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Aquacultural Engineering 1 ( 1 9 8 2 ) 1 3 9 - 1 5 1

R E C I P R O C A T I N G B I O F I L T E R F O R W A T E R R E U S E INA Q U A C U L T U R E

M . H . P A L L E R a n d W . M . L EW I SFisheries Research Laboratory and Department o f Zoology, Southern Illinois University,Carbondale, Illinois 62901, USA

ABSTRACTThe rec iprocat ing b io f i l t er i s autom at ical ly dew atered a t regular and f re qu en t intervals ,in contras t to the co nven t ional upf lo w subm erged b io f i l t er which is cont inu al ly in-undated . Rec iprocat ing b io f i l t ers were compared wi th submerged b io f i l t ers in t erms o fab i l i ty to m aintain wa ter qu al i ty in small-scale f ish ho m ing u ni ts . In the f irs t trial therec iprocat ing f i l t er sys tem s averaged 35 % mo re f ish in t erms o f numbers , 59 % mo ref i sh in t erms o f weight , an d a 45 % greater f eed ing ra te . In the second t rial the rec ipro-cat ing f i l t er sys tem s averaged 29 % mo re fi sh in t erms o f numbers , 33 % m ore f i sh int e rm s o f we i gh t, and a 2 9 % grea te r f e ed i ng rate . Super i or pe r f orm ance o f t herec iprocat ing f i l ters appeared to be the resul t o f res is tance to c logging and imp rovedaerat ion o f the f i l t er subs tra te .

I N T R O D U C T I O NThe subm er ged t ype o f b i o f i l t e r is com m onl y used t o m a i n t a i n w a t e r qua l i ty in c lo sedf ish cu l t u r e sys t em s ( Bur r ow s and Com bs , 1968 ; Meade , 197 4) , de sp i t e t he f ac t t ha t i to f t en su f f e r s f r om t w o p r ob l em s ( Lew i s et al . , 1981a) . One problem i s tha t the sub-me rged des ign i s ra the r inef f ic ient a t supply ing ox yge n to the aerobic t r i t e r bac te r ia .The s econd p r ob l em is t ha t t he i n t e rs t ic e s o f t he f i lt e r subs t r a te beco m e o bs t r uc t ed bypa r t i cu l a t e m a t t e r and bac t e r i a l g r ow t h . P r i o r w or k i n ou r l abo r a t o r y (Lew i s et al . ,1978) sugges t ed t ha t t he se p r ob l em s cou l d be r educed by t he u se o f a b i o f i l te r w i t h ar ec i p r oca t ing pa t t e r n o f w a t e r f l ow ( i .e . a l t e r na t e f l ood i ng and d r a in i ng o f t he b io -f i l t e r ) . To tes t th i s hypothes i s , we compared b iof i l t e r s wi th a rec iproca t ing f lowpa t t e r n ( he r ea f t e r r e f e r r ed t o a s ' r e c i p r oca t ing b i o f i l te r s ' ) and con ven t i ona l subm er gedb i o f i l t e r s w i t h an up f l ow pa t t e r n , i n t e r m s o f ab i l i t y , t o m a i n t a i n w a t e r qua l i t y i nsmall-scale f ish ho ldin g uni ts . 139Aquaeultural Engineering 0 1 4 4 - 8 6 0 9 / 8 2 / 0 0 0 1 - 0 1 3 9 / $ 0 2 . 7 5 A p p l i e d S c ie n c e P u b l is h e rs L t d ,E n g l a n d , 1 9 8 2P r i n t e d i n G r e a t B r i t a i n

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140 M.H. PALLER, W. M. LEWISMETHODS AND MATERIALS

T h i s s t u d y c o n s i s t e d o f t w o t r ia ls . T h r e e r e c i p r o c a t i n g b i o f i l t e r s y s t e m s w e r e c o m p a r e dw i t h t h r e e s u b m e r g e d b i o f i l t e r s y s t e m s i n e a c h t ri al . A l l s y s t e m s w e r e 1 0 0 % r e c y c l e ,w i t h t h e e x c e p t i o n o f t h e w a t e r e x c h a n g e t h a t r e s u l te d f r o m b a c k f l u s h i n g t h eb io f i l t e r s . W a te r f l o w r a t e s a n d b io f i l t e r d im e n s io n s we r e v a r i e d b e twe e n t r i a l s .

T h e p r o c e d u r e f o r e a c h o f t h e t r ia ls w a s t o r e g u l a rl y m o n i t o r w a t e r q u a l i t y i n e a c hs y s t e m , w h i l e g r a d u a l l y i n c re a s i n g t h e s t o c k i n g a n d f e e d i n g ra t e s u n t i l t h e c a r r y i n gc a p a c i t y o f e a c h s y s t e m w a s d e te r m i n e d . T h e s y s t em s w e r e s t o c k e d w i t h l a b o r a t o r y -a c c l i m a t e d g o l d f i s h f e d S i lv e r C u p n u m b e r 2 t r o u t f o o d . T h e s y s t e m s w e r e c o m p l e t e l yd r a i n e d b e t w e e n t h e t r ia l s, w h i c h w e r e s e p a r a te d b y a l i tt l e u n d e r f o u r m o n t h s .

S h a l l o w b io f f l t e r s we r e u s e d d u r in g t h e f i r s t t r ia l ( F ig . 1 ) . E a c h g r a v e l b e d wa s3 2 c m w i d e , 4 3 c m l o n g , a p p r o x i m a t e l y 8 c m d e e p a n d c o n t a i n e d 1 % 5 k g o f g ra v el .T h e g r av e l p a rt i c le s a v e r a g e d 9 . 7 m m i n d i a m e t e r . W a t e r f r o m a f is h t a n k e n t e r e d e a c hb i o f i l t e r t h r o u g h a m a n i f o l d l o c a t e d b e l o w t h e g r a v el b e d . A n e m p t y s p a c e b e l o w t h em a n i f o l d p e r m i t t e d t h e p a r t i c u l a te m a t t e r t o s e t tl e . T h e w a t e r t h e n f lo w e d u p w a r d

. . . . o . / /

o F I G F

J JFig. 1. D es ign of the reciprocating and submerged biof 'flter systems used in trial 1. Com ponentscom m on to bo th systems include: fish tank (A), air stone ( B) , standpipe (C), valve (D), biof 'dter(E), gra vel bed (F ), m anifold (G), settling basin (H ), biof'flter standpipe (I), and pu m p (J). Th e

reciprocating systems also ineludes a perforated pipe housing a float switch and float (K).

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R E C I P R O C A T I N G B I O F I L T E R F O R W A T E R R E U S E 1 4 1

t h r o u g h t h e g r av e l a n d s u b s e q u e n t l y d r a i n e d t o a p u m p . T h e w a t e r w a s p u m p e d u p t oa 75 l i t r e f i sh t ank , whe re i t was v igo rous ly ae r a t ed w i th compres sed a i r . Wa te r f romthe f i sh t ank f l owed d ow n to t he b io f i l t e r a t t he r a t e o f 8 .5 l i t r e min -1.

The pa t t e rn o f f l ow in t he r ec ip roca t i ng f i l t e r s was d i f f e r en t f rom tha t i n t he sub -m e r g e d f il te r s. W a te r f l o w e d i n t o a n d w a s p u m p e d o u t o f ea c h s u b m e r g e d b i o f il t e r a te q u a l a n d c o n s t a n t r a te s , t h u s k e e p i n g e a c h s u b m e r g e d f i lt e r b e d c o n s t a n t l y i n u n d a t e d .W a te r f low ed in to each r ec ip roca t i ng b io f i l t e r a t a cons t an t r a t e , bu t was pe r iod i ca l l yp u m p e d o u t a t a m u c h h i g h e r r a t e , t h u s d e w a t e r i n g e a c h re c i p r o c a ti n g t r it e r b e d a tr egu l a r in t e rva ls . A f l oa t sw i t ch deac t i va t ed t he pum p wh en the f i l te r bed was fu l l yd ra ined , an d a c t i va t ed t he pu m p w hen the wa te r r o se t o a l eve l s eve ra l c en t im e t r e sabove t he g r avel . Each cyc l e o f f il li ng and d ra in ing t oo k app rox im a te ly 2 r a in to com -p le t e . T he t o t a l wa t e r vo lum e in each subm erged f al te r sy s t em ( i. e . f il t e r and f i sh t ank )was a pp rox im a te ly 100 l i tr e s , and ap p ro x im a te ly 93 l i tr e s i n e ach r ec ip roca t i ng f i l te rs y s t e m .

Deep b io f i l t e r s we re u sed i n t he s econd t r ia l (F ig . 2 ). Each g rave l bed was cy l i nd r i ca l,49 cm deep , 15 .35 cm in d i am e te r , and con t a in ed 15 .9 kg o f g r avel . W a te r f l owed in to

;0E / I

i ) lF i g . 2 . D e s i gn o f t h e re c i p r o c a t i n g a n d s u b m e r g e d b i o f i lt e r s y s t e m s u s e d i n t r ia l 2 . C o m p o n e n t sc o m m o n t o b o t h s y s t e m s i n c l u d e : f i s h t a n k ( A ) , a i r s t o n e ( B ), s t a n d p i p e ( C ) , v a lv e (D ) , b i o f i l t e r( E ) , g r a ve l b e d ( F ) , s e t t l i n g b a s i n ( G ) , b i o f i l t e r o u t l e t ( H ) , a n d p u m p ( I ) . T h e r e c i p r o c a t i n g s y s t e ma l so i n c l u d e s a s e p a r a t e c o n t a i n e r a t t a c h e d t o t h e b i o f i l t e r w h i c h h o u s e s a f l o a t s w i t c h a n dfloat (J).

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142 M.H . PALLER, W. M. LEWIS

each b io f i l t e r bed a t a r e l a t i ve ly l ow r a t e - 2 . 5 l i tr e min -1. A s ingl e r ec ip roca t i ngc y c l e t o o k 3 - 4 m i n t o c o m p l e t e . T o t a l w a t e r v o l u m e in e a c h fi lt e r s y s t e m w a sap p ro x im a te ly 87 l it r es . The o the r f e a tu r e s o f t he d eep b ed b io f i l t e r s we re s imi la r t othose o f t he sha l l ow bed b io f i l t e rs , w i th one exc ep t ion : t he f l oa t and f l oa t sw i t ch o feach deep bed r e c ip roca t i ng b io f i l t e r we re co n t a ined i n a s epa ra t e hous ing wh ich wasconnec t ed t o t he con t a ine r enc lo s ing t he f i l t e r bed . Th i s connec t i on pe rmi t t ed t hewa te r leve ls t o equa l iz e w i th in t he pa i r ed con t a ine r s .

I t w a s n e c e s sa r y t o b a c k f l u s h t h e d e e p b e d b i o f il t er s t o m a i n t a i n u n i m p e d e d f lo w .B a c k f lu s h in g w a s a c c o m p l i s h e d b y c o n n e c t i n g a n e x t e rn a l w a t e r s o u r ce t o t h e b o t t o mof t he t r it e r hous ing and d i r ec ti ng a r e l a t ive ly pow er fu l w a t e r s t r e am upw ard t h roughthe g r ave l . Th i s p rocedu re d i s l odged accumula t ed de t r i t u s , wh ich f l owed up and ove rthe edge o f t he f i l te r hous ing . T he g r ave l was s ti r r ed w i th a rod du r ing back f lu sh ing t ohe lp d i s l odge pa r t i cu l a t e ma t t e r .T h e c h e m i c a l a n d p h y s i c a l p a r a m e t e r s m e a s u r e d i n t h e s t u d y a re l is t ed b e l o w .

(1 ) D i s so lved ox ygen con cen t r a t i on (DOC ) (r ag l i tr e - 1) was m easu re d wi th a YSIpo la rog ra ph . M easu rem en t s w e re m ade i n the f ish t anks du r ing t he f ir s t t r ia l ,and i n t he f i sh t anks and b io f i l t e r e f f l uen t s du r ing t he s econd t r i a l . F i l t e re f f l uen t was co l l e c t ed f rom a s am pl ing po r t l oc a t ed a t t he ba se o f e ach deepb e d r e c i p r o c a t i n g b i o f i l t e r c o l u m n t o a v o i d c o n t a m i n a t i o n b y t h e u n f i l t e r e dwa te r i n t he a s soc i a t ed con t a ine r hous ing t he f l oa t and sump pump swi t ch .T h i s p e r m i t t e d c o m p a r i s o n w i t h f il te r e f f l u e n t f ro m t h e s u b m e r g e d b i o ff l te r swh ich was co l l e c t ed f rom the ou t f l ow in to t he f i sh t ank .

( 2 ) T o t a l a m m o n i a - n i t r o g e n ( m g l it re -1 ) w a s m e a s u r e d w i t h a n O r i o n a m m o n i ae l ec t rode 9 5 -1 00 and O r ion 901 M ic rop roces so r Iona ly ze r (USEPA , 1974) .S a m p l e s w e re t a k e n f r o m t h e t a n k s a n d f r o m t h e b i o f i lt e r ef f l u e n ts d u r in g b o t ht ri a ls . F i l t e r e f f l uen t s amp le s we re d r aw n f rom sam ple po r t s i n t he deep bedrec ip roca t i ng b io f 'f l te r co lumns .

( 3 ) N i t r i t e - n i tr o g e n ( m g l i t r e - 1) w a s m e a s u r e d w i t h a H a c h s p e c t r o p h o t o m e t e r .Sample s we re t aken f rom the f i sh t anks .

( 4 ) N i t r a t e - n i t r o g e n ( r ag l it r e -1 ) w a s m e a s u r e d w i t h a H a c h s p e c t r o p h o t o m e t e r .Sample s we re t aken f rom the f i sh t anks .

(5 ) Te m pera tu r e ( C) was m easu red i n t he f ish t anks .( 6 ) p H W a s m e a s u r e d w i t h a C o m i n g M o d e l 1 2 p H m e t e r a n d e l e c tr o d e s . S a m p l e s

were t aken f rom the f i sh t anks .(7 ) A lka l i n i t y (ac id b ind ing cap ac i t y ) was de t e rm ined acco rd ing t o S t and a rd

M e t h o d s p r o c e d u r e 4 C ( A P H A , 1 9 7 5 ). T h e e n d p o i n t w a s p H 4 . 4 . S am p l e sw e r e t a k e n f r o m t h e f is h t an k s .

RESULTSA m m onia -n i t ro gen l eve l s r em a ined r e l a t i ve ly l ow in each sys t e m un t i l a c r it i c a l l oad ingr a t e w a s e x c e e d e d b u t t h e n r a p i d ly i n c re a s e d t o 1 0 - 4 0 m g l i t r e -1 ( F ig s 3 a n d 4 ) .

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R E C I P R O C A T IN G B I O F IL T E R F O R W A T E R R E U S E 143

N u m b e r o f f i s h 0 2 7 1 2 1 7 2 2 2 2 2 2 2 7 3 2 3 7 4 2F o o d ( g ) 0 :15 6 8.5 11 1 7 . 5 20 23 26 29 29

- -~" t I . 5.0 II t I. E E I I Ic I. 0 4 0 I ~ :1

I 1

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t t I i2.0 /~ l I :. _ d ' : : i i l lz : ' : j ,I V10 I !1

< o o - - - i l : : i0 10 20 30 40 50 60 70D a y s

F ig . 3 . C h a n g e s i n to t a l a m m o n i a - n i t r o g e n c o n c e n t r a t i o n i n t h e f is h t a n k s o f t h e r e c i p ro c a t in gb i o f i l te r s y s t e m s ( e ) , a n d i n t h e f i s h t a n k s o f t h e s u b m e r g e d b i o f i l te r s y s t e m s ( o ), v e r s u s t i m e a n dc h a n g e s i n l o a d i n g r a t e ( t ri a l 1 ) . E a c h d a t a p o i n t i s a n a v e r a g e d e r i v e d f r o m t h r e e s y s t e m s . C h a n g e si n l o a d i n g r a t e a r e i n d i c a t e d b y b r o k e n l in e s .

N u m b e r o f f i s h 0 2 5 15 2.5 30 3.5 40 50 4 5F o o d (g~ 0 I 2 .5 Z5 12.5 15 1.75 20 25 22.5

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< o.o 0 10 20 30 40 50 60 70 80D a y s

F i g . 4 . C h a n g e s i n t o t a l a m m o n i a - n i t r o g e n c o n c e n tr a te , o n i n t h e fi s h t a n k s o f t h e r e c i p r o c a t i n gb i o f i l t e r s y s t e m s ( o ) , a n d i n t h e f is h t a n k s o f t h e s u b m e r g e d b i o f i l t e r s y s t e m s ( o ), v e rs u s t i m e a n dc h a n g e s i n l o a d i n g r a t e ( t ri a l 2 ) . E a c h d a t a p o i n t i s a n a v e r a g e d e r i v e d f r o m t h r e e s y s t e m s . C h a n g e si n l o a d i n g r a t e a r e i n d i c a t e d b y b r o k e n l in e s .

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144 M .H. PALL ER, W. M. LEWIST h e r e f o r e , t h e c a r r y i n g c a p a c i t y o f e a c h s y s t e m i s d e f i n e d a s t h e m a x i m u m s t o c k i n gd e n s i t y a n d f e e d i n g r a t e a t t a i n e d b e f o r e t h e d e v e l o p m e n t o f a l a rg e i n c re a s e i na m m o n i a - n i t r o g e n c o n c e n t r a t i o n . T h i s d e f i n i ti o n c o u l d n o t b e a p p l i e d t o s u b m e r g e df i lt e r s y s t e m 1 o f t r i a l 2 , w h e r e s e v er e c l o g g in g o f th e g r a v e l b e d p r e v e n t e d n o r m a lw a t e r f l o w d e s p i t e r e p e a t e d b a c k f l u s h i n g . I n t h i s c a s e, c a r r y i n g c a p a c i t y i s d e f in e d ast h e m a x i m u m s t o c k in g d e n s i t y a n d f e e d in g ra t e a t t a i n e d b e fo r e t h e o c c u r re n c e o fs e v er e i r r e m e d i a l f i lt e r c l og g i n g. T h e c a r r y i n g c a p a c i t y o f r e c i p r o c a t i n g f il t er s y s t e m 2w a s n o t d e t e r m i n e d d u r i n g t r ia l 1 b e c a u s e o f a m e c h a n i c a l f a i l u re w h i c h fo r c e d it sp r e m a t u r e s h u t d o w n .

T o p r e v e n t t h e i r l o s s , t h e f i sh w e r e u s u a l l y r e m o v e d f r o m e a c h s y s t e m s o o n a f t e rt h e d e v e l o p m e n t o f th e a m m o n i a - n i t r o g e n i nc r ea s e . H o w e v e r , th i s w a s n o t d o n e i n t h et h r e e r e c i p r o c a t i n g f i l t e r s y s t e m s d u r i n g t ri a l 2, w h e r e s t o c k i n g d e n s i t i e s a n d f e e d i n gr a te s w e r e i n s t e a d d e c r e a s e d t o c a r r y i n g c a p a c i t y le v e ls s o o n a f t e r t h e a m m o n i a -n i t r o g e n i n c re a s e b e g a n t o d e v e lo p . T h e r e t u r n o f a m m o n i a - n i t r o g e n c o n c e n t r a t i o n st o l o w l e v el s f o l lo w i n g t h i s a c t i o n ( F i g . 4 ) c o n f i r m e d t h e c a r r y i n g c a p a c i t y a ss e s s m e n ti n t h e s e s y s t e m s .

C a r r y i n g c a p a c i t ie s in t h e r e c i p r o c a t i n g f i l t e r s y s t e m s w e r e h i g h e r t h a n i n t h e s u b -m e r g e d f i l t e r s y s t e m s ( T a b l e 1 ). I n t r i a l 1, t h e r e c i p r o c a t i n g f i l te r s y s t e m s a v e r a g e d5 5 % m o r e f is h i n t e r m s o f n u m b e r s , 5 9 % m o r e f is h i n t e rm s o f w e i g h t a n d a 4 5 %g r e a t e r f e e d i n g r a t e . I n t r i a l 2, t h e r e c i p r o c a t i n g f il t e r s y s t e m s a v e r a g e d 2 9 % m o r e f i s hi n t e rm s o f n u m b e r s , 3 3 % m o r e f is h in t e r m s o f w e i g h t a n d a 2 9 % g r e a t e r f e e d in g r a te .

TABLE 1Re lative capa bili ty of recipro cating and submerged biof 'f l ters in terms o f carrying cap acityReciprocating Submergedfilter systems filter systems

1 2 3 1 2 3Trial I b

Nu mb er of f ish 37 c 52 27 27 32Cumulative weight of f ish (g) 774 _c 1099 559 533 675Qu ant i ty of food per day (g) 26 _c 35 20 20 23Trial 2 d

Nu mb er of f ish 45 45 45 35 35 35Cum ulative weight of fish (g) 1006 1049 995 754 769 767Qu an tity of foo d per day (g) 22.5 22.5 22.5 17.5 17.5 17,5a . .b Higher loading ra tes caused am mom a concen trat ions to increase to poten t ia l ly toxic levels .Sha l low b io f i l te r s em ployed .e Mechanical fa i lure caused shutdown before the car rying capaci ty could be determined; 37 f i sh~(eighing 737 g fed 26 g at t im e o f shutdown.

Deep biof i l ters emp loyed,

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RECIPROCATING BIOFILTER FOR WA TER REUSE 145

The ave rage am m o ni a - n i t r og en va lue s i n t he r ec i p r oca t i ng f il t e r sys t em f i sh t anksa f t e r t he r ec i p r oca t i ng f i lt e rs becam e ac t i va t ed , bu t be f o r e t he f i l te r c a r r y ing capac it ie sw er e exce eded , w e r e 0 . 28 m gl i t r e -1 du r i ng t ri a l 1 and 0 - 5 8m gl i t r e - ' du r i ng t ri a l 2 ;co r r e spon d i ng va lue s f o r t he subm er ged f i lt e r sys t em s w er e 0 . 25 and 0 . 07 m g li tr e - 1,r e spec t i ve l y . D ur i ng t r i a l 2 , am m oni a - n i t r ogen concen t r a t i ons i n t he r ec i p r oca t i ngf i l t e r sys t em s a r e i n i t i a l l y som ew ha t h i ghe r t han i n t he subm er ged f ~ t e r sys t em sbecause o f occas iona l c logging o f the grave l in te rs t i ces in the rec iproca t ing f i l te rco l um ns , w h i ch caused w a t e r t o be channe l ed i n t o the co l um n s hous i ng t he f l oa tsw i t ches . Th i s g r ea tl y r educed t he f low r a t e s t h r oug h t he r ec i p r oca t i ng b i o f i l te r hedsand r e su l t ed i n am m oni a - n i t r ogen peaks a t days 38 and 55 ( F i g . 4 ) . Th i s p r ob l em i sno t i nhe r en t in t he de si gn and o pe r a t i on o f r ec i p r oca t ing f il t e r sys t em s and r e su l tedi n a conse r va t i ve e s t im a t e o f t he r e la t ive pe r f o r m ance o f t he r ec i p r oca ti ng f i lt e rsdur ing t r i a l 2 .D i s sol ved ox ygen co ncen t r a t i ons i n the f ish tanks o f t he r ec i p r oca t i ng f i lt e r sys t em swere h igh er th an in the f ish t anks of the subm erged f i l t e r sys tem s (Figs 5 and 6) . Thisd i f fe renc e becam e p rogress ive ly grea te r as s tocking dens i t i es and feed ing ra tes beca megr ea t e r. A dd i t iona l l y , d is so lved oxy gen m easu r em en t s m ade i n t he b i o f i l t e r e f f l uen t sdur ing t r i a l 2 indica ted th a t oxy gen level s w i th in the rec iproca t ing b iof i l t e r s remaine dabove 3-0 mg l i tre -1, b ut fe l l to app rox im ate ly 0-5 mg l i tre -1 wi th in the subm ergedbiofi l ters (Fig. 7) .

Average n i t r i t e -n i t rog en con cen t ra t ion s w ere s imi la r in the rec ipro ca t ing and sub-subm erged f i l t e r sys tem s dur ing t r i a l 1 (1 .41 and 1 .22 m gl i t re -1, respec t ive ly) .N i t r i t e - n i t r ogen co ncen t r a t i ons peaked i n bo t h t he rec i p r oca t i ng and subm er ged fi l te rsys t em s a f t e r t he f i r s t f ew f i sh w e r e s t ocked , bu t peaked h i ghe r and l a t e r i n t her ec i p r oca t ing f i lt e r sys tem s ( F ig . 8 ) . N i t r it e - n i t r ogen conc en t r a t i ons du r i ng t r ia l 2

N u m b e r o f f i s h 0 7 12 17 22 22 22 27 32 37 42F o o d (g) 0 a5 6 ~.5 11 lz5 20 23 26 29 29

- i ! ! ! i i !

~ 6.0 -. -e

~ C : : i i : : : i i i :1.2 OO 0 }0 20 30 40 50 60 70D a y s

F i g . 5 . C h a n g e s i n d is s o l v e d o x y g e n c o n c e n t r a t i o n i n t h e f i s h ta n k s o f t h e r e c i p r o c a t i n g b i o f f i t e rs y s te m s ( e ) , a n d i n t h e f i s h t a n k s o f t h e s u b m e r g e d b i o f J l t e r s y s t em s ( o ) , v e r su s t i m e a n d c h a n g e s i nl o a d i n g r a t e ( t r i a l 1 ) . E a c h d a t a p o i n t i s a n a ve r ag e d e r i v e d f r o m t h r e e s y s te m s . C h an g e s in lo a d i n g

r a t e ar e i n d i c a t e d b y b r o k e n l in e s .

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1 4 6 M . H . P A L L E R , W . M . L E W I S

N u m b e r o f f i s h 0 2 5 1 5 2 5 3 3 5 4 0 5 0 4 5F o o d ( g ) 0 1 2 .5 7 .5 12.5 15 1z5 20 25 22.59 i ~

/ . _ - 8 o - - L ~ : - - :~

o 8 5 ~ ' ~ - " " - - )'g82

10

10 20 30 40 50 60 70 80D a y s

F ig . 6 . C h a n g e s i n d i s s o l v e d o x y g e n c o n c e n t r a t i o n i n t h e f i s h t a n k s o f t h e r e c i p r o c a t i n g b i o f i lt e rs y s t e m s ( o ) , a n d i n t h e f i s h t a n k s o f t h e s u b m e r g e d b i o f i l t e r s y s t e m s ( o ), v e r s u s t i m e a n d c h a n g e si n lo a d i n g r a t e ( t r ia l 2 ) . E a c h d a t a p o i n t i s a n a v e r a g e d e r i v e d f r o m t h r e e s y s t e m s . C h a n g e s i nl o a d i n g r a t e a re i n d i c a t e d b y b r o k e n l i n e s.

N u m b e r o f f i s h 0 2 5 ]5 2 5 3 0 3 5 4 0 5 o 4 5F o o d ( g / 0 1 ;5 .75 125 15 l z5 20 25 22.5

~ . 8 Q . : :

o _ 5

-~ 2a3~0 i i

0 10 20 30 40 50 60 70 80D a y s

F i g. 7 . C h a n g e s i n d i s s o l v e d o x y g e n c o n c e n t r a t i o n i n t h e b i o f i lt e r e f f l u e n t s o f t h e r e c i p r o c a t i n gb i o f i l te r s y s t e m s ( o ), a n d i n t h e b i o fi l t e r e f f l u e n t s o f th e s u b m e r g e d b i o f i lt e r s y s t e m s ( o ), v e r s u st i m e a n d c h a n g e s i n l o a d i n g r a t e ( t ri a l 2 ). E a c h d a t a p o i n t i s a n a v e r a g e d e r i v e d f r o m t h r e e s y s t e m s .C h a n g e s in l o a d i n g r a te a r e i n d i c a t e d b y b r o k e n l i n e s .

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R E C I P R O C A T I N G B I O F I L T E R F O R W A T E R R E U S E 147

N u m b e r o f fi s h 0 2 7 12 17 22 22 22 27 32 37 42F o o d { ,g ) o ~ ~ .5 6 8 .~ , 17.52o 23 26 ~929

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D a y sFig. 8. C ha ng es in nitrite-nitrogen concentration in the fish tanks of the reciprocating biofiltersystems (o), and in th e fish tanks o f the subm erged biofilter systems (o), versus time and changes inloading rate (trial 1). Each data poi nt is an average derived from three sy stems. Changes in loadingrate are indicated b y broke n lines.

w e r e u n i f o r m l y l o w i n b o t h t h e r e c ip r o c a ti n g ( m e a n = 0 . 3 4 m g l i t r e - 1, r an g e =0 . 0 1 - 1 . 0 0 ) a n d s u b m e r g e d ( m e a n = 0 - 1 8 m g li tr e -1 , r a ng e = 0 - 0 3 - 0 . 8 5 ) f i lt e r s y s t e m s .

N i t r a t e - n i t r o g e n c o n c e n t r a t i o n s s h o w e d t h e s a m e p a t t e r n o f c h an g e i n all re c ip r o -c a t i n g f i l t e r s y s t e m s d u r i n g b o t h t r i a l s . C o n c e n t r a t i o n s w e r e l o w , t h e n c l i m b e d s t e a d i l yt o a p p r o x i m a t e l y 1 5 m g l it r e -1 a n d s u b s e q u e n t l y d e c l i n e d t o a p p r o x i m a t e l y 3 - 6 m gl it re -1 . T h e d e c l in e w a s p r o b a b l y d u e t o t h e o c c u r r e n c e o f d e n i t r i f i c a t i o n i n t h ea n o x ic s e t t l i n g b a s in s l o c a t e d b e n e a th t h e f i l te r s . N i t r a t e i n c r e a s e s a l s o o c c u r r e d i n t h es u b m e r g e d f i lt e r s y s t e m s b u t w e r e n o t f o l l o w e d b y c o n s p i c u o u s d e c re a s es a s i n t h er e c i p r o c a t i n g f i lt e r s y s t e m .

T e m p e r a t u r e s i n t h e s u b m e r g e d a n d r e c i p r o c a t i n g f il t er s y s t e m s w e r e s im i l ar w i t h i nt ri a ls . M e a n t e m p e r a t u r e s d u r i n g tr ia l 1 w e r e 2 2 . 7 C ( 2 1 . 5 - 2 3 . 5 C ) i n t h e r e c ip r o -c a t i n g f il te r s y s t e m , a n d 2 3 . 7 C ( 2 2 . 0 - 2 5 . 0 C ) i n th e s u b m e r g e d f i lt e r s y s t e m s . M e a nt e m p e r a t u r e s d u r i n g t r ia l 2 w e r e 2 5 . 8 C ( 2 3 . 5 - 3 1 . 0 C ) i n t h e r e c i p r o c a t i n g s y s t e m sa n d 2 6 . 1 C ( 2 3 . 5 - 2 9 . 0 C ) i n t h e s u b m e r g e d f i l te r s y s te m s .

A l k a l i n i t y c h a n g e s f o l l o w e d t h e s a m e p a t t e r n i n a ll s y s t e m s d u r i n g b o t h t ri al s.A lk a l i n i t y a v e r a g e d 4 5 m g li t re -~ in t h e r e c ip r o c a t i n g f i l te r s y s t e m s a t t h e b e g in n in g o ft r i al 1 , 5 7 m g l i t re - 1 i n t h e s u b m e r g e d t r it e r s y s t e m s a t t h e b e g in n in g o f t r ia l 1 , 8 6 m gl i tr e - 1 i n t h e r e c ip r o c a t i n g f i l te r s y s t e m s a t t h e b e g in n in g o f t ri a l 2 a n d 6 4 m gl i tr e - 1 in t h e s u b m e r g e d f i l te r s y s t e m s a t t h e b e g in n in g o f t r i al 2 . A s t h e t r ia l sc o n t i n u e d , a l k a l i n i t y l e ve ls p r o g r e s s iv e ly d e c l i n e d t o v a lu e s a s l o w a s 1 6 m g l i tr e - 1.W h e n l o w l ev e ls w e r e r e a c h e d , a l k a l i n it y w a s r e s t o r e d b y t h e a d d i t i o n o f 5 - 5 0 g o fs o d i u m b i c a r b o n a t e t o e a c h s y s t e m . A l k a l i n it y w a s r e s t o r e d t h r e e t i m e s i n t ri a l 1 a n d

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148 M . H . P A L L E R , W . M . L E W I Stw ice i n t r ia l 2 . A lka l i n i t y l eve ls r o se as h igh a s 310 m g l i t r e - ' a f t e r r e s to r a t i on wi ths o d i u m b i c a r b o n a t e .

F luc tua t i ons i n a lka l i n i t y we re a s soc i a t ed w i th f l uc tua t i ons i n pH. pH Var i edbe tw een 7 .2 and 8 .4 i n t he r ec ip roca t i ng f i lt e r sy s t em s du r ing tr i al 1 , be tw een 6 .7and 8 .1 i n t he su bm erged f i lt e r sy s t ems du r ing t r ia l 1 , be tw een 6 .7 an d 8 -2 i n t her ec ip roca t i ng f i l te r sy s t em s du r ing t ri a l 2 and be tw een 6 .7 an d 7 -7 i n t he sub me rgedf i l t e r sy s t ems du r ing t r i a l 2 . pH Usua l ly r ema ined above 7 .2 i n a l l sy s t ems . F luc tua -t i ons in pH a nd a lka l i n i t y we re no t a s soc i a t ed w i th o th e r changes i n wa te r qua l i t y o rwi th changes in f ish behav io r .

No ne o f t he sha l l ow b io f i l te r s em p loy ed du r ing t ri a l 1 we re back f lu shed , s incew a t e r f l o w t h r o u g h t h e f i lt e rs a p p e a r e d t o b e u n i m p e d e d . H o w e v e r , t h e m u c h d e e p e rc o l u m n a r f il te r s e m p l o y e d d u r i n g t ri al 2 o f t e n b e c a m e b l o c k e d a n d r e q u ir e d r e p e a te db a c k f lu s h i n g . T h e s u b m e r g e d c o l u m n a r b i o f il t e rs r e q u i r e d m o r e b a c k f l u s h in g t h a n t h er e c i p r o c a ti n g c o l u m n a r b i o f d t e r s . F o r t y - t h r e e b a c k f l u sh i n g o p e r a t i o n s w e r e d i s tr i b u t edamong the t h r ee submerged b io fx l t e r s . C logg ing was so s eve re i n submerged f i l t e r 1t h a t i t b e c a m e i m p o s s i b l e t o m a i n t a i n t h e w a t e r f l o w , d e s p i t e r e p e a t e d b a c k f l u s h i n g sa t 8 h in t e rva ls . I n con t r a s t , o n ly 24 back f lu sh ing o pe ra t i ons w e re neces sa ry am ong theth r ee r ec ip roca t i ng f i lt e rs . S ince t he r ec ip roca t i ng f i lt e r s ope ra t ed l onge r t han t hesubm erged f i lt e rs , t h is a m ou n t s t o 0 .29 back f lu sh ings pe r day am on g the r ec ip roca t i ngf il te r s y s te m s , a n d 0 . 6 3 b a c k f lu s h i n g s p e r d a y a m o n g t h e s u b m e r g e d f il te r s y st e m s .

M i c r o b i a l g r o w t h a p p e a r e d t o d i f f e r b e t w e e n t h e s u b m e r g e d a n d r e c i p r o c a t i n gf il te r s . Th i s w as mo s t no t i c eab l e wh en v i ewing t he t ops o f t he g r ave l beds i n t hesha l l ow b io f i l t e r s . Mic rob i a l g rowth appea red t o fo rm a r a the r c lo se ly adhe r ing l aye ra roun d ind iv idua l g r ave l pa r t i c l e s in t he r ec ip roca t i ng f i l t e r beds , bu t f o rm ed loosea s soc i a t ions co l l e c t i ve ly cove r ing a nu m be r o f g r ave l pa r t i c le s i n t he su bm erged f i lt e rb e d s .

DISCUSSION

L a c k o f o x y g e n c a n l i m i t t h e p e r f o r m a n c e o f a e r o b i c b i o f il t e rs ( S e m m e n s , 1 9 7 6 ) .R e c o g n i t i o n o f t h is f a c t ha s le d t o t h e d e v e l o p m e n t o f v a r io u s m e t h o d s f o r s u p p l y i n go x y g e n t o b i o f' d te r s. T w o s u c c e s sf u l m e t h o d s a r e t h e a d d i t io n o f a ir o r o x y g e n t o t h eb io f i l t e r i n f l uen t (Meade , 19 74 ; Lewi s e t a I . , 1981b) , and t he u se o f revo lv ing c i r cu l a rp l a t e s wh ich cyc l i c a ll y expose t he a t t a che d mic rob i a l g rowth t o t he f i lt e r i n f l uen t andto t he a tm osp he re (Lewis and Bu ynak , 1976 ) . The r ec ip roca t i ng b io f 'l l te r p rov idesoxy gen t o t he f i l te r bac t e r i a by pe r iod i ca l l y dewa te r ing t he f i lt e r bed . Th i s p roces sc r ea t e s a vacu um wh ich d r aws a i r i n to t he g r ave l i n t e r s ti c e s and oxyge na t e s t he t h in f i lmo f w a t e r s u r r o u n d i n g t h e g r av e l p a r ti c le s . O x y g e n a t i o n o c c u r s u n i f o r m l y t h r o u g h o u t t h ef i l te r bed . As a r e su lt , o xyg en l eve ls in r ec ip roca t i ng f 'd t e r e f f l uen t s t end t o be h ighe rthan i n submerged f i l t e r e f f l uen t s .

The oxy gen a t ing ac t i on o f t he r ec ip roca t i ng f i l te r i s pa r t i cu l a r l y im por t an t a t h ighl o a d i n g r a t e s . U n d e r s u c h c o n d i t i o n s , e x t e n s i v e o x y g e n c o n s u m p t i o n b y f i s h a n d

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R E C I P R O C A T I N G B I O F I L T E R F O R W A T E R R E U S E 149bac t e r i a i n t he f ish t ank r e su lt s in l ow oxy gen l eve ls i n t he b io f i l t e r i n f l uen t . Con-cu r r en t l y , oxy gen dem and wi th in t he b io f i l t e r i nc r ea ses because o f t he l a rge quan t i t i e so f o r g a ni c m a t t e r a n d a m m o n i a e n te r i n g t h e f i lt e r. A s o x y g e n c o n c e n t r a t i o n s a p p r o a c hl imi t ing leve ls , the e f f ic ie ncy o f the f 'f lt er be d dec l ines and th e qu a l i ty of the f i l t e re f f l uen t de t e r i o r a t e s . The re fo re , an even g rea t e r l oad i s imp osed upo n the f i l te r t hene xt t im e the w ate r i s recyc led . Th e resu l t i s a v ic ious c i rc le caus ing the co l lapse off i l te r p e r fo rm anc e and a re su l t ing r ap id i nc rea se i n am m onia conc en t r a t i on . T h i sco l l apse is de l ayed by t he ae r a t i ng ac t i on o f t he r ec ip roca t i ng f i lt e r , wh ich help sr ep l en i sh t he oxy gen con sum ed by t he f i lt e r bac t e r i a . Fu r th e rm ore , a s oxy gen leve lsb e c o m e p r o g r es s iv e l y u n d e r s a t u r a t e d , a e r a t io n b e c o m e s m o r e e f f ic i e n t d u e to f a s t e rt r anspo r t a c ros s t he a i r -wa te r i n t e r f ace s i n t he dewa te r ed f i l t e r bed . As a r e su l t o ft he se f ac to r s , t he ae r a t i ng e f f ec t s o f t he r ec ip roca t i ng f il t e r a r e no t f u l l y ma n i f e s t edun t i l h igh f i lt e r l oad ing r a t e s a r e a t t a ined .The c logg ing o f b io f i l t e r s w i th mic rob i a l g rowth and pa r t i cu l a t e o rgan i c ma t t e r i sano the r f ac to r t ha t t yp i ca l l y lim i t s b io f i l t e r pe r fo rm anc e . C logg ing o f t en cause schann e l i z a t i on o f t he wa te r t h ro ugh loca l i z ed a rea s o f t he f i l te r bed and cancom ple t e ly ob s t ruc t t he wa te r f l ow . C logging i s t yp i ca l l y con t ro l l ed by occas iona l l yr eve r s ing t he wa te r f l ow and back f lu sh ing t he f i l t e r bed . C logg ing i s appa ren t l yr educed i n t he r ec ip roca t i ng f i l te r s by t he two -wa y wa te r fl ow , wh ich exe r t s a con -t i nuous l ow l eve l back f lu sh ing ac t i on a nd r educes t he occu r r ence o f channe l i z a t i on . I ti s a lso pos s ib l e t ha t t he r ec ip roca t i ng pa t t e rn o f wa t e r f l ow l eads t o fo rm s o f mic ro b i a lg r o w t h a n d o r g an i c m a t t e r a c c r e t io n w h i c h m i n i m i z e cl o gg i ng a n d c h a n n e l iz a t io n .These f ac to r s a r e p robab ly r e spons ib l e fo r t he r educed back f lu sh ing r equ i r emen t s i nthe co lumnar r ec ip roca t i ng f i l t e r s .

Un l ike t he co lu mn ar b io f i l te r s , t he sha l l ow b io f i l t e rs d id no t app ea r t o r equ i r eback f lu sh ing . Re duc ed occ lu s ion in t he sha l low b io f i l te r s was p ro bab ly due t o t hed i s t r i bu t i on o f i ncom ing w a te r and suspended so li d s ove r a r e l a ti ve ly l a rge a r ea. How-eve r , we be l ieve t ha t t he d eep co lum nar b io f i l t e r s m ore acc u ra t e ly s imu la t e a c tua lcond i t i ons i n fu ll -s ca le b io f f l te r s . The sup e r io r pe r fo rm anc e o f t he r ec ip roca t i ngf il te r s u n d e r b o t h t y p e s o f c o n d i t i o n s s u g ge s ts th a t t h e r e c i p r o c a ti n g m o d e o fo p e r a t i o n m a y b e w i d e l y a p p l ic a b l e to m a n y t y p e s o f fi sh c u l tu r e b i o f fl t r a ti o ns y s t e m s .

I n c o m p a r i n g t h e r e su l ts o f t h e t w o t ri al s, i t is n o t e w o r t h y t h a t t h e s u p e r i o r it y o fthe r ec ip roca t i ng f i lt e r s was more ev iden t du r ing t ri a l 1 , wh en ne i t he r t he r ec ip roca t i ngn o r t h e s u b m e r g e d f il te r s w e r e b a c k f l u s h e d . T h e p r o n o u n c e d s u p e r i o r it y o f t h er ec ip roca t i ng f i l t e r s du r ing t r i a l 1 m ay have been t he r e su l t o f channe l i z a t i on i n thesubm erged f i lt e r s, w h ich r e su l t ed i n unde r -u t i l iz a t i on o f som e o f the f i l t e r subs t r a t e .Conve r se ly , t he f ac t t ha t t he submerged b io f i l t e r s we re back f lu sh ing more t han tw icea s o f t en a s t he r ec ip roca t i ng b io f i l te r s d u r ing t r ia l 2 m ay have r e su lt ed i n t heimp os i t i on o f a heav i e r o rgan ic l oad upo n the r ec ip roca t i ng b io fd t e r s , s ince back -f l u sh ing r emoves l a rge amoun t s o f o rgan i c ma t t e r . The re fo re , i n t r i a l 2 t her e c i p r o c a t in g s y s t e m s m i g h t h a v e s h o w n g r e a te r s u p e r i o r i ty i f t h e y h a d b e e n b a c k -f l u shed a s o f t en a s the su bm erged sys t ems .

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150 M.H. PALLER, W.M. LEWISA n o t h e r f a c t o r w h i c h c a n i n f lu e n c e t h e p e r f o r m a n c e o f t h e r e c i p r o c a ti n g f il te r is

t he du ra t i o n o f t he r ec ip roca t i ng cyc l e , i .e . t he t im e needed t o f il l t he e m p ty f i l t e r andthen com ple t e ly d r a in i t aga in . P r i o r expe r i ence i n ou r l abo ra to ry suggests t h i s is af l ex ib le de s ign p a r am e te r and r ec ip roca t i ng cyc l e s l a s t ing 1 h have been succes s fu l lye m p l o y e d . H o w e v e r , th e d i ss o lv e d o x y g e n c o n t e n t a n d o x y g e n d e m a n d o f t h e f il te ri n f l uen t u l t im a t e ly imp ose l im i t a t ions o n t he l eng th o f t he r ec ip roca t i ng cyc le , si ncer ec ip roc a t i on shou ld be f r equen t en ough t o ma in t a in f i lt e r oxy gen l evel s above3 m g l i t re -1 .

R e c i p r o c a t i n g f i l te r s m a y b e i n f e ri o r to s u b m e r g e d f i lt er s i n te r m s o f th e a m o u n t o ft ime r equ i r ed fo r a c t i va t i on . Th i s i s sugges t ed by t he r e su l t o f tr i a l 1 , wh ich show th a tam m onia -n i t r o gen c once n t r a t i ons i n i ti a l ly r o se t o f a i r l y h igh leve ls i n t he r ec ip roca t i ngf i lt e r sy s t ems , bu t r ema ined l ow in t he sub me rged f i l te r sy s t ems . Add i t i ona l l y , n i t r i t e -n i t r o g e n c o n c e n t r a t i o n s s h o w e d e a r ly p e r i o d s o f e l e v a ti o n in b o t h r e c ip r o c a t in g a n dsubm erged f i lt e r sy s t ems , bu t t he pe r i od o f e l eva t i on in t he r ec ip ro ca t i ng f i l te r sy s t em soccu r r ed l a t e r an d l a s t ed l onge r. S imi la r i nc r ease s in am m on ia and /o r n i t r i t e -n i t r ogenc o n c e n t r a t i o n w e r e n o t o b s e r v e d in a n y s y s t e m d u r in g t ri a l 2 , p r o b a b l y b e c a u s e th ef i l t e r subs t ra te used in t r ia l 1 was a l so used in t r ia l 2 . Al though the subs t ra te waswash ed an d dra in ed be tw een t r ia l s , it is p ro bab le tha t l iv ing bac te r ia in an inac t ivefo rm r em a ined i n t he g rave l. I f t he i nocu lum was heavy enough p r eac t i va t i on wou ldoccu r and an i n it i al b r eak ing - in pe r i od wou ld n o t be obse rved .

Rec ip roca t i ng f i l t e r sy s t ems may have a po t en t i a l eng inee r ing advan t age i n t e rms o fi n su r ing t he even d i s t r ibu t i on o f w a t e r t o a l l pa r t s o f the f i lt e r bed . W a te r d i s t r i bu t i onsys t ems such a s man i fo ld s and r evo lv ing a rma tu re s a r e a ma jo r expense i n t he con -s t ruc t i on o f mos t b io f i l t e r s . I n con t r a s t , t he wa t e r i s au toma t i ca l l y d i s t r i bu t ed t o a l lpa r t s o f t he r ec ip roca t i ng b io f i l t e r a s i t en t e r s t he dewa te r ed f i l t e r bed . Th i s f e a tu r emig h t pe rm i t t he c ons t ruc t i on o f r e l a t ive ly s imp le , e con om ica l , ye t f unc t i ona lb iof i ! te rs .

The re a r e va r ious o the r eng inee r ing a spec ts o f r e c ip roca t i ng b io f i l te r s wh ich we reno t i nves t iga t ed du r ing t h i s s tudy . A mechan i ca l f l oa t va lve can be u sed t o f l u sh t her ec ip roca t i ng b io f i l t e r s (Lewi s e t a l . , 1981a ) . Th i s pe rm i t s t he e l imina t i on o f t hee l ec t ron i c f l oa t sw i t ch , and , more im por t an t l y , pe rm i t s t he u t i l i z a t ion o f a l owercapac i t y pump . I t i s a l so pos s ib l e t o ob t a in a con t i nua l d i s cha rge f rom the b io f i l t e r s ,r a t he r t han t he i n t e rm i t t en t d i s cha rge cha rac t e r i s ti c o f t he r ec ip roca t i ng f il t e r sy s t em sused i n t h i s s t udy . Th i s is a ccom pl i shed b y s i t ua t i ng one r ec ip roca t i ng b io f' il t er abovea n o t h e r ( L e w i s e t a l . , 1981a ) . W a te r is con t i nua l l y pum ped t o t he up pe r f il te r . Whenthe f i l t e r is f il l ed , a f loa t va lve rap id ly dra ins i t tho ugh a la rge d ia m ete r p ipe in to thelower f i lt e r . The l ower f i lt e r is de s igned t o d r a in mo re s lowly so t ha t it app roach esem pt ine s s a s t he upp e r f i l t e r en t e r s i ts nex t f l u sh ing cyc l e.

Th i s s t ud y i nd i ca t e s t ha t t he de s i rab l e a t t r ibu t e s o f t he r ec ip roca t i ng b io fd t e ri nc lude : ( 1 ) ab i l i ty t o m a in t a in r e l a ti ve ly h igh oxy gen l eve ls w i th in t he f i l t e r subs t r a t e ,( 2 ) r e s i st ance t o c logg ing, and (3 ) au tom a t i c d i s t r ibu t i on o f t he wa t e r t o a ll pa r t s o ft he f i lt e r bed . The r ec ip roca t i ng f i lt e r pe r fo rm s t he se func t i ons s im p ly , ea s il y anda u t o m a t i c a l l y b y e f f i c i e n t u s e o f t h e e n e r g y o f f l o w i n g w a t e r . O t h e r m e t h o d s o f

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a c c o m p l i s h i l lg t h e s e t a s k s u s u a l l y r eq u ir e a d d i t i o n a l e n e r g y e x p e n d i t u r e s a n d / o r m o r ee l a b o r a t e a n d e x p e n s i v e d e s i g n s. T h e r e f o r e , t h e r e c i p r o c a t i n g d e s i g n m a y b e m o s ta d v a n t a g e o u s i n s i m p l e , e c o n o m i c a l s y s t e m s , a n d l e a s t a d v a n t a g e o u s i n h i g h l ye n g i n e e r e d s y s t e m s i n c o r p o r a t i n g s u c h e x t r a s a s b o t t l e d o x y g e n a n d e l a b o r a t e f il te rc l e a n i n g s y s t e m s , a l t h o u g h f u r t h e r r e se a rc h w i t h l ar g e- sc a le s y s t e m s i s n e e d e d t o c l a r i f yt h e s e p o i n t s . I t i s a l s o n o t e w o r t h y t h a t r e c i p r o c a t i n g f i l t e r s m a y p o s s e s s i n h e r e n td i s a d v a n t a g e s in t e r m s o f t h e t i m e r eq u ir ed f o r a c t iv a t i o n . T h is c o u l d b e i m p o r t a n ti n a p p l i c a t i o n s i n v o l v i n g t h e i n t e r m i t t e n t u s e o f b i o f i l t r a t i o n .

R E F E R E N C E SA P H A ( 1 9 7 5 ) . Standard Methods for the Examination of Water and Wastewater, 1 4 t h e d n .

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