54 and 3 .81 cm ou t sMe d iameter ) ,Tr i-pac sphere s (4 .6 cm o utsM e diame ter ) , or Ol in 12-gauge shotcups .P lunges an d screens prov ided b io log ica ll y i t ,adequa te degass ing whenh y p e r b a r i c p re s s u re ( A P ) e x c e e d e d a b o u t 5 0 m m H g ( a b o u t 1 0 6% b a ro -me tr ic pres sure ). Ba l las t r ings o f 2 .5 4 cm s i z e prov ed mo re e f f e c t i ve thanthe o the r s i zes or the Tri -pac spheres or sho tcups. The e f f l ue n t gas l evel sw e r e a f u n c t i o n o f in c o m i n g l e v e l o f s u p e rs a tu r a ti o n , c o l u m n h e ig h t,p a c k i n g t y p e a n d s iz e, w a t e r ) l o w r a te a n d f l o w r a te p e r e m 2 e o h l m n .W e c o n c l u d e d t h a t p a c k e d c o l u m n s c a n g e n e r a l l y p r o d u c e b i o l o g i c a l l yaccep tab le l eve l s o f d i s so lved gases { A p


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160 G . R . B o u c k , R . E . K i n g . G . L . B o u c k - S c h r n i d tINTRODUCTION

Water from lakes, streams, springs and wells is commonly supersaturatedwith dissolved gases for at least part of the year (Bouck, 1980a; Weit-kamp and Katz, 1980). Recognition of this problem has been slow, inpart because analyses o f total dissolved gases were difficult and becausethe impacts of low level supersaturation can be subtle, variable andconfused with other problems (Bouck. 1980b). Frequently, super-saturation is recognized only after conditions have become seriousenough to produce morbidity or catastrophic mortality. As one result,there is a widespread need to degas water before it is used in fishhatcheries and fishery research labs. In many cases, degassing must beaccomplished by treatment that is non-centralized, inexpensive andreliable. Therefore, we investigated several methods of degassing waterby aeration, evaluated their effectiveness at different tlows and gaslevels and reported tile data in mm Hg hyperbaric gas pressure (AP).

Several methods of degassing have been reported in the literature:(1) an aeration cascade (Rucker and Tuttle, 1948); (2) airblowers(Dennison and Marchyshyn, 1973): (3) siphons (Monk e t a l . , 1980);(4) vacuums (Mount, 1961): (5) vacuum U-tubes (Speece, 1981);(6) surface agitators (Wold, 1973); and (7) packed-columns (Owsley,1981). Of these methods, packed columns have achieved considerablepopulari ty because they can be operated effectively and inexpensivelyby gravity. Hackney and Colt (1982) concluded that packed columnswere technically feasible for reoxygenating water. McLean andBoreham (1980) who investigated various aeration towers tbr increasingdissolved oxygen and removing excess nitrogen, reported that packedcolumns seemed most desirable. However, we are aware that bothpacked columns and vacuum degassers have been considered unsuccess-ful at some locations (Marking e t a l . , 1983). Hence, we agree withMcLean and Boreman (1980) that packed columns (and other de-gassers) have limitations that need to be evaluated in a broader arrayof water qualities, gas mixtures and other circumstances.

Aeration-type or isobaric degassers all operate on tile same generalprinciples, although they differ widely in appearance and efficiency.Each method exposes a large amount of air-water surface area atambient air pressure; this passively allows the dissolved hyperbaricgases to diffuse into and approach equilibrium with their individualpressures in air. For each gas, the driving force is the diffusion pressure


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Removal o f gas supersaturation by plunges, screens and packed columns 161d i f f e r e n t i a l b e t w e e n t h e d i ss o lv e d p h a s e a n d t h e g a se o u s p h a s e . A t h i ghl e v e l s o f s u p e r s a t u r a t i o n , t h e h y p e r b a r i c g a s p r e s s u r e ( . S P ) r e p r e s e n t sc o n s i d e r a b l e d i f f u s i o n p r e s s u r e , w h i c h f a c il it a te s t h e e x i t o f g a se s f r o mw a t e r . H o w e v e r , as t h e s u p e r s a t u r a t i o n d e c li n es , t h e a m o u n t o f h y p e r -b a r i c g a s p r e s s u r e a l s o d e c l i n e s a n d t h e p r e s s u r e t o d r i v e d i f f u s i o nd e c r e a s es p r o p o r t i o n a l l y . T h e r e f o r e , o n e m u s t a lw a y s e x p e c t a t y p i c a lp a c k e d c o l u m n t o d e g as w a t e r i n c o m p l e t e ly , i .e . t h e p r o d u c t w a t e r w illb e s l ig h t ly s u p e r s a t u r a t e d .

O p e r a t in g c o n d i t i o n s c a n h a v e a c o n s i d e ra b l e e f fe c t o n t h e p r o d u c to f a e r a t i o n - t y p e d e g a s s e r s . C o n t a c t a r e a b e t w e e n a i r a n d w a t e r s h o u l db e r e l a t i v e l y l a rg e a n d t h i s a r e a c a n b e e n l a r g e d b y d i s p e r s i n g s m a l l a irb u b b l e s in t h e w a t e r o r b y d i s p e rs i n g w a t e r d r o p l e t s i n a ir . N o n - w e t t a b l ec o n t a c t s u r fa c e s m a y h e lp l o w e r th e e n e r g y n e e d e d t o o v e r c o m e t h es u r f a c e t e n s i o n o f w a t e r . E f f e c t iv e w a t e r d e p t h o r p re s su r e s h o u ld b el o w : p r e f e r a b l y t h e p r e s s u r e in t h e b u b b l e s s h o u l d b e n e a r l y e q u a l tot h e a m b i e n t a i r p r e s s u re . C o n t a c t t i m e w i t h a i r s h o u l d b e as l o n g a sp o s s ib l e t o f a c il i t a te t h e a t t a i n m e n t o f e q u i l i b r i u m b u t , o b v i o u s ly , l o n gc o n t a c t is a l u x u r y t h a t m u s t b e t e m p e r e d b y o p e r a t i o n a l l im i t a ti o n s.

A e r a t i o n d e g a s s in g a t i s o b a ri c c o n d i t i o n s c a n g r e a t l y d e c r e a s e t h ele ve l o f s u p e r s a t u r a t i o n in w a t e r. L a r g e p a c k e d c o l u m n s ca n re d u c e ga sle v els f r o m o v e r 1 3 0% b a r o m e t r i c p r e s s u r e to n e a r l y 1 0 0% in a s i n ~ ep a s s, g iv e n a h i g h - p r e s s u r e w a t e r s u p p l y (O w s l e y , 1 9 8 1 ). H o w e v e r ,p e r so n a l c o m m u n i c a t i o n s f r o m c o ll ea g u es an d o u r o w n e x p e r i e n c ei n d ic a t e t h a t s u c h h ig h p e r f o r m a n c e is n o t a c h i e v e d ro u t i n e l y a n d t h ed i f f e r e n c e s c a n b e c r it ic a l t o f is h es . I f o p e r a t i n g c o n d i t i o n s b e c o m es u b o p t i m a l , t h e r e si d u a l s u p e r s a t u r a t i o n w o r s e n s a n d m a y r e a c h b io -l o g i c a l l y u n a c c e p t a b l e l e v e l s . I n a p p r o p r i a t e e f f o r t s t o a e r a t e c a n b ec o u n t e r p r o d u c t i v e ( C o l t a n d W e s te r s. 1 9 8 2 ) a n d r e s u l t in s u p e r s a t u r a t e dgases .

T h e m a x i m u m g a s le v el a l lo w a b l e i s u n c e r t a i n f o r m a n y se n s it iv es p e c ie s , b u t i t a p p e a r s t o b e v e r y cl o se t o t h e l i m i t o f t r e a t m e n t t h a tp a c k e d c o l u m n s c a n a c h ie v e . T h e r e f o r e , i t is c r i ti c a ll y i m p o r t a n t tou n d e r s t a n d h o w v a r i ou s o p e r a t io n a l c o n d i t io n s o f p a c k e d c o l u m n sc a n a f f e c t t h e r e s u l t a n t g a s le v els , a n d h e n c e t h e h e a l t h o f f is h.

M A T E R I A L S A N D M E T H O D SW a te r f o r t h e s e e x p e r i m e n t s w a s d r a w n f r o m L a k e W a s h in g t on ( S e a t t le ,W a s h i n g to n ) a n d p r o c e s s e d t h r o u g h t h e S e a t t l e N a t i o n a l F i s h e r y


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162 G . R . B o u c k . R . E . K i n g , G . L . B o u c k - S c h m i d t

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\ ' 6F ig . 1 . Schem at ic rep resen ta t ion o f wa te r supp ly , supersa tu ra t ing sys tem , anddegass ing eva lua t ion sys tem. Componen ts a re as fo l lows: ( I ) l ake wa te r supp lysystem; (2) aera t ion column packed with 2 .54-cm o.d . Bal las t r ings to s tabi l izedissolved gas levels; (3) 1000-liter reservo ir; (4) 1.5-h.p. cen trifu gal pu m p; (5) w aterpressure gauge an d f low m eter ; (6) regula ted com pressed a ir sup ply; (7) 33-m coi lo f 2 -54-cm i .d . p o ly e thy lene p ipe ; (8 ) bubb le de can t ing co lu m n w i th gas ven t ingvalve; (9) w ater pressure gauge a nd f low m ete r ; ( I 0) in- line G asom eter with pressuregauge and f lo w m ete r ; (11) by-pass re tu rn to ae ra t ion co lu m n; (12) te s t co lum n, s izeand packing as required; (13) degassed water reservoir with portable Gasometer

and dra in . Co lum n (12) w as rem oved f or tes ts requir ing screens o r p lunges .R e s e a r c h C e n t e r ' s l a b o r a t o r y w a t e r s u p p l y s y s t e m ( F ig . 1). G a s le ve lsw e r e s t a b i l iz e d i n i t ia l ly b y p a s s in g t h e w a t e r t h r o u g h a p a c k e d c o l u m n( 1 2 0 c m h i g h f il le d w i t h 2 - 5 4 - c m b a l la s t r i n gs ( G l i t c h C o . , D a l la s ,T e x a s * ) ) a n d i n t o a 1 0 0 0 - l it e r r e s e rv o i r . T h e w a t e r w a s t h e n s u p e r -* Refe rence to t rade nam es is fo r iden t i f ica t ion on ly and does no t im ply USGo v e rn m e n t e n d o r se m e n t o f c o mme rc i a l p ro d u c t s .


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R e m o v a l o f g a s s u p e r s a t u r a ti o n b y p l u n g e s, s c r e e n s a n d p a c k e d c o i. um n s 163s a t u r a t e d a s d e s c r i b e d b y B o u c k a n d K i n g {11983), b y p u m p i n g w a t e ra t a p r e s su r e o f a b o u t 2 0 0 k P a a n d m i x i n g i n m e t e r e d a m o u n t s o fc o m p r e s s e d a ir . U n d i s s o l v e d b u b b l e s w e r e d e c a n t e d w i t h a g a s v e n t in gv a l u e. H y p e r b a r i c g a s l ev e ls ( & P ) in t h e r e s u l t in g w a t e r s u p p l y w e r em o n i t o r e d b e f o r e a n d a f te r t r e a t m e n t a s d e s c r i b e d b y B o u c k ( 1 9 8 2 ).A i r p re s s u r e w a s r e a d f r o m a m e r c u r y b a r o m e t e r . D i ss o lv e d o x y g e nc o n c e n t r a t i o n ( m g l it e r -~ ) w a s d e t e r m i n e d b y t h e a z i d e m o d i f i c a t i o no f W i n k le r ' s m e t h o d ( A m e r i c a n P u b l ic H e a l t h A s s o c ia t io n e r a l . , 1 9 7 1 ) ,e x c e p t t h a t p h e n y l a r s i n e o x i d e w a s s u b s t i t u t e d f o r 0 - 0 2 5 N t h i o s u l f a tea n d t hi s w a s s t an d a r d iz e d w e e k l y . O x y g e n p r e ss u re w a s c o m p u t e d f r o mt he m e t h o d d e s c r ib e d b y B o u c k ( 1 9 8 2 ) . W a t er t e m p e r a t u r e w a sm e a s u r e d w i t h h a n d - h e l d m e r c u r y - f il l e d t h e r m o m e t e r s . W a t e r f l o w sw e r e d e t e r m i n e d w i t h in -lin e f l o w m e t e r s a n d w a t e r p r e s su r e d r o p p e d t oa b o u t 2 0 k P a a b o u t 6 0 c m f r o m t h e s i ng le o u t l e t w h i c h su p p l i e d t h ec o l u m n . C o l l a t e r a l t e s t s e s t a b l i s h e d t h a t d e g a ss in g w i t h a p a c k e dc o l u m n w a s e q u a l l y e f f e c t iv e w h e n s u p p l i e d b y g r a v i ty t l o w o r u n d e rp r e ss u r e . (I n a p e r s o n a l c o m m u n i c a t i o n , D a v id O w s l e y i n d i c a te d t h a tg r a v i t y s u p p l y s y s t e m s t o l a r g e - s c a l e p a c k e d c o l u m n s d o n o t a c c o m p l i s ha s m u c h d e g a s s in g a s h ig h p r e s s u r e s u p p l y s y s t e m s . T h e c a u s e s ar eu n c e r t a i n , a n d t h e r e f o r e t h i s p o i n t w a r r a n t s c a u t i o n . )

D e g a s s in g m e t h o d s t e s t e d w e r e a s i m p l e 1 2 0 - c m p l u n g e i n t o a1 6 - li te r c o n t a i n e r a b o u t 3 0 c m d e e p , a p l u n g e th r o u g h 12 s c r e e n s( a b o u t 4 - m m m e s h ) s u s p e n d e d a b o u t 2 c m a b o ve e a c h o t h e r ( a b o v e t h e1 6 - li te r c o n t a i n e r ) a n d a p a c k e d c o l u m n 1 0 c m i n si d e d i a m e t e r ( i. d.) o fw l r i o u s h e g h t s a n d p a c k i n g m a t e r ia l s w h i c h a l so s p il le d i n to t h e 1 6 -l it erc o n t a i n e r . P a c k i n g m a t e r ia l ( F ig . 2) i n c lu d e d p o l y p r o p y l e n e B a lla str in g s , 1 . 5 8 , 2 . 5 4 a n d 3 .8 1 c m o u t s i d e d i a m e t e r ( o . d . ) a s d e s c r i b e d inT a b l e 1 ; p o l y p r o p y l e n e T r i- p a c s p h e r e s, 2 . 5 4 c m o . d . ( J a e g e r C o . . C o s t aM e s a , C a l i f o r n i a ) ; G l o b e f l u x p a c k i n g ( M i l a n o , I t a l y ) a n d p o l y e t h y l e n e12 g a ug e W A A 1 2 R s h o t c u p s ( W i n c e s te r -W e s te r n D i v is io n o f O li n. N e wH a v e n , C o n n e c t i c u t ) . C o l u m n s a n d s c re e n s w e r e p o s i t i o n e d a b o u t 5 c ma b o v e t h e s u r f a c e o f a 1 6 -l ite r t a n k ( a b o u t 3 0 c m d e e p ) . A s u b m e r s i b lep u m p t h e re i n ( m o d e l 4 E - 3 4 N R , L i t tl e G i a n t C o r p . ) s u p p l ie d w a t e r t o ad e v i c e t b r m e a s u r i n g d i ss o l v ed g as ( G a s o m e t e r ) d e s c r i b e d b y B o u c k( 1 9 8 2 ) . C o l u m n s w e r e a ls o t e s t e d a s 3 0 c m s e g m e n t s o f v a r i ab l e s id e -w a l l h e ig h t . F l o w r a t e s th r o u g h a ll p a c k e d c o l u m n s w e r e e x p r e s s e d a st i t e r s m i n -~ c m -2. F l o w r a t es t h r o u g h s c r e e n s o r i n a p lu n g e w e r ee x p r e s s e d o n l y a s l i te r s r a in -~ a n d d i d n o t l e n d t h e m s e l v e s t o e x p r e s s i n gi n t e r m s o f ar e a.


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Re m ova l o f gas super sa tura tion by p lunges , s creens and paCked co lum ns 165T A B L E 1

Physical Characteristics o f Plastic Ballast RingsaCharacter ist ic Pac king s ize (cm , o .d . )

1 . 5 9 2 . 5 4 3 . 8 1 5 . 1 8Packing variable

Unit surface area tcm 2) 13-8 39-0 87.5 147.8Space occupied (cm 3) 3.2 12.6 43.1 98.2Area/volume ratio 4.3 3.2 2-0 1.5Free space t%) 86 90 91 92

Square meters surface per lOcm i.d. column6 0c m high 53.2 33.3 20.5 16.4120 cm high 106.4 66.5 40.9 32-8

a Variot, s m anu facturers have similar pro du cts with oth er names.

T h e t e s ti n g p r o c e d u r e b e g a n b y a d j u s ti n g t h e e q u i p m e n t t o a c h ie v et h e d e s i r e d w a t e r f l o w a n d g a s l ev e l. T h e g a s l e v el s w e r e t h e n r e a d 1 hl a te r a t p o i n t s i m m e d i a t e l y b e f o r e a n d a f t e r t r e a t m e n t . C o l l a te r a lt e s ti n g w a s c o n d u c t e d o v e r l o n g e r p e r i o d s , b u t g a s l ev e ls w e r e r e ad a th o u r l y i n te r v a ls . T h e l o n g e r t e s t s p r o v i d e d i n s ig h t i n t o p o s s i b l e v a ri a-b i l i t y ( w h i c h p r o v e d t o b e n e g li g ib l e) .

G a s c h a n g e s w i t h in t h e p a c k e d c o l u m n w e r e in v e s ti g a te d b y c o m -p a r i n g t h e l e ve l s o f i n d i v i d u a l g a s e s i n w a t e r a f t e r i t p a s s e d t h r o u g hv e n t e d p a c k e d c o l u m n s 3 0 , 6 0 , 9 0 a n d 1 20 c m h ig h p a c k e d w i th2 . 5 4 - c m B a l l a s t r i n g s .

S t a ti s ti c al a n a l y se s o f t h e d a t a w e r e p e r f o r m e d a c c o r d i n g t o Z a r r( 1 9 7 4 ) a n d i n c l u d e d r e g r e ss i o n a n a l y si s , c o m p a r i s o n o f s l o p e s a n d i n te r -c e p t s , a n d a n a l y s e s o f v a r ia n c e .

U s i n g i n f o r m a t i o n i n t h e l i te r a t u r e , w e a r b i t ra r i ly e s t a b l i s h e d ~ P =2 5 m m H g ( a b o u t 1 03 % 7 6 0 m m H g ) as a t h e o r e t ic a l m a x i m u m a llo w -a b l e l e v e l o f g a s f o r s a l m o n i d s ; d e g a s s i n g t h a t f a i le d t o a c h i e v e th i s o r al o w e r l ev el , w a s j u d g e d u n a c c e p t a b l e . H o w e v e r , i n as m u c h a s r e c e n td a t a b y C o r n a c c h n i a a n d C o l t ( 1 9 8 4 ) d e m o n s t r a t e d t h a t a .X P o f2 2 m m H g p r o d u c e d c li n ic a l s ig n s o f g a s b u b b l e d i s ea s e in la rv a l s t ri p e db a s s M o r o n e s a x a t i l i s , t h e m a x i m u m a l l o w a b le A p l ev el o f 2 5 m m H gs h o u l d b e c o n s i d e r e d p r o v i s i o n a l u n t i l t e s t e d f o r t h e s p e c ie s o f c o n c e r n .


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166 G. R. Bouck. R. E. King. G. L. Bouck-SchmidtRESULTS

S i m p l e p l u n g e d e g a s s in gRoughly 50% of the AP (200 mm Hg) was degassed by a simple plungeat flow rates varying f rom 8 to 64 liters rain -l ~Fig. 3). The post-tr ea tmen t to tal gas level was abo ut 112% of atmospher ic pressure - alevel not cons idered safe for fish in shallow water. Regression analysesshowed that the slopes and elevations were not significantly diff eren tacross a flow range f rom 8 to 64 liters mi n- ': hence the rate ofdegass-ing was independent of these flow rates. This method is not satisfactoryfor removing excess dissolved gas.

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0UT- FLOW WATERComparison of degassing capacities by a plunge, screens and various

packed columns.


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Removal of gas supersaturation by plunges, screens and packed columns 167D e g a s s i n g w i t h s c r e e n sP a s sa g e t h r o u g h t h e 1 2 s c r e e n s c o n v e r t e d s o m e w a t e r i n t o a s p r a y o ro t h e r w i s e b r o k e a n d a e r a t e d t h e w a t e r c o l u m n . A l t h o u g h t h e r e s u l t w a sv i su a l ly i m p r e s s iv e , d e g as s in g b y t h is m e t h o d w a s o n l y s l ig h t ly m o r ee f f e c t i v e t h a n t h a t e f f e c t e d b y a s i m p l e p l u n g e ( F ig . 3 ). D e g a s s in ga p p e a r e d t o b e f l o w d e p e n d e n t : ,.x p d r o p p e d 5 5 % a t 1 6 l it er s r a in - t a n d7 0 % a t 6 4 l it er s m i n - t . H o w e v e r . t h e s l o p e s w e r e n o t s i g n if i c a n tl yd i f f e r e n t o v e r t h e r a ng e o f f l o w s t e s te d .

M o r e i m p o r t a n t , t il e s c r e e n s f a il ed to r e m o v e s u f f i c i e n t g as t o r e n d e rt h e w a t e r s a f e tb r s e n s it iv e a q u a t i c l if e w h e n t h e w a t e r s u p p l y h as aZ IP o f 1 0 0 m m H g o r h i g h e r, a l b e it t h e u s e o f m o r e s c r e e n s m i g h ta c c o m p l i s h t h e d e s i r e d e n d .

D e g a s s i n g w i t h p a c k e d c o l u m n sT h e i ni tia l u s e o f a 1 2 0 - cm p a c k e d c o l u m n p r o v e d t o b e a m o r e e f fe c t iv em e t h o d o f d e g a s s in g t h a n s c r e e n s o r s im p l e p l u n g e ( F ig . 3 ). T h e g a sl ev e l in t r e a t e d w a t e r w a s d e p e n d e n t o n t h e g a s l ev e l in u n t r e a t e d w a t e r( T a b l e 2 ) , i n d ic a t in g t h a t t r e a t m e n t n e e d e d c o u l d be p r e d i c t e d a n dd e s i g n e d . T h e e x t r a p o l a t e d r e g r e s si o n li ne m e e t s t h e v e rt ic a l a x is a ta b o u t t h e v a l ue o f w a t e r v a p o r p r e ss u r e a t t h e s e t e m p e r a t u r e s . E q u a l l yi m p o r t a n t , i t w a s o b v i o u s f r o m t h e se r e s u l ts t h a t t h e g as r e m o v e d b y ap a c k e d c o l u m n w a s r e la t e d t o v a r io u s o p e r a t i o n a l c h a r a c t e r is t ic si n c l u d i n g t h e c o l u m n h e i g h t, t h e p a c k i n g m a t e r i a l s i ze , t h e f l o w r a tet h r o u g h t h e c o l u m n a n d t h e Z IP o f t h e w a t e r s u pp l y .

C o l u m n p a c k i n g h e ig h t in f l u e n c e s a c o m p l e x in t e r a c t io n b e t w e e ns e v e ra l f a c t o r s i n c l u d i n g t r e a t m e n t t i m e , s u r f a c e a r e a a n d p r o b a b l yo t h e r s . F i g u r e 4 d e p i c t s t h e g e n e r a l r e l a t i o n , s h o w i n g a l ar g e l o ss o fz i P i n e a ch o f t h e f ir st t w o 3 0 - c m s e g m e n t s ; t r e a t m e n t t h e r e a f t e rb e c o m e s p r o g r e s s i v e l y le ss b e n e f i c i a l w i t h p r o g r e s s i v e l y g r e a t e r c o l u m nh e i g h t. T h e c h a n g e in Z I P w i t h p as s a ge t h r o u g h t h e c o l u m n w a s a l o gf u n c t i o n g e n e r a l l y d e s c r i b e d b y t h e e q u a t i o n :

in z i P h = a + b hw h e r e i n t h e i n it ia l A p = 2 1 0 m m H g , zi P h = A P a t h e i g h t h in c m ,a = 5 . 2 4 9 4 ( i n t e r c e p t v a lu e ) , b = - - 0 - 0 6 2 6 ( s lo p e ) a n d r a = 0 . 9 7 2 8 . Ing e n er a l, w e s ee l i tt le r e a so n t o e x t e n d c o l u m n h e i g h t m u c h b e y o n d


10/18

168 G . R . B o u c k , R . E . K i n g , G . L . B o u c k - S c h m i d rT A B L E 2

Summary of Regression Analyses for Inlet (x) on Outlet ~v) Pressures of TotalHyperbaric Gas (&P) Removal by Plunge. Screens and Packed ColumnsT r e a t m e n t Y i n t e r c e p t S l o p e C o r r el a ti o n n

{ a ) ( b ) c o e f f i c i e n t , rSimple plunge 17.83 1.70 0-99 a 12Twelve screens 20-00 2."5 0.95 a 12P a c k e d c o l u m n , 6 0 c m h ig hPacking size (cm, o.d.)

1-59 22.06 2 .55 b 0.96 a 162 54 9-44 3 "81 0'94 a 163"81 13'50 3"01 0-96 a 16

P a c k e d c o h t m n , 1 2 0 c m h i ghPacking size (cm, o.d.)1.59 3.22 4.34 0-86 a 162.54 16.70 8.73 0.91 a 163.81 33-58 3.82 t' 0.88 a 16

a Significant correlation at 0.975 level.b These values were significantly different between different flows in their category.

120 cm, which represents a practical balance between head loss. over-head clearance, column supports and treatment needed. This height wasjudged adequate to render water with a AP of 210 mm Hg (over 125%saturation, depending on elevation/air pressure) relatively safe forgeneral fish cultu re (~< 25 mm Hg). The impact of colunm sidewallheight was investigated by comparing the degassing in solid sidewallcolumns with tha t it,. columns segmented at 30-cm intervals. Segmentedcolumns had slightly better degassing performance but presented struc-tural support problems and were not studied further. As an untes tedalternative, we surest vents in the sidewalls of packed columns.

Three col umn packing materials were tested for degassing capabilityat different flow rates (Fig. 5). At the lowest flow rates tested (0-I litermin -~ cm-2), shotcups reduced AP from 200 to 3 0 m m Hg. Thisperfo rmance was judged highly beneficial, but inadequa te; shotcupswere dropped from further study. The performance of Ballast ringsdepended both on flow (considered later under flow ef fe ct s) an d


11/18

R em o v a l o f ga s su p e r s a t u r a t i o n b.v p l u n g es, sc r een s a n d p a c k e d c o l u m n s 169

2 O OEE

~c

',' 1oocco.

~ _ 5 0c~ 4 Om 3 0 ~'~' 2 O~. IOx" 0

k. ' \ \

' \

\

0 3 0 6 0 9 0 1 2 0 150CO L UM N PACKING HEIG H T (cm. )

Fig. 4. Effect of column height on degassing.

on ring size. As judged from da ta in Fig. 5, the 2.54-cm rings weremuch more efficient than either the small or larger rings (see slopevalues, Table 2). The degassing ability of Tri-Pac spheres were co mpa redwith that of the 2 .54-cm Ballast rings as tested in a col umn of 10 cmi.d. Usage in such a small diameter column was not recommended bythe supplier, but as can be seen in Fig. 5. its degassing was good andonly slightly less than by the 2,54-cm rings.

Flow has a variable effect on degassing, as shown in both Figs 5 and6. When inpu t gas levels were A P = 200 mm Hg and flows werebe tween 0.1 and 1.0 liter min -I c m -2 , o u t p u t . ~ a s levels were at orbelow A P = 25 mm Hg and in de pend en t of flow rates. Above this flowrate, output gas levels were dependent on flow and AP rose from3 0m mH g at 1.5 liter smin -~c m -2 to 7 0m m Hg at 2.3 liter smin -1cm - 2 - bo th levels biologically unac cept able . When input gas levelswere low (AP = 75 mm Hg) a definite optimum flow existed between1 -0 and 1.5 l iters min -t cm -~- which pr od uc ed a & P of about 10 mm Hg.Above this flow. ou tp ut gas levels rose, but still remained belowzXP= 25 mm Hg - even at 2.3 liters min -~ cm- :


12/18

170 G . R . B o u c k , R . E . K i n g , G . L . B o u c k - S c h m i d tI0 09O

0 8 OO~ 7 0

5o

.o

r,- 3o

uJ 20~ElO

0 0

I 12 ga ug e shotcups 2 . . 5 4 c m . d i a . B a l l a s t r in g s 5 . 08 c rn . d io . T r i - p oc s p h e r e s

A

I I I I I.5 f.O 1.5 2 0 2.5F LO W R A T E

Fig. 5. Comparison of degassing by three types of column packing at differentflow rates. Flow rate is in liters rain -I cm -2.

Supers aturat ion level influenced the degassing capacity of the packedcolumn at 1 liter min -~ cm -~ (Figs 3 and 6), but this capaci ty m aydiffer between columns of different designs; hence we recommenddue consid eratio n of this variable.Gas changes within packed columnThe levels of oxygen and nitrogen changed during the downwardpassage of water through a packed column (Table 3). Performancedep ended in part on flow rate. which in this example was 0-4 literrain -Ecm -2 and degassed the water in a single pass from AP" 212mm Hg to A P " - 2 5 mm Hg. In d iffer ent tests at higher flows, we


13/18

Removal of gas supersaturation by plunges, screens and packed columns 1718 O

- ; 7 0w>- J 6 0u' )

5 0

> -" r

2 Oz

00

J

JI , i i.S 1.0 1.5 2 .0

C O L U M N F L O W R A T E

I 09

8 "r"E

7 e06

5 u,.0

4 I,-,-3

I

2 . 5

F ig . 6 . E f f e c t o f w a t e r f l o w a n d i n p u t g a s l e v e l o n t h e o u t p u t g a s l e v e l i n p a c k e d -c o l u m n d e ga ssi ng . In p u t g a s le v els : ( A ) 7 5 m m H g ; ( e ) 2 0 5 m m H g . Th e c o l u m n

flow rate i s in l i ters rain -~ cm -2.

a c h i e v ed e v e n b e t t e r d e g a ss in g a n d r e a c h e d a A p a s l o w a s 1 2 - 1 4 m m H gw i t h a v e n t e d , 1 2 0 - c m c o l u m n a n d a s in g le p a s sa g e . In d i v i d u a l g a s esw e r e r e m o v e d a t d i f f e r e n t r a t e s, w i t h o x y g e n n e a r l y a c h ie v i n g a ir e q u i -l i b r i u m w h i l e d i s s o l v e d n i tr o g e n r e m a i n e d 1 4% a b o v e a ir e q u i l i b r i u mv a l u e s .R e g r e s s i o n a n a l y s e s w e r e p r e p a r e d f o r p r e d i c t i n g t h e r e s u lt in g l ev e lso f n i tr o g e n a n d o x y g e n a f t e r l is te d t r e a t m e n t s o f a ir s u p e r s a t u r a t e dw a t e r w i t h A p v a lu e s r an g in g f r o m 2 0 t o 2 0 0 m m H g ( T a b le 4 ) . W ec a n n o t a c c o u n t f o r th e w i d e r a n g e o f c o r r e l a t i o n c o e f f i c i e n t s a n db e l ie v e f u r t h e r s t u d y is n e e d e d a t a w i d e r ra n ge o f b a r o m e t r i c p r e s s u r e sa n d o p e r a t i o n a l c o n d i t i o n s .

D I S C U S S I O ND e g a s s i n g b y s i m p l e p l u n g e , o r p a s s a g e t h r o u g h 1 2 l a y e r s o f s c r e e n s , d idn o t a c h i e v e a f in a l A P b e l o w 2 5 m m H g w h e n t h e A P o f t h e i n c o m i n g


14/18

TABLE3

C

nsovGWheAu

uaeWaeP

To~aP

Cum(oumWee11cmd

P

Wih25cmodBaRnaSeWihWaea04eran-cm-2aIIWhcIaBS

suaeWihAaPeueTAVuResBhhDuoPeueadIybcC

2

~:~

CnmTaaH

bcgpu

O

Nro

hg

g

(amH

(amH~

(amgc

(eapu

(aH

Ine

Oe

WaesuyCmoe

Waesuy

A%saA%saPA2PA

%loPA

Cnoe

P

A

%O

e C

3

9

2

141

171

4

1

2

4

7

1

6

9

4

6

9

2

176181

4

1

1

6

7

1

6

4

7

9

9

2

143101

4

1

68

7

1

6

3

8

1

9

2

152131

4

1

19

7

1

6

2

8

r~

aTagpeue=amebomcpeue+amehbcgpeueo7mIg+2mIIg=~

bAeo

peuena(Pwccaeoba

1nH

eAenopcuena(PNwccaeoba

5mIg


15/18

Re m ov a l o f gas supersa tura t ion by p lunges , screens and packed co lum ns 173T A B L E 4

Regress ion Ana lys i s o f In l e t (x ) on O ut l e t (y ) P ressu res tb r Po , . and Py : Af te rTh r e e M e t h o d s o f D e g a s s i n g A i r S u p e r sa t u r a t e d W a t e r . ~P R a n g e d f r o m 2 0 t o

200 m m Hg, F lows Ranged F ro m 8 to 64 l i te r s min - ITre a tm en t Y S lope Corre la tion N um be r o f

in t ercep t (b ) coe f f i c i en t observa t ions(a) (r)

Simple p lungeP o : - 0 " 0 4 1 8 1 '4 6 - -0 " 0 4 12PN 2 1 .67 - -3 81 .43 0-99 a 12

Tw elve sc reensP o : 1 -7 80 - 1 1 5 .8 3 0 -9 1 a 1 2PN: 2 -18 - -677 .11 0 .94 a 12

Coh tmn 120 era, pa cke d wi th r ingsRings 1-59 cm o.d .

P o : 2 .045 - -15 3-70 0 -61 a 16PN : 3-00 - -1 122.72 0-85 a 16

R i n g s 2 - 5 4 c m o .d .PO: 1 .62 - -83 .88 0 .35 16PN , 5-97 12 930 -00 0 .76 a 16

Rings 3 .81 cm o .d .P o : 1 .07 2 .24 0 -36 16PN : 2.98 --1 125.21 0-87 a 16

a Signi f icant a t 0-97 5 level.

w a t e r e q u a l e d o r e x c e e d e d 5 0 m m H g . C o n v e r s e l y , a c o l u m n 1 20 c mh i g h w i t h 2 . 5 4 - c m p a c k i n g r e d u c e d t h e e xP f r o m 2 0 0 t o 2 5 m m H g o rle ss in a s in g le p a s s . T h u s , p a c k e d c o l u m n s c a n n o t o n l y d e g a s w a t e r t ol ev e ls a c c e p t a b l e f o r m o s t a q u a c u l t u r a l a p p l i c a t i o n s , b u t c a n a ls op r o v i d e r e s e rv e c a p a c i t y t o h a n d l e u n f o r e s e e n r i se s in e xp . S u c h r e s e rv ec a p a c i t y m a y b e n e e d e d w h e n s u p p l y s y s t e m s m a l t 'u n c t i o n o r w h e ns e a s o n a l v a r i a t i o n s i n e xP o c c u r . A n o t h e r a p p l i c a t i o n o f a p a c k e dc o l u m n is a s p r e t r e a t m e n t f o r w a t e r e n t e r i n g a v a c u u m d e g as s e r ; t h isc o u l d r e d u c e b o t h t h e lo ss in w a t e r p r e s s u re a n d o p e r a t i o n a l c o s t s .

C e r t a i n o p e r a t i o n a l c o n d i t i o n s in p a c k e d c o l u m n s a re c l e a r l y s u p e r i o rt o o t h e r s . F o r e x a m p l e , t h e 2 - 5 4 - c m B a l l as t r in g s d e g a s se d w a t e r m o r e


16/18

174 G . R . B o u c k , R . E . K i n g , G . L . B o u c k - S c h m i d tT A B L E 5Degassing Capacity of Different Diameter Columns When Packed With 2.54 cm

Ballast RingsC o l u m n i .d . C r o ss -

s e c t i o n a li n c m a r ea , c m 2

D e g a s si n g c a p a c i t y al i t e r s m i t t - t cm-: g a l l o n s r a i n - '

3 7.6 45.6 45-6 124 10.2 81.1 81.1 216 15-2 182.5 182.5 478 20-3 324.3 324.3 84

12 30-5 729.7 729.7 19024 60.9 2 918-6 2 918.6 758

a At 1 liter min -l cm-:.

efficiently than the other packings we tested. Segmented or ventedsidewalls were slightly better than their solid counterparts. Water flowscan have a sign ificant impac t on degassing and about 1 liter rain -t cm -2provided optimum degassing (Table 5), but this optimum also dependedon the Ap of the water supply. Taller columns removed more Ap th andid shorter columns, but the required column height depends on theinteractions of maximum A P , flow rates and column packing. There-fore, columns in series may be necessary to achieve the required degreeof treatment.

Differences in operating con dit ions can make relatively small butbiologically significant differences in resulting dissolved gas levels. Forexample, if a packed column produces a Ap above 25 mm Hg, gasbubbles could form in freshwa ter as deep as 34 cm; at /xp = 15 mm,bubbles could form to a freshwater depth of 20 cm. This condition maybe unacceptable for incubating eggs and fry (larvae) in typical hatcherytroughs or in shallow water niches in rivers or lakes. Allowing fordeeper submersion can give an additional protection, provided the fishelect to use it.In the long run, it appears wise to regard the ope rat ion of a packedcolumn degasser as having maintenance and monitoring requirements,like other equipment. If designed and operated properly, packedcolumns can usually achieve gas levels which are safe for most life stages


17/18

Removal of gas supersaturation by plunges, screens and packed columns 175of fish. However, changes in water flow, inco ming 2xP, or othe r uniden-tified factors can decrease degassing efficien cy and raise the ou tp utAp, which in turn may create biolo~cally unsafe conditions. Therefore,packed columns must be monitored regularly to ensure that they areoperating properly and providing the level of safety assumed.

AC KNOWLEDGEMENTThis research was funded in part by the US Fish and Wildlife Service.

REFERENCESAmerican Public Health Association, American Water Works Association & Water

Pollution Control Federation (1971). Standard Methods for the Examinationof Water and Wastewater. APHA, New York, USA.

Bouck, G. R. (1980a). Air supersaturation in surface water: a continuing engin-eering and biological problem. Proc. Syrup. Surface Water Impoundments,June 2-5, 1980, ASCE, Minneapolis, pp. 1542-51.Bouck, G. R. (1980b). Etiology of gas bubble disease, Trans. American FisheriesSot., 109,703-7.

Bouck, G. R. (1982). Gasometer: an inexpensive device for continuous monitoringof dissolved gases and supersaturation, Trans. American Fisheries Soc., 111,505-16.

Bouck, G. R. & King, R. E. (1983). Tolerance to gas supersaturation in fresh waterand sea water by steelhead trout Sahno gairdneri Richardson, J. Fish Biology,23,293-300.Colt, J. & Westers, H. (1982). Production of gas supersaturation by aeration, Trans.American Fisheries Soc., 111,342-60.

Cornacchia, J. W. & Colt, J. E. (1984). The effect of dissolved gas supersaturationon larval striped bass (Morone saxatilis),J. Fish Diseases,7, 15-27.Dennison, B. A. & Marchyshyn, M. J. (1973). Device for alleviating supersaturationof gases in hatchery water supplies, Progressive Fish-Culturist, 35, 55-8.

Hackney, G. E. & Colt, J. E. (1982). The performance and design of packed columnaeration systems for aquaculture, Aquacultural Engineering, 1,275-95.

McLean, W. F. & Boreham, A. L. (1980). The design and assessment of aerationtowers, Pro~cot Report, Canada Department of Fisheries and Oceans.Marking, g. L., Dawson, V. K. & Crowther, J. R. (1983). Comparison of columnaerators and vacuum degasser for treating supersaturated culture water, Pro-gressive Fish-Culturist, 45, 81-3.


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176 G. R. Bouck, R. E. King. G. L. Bouck-SchmidrMonk, B. H. , Long. C. W. & Dawley. E. M. (1980) . Feasib i l i ty of s iphons for degass-

ing w a te r , Trans. American Fisheries Soc., 1 0 9 , 7 6 5 - 8 .M o u n t . D . ! . ( 1 9 6 1 ) . D e v e l o p m e n t o f a sy s t e m f o r c o n t r o ll i n g d is so lv e d - o x yg e n

c o n t e n t o f w a t e r, Trans. American Fisheries Soc., 9 0 , 3 2 3 - 7 .Owsley , D . E . 11981) . Ni t rogen gas r emova l us ing packed co lumns . Proc. Bio-Engineering Symposium for Fish Culu~re, eds k . J . Al len and E. C. Kinney,A m e r i c a n F i sh e ri e s S o c i e t y , B e t h e sd a , M a r y la n d , p p . 7 1 - 8 2 .

R u c k e r , R . R . & T u t t l e , E . M . ( 1 9 4 8 ) . R e m o v a l o f e x c e s s n i t ro g e n m a h a t c h e r yw a t e r su p p ly , Progressive Fish-Culturist. 9 , 8 8 - 9 0 .

Spee ce , R . E . (198 I ). M anagem ent o f d i sso lved oxy ge n and n i t rogen in f ish ha tcherywate r s . Proc. Bio-Engineering Symposfllm for Fish Culture, eds k. J . Allen andE. C . Kinney , Am er ican F i she r ies Soc ie ty , Be thesda , Mary land , pp . 53 -6 2 .

W ci tkamp, D . E . & Katz . M. (198 0) . A r eview o f d isso lved gas super sa tu ra t ionl i t e r a tu re , Trans. American Fisheries Soc., 1 0 9 , 6 5 9 - 7 0 2 .

Wold , E . (1973) . Sur f ace ag i t a to r s a s a means to r educe n i t rogen gas in a ha tcheryw a t e r su p p ly , Progressive Fish-Culturist, 3 5 , 1 4 3 - 6 .

Zar r , J . H . (1974) . Biostatistical Analysis, Prent ice-Hal l Inc . , Englewood Cl i f f s ,New Je r sey .

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