The Activated Sludge Process Part 2. Contact Stabilization Process

7/29/2019 The Activated Sludge Process Part 2. Contact Stabilization Process

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WmtT gaem.t'k Vol. 14, pp. 1737 o 1747INwlptmon Prms Ltd 1990. Printed in Great Britain

T H E A C T I V A T E D S L U D G E P R O C E S S P A R T 2.A P P L IC A T I O N O F T H E G E N E R A L K I N E T IC

M O D E L T O T H E C O N T A C TS T A B I L I Z A T I O N P R O C E S SW . V. ALEXANDER , G . A . EKAM A a n d G . v . R . M ARXtS~

1Scot t & de Wanl Inc . , Johannesburg and" W a te r R e s o u r c e s a n d P u b l i c He a l th En g in e e r in g , Un iv e r s i ty o f Ca p e To w n ,Ro n d e b o s c h , S o u th Af r i c a(Received A ugust 1979)

Ak atzag t- -Th e gene ra l ae ro bic ac t iv a ted s ludg e m ode l inc lud ing n i t r i f ica t ion for pr _oo~__~s t r ea t ing pr in-c ip a l ly mu n ic ip a l wa s te wn te r is a p p l i e d to th e c o n ta c t s t a b il i z a tio n p r o c e s s t r e a t in g mu n ic ip a l wa s te -wa te r s . Th e a p p l i c a tio n in v o lv e s two c h a n lp = to th e m o d e l : ( i) a c h a n g e in o n e o f th e v a lu e s o f th ek i n et ic c o n st a nt s i n t h e e x ~ o f t h e s ub st ra ta u t il iz a ti o n r a te s ; (fi) a c ha n ge in t h e e nm e s h m e ntm a : h m i g m b y a ~ t h a t a f ra c ti o n o f t h e p a rt i cu l at e C O D w h i c h i s m ot a d m r b e d o n t o t h e a c ti v em ' g a n i lm~ d o e s n o t . b e c o me m m e s l m i in th e s lu d g e flo e s a n d e s c a p e s w i th th e e f flue n t. Ac c e p t in g o n lyth e s e d u mg e s i t wa s f o u n d p o s s ib le to s a t i sf a c to r i ly s imu la te th e b e h a v io u r o f th e c o n ta c t a n d s t a b il i z -a t io n r e a z t o ~ o f th e p ro e ss u n d e r b o th c o n s ta n t a n d c y c l ic c o n d i t io n s o f lo a d in g .Fo r des ign , the genera l ac t iva ted s lud ge model , as appl ied to the conta c t s tab i l iza t ion process , r equiresthe W oet~ 8 conf igu ra t ion to b e comp le te ly spec if ied . To a id in the in i t ia l des ign o f the process , ap r 0 1 i l~ d e s ig n p r o c e d u r e i s p r e s e n ted b y me a n s o f wh ic h th e v o lu me s , s lu d g e c o n c e n t r a t io n s a n dr e te h t io n t ime s o f th e c o n ta c t a n d s t a b i l i z a t io n r e a c to r s ma y b e d e te r min e d f r o m f iv e in d e p e n d e n ti a m t m e t e r s w h i s t a r e m u m e d t o g o v e r n th e p r o e e ~ Th e s e a r e th e s lu d g e a g e , re c y c le r a tio , f r a c t io n a ld i s t r ib u t io n o f th e s lu d g e ma s s b e twe e n c o n ta c t a n d s t a b il i z a tio n r e a c to r s , d a l ly CO D ma s s lo a d a n d th eaverage s ludge concentra t ion in the process ,

NOM ENCLATURE 0 - - g e n e r a l s y mb o l f o r o x y g e n c o n s u m p t io n r a t e(mgO1-1 h-X) . Subscr ip ts c , n and t r e fe r tobh* - a n g u s n~q. i r a t ion ra te (d - l ) ca rbonace ous , n i t r i f ica t ion and to ta l va lues re -CM AS P - Co m p le te ly M ix e d Ac t iv a te d S lu d g e P r o c e s s s p e c tiv e ly . Ad d i t io n a l s u b s cr ip ts c , p a n d sC S A S P , - C o n t a c t S t ab i li za ti o n A c t iv a te d S lu d ge P r o - r ef er ~ e l y t o v a lu e s i n t h e c o n ta c t r ea c-osss to t , overa l l p rocoss and s tab i l iza t ion reac torf m a n b i o d e g r ud a b l e f ra c ti o n o f t h e a c ti v e m a s s P -= C O D t o V S S r a t i o ( m g C O D m g V S S - t )( m g V S S m g V S S - x ) Q = i n fl u en t f lo w r a t e ( l d - s )f , I , t fi t ro lpm f rac t ion of the s ludge mass (mg N m g q = wa s te f lo w r a te 0 d - x )VSS - s ) Rt = hydrauf ic re ten t ion t ime (d) . Subscr ip ts a or nf up " f r a c tio n o f to t a l i n f lu en t C O D wh ic h i s in th e r e f er to a c tu a l o r n o min a l m e n t io n t ime s r e -par t icu la te unbio degrada ble form spec t ive ly . Ad dit iona l subscr ip ts c or s r e fe r tof . , , = f mc t /o n o f th e to t a l i n f lu e n t C O D wh ic h i s in v a lu e s in th e c o n ta c t o r s t a b i li z a t io n r e a c to rth e s o lu b le u n b io d e g r a d a b le f o r m r e s p ec t iv elyKin* =, m axim um spec if ic subs tra ta u t i l iza t ion ra te for R, = s ludge age (d)r e a d ily a s s imi la b le CO D ( mg C O D mg VS S - x r = r e c y c le r a t io w i th r e s p e ~ to a v e r a g e in f lu e n t

d - ~ ) f lowM = p r e fix d e n o t in g ma s s , i .e . S = g m e r a l s y mb o l f o r C O D c o n c e n t r a t io n( rag C O D I - s) . Subscr ip t t r e fe r s to to ta l con-M(Su) = Su" Q centra t ion L Ad dit iona l subscr ip ts c , e , i , s r e fe r== d a i ly C O D ma s s lo a d respec t ive ly to the va lues in the contac t r eac-( ms C O D d - x ) to r , e lHu en t, i n f lu en t a n d s t a b i li z a t io n r e a c to rM ( X ~ ) = X , . , V p T = t e mp e r a tu r e in Cmass of s ludge in process V = genera l sym bol for vo lu me 0 .~ Subscr ip ts c , p( rag VSS) and s r e fe r tO con tac t r eac tor , overa l l p rocessN =. genera l sym bol for n i t rogen conce ntra t ion and s tab i l iza t ion reac tor r espec t ive ly( mg N 1= xg Subscr ip ts n or t r e fe r to n i t r a te or w = was te I ]ow ra t io with respec t to averageT K N c o n c e n t r a t io n s r e sp e c t iv e ly . Ad d i t io n a l in f lu e n t f lo ws u b g r ip t s c~ e , i o r s r e fe r r e s p e ct iv e ly to th e X = s m e f a l s y mb o l f o r v o la t i l e s lu d g e ~ c e n -

va lues in the cont ac t r eac tor , e f fluent , in f luent t r a t io n (m gV SS l-1) . Subser ip ts a , n and vor s tab i l iza t ion reac tor r e fe r to ac t ive vola t i le , n i t r i f ie r and to ta l vo la -t i le s ludge concentra t ions respec t ive ly . Ad-di t iona l subscr ip ts c , p and s r e fe r r espec t ive ly* Ad d i t io mt l s u b e c ~p t To t 2 0 r e fe r s to th e v a lu e a t TC to v a l t ie s in th e c o n ta c t r e a c to r, o v e r a l l p r o c e ssor 200C. and s tab i l iza t ion reac tor1737


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1738 W.V. ALEXANDER,G. A. EKAMAand G. v. R. MARAISYh = yield coefficient (rag VSS m g C O D - ~) ,o WASTE= fraction of m ass sludge in process in the con - CONTACT & FLOWtact reactor R E A C T O R / S E T T L I N G0 = temperature sensitivity coefficient INft.U~T ~ [ TANK EFFLUENT~ , , * = m a x im u m s pe ci fi c g r o w th r a t e o f t h e ~ x ~ * ~ ] " ~ - ' - ~ )"nit riflers (d - ') o, sti t i -.~ o, steT II ~ I R E C Y C L E

I f ~ , j . F LOWL ( : : . ) J , 0NTRODUCTION S T A S I L t Z A T I OIn the l i t e ra tu r e , the k ine t i c theor i e s des c r ib ing the nEACTO~b e h a v i o u r o f t h e c o n t a c t s t a b i l i z a t io n a c t i v a t e d s l u d g ep r o c e s s ( C S A S P ) h a v e b e e n e s s e n t i a l l y o f a n a d h o c Fig. 1 . Diagram ma tic representa t ion of the co ntact s tabi liz-a t ion process .n a t u r e . N o a t t e m p t t o d e s c r i b e t h e b e h a v i o u r o f t h i sp r o c e s s b y a g e n e r a l a c t i v a t e d s l u d g e k i n e t i c t h e o r y s l u d g e m a s s i n t h e C S A S P s y s t e m i s c o n t a i n e d in t h eh a s b e e n s u c c es s fu l , p r i n c i p a l l y b e c a u s e t h e t h e o r i e s s t a b i l i z a t i o n r e a c t o r i n a d e n s i f ie d f o r m , g i v i n g a c o r -p r o p o s e d t o d a t e a r e d e f i c ie n t i n th e m s e l v e s. H o w - r e s p o n d i n g l y s m a l l e r v o l u m e t o t h e w h o l e s y s te m .e v e r . t h e g e n e r a l h i - s u b s t r a t e d e a t h - r e g e n e r a t i o n a e t i- F u r t h e r m o r e , i f t h e e x ce s s s l u d g e is w a s t e d f r o m t h ev a t e d s l u d g e k i n e t i c t h e o r y f o r t h e p r o c e s s t r e a t i n g c o n t a c t r e a c t o r , a s s h o w n i n F . if , 1 , t h e o v e r a l l p r o c e s sp r i n c i p a l l y m u n i c i p a l w a s te w a t e r , p r o p o s e d b y D o l d o x y g e n d e m a n d i s s l ig h t l y r e d u c e d b e c a u s e t h e s l u d geet al . ( 19 8 0) , a p p e a r s t o b e s u f f i ci e n t ly c o m p r e h e n s i v e m a s s i n t h e c o n t a c t r e a c t o r c o n t a i n s a r e l a t i v e l y h i g ht o e n c o m p a s s a l s o t h e k i n e ti c b e h a v i o u r o f t h e f r a ct i o n o f u n m e t a b o l i z e d C O D .C S A S P t r e a t i n g m a i n l y m u n i c i p a l w a s t e w a t e r s . T h i s T h e C S A S P i s a s p a t i a l l y d e l ) e n d c a t s y s te m e v enp a p e r d e s c ri b e s t h e i n t e g r a t io n o f t h e C S A S P w i t h t h e u n d e r s t e a d y f lo w a n d l o a d c o , d i t io n s . T h e c o n c er t -g e n e r a l a c t i v a t e d s l u d ge t h e o ry , t r a t i o n o f s l u d g e a n d t h e f o o d / m i c r o - o r g a n i s m r a t i oI n p r e s e n t in g th i s p a p e r , i t i s a s s u m e d t h a t t h e d i ff e r s i g ni f ic a n t ly i n t h e c o n t a c t a n d : s ~ l i z a t i o nr e a d e r h a s s t u d i e d t h e p a p e r s e t ti n g o u t th e g e n e r a l r e a c t o r s ; t h e s e p a r a m e t e r s a r e d e p f n d e a t o n t h ea c t i v a t e d s lu d g e t h e o r y ( D o l d et al . , 1 9 8 0 ) - - o n l y t h e s l u d g e r e c y c l e r a t e a n d t h e f r a c t i o n a l d i s t r i b u t i o n o fa s p e c t s n e c e s sa r y fo r t h e p a r t i c u l a r r e q u i r e m e n t s o f t h e m a s s o f s l u d g e b e t w e e n t h e t w o r e a c to r s . T h e r e -t h e C S A S P w i l l b e p r e s e n t e d , f o r e, u n l i k e t h e C M A S P w h i c h e s s e n t i a l l y i s d e f i n e d

b y t h e sl u d g e a g e a n d t h e o r g a m c t o a d i n t h e p r o c e s s ,I N IT IA L P R O CE S S D E SI GN i n t h e C S A S P t h e p r o c e s s d e f i n i t i o n m u s t i n c l u d e , i n

a d d i t i o n t o t h e a b o v e p a r a m e t e r s , t h e r e c y c l e r a t i o , r ,A d i a g r a m m a t i c r e p r e s e n t a t io n o f t h e C S A S P i s a n d s o m e p a r a m e t e r f ix i ng t h e f r a c t i o n a l d i s t ri b u t i o ng i v e n i n F i g . 1 . T h e c o n t a c t r e a c t o r r e c e i v e s t h e o f t h e m a s s o f s l u d g e b e t w e ~ l t h e c o n t a c t a n d s t a b i l -i n f l u e n t f l o w a n d t h e r e c y c l e f r o m t h e s t a b i l i z a t i o n i z a t i o n r e a c t o r s . O h r o n & J e n k i n s ( i 9 7 2 ) s e l e c t e d . t h er e a c t o r . I n t h e c o n t a c t r e a c t o r , w h i c h h a s a s h o r t f r a c t i o n o f t h e m a s s o f s l u d g e i n c o n t a c t r e a c t o r r e l a -n o m i n a l h y d r a u l i c r e t e n t i o n f , a s m a l l f r a c t i o n o f t h e t iv e to t h e t o t a l m a s s o f s lu d g e i n t h e p r o c e s s , ~ , t os l u d g e m a s s r e m o v e s t h e b u l k o f t h e c a r b o n a c e o u s d e f i n e t h e f r a c t i o n a l p a r a m e t e r , i .e .m a t e r i a l f r o m t h e i n f l u e n t - - p r i n c i p a l l y b y a d s o r p t i o nand enm es hm ent . A m ino r f rac t ion o f the ad s o rb ed ~t = VcXvc /(VcX, ~ - V , X~ ,) {1)o r g a n i c m a t e r i a l i s m e t a b o l i z e d i n t h e c o n t a c t r e a c to r w h e r ea n d c o n s e q u e n t l y t h e s l u d g e m a s s c o n t a i n s a h i g hp r o p o r t i o n o f u n m e t a b o l i z e d C O D . T h e e f fl u en t f r o m V c, V s = v o l u m e s o f t h e c o n t a c t a n d s t a b i li z a t i o nt h e c o n t a c t r e a c t o r i s d i s c h a r g e d t o t h e s e t t l in g t a n k . r e a c t o r s r e s p e c t i v e l y (1 .)T h e s e t t l i n g t a n k o v e r f l o w l ea v e s t h e p r o c e s s a s e f fl u - X ,0 , X , , = v o l a t i l e s l u d g e c o n c e n t r a t i o n s i n t h e c o n -e n t ; t h e u n d e r t o w i s d i s c h a r g e d t o t h e s t a b i l iz a t i o n t a c t a n d s t a b i li z a t i o n r e a c t o r s r e s pe c t -r e a c t o r , w h i c h h a s a l o n g n o m i n a l r e t e n t i o n t i m e i n i v e l y ( m g V S S 1 -1 ).w h i c h t h e d e n s i f l e d m i x e d l i q u o r i s s t a b i l i z e d . S t a b i l - I t w i l l b e a p p a r e n t f r o m t h e a b o v e , t h a t t h e d e s ig ni z e d s l u d g e i s r e t u r n e d t o t h e c o n t a c t r e a c t o r v i a a o f a s p ec i f ic C S A S P is n o t a s s t r a i g h t f o r w a r d a s t h es lu d g e r e c y c le f lo w . C l o s e c o n t r o l o f t h e s lu d g e ag e o f C M A S P a n d m u s t p r o c e e d o n a t r ia l a n d e r r o r b a s is .t h e p r o c e s s i s p o s s i b l e b y h y d r a u l i c c o n t r o l , i.e . b y I n d e s i g n i n g a C S A S P c o n f i g u r a t i o n it i s p a r t i c u l a r l yw a s t i n g a n a p p r o p r i a t e v o l u m e o f m i x e d l iq u o r d i - u s e f u l t o h a v e a p r o c e d u r e f o r i n i t i a l l y e s t i m a t i n g t h er e c t l y f r o m e i t h e r t h e c o n t a c t o r s t a b i l i z a t i o n r e a c t o r , s i z es o f t h e r e a c t o r s a n d t h e i r r e s p e c ( i v e s l u d g e c o n -

T h e a d v a n t a g e o f th e C S A S P o v e r th e C o m p l e t e l y o e n t r a t io n s i n o r d e r t o d e c i d e w h e t h e r t h e l i q u i d s o l idM i x e d A c t i v a t e d S l u d g e P r o ~ e ss ( C M A S P ) s t em s s e p a r a t i o n e f fi c ie n cy i n th e s e t tl i n g t a n k a n d t h ef r o m i ts c o n f i g u r at i o n . T h e m a j o r f r a c ti o n o f t h e a c t u a l h y d r a u l i c r e t e n t i o n t i m e i n t h e c o n t a c t re a c t o ra r e w i t h i n a c c e p t a b l e l i m i t s .

~"N ~ i n a l h y d r a u li c r e te n t io n t i m e is g iv en b y t h e A b a s i s f o r m a k i n g a n i n i t i a l e s t i m a t e o f t h e r e a c t o rvolume -~ the reactor divided by the dai ly average influent s izes is to ac ce pt tha t the C S A S P p r o d lg ~ : tk e '~ a m eflow. m a s s o f sl u dg e a s t h e C M A S P f o r t h e sa m e s l u dg e a g e,


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The activated sludge pro cess --Pa rt 2 1739R,, and da i ly organic mass load, M(S~) . T h i s a s s u m p - a d v a n t a g e , H i g h e r X , p c o n c e n t r a t i o n s f o r t h e C S A S Pt ion i s ba sed on the f i nd ings o f Gu je r & Jenk ins a re pe rmiss ib le because t he se t t li ng t ank o f t he p ro-(1975a) and has been ve r i fi ed dur ing t h i s i nves t iga - ce ss has t o dea l wi th t he re l at i ve ly low conc en t ra t i ont ion . The re fore , an e s t ima te o f t he t o t a l ma ss o f s lUdge o f sl udge f rom the con tac t r eac to r .i n t h e p r o c e s s M ( X ~ ) c a n b e o b t a i n e d f r o m t h e s t e a d y W h e r e t h e s l u d g e ag e , R , , o f t h e C S A S P i s c o n -s t a t e equaU'ons i 'o r t he C M A SP (M ara i s & Ek am a , t ro l led by was t ing s ludge d i rec t l y f rom the con tac t1976) , i .e . r eac to r , i . e . hydra u l i c con t ro l o f t he s ludge age , t hes ludge age o f t he p rocess i s g iven by{ ~ ( l + f b h r R , )M ( X . p) = M ( S , ) R , + b h rR , ) R, = mass o f s l udge in p rocess

m a s s o f s lu d g e w a s t e d p e r d a y3x (1 - fu. - fup P) + fup (2) - - - X v p V p / ( X v c q ) (6 )w h e r e w h e r eM ( X v ) - - m a s s o f v ol a ti le s l u d ge i n p ro c e s s( rag VSS) q = was t e f l ow ra t e (1 d - t )

= XvpVp In t he ana lys i s o f the CS AS P s ign i fi can t s impl if ica -Xvp = ave rage p rocess vo l a t il e s l udge conce r t- t i on i n t he fo rm ula t i ons i s a t t a ined b y express ing q a st ra t i on ( rag VSS 1- J )a f r ac t i on o f t he ave rage i n f luen t f l ow ra t e , i .e .Vp = vo lum e of p rocess (1.)M ( S tl ) = d a i l y o r g a n i c m a s s l o a d ( m g C O D d - l ) q = w Q (7 )

= Q" Sit wh ereSti - t o t a l i n f luen t C O D conc en t ra t i on( rag C O D 1-1) Q = ave rage i n f luen t f l ow (1 d - 1 )R, = s ludge age (d ) w = s ludge was t e f l ow ra t e f rom the con tac t r eac to rYh - y i e ld coe f f ic i en t ( r ag VSS m g C O D -1 ) a s a f r ac t i on o f t he ave rage i n f luen t f l ow ra te .

= 0.45f = unb iodegra dab le f rac t i on o f t h e ac t i ve Hence , fo r a g iven s ludge age , w i s g iven byvo la t il e mass (mg VSS m g V SS - 1 ) w - - XvpVp/(RsQXvc (8 )

= 0 .20 In o rde r t o s ize t he two reac to r s o f t he CSA SP, abhr = end ogen ous re sp i ra t i on ra t e a t T C (d ) dec i s ion on the two add i t i ona l i ndepen den t pa r -_. _ b h 2 o ( T T - 2 o )bh2o - end ogen ous re sp i ra t i on ra t e a t 20C ame te r s ~ and r i s r equ i red . Kn ow ing ~ , r , R8 and X,p= 0 .24 a s we ll a s Vp , which can be de t e rmined f rom equa -

0 - t empe ra tu re sens i ti v i ty coe f f ic i en t t i ons (2 ) and (3 ) , t hen by do in g a s ludge mass ba l ance= 1 .0 29 a r o u n d e a c h r e a c t o r a n d m a n i p u l a t i n g e q u a t i o n sT = t empe ra tu re ( C) (4 )- (7 ) , t he conc en t ra t i on o f t he s ludge in , and the

fu , = so lub l e unb io degrad ab le f rac t i on o f t he vo lum es o f t he two reac to r s can be expressed in t e rmsi n fl u e n t C O D ( ra g C O D m g C O D - 1 ) o f t h e f iv e k n o w n p a r a m e t e rs , i. e.

f~p = pa r t i cu l a t e unb iodegra dab le f rac t i on o f Vc = Vpa(1 + r - w)/(~t + r - otw) (9 )t h e i nt tu e n t C O D ( ra g V S S m g C O D - i )P = C O D t o V S S r a t i o ( m g C O D m g V S S - 1 ) V , = V p r( 1 - ~ )/ (~ + r - ~ tw ) ( 10 )X,c = Xvp(~ + r - uw)/(1 + r - w) '(11)On ce an e s t ima te o f M(Xvp) i s ava i lab l e , a dec i s ionm u s t b e m a d e o n t h e o v e r a l l m e a n s lu d o e c o n ce n - X ~ , = X ~ ( ~ + r - ~ w ) / r (12)t ra t ion , Xv t# f o r t h e p r o c e s ~ T h e o v e r a ll v o l u m e o f t h e X , , = X ~ (1 + r - w) / r . (13)C S A S P , V p , i s d e te r m i n e d b y An express ion fo r w in equa t ions (9 ) - (13) can be de -

V p - -- M ( X ~ p ) / X v p , (3) r ived as fol low s:w h e r e S u b s t i tu t in g e q u a t i o n ( 5 ) f o r VpXvp i n equa t ion (8 ) :

Vp - V , + V ~ ( 4 ) w = (X~cV + X ~ , V , ) / ( R , X v c ) . ( 1 4 )a n dS u b s t i tu t i n g e q u a t i o n ( 1 3 ) f o r X v , a n d e q u a t i o n ( I 0 )VpXvp = VcXvc + VtXvs. (5) fo r V, i n to equa t ion (14) and rea r rang ing , y i e lds :

I f X , p f o r th e C S A S P i s c h o s e n t o b e t h e s a m e a st h a t f o r a C M A S P t h e v o l u m e s o f t h e t w o p r o c e s s e s w = V p (1 + r - w ) / { R , Q ( , , + r - ~tw)}. (1 5)wi l l be ident ica l , resu! tin_g " n o space sa ~ Co nse -quen t ly , ~ ,p fo r t he ' CS AS P usua l ly i s se l ect ed t o be Un de r norm a l ope ra t i ng cond i t i ons , ~ov i s sma l l r e l a -apprec i ab ly h ighe r ~ than tha t norm a l ly accep ted fo r f i ve t o (c( + r~ Con sequen t ly , i gnor ing ~,w in equa t iont h e C M A S P in o r d e r t o g a i n a p l a n t v o l u m e t r i c ( 1 5 )and so lv ing fo r w, an ex press ion fo r w in t e rms o f


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1 7 4 0 W . V. ALEXANDER,G. A. EKAMAand G. v. R. MARAISth e k n o w n p a r a m e te r s i s o b t a in e d , i .e . c a r b o n a c e o u s s u b st r at e , wh ic h f o r ms th e b u lk o f th ein f lu e n t COD, i s e i th e r a d s o r b e d b y th e o r g a n i s ms o rw = (1 + r) /{ 1 + (~ + r)QRs/Vp}. (16) enmeshe d in the s ludge f loes and remo ved f rom the e f fluentin the settling tank.F r o m e q u a t io n s ( 9) a n d ( 10 ), t h e a c tu a l a n d Ad s o r p t io n a n d e n me s h me n t a r e r a p id p ro ce ss es , s o th a tn o m i n a l r e t e n t i o n t i m e s i n t h e t w o r e a c t o r s c a n b e t h ey a r e e x p ec t ed t o b e n e a r c o m p l e t io n e v e n i n r el a ti v el yfoun d, i .e . shor t ac tua l hydraul ic r e ten t ion t imes . The remo val of the

so luble COD f rac t ion in the inf luent by the contac t r eac torR ~ c = V p ( 1 + r - w)/[(1 + r)(0c + r - 0~w )~] (17) is affected by the length of the contac t t im e: any so luble CO D c o n c e n t r a t io n n o t me ta b o l i z e d d u r in g th e c o n ta c tRha = Vp (I - ~t) /[(~ + r - 0tw)Q] (18) perio d in the con tact reac tor escapes with the eff luent f low.Rh. = (1 + s )" Rhoc (19) How ever , as on ly abou t 25% of the b iodegrad able C O D isin so luble form (D old et al . , 1980) and the rem oval is r ap id ,Rhns = Rh~ (20) the escape of the so luble C O D concen tra t ion is un l ike ly toaffect the ov era l l CO D rem oval e f fic iency.wh ere With regard to ~ , a va lue of 0 .1 was se lec ted as with th isva lue only one ten th of the mass of s ludge in the process isRh = h y d r a u l i c r e t e n t io n t im e ( d) . S e c o n d s u b - r e s p o n sib le f o r th e in i ti a l r e mo v a l o f th e c a r b o n a c e o u s ma -scr ip ts a or n re fe r to ac tua l or no m in a l te r ia l f rom the influent . I t r epresents , there fore , an ex tremes i tua t ion and cons t i tu tes a severe tes t o f the pred ic t ive ca-v a l u e s r e s p e c ti v e l y . A d d i t i o n a l s u b s c r i p t s c p a ci t y o f t h e m o d e l

o r s r e f e r to c o n t a c t o r s t a b i l i z a t io n r e a c to r s W i th re g a r d to th e c h o ic e o f r , a h ig h v a lu e ( r > 4 ) in th erespe c t ive ly . CS AS P resu l ts in a smalle r r e la t ive d iffe rence be tween theconcen tra t ions of s ludge in the conta c t and s tab i l iza t ionF r o m t h e e q u a t i o n s s e t o u t a b o v e e s t i m a t e s o f Vc , r e ac t o rs a n d t h e p r oc e ss a p p r o x im a t e s t h e C M A S P w hi c hV, , X , o , a n d X , , i n t e r m s o f th e v a lu e s a , r , M ( S t, g X~p wo u ld t e n d to d e t r a c t f ro m th e u n iq u e n e ss o f th e CS A S Pa n d R , a r e n o w a v a i l a b le a n d c o n s t i tu t e a n in i t i a l c o n f ig u r a t io n . I n c o n t r as t , a lo w r e cy c le r a t io ( r < 1 )d e s ig n . Ch e c k s c a n b e m a d e to e n s u r e th a t t h e a c tu a l r e su l t s in a lo w p r o c e ss n i t r if i c a tio n e ff ic ie nc y , a s o n ly asmall f r ac t ion of the inf luent ammonia is r ecyc led to theh y d r a u l i c r e t e n t i o n i n t h e c o n t a c t r e a c t o r i s a d e q u a t e s ta b il i za t io n re a c to r i n w h i ch t h e b u l k o f t h e s l ud g e m a s s i sa n d th a t th e d e n s i f i c a t io n r a t i o in th e s e t t l in g t a n k i s r e t ain e d ( Gu je r & Je n k ins , 19 7 5b ). Co n s e q u e n t ly , a r e cy c leno t to o grea t , r a t io of 2 :1 with respec t to average f low was se lec ted .E x p e r i e n c e w i t h e x p e r i m e n t a l p l a n t s a n d t h e g e n - T h e e x p e ri m e n t al i n v es t ig a ti o n w a s t o b e u n d e r ta k e n a te r a l a c t i v a t e d s lu d g e m o d e l a s a p p l i e d t o t h e C S A S P , 2 0 C. T o e n s ur e c o m p l e t e n i t r if i ca t io n o f t h e r e c y c le dammonia in the s tab i l iza t ion reac tor , s ludge ages longerh a s i n d i c a t e d t h a t t h i s p r e l i m i n a r y d e s i g n p r o c e d u r e t h an a b o u t 4 d a y s a r e r eq u ir e d. C o n s eq u e n tl y , s lu d g e a g es

p r e d ic t s X~c a n d X , , a c c u r a te l y . A l s o , th e a v e r a g e p r o - o f 6 a n d 1 0 d a y s we r e s el ec te d.c e s s c a r b o n a c e o u s o x y g e n d e m a n d f o r t h e s y s t e m a p - I f t h e C S A S P o p e ra t e s u n d e r c y cl ic f lo w c o n d it i on s , t h ep r o x i m a t e l y e q u a l s t h a t f o r t h e e q u i v a l e n t C M A S P a c tu a l h y d r au l i c r e te n t io n ti m e i n t h e c o n t a c t r e ac t o r i sreduced dur ing the peak f low per iod . Should the ac tua l[ i. e. f o r t h e s a m e R , a n d M ( S ~ ) ] . H o w e v e r , t h e p r o - h y d r au l i c r e t en t io n t i m e u n d e r p e a k f lo w c o n d i ti o n sc e d u r e p r o v i d e s n o e s t i m a t i o n o f t h e r e l a t i v e O x y g e n b e c o m e t o o s h o rt , p a r ti c u la t e C O D r e m o v a l b y a d s o rp t i o nd e m a n d s in t h e t w o r e a c t o rs , n o r th e C O D , T K N a n d a n d e n m e sh m e n t a nd s ol ub le C O D r e m o va l b y m et ab -nitrate concentration in the r e a c t o r s o r e~uent under olism will be insuff icient, resultin$ in a ~ o r effluentc o n s t a n t o r c y c li c l o a d i n g c o n d i t i o n s . U n d e r c o n s t a n t q u a li t y. C o n s e q u en t l y a l o w e r l i m it t o / l i e ~ i h / U / f i a~ c/ ~-ihydraul ic r e ten t ion t ime in the contac t r eac tor needs to bef lo w c o n d i t io n s th e s e p r o c e s s v a r i a b le s m a y b e e s t i - s et . To sa ti sf y th i s lo we r l imi t , i t w i l l b e f o u n d th a t lo we rm a t e d f r o m m o d e l s w i t h c o n s t a n t s d e t e r m i n e d o n a n v a lu e s o f X ~p o r hi g h er v a l ue s o f a n e e d t o b e c h o s en a s t h ea d h o c b a s i s ( G u je r & J e n k in s , 1 9 7 5 a , b g Ho w e v e r , p e a k /a v e r a g e f low r a t io inc re as es , th a n wo u ld b e r e q u i r e du n d e r c y c l i c l o a d i n g c o n d i t i o n s t h e o n l y r e l i a b l e p r o - u n d e r t h e a v e ra g e f lo w c o n di ti o n s.I n th i s e x p e r ime n ta l in v e s t ig a t io n , th e min imu m a c tu a lc e d u r e f o r d e te r m in i n g t h e s e p r o c e s s v a r i a b le s i s b y h y d r a u l ic r e t e n t io n time in th e c o n ta c t re a c to r wa s se t a tu t i l i z in g t h e g e n e r a l a c t i v a t e d s l u d g e m o d e l a s a p p l i e d a b o u t 2 0 m i n u n de r a n ex p e ct e d p e a k /a v e ra g e fl o w r a t io o fto the CS A SP . 1 1 .5 . With th is r es t r ic tion , the chosen des ign param eter s a ,r , R, and M(S, t) , the des ign of the exper imen ta l CS AS Punits was obta ined by a t r ia l and e r ror procedure . All the

EXPERIMENTAL INVESTIGATION condi t io ns could be met on ly when X~p was less than abou t2000 mg VSS 1-1 a re la t ive ly low va lue . By dou bl ing a toI t h a s b e e n s h o wn th a t th e r e a r e fiv e ma jo r p a r a me te r s 0 .2 , a n d k e e p in g th e min im u m a c tu a l h y d r a u l i c r e t e n t io nwh ic h in f lue n c e th e re s p o n s e o f th e CS AS P , i .e . d m 'ly C O D t ime in th e c o n ta c t r e a c to r a t 2 0 min u n d e r th e p e a k /a v e r -load . M(S~), s ludge age, R, , tem pera ture T, s ludge recyc le age f low ra t io of 1 :1.5 . X ~ can be increased tora te , r , and the f ra~ ioa a l d la ir ibe t io n of s ludge m ass 3800 mg VSS 1- : , g iv ing a mo re prac t ica l design . How ever ,be tween the con tac t and s tab i l iza t ion reac tor s in te rms of , f rom a model ver i f ica t ion poin t of v iew, the la t te r des igna . F o r th i s in v e s t ig a t io n i t wa s ~ th a t th e t e mp e r a - wo u ld n o t h a v e p r o v id e d a s s e v e r e a t e s t o n th e p r e d ic tiv etu r e, r a n d a w o u ld b e k e p t c e z f, a a t , wh i l e th e :O~ lg e a g e c a p a o ty o f th e mo d e l a s th e fo r me r .a n d d a i ly C O D lo a d p a t t e r n w o u ld b e v a r i ed . Th e r e a so n Th e c h o s e n d e s ig n p a r a me te r s o f th e e x p e r ime n ta l u n i t sf o r ma in ta in in g r a n d a c o n s ta n t wa s th a t Gu je r & J e n k in s a r e g iv e n in Ta b le 1 a n d a d ia g r a mma t ic l a y o u t o f th e u n it s(1975a) repor te d tha t these two para m ete r -h av e less e f fec t is shown in Fig . 2on the e f fic iency of ca rbon aceous mater ia l r em oval by the The exp er imenta l inves t iga t ion was d iv ide d in to twop ro oe M t h a n t h e' O th er t iw e e. ~ ~ v t [ i l i ~ ~ - p h a se s : ( a) a p h a se d u r i ng w h i c h t h e t w o e x p er i m e n ta lt io n i s a l s o c o n s i s te n t w i th t h e b i - m ~ l ~ m t ~ ~ e t u n i t s we r e o p e r a te d u n d e r c o n s ta n t f lo w a n d lo a d c o n -al., 1980) , Alth ough r and 0c ha ve a mark ed ~ 9~a the d i t ions , and (b) a phase dur ing which one of the uni ts wasc o n ta c t r e a c to r h y d r a u l ic r e t e n t io n t ime , th e i~ ' t t e u la t e o p e r a te d u n d e r c y c l ic f lo w a n d lo a d c o n d i tio n s .


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Th e a c t iv a te d s lu d g e p r o c e s s - - P a r t 2 1 74 1Ta b le 1 . P r o c e s s d e sig n p a r a me te r s f o r th e La b o r a to r y C S A S P u n i t s

P a r a m e t e r S l u dg e a g eNa me S y mb o l Un i t s 6 d a y s 1 0 d a y s1 . Sewage type Unse t t led Un se t t led2 . I ~ f lu en t C O D S ti mg CO D 1 -1 5 0 0 5 0 03. in f luen t T K N Nil mg N 1-1 ~ 50 ~ 504 . Ave rage inf luent f low Q I d- ~ 36 245. Tem pera tu re T C 20 206. S ludge age R, d 6 107 . Wa s te f low ra t io w - - 0 .092 0 .0838. Frac t iona l d is t r ib i t ion a - - 0 .10 0 .10of s ludge in process9 . Recycle ra t io r - - 2.0 2.010. Vo lum es 1.Pro ces s Vp 1. 14.3 14.3Co nta ct reac tor Vc 1. 2.0 2.0Stab ilizat ion reac tor V, 1. 12.3 12.311. Averag e hydrau l ic Rh hr e te n t io n t ime sC o n t a c t:

Ac tua l Rt, h 0.44 0.67No m ina l R~c h 1 .32 1 .98Stabi l iza t ion :Ac tual R~,, h 4.07 6.27No mi na l R~, , h 4 .07 6 .2712. Leng th of tes t - - d 23 25p e r io d

(a) Co ns tan t f low and load condi t ion s were es tab l ished d iv id ing the da i ly inf luent f eed volu me of 361 . be tweenb y f e e d in g th e r e q u i r e d v o lu m e o f s e wa g e f e e d p e r d a y a t a two p u m p s - - o n e o p e r a t in g a t a r a t e o f 1 81 d - : o v e rc o n s ta n t r a t e o v e r th e d a y . Th e av e r a g e v a lu e s o f th e d a i ly th e f u ll 2 4 h p e r io d a n d th e o th e r a t 3 6 1 d - 1 f o r ame a s u r e me n t s o f th e p r o c e s s v a r i a b les in th e c o n ta c t a n d p e r io d o f 1 2 h o n ly .s tab i l iza t ion reac tor s and in the e f f luent of the 6 and 10 Mea surem ents of the process var iab les in the con-day s ludge uni ts a re g iven in Tab le 2 , the length of the tes t tac t and s tab i l iza t ion reac tor s and in the e f fluent werep e r io d h a v in g b e e n a b o u t 2 5 d a ys . ma d e a t r e g u la r in t e r v al s o v e r a 2 4 h p e r io d - - a ty p ic a l(b) In the cyfic loadin g tes ts the 6 day s ludge age uni t se t o f da ta is shown in Fig . 3 .was opera t ed under two d if fe ren t in f luent cyc l ic f low pa t- ( fi ) The s i ne wa ve i oadin 0 p a t t ern wa s a c h ie v e d b yte rns : ( i) a square wave pa t te rn , and (i i) a s ine wav e pa t- mean s of a spec ia l ly des igned pum p tha t prod uced ante rn . I n b o th c a s e s th e d a i ly in f lu en t C O D ma s s wa s th e a p p r o x ima te s in e wa v e flo w wi th a n a mp l i tu d e o fsame and equa l to tha t fed dur ing the cons tan t f low test , abou t 0 .5 t ime s the average f low. A typica l se t o f da tai .e . 1 8 , 0 0 0 m g C OD d - t . o b s e r v e d o v e r a 2 4 h c y c le u n d e r s in e wa v e in f lu e n t( i) Th e square wave loadino pa t t ern wa s o b ta in e d b y f lo w c o n d i t io n s i s s h o wn in F ig . 4 .

i i "CONTACT|ACTO~I S E T T L I r ~E F F L U E N T

R I ~ C ~ CEFig . 2 . Dia gram ma tic representa t ion of the labora t ory sca le cont ac t s tab i l iza t ion process.


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1742 W.V. ALEXANDER,G. A. EKAMA and G. v. R. MARAISTable 2 . Averages of process var iab le measurem ents observed in labo ra tory scale contac ts tab i l iza t ion uni ts . The cor responding predic t ions of the genera l model a re a lso g iven

Co n s ta n t f lo w a n d lo a d c o n d i t io n sSludge ageP r o c e s s v a r i a b le 6 Da y s l0 Da y sP o s i t io n S y mb o l M e a s u r e d P r e d ic te d M e a s u r e d P r e d ic te dC O D m g C O D l -Inf luent S t i* 512 512 496 496Co nta ct Stc 62 60 52 57Effluen t St* 90 81 61 75Stab ilizatio n Sts 54 45 42 43T K N m g N 1 - lInfluent Nti* 56 56 47 47C on tac t Ntc 11 15 10.5 12Effluent Nte* 15 15 12 12Stab ilizatio n Nts 1.7 3 4.8 3Ni t r a t e mg N 1 - lInflu ent Nn~ 0.0 0.0 0.0 0.0Con tac t N,c 27 27 22 23Effluent N ,, 27 27 23 23Stab ilizatio n N,s 36 37 29 31M L V S S m g V S S 1 -1C on tac t X, 1301 1489 1478 1389Stab ilizatio n X , 1772 2099 2073 1989Pro ces s X,p 1706 2016 1990 1903O x y g e n D e m a n d ( m g O 1~ z h - l )Con tac t O ~ 31 29 24 23C arb on ~ Stab ilizatio n O~ 22 23 16 16I Proc ess Oep 23 24 17 17

Co ntac t On, 30 24 18 19Nitr if . ~ Stab iliza tion O,~ 10 11 5 5[ . Proc ess O.p 13 13 7 8( Co nta ct Ot~ 61 54 42 42

To tal ~ Stab ilizatio n Ot~ 32 34 21 22{ , Process O , , 36 37 24 25Mass ba lances CO D+ 96% 100% 97% 100%N:~ 94% 100% 96% 100%

* Refer s to unf i l te red samples .t " Ba s e d o n a CO D /V S S r a t io o f 1 .4 8.:~ Based o n T KN /V SS ra t io ( f , ) o f 0.10 .

M O D E L V E RIF IC A TIO N d i c t e d T K N a n d n i t r a t e c o n c e n t r a t i o n s . F r o m a c o m -p a r is o n o f m a n y e x p e r i m ~ t a t a n d p r e di c te d r e-T h e g e n e r a l k i n e t i c s o f t h e a c t i v a t e d s l u d g e p r o c e s s s p o n s e s , i t b e c a m e e v i d e n t t h a t t h e v a l u e o f / q , ,z 0

i n c l u d i n g n i t ri f i c at i o n d e s c r i b e d b y D o l d e t a l . ( 19 80 ) o f t en c h a n g e s fo r e a c h b a t c h o f s e w a g e p r o b a b l y d u ew e r e i n c o r p o r a t e d i n t o a g e n e r a l m o d e l f or t h e C S A S P t o c h a n g e s i n t h e c o n t e n t o f in h i b i t o r y s u b s t a nc e s . I n( in t h e f o r m o f a c o m p u t e r p r o g r a m ) . W h e n t h i s t h i s i n v e s t i g a t i o n ; i t w a s f o u n d t h a t t h e b e s t o v e r a l lm o d e l , a s w e l l a s i t s a s s o c i a te d k i n e ti c c o n s t a n ts , w a s c o r r e s p o n d e n c e b e t w e e n e x p e r i m e n t a l a n d p r e d i c te da p p l i e d t o s i m u l a t e t h e b e h a v i o u r o b s e r v e d i n t h e e x - T K N a n d n i t r a t e c o n c e n t r a t i o n s w a s o b t a i n e d w h e np e r i m e n t a l C S A S P u n i t s d e s c r i b e d a b o v e , t h e f o l l o w - P~=2o = 0 . 5 5 d - ~ . A l l t h e C S A S P s i m u l a ti o n s a r ei n g o b s e r v a t i o n s w e r e m a d e : I m s e d o n t h i s v a lu e .

W i t h r e g a r d t o K , , 2 o , D o l d e t a l . f o u n d t h a t aC h a r u 3 e s i n k i n e t i c c o n s t a n t s v a l u e o f 8. 0 n ag C O D m g V S S d - 1 g a v e t h e b e s t c o r r e -T h e p re d i c t i o n s o f t h e r e s p o n s e o f t h e c o n t a c t r ea c - s p o n d e n c e b e t w e e n e x p e r i m e n t a l a n d p r e d i c t e d o x y -

t o t i s v e r y s e n s it i v e t o t h e v a lu e s o f t h e m a x i m u m g e n c o n s u m p t i o n r a t e s fo r s i n gl e a n d s e r i es C M A Ss p e c i ti c g r o w t h r a t e o f t h e n i t r i f i e rs ( /~ m 2o ) a n d t h e p r o c e s se s . W h e n t h i s v a l u e fo r K m ~2 o w a s u t i l i z e d t om a x i m u m u t i l iz a t i on r a t e o f s o l u b le c a r b o n a c e o u s s i m u l a t e t h e b e h a v i o u r o f t h e C S A S P u n d e r c y c l ics u b s t r a t e (K m ~ zo ). l o a d i n g c o n d i t i o n s i t w a s f o u n d t h a t t h e p r c gi i ct e dW i t h r e g a r d t o P ~ m2 0, D o l d e t a l . f o u n d a n a v e r a c j e o x y g e n c o n s u m p t i o n r a t e i n t h e c o n t a c t r c a ~ t o r (O tc )v a l u e o f 0 .6 5 d - ~ f o r d a t a. T h e v a l u e o f / z w z o a s so c i - s h o w e d a m a r k e d c y c l ic v a r i a t i o n o v e r t h e d a y - - aa t e d w i t h e a c h b a tc h o f s e w a g e c a n b e d e t e r m i n e d b y r e su l t in c o n fl i c t w i t h t h e e x p e r i m e n t a l o b s ~ v a t i o n s .o b t a i n i n g t h e b e s t f it b e t w e e n e x p e r i m e n t a l a n d p r e- A l s o , t h e v a r i a t i o n i n t h e p r e d i c t e d s o lu b l e C O D c o n -


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The activated sludge proc ess--P art 2 1743C~SON OF DATA SOUAREWAVELOAD %~RhWTON (X)III!ARISON OF DATA SQUARE~.LOAD VARATON~ A N N ~ C O 0 5 0 0 M G -C O D /L S L UDG E UAR~E .8 DAYS TE ST NUMBER 06 SLLE )GE AG E 5.SDAYSI N F L U IE N T r K N 4 6 0 M G N / L T E M = E R A T U E 1 95 = C T E M PE R A T U R E 1 9 5 C8 II ~_~_ ~ PH.zzo ~EA~O o START STOP PH=720 STARTw L ~ I FF~ .O r=cu r = = B O F E E D F E E D F E E D ~ .

--l-i8 ~ ~ ~ 8 "" " ' 8 ~. F


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1744 W .V . ALEXANDER,G. A. EKAMAand G. v. R. MARAISCOMIRM~ OF~'ll S~.~ ~DVAF~AT(H~ COVF~RSONOFDATASNEWAVELOADVARATO~M ~ N T COD 5 0 3 MQ-C OO /L S I ~ A Q E 5.9I~W S T E ST NUMBERC5 SLU[X~ AGE5,9DAYSMEAN INFLUENTTKN 380 MG-N/LTEMPERATLI tE20.0 "C TEMPERATURE20.0ocl~ l~ PH--720 o o PH= 72 0o ~ ~ w MiNF .~)W~

i s,,,..., o o

~II .IP:-l~ Q -,',,i . . . . . ,, . . . - . , , , . . I I 8o o o I .o o i o o k . o o llt .o o ~ o o 2 ~ ,. o o 2 .. o o o o o 4 ~ o ~ = ~ o 1 6 . o o l o ~ ~ i ~ o oTIME 'HO URS' TIME


9/11

The activated sludge pro cess --P art 2 1745measu red dur ing t h i s pe r iod a re l i ke ly t o be a conse - da i l y cyc l e , t he recyc l e ratio va r i ed ove r t he cyc l equence o f t he va r i a t i on i n behav iour usua l ly a ssoc i - r e su l ti ng i n f l uc tua t ing M LV SS concen t ra t i ons in thea t ed wi th b io log i ca l sys t ems, con tac t s t ab i l iza t i on reac to r s . Fo r example , unde r t he

The ove r -p red i c t i on o f t he T K N concen t ra t i on i n squa re wave loa d in g cond i t i ons (F ig . 3 ), t he recyc l ethe con tac t r eac to r , Ntc, appe a rs t o i nd i ca t e t ha t more ra t i o was 4 :1 (2 x 36 /18) dur ing t he l ow f low pe r iodni t r i f ica t ion to ok place in the expe rime nta l uni t than and 1.33:1 (2 x 36/54) durin g the peak f low period.t he mod e l p red i ct s . How ever , t h i s i s no t r e f lec t ed i n The pe r iod o f h igh recyc l e r a t i o (4 :1 ) caused thet h e m e a s u r e d a n d p r e d ic t e d n i t ra t e c o n c e n tr a t io n s . C S A S P t o a p p r o a c h t h e C M A S P i n b e h a v i o u r s o t h a tThe rea son fo r t h i s d i sc repancy is t ha t t he n i t rogen the d if fe rence be tween the s ludge concen t ra t i ons i nf rac t ion o f t he was t e s ludge , f, , i n t he expe r imenta l t he con tac t and s t ab i l iza t i on reac to r s was sma l l . Inun i t w as s l i gh tly h ighe r , i .e . 0 .1 2m gN m gV S S - l , con t ra s t , t he l ow recyc l e r a t i o pe r iod causes a p ro -than the va lue i ncorpo ra t ed i n t he gene ra l mode l , i .e . nounce 'd dev i a t i on f rom the CM AS P so t ha t the d i f-0 .10 mg N mg VSS-~. Th i s r e su l ts i n a g rea t e r mass o f ' f erence be tween the s ludge concen t ra t i on i n t he con-n i t rogen b e ing remov ed f rom the expe r imenta l un i t t a c t and s t ab i l iza t i on reac to r s i s l a rge .v i a t he was t e s ludge than p red i c ted by the mode l . 3 . Oxygen consumption rate . In t he con tac t r eac to r ,Th i s d if fe rence i s r e f lec t ed i n t he e ff l uen t T K N con- t he expe r imenta l t o t a l oxygen consum pt ion ra t e , Otccen t ra ti ons . Fur the rmo re , by a ssuming an fn va lue o f r ema ins v i r t ua l ly cons t an t t h roug hou t t he cyc l e .0 .12 mg N mg VS S- ~ fo r t he expe r imenta l un it , an im- Co mp ared to t he CM AS P, such a h igh degree o f a t-p roved n i t rogen ba l ance wi l l be ob t a ined (Tab le 2 ). t enua t ion in t he oxygen dem and i s unexpec t ed unde rThe expe r im enta l n i t r if i ca ti on oxygen dem and m ay cyc li c i n f luen t load ing cond i ti ons~ How ever , cons ide r -be ca l cu l a t ed f rom the expe r imen ta l da t a by cons ide r - i ng squa re w ave load ing cond i t i ons (F ig . 3 ) th i s be -ing a n i t r a t e ma ss ba l ance ove r t he con tac t o r s t ab il l z - hav iou r can be exp la ined a s fo l l ows: Up on com-a t ion reac to rs . The ca rbonaceo us va lue i s g iven by the menc emen t o f t he peak f l ow pe r iod , t he ava i l ab i l it y o fd i f fe rence be tween the t o t a l and n i t r if i ca ti on va lues . A ca rbon aceous and am mo niaca l subs t ra t e s i nc reases i ncom par i son be tw een the expe r imen ta l and p red i c t ed t he con tac t r eac to r , r e su l t ing i n a g rea t e r ac t i v it y o fva lues i s g iven in Tab le 2 . The con tac t n i t ri f i ca t i on t he he t e ro t roph ic and n i t r ify ing o rgan i sms. How ever ,oxygen deman d (One) ca l cu l a ted f rom the n i t r a t e conco mi t an t wi th t he h ighe r s ludge ac t i v it y , themeas urem ent s (N~c) , i s ex t reme ly sens it i ve t o changes s ludge co ncen t ra t i on (X~,X~) i s reduced. Th e n etin t he va lues o f t he se me asu rem ent s - -a 1 mg N 1- ~ re su l t i s a re l a t ive ly cons t an t t o t a l oxygen consum p-difference in N,c resul ts in a 1 0 . 7 m g O 1- ~ h - ~ di ffer- t ion ra te .ence in O~c . In con t ra s t , t he n i t r if i ca ti on oxygen Div id ing the expe r imenta l t o t a l oxygen consu mp-dem and in t he st ab i l iza t i on reac to r (O, , ) i s r e l a ti ve ly t i on ra t e me asured in t he con tac t r ea c to r (Or) in toinsens it i ve t o changes i n t he i npu t and ou tpu t n i t r a t e t he ca rbon aceou s (Oc) and n i t r if i ca t ion (O~ ) corn-concen t ra t i ons . Tak ing in to ac coun t t he se fac to rs , i t i s ' ponen t s by deduc t ing One f rom Or l ed t o wide ly f luc -c lear tha t there i s a go od corre spo nde nce between the tuat ing resul ts----O~c is ext rem ely sensi t ive to smal lmod e l p red i c t i ons and expe r imen ta l obse rva t ions , va r i a t i ons i n t he measu red n i t r a t e concen t ra t i on .

Consequen t ly , on ly t he p red i c t ed and expe r imenta lCycl ic f low and load condi t ions (Figs 3 an d 4) Ot~ re sponses cou ld be re l i ab ly compared .In gene ra l the cor re spondenc e be tween the p re - The mod e l sa t is fac to ri l y p red i c t s t he obse rved a t -

d i c t ed and expe r imen ta l r e sponses unde r bo th squa re t enua t ed Ot~ re sponse . How ever , unde r squa re waveand s ine wave load ing cond i t i ons i s ve ry good . A load ing cond i t i ons (F ig . 3 ), t he mod e l i ncor rec t l y p re -de t a i led d i scuss ion o f t he comp ar i son o f each p rocess d i c ts an i nc rease in Ot~ imme dia t e ly a f t e r ce ssa t i on o fva r i ab l e i s g iven be low, t he peak f l ow pe r iod . In t he t heore t i cal mod e l t h i s

1. COD concentration. As in t he cons t an t f l ow and . i nc rease i s caused by the i nc rease i n s ludge concen-load t e st s , a d i f fe rence be tween the f i lt e red C O D con- t r a t i on (X~, X~ ) a t a t ime when the c a rbon aceouscen t ra t i on in t he con tac t r eac to r and the unf i l te red and am mo niaca l subs t ra t e s concen t ra t i ons a re s t il lva lue in t he e f f l uen t i s appa ren t . I t appe a rs t ha t t he h igh f rom the peak f l ow pe r iod . Cons ide r ing t he d if fl -empi r i ca l pa r t i a l enm eshm ent modi f i ca t i on i n t he cu l ti e s a ssoc i a t ed wi th accura t e ly measur ing oxygenCSA SP mode l a l l ows accura t e p red i c t i ons o f t he e f fi u - consu mpt ion ra t e s i n t he con tac t r eac to r (Ekam a &ent CO D concen t ra t i on (bo th fi l te red and unf i lt e red) Mara i s , 1979) , t he d i sc repancy be tween the p red ic t edeven und er di fferent cycl ic loadin g condi t ions, and expe rimen ta l Or respon ses i s as l ike ly to be a

2 . M L V S S concen t ra tion . The cor re spond ence consequence o f a de f ic i ency in t he t heore t i ca l mod e lbe tween the p red i c t ed and expe r imenta l ML VS S con- a s expe r imenta l e r ro r .cen t ra t i ons i s exce ll en t i n bo th cyc li c t e st s . An in t e r - In bo th cyc l ic t e st s , t he t o t a l oxygen consum pt ionesting_L obs erva t ion i s the signi f icant "w ashin g out " ra te in the stabi l iza t ion reacto r (Oa) is s l ight ly over-e f fec t o f t he M LV SS f rom the con tac t r eac to r dur ing p red i c ted . Div id ing the expe r imen ta l O s re sponsethe peak f l ow pe r iod . I t was s t a ted abo ve tha t a s t he i n to t he ca rbo naceo us (Oo. ) and n i t r if i ca ti on (On, )recyc le r a ti o i nc reased in t he C SAS P, t he p rocess com pon en t s is poss ib l e because On . is r e l a ti ve ly i n -app roac hed the CM AS P. As t he recyc le f l ow was kep t sens it ive t o sma l l v a r i a t i ons i n t he n i t r a t e conce rt -a t twice t he ave rage i n f luen t fl ow th roug hout t he t r a t i on (N. . ). Cons ide r ing t he squa re wave te s t , a


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1746 W.V. ALEXANDERG. A. EKAMAand G. v. R. MARAIS

START E N D5 4 I / d 18 I / dP E A K F L O W P E A K : L O W4 0 l I I I I l t I I

,~ 35 ~ , ~ T h e o r e t ic a l \_ / e ~ e . . . . , E x p e r i m e r t ~ l

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T I M E ( h )Fig. 5. Com parison of the experimental and predic ted carbonaceou s nit r if ica t ion and tota l oxygenconsumption ra tes in the s tabi l iza t ion reactor of the CSASP under square w ave loading condi t ions a t20C.c o m p a r i s o n o f t h e p r e d i c t e d a n d e x p e r i m e n t a l O r ,, s t r a t e u t i l i z a t i o n r a t e s ; a n d ( 2) a c h a n g e i n t h eO , , a n d O c , r e s p o n s e s i s g i v e n i n F ig ~ 5 a n d s h o w s a e n m e s h m e n t m e c h a n i s m b y a c c e p t i n g t h a t a f r a c t i o ns a t i s f a c t o r y c o r r e l a t i o n , o f t h e p a r t i c u l a t e C O D w h i c h d o e s no t b e c o m e

4. T K N and nitrate concentrations. I n g e n e r a l , t h e a d s o r b e d o n t o t h e a c t i v e m a s s , d o e s not becomet h e o r e t i c a l m o d e l s a t i s f a c t o r i l y p r e d i c t s t h e e x p e r i - e n m e s h e d i n t h e s l u d g e f l o e s a n d e s c a p e s w i t h t h em e n t a l T K N a n d n i t r a t e c o n c e n t r a t i o n s in t h e c o n - e ff lu en t.t a c t a n d s t a b i l i z a t i o n r e a c t o r s u n d e r b o t h c y c l i c W i t h t h e s e c h a n g e s i t w a s f o u n d t h a t t h e m o d e l i ss q u a r e a n d s i n e w a v e l o a d i n g c o n d i t i o n s . I n t h e s u f fi c i en t l y g e n e r a l t o g i v e a g o o d d e s c r i p t i o n o f t h ef o r m e r t e s t ( F i g . 3 ), t h e o v e r - p r e d i c t i o n o f t h e n i t r a t e b e h a v i o u r o f t h e C S A S P u n d e r c o n s t a n t a n d c y c l icc o n c e n t r a t i o n i n t h e c o n t a c t ( N , c ) a n d s t a b i l i z a t i o n c o n d i t i o n s o f l o a d i n g .r e a c t o r (N ,~ ) i s m o s t l i k e ly a t t r i b u t a b l e to a s l i g h t l y T h e i n v e s t i g a t io n i n t o t h e c y c l i c b e h a v i o u r o f t h eh i g h e r n i t r o g e n f r a c t io n i n th e sl u d g e ( f ,) o f t h e e x - C S A S P i n d i c a t e d t h a t t h e p e a k f lo w p e r i o d s r e d u c ep e r i m e n t a l u n i t t h a n a s s u m e d t h e o r e t i c a l l y . I n t h e l a t - t h e c a r b o n a c e o u s m a t e r i a l r e m o v a l a n d n i t r i f i c a ti o nt e r t e s t ( F ig . 4 ). a n i m p r o v e d p r e d i c t i o n o f t h e T K N e f fi c ie n c ie s o f t h e p r o c e s s i n tw o w a y s , i .e . b y r e d u c i n ga n d n i t r a t e c o n c e n t r a t i o n s i n c o n t a c t r e a c t o r ( N t c , ( i) t h e a c t u a l h y d r a u l i c r e t e n t i o n a s w e l l a s { i i) t h eN ~ c) w o u l d b e o b t a i n e d w i t h a s l i g h t l y h i g h e r ~ m 2 0 S l u d ge c o n c e n t r a t i o n i n t h e c o n t a c t r e a c t o r . T h i s b e -t h a n a s s u m e d ( 0.5 5 d - ~) f o r t h i s p a r t i c u l a r b a t c h o f h a v i o u r o f t h e p r o c e s s r e s u l t s i n t h e e f f lu e n t C O D a n ds e w a ge . T K N c o n c e n t r a t i o n s b e i n g v e r y s e n s i ti v e t o c y c l i c

l o a d i n g c o n d i t i o n s . M o r e s t a b l e e f f l u e n t q u a l i t i e su n d e r c y c l ic l o a d i n g c o n d i t i o n s m a y b e p r o d u c e d b yC O N ( 1 U $ 1 O N S i n c r ea s i n g t h e d e s ig n p a r a m e t e r s a n d r a n d r e d u c i n g

T h i s p a p e r p r o v i d e s a m e a n s w h ~ b y , i ~ i t i a l s ti - X ~ . H o w e v e r , s u ch c h a n g e s r e d u c e t h e s p e ci f ic a d v a n -m a t e s c a n b e m a d t ag e a t h a t t h e C S A S P h a s o v e r t h e C M A S P , a n d p r o -c o n c e n t r a t i o n s i n d a c e c o n f i g u r a t io n s c o a f o n n i n g m o r e t o t h e b e h a v -to rs o f a C S A S P f ro m t h e s ix i n ~ t : ~ p a r - i o ar a l c h ar a c te r i s t i c s o f t h e ~ S P .a m e t e r s a ss u m ed to g ov er n th e i a ch a ~ o u r ' d t ~ p ro - A m a jo r ~ l t y in a ~ , ~ g c ar b on -c es s. i.e . s lu d g e a g e , R , , r e cy c le r a ti o , r , ~ a t d i s- a c e ou s m a t e r i a l r e m o v a l a n d n it r if i ca t io n m o d e l s f o rt r i b u t i o n o f t h e s l u d g e m a s s - b o t w c e n t h e t w o r e a c t o r s t h e C S A S P , f o r x a~d e f i n e d b y ~, d a i l y C O D m a s s l o a d , M ~ ) a n d t h e J e n k i n s ( 19 7 5a , b ) , t (a v e r a g e p r o c e s s s l u d g e c o n c e n t r a t i o n , X ~ . T h e s ix t h t h e v a l u e s o f t h ed e s ig n p a r a m e t e r , i.e , t e m p e r a t u r e , w a s n o t co n s i d e r e d m o d e l s u n d e r t h e e n v i r o n n m a t a l c m a l i t i o t t s i n t h ei n t h i s p a p e r , f i e ld . T h e s e d i f fi c u lt i es a r e a l s o m c o t m ~ ~ . 4 h eT w o c h a n g e s t o t h e g e n e r a l a c t i v a t e d ~ a d p k ix m tic a p p l i c a t i o n o f t h e c o m p u t e r m ,m o d e l p r o p o se d b y D o l d et aL ( 19 8 0) w ~ e ~ a l t l t o u g h t h e kinetic constants o fi .e . ( 1) a c h a n g e i n t h e v a l u e o f o n e o f t h e k i n e t i c m a t e r i a l r e m o v a l m c c h a n i m a sc o n s t a n t s i n th e e x p r e s s m n s o f t h e c a r b o n a c e o u s s u b - u n c h a n g e d f o r d i f f e re n t d o m e s t i c s e w a g e s , t h e p r o c e s s


11/11

The act ivated s ludge pro ces s--P art 2 1747r e s p o n s e d o e s d e p e n d o n t h e i n f l u e n t s e w a g e c h a r a c - v a r i a b le s a n d s u b s e q u e n t a n a l y si s o f th e d y n a m i c b e-t e ri s fi c s, s u c h a s t h e u n b i o d e g r a d a b l e s o l u b l e a n d p a r - h a v i o u r o f t h e p r o c e s s b y c o m p u t e r m o d e l s s e e m s at i c u la t e C O D f r a c ti o n s ( f , a n d f up r e sp e c ti v e ly ) . A l s o , s a t is f a c to r y c o m p r o m i s e a n d i n d e e d a p p e a r s t o b e at h e m a x i m u m s p ec i fi c g r o w t h r a t e o f th e n i t r if i e rs a t l o g i c al a p p r o a c h t o de s ig n .2 0 C ( / ~ m , o ) h a s b e e n f o u n d t o v a r y c o n s i d e r a b l yb e t w e e n d i f f e r e n t s e w a g e s . I t a p p e a r s t h a t t h e o n l y A c k n o w l e d g e m e n t s - - T h i s research was carr ied out underway o f ov e rco m ing thes e d i f f i cu l t ie s i s to de t e rm ine con t rac t wi th the Wate r Res ea rch Commiss ion o f Sou ththes e con s tan t s in l ab or a to ry s ca le inve s t iga t io ns u t i - Afri ca . T he au thors wis h to thank the Commiss ion fo rl i z ing the wa s tew a te r to be t rea ted , pe rmiss ion to pub li sh th i s pape r .

A f u r t h e r d i f f i c u l ty i n a p p l y i n g t h e e x i s t i n g m o d e l so f t h e C S A S P t o f u l l s c a l e d e s i g n i s a n e s t i m a t i o n o f R EF ER EN CE Sthe in f luence o f cyc l i c loa d in g cond i t ion s on the p ro - Dold P . L ., E kama G. A . & Mara i s G . V . R . (1980) T heces s beha v iou r . Onc e the s ew age cha rac te r i s t i c s have ac t iva ted s ludge p rocess Pa r t I . A genera l mode l fo r thebeen e s t ab l i s he d , the gene ra l ac t iv a ted s ludge theo ry ac t iva ted s ludge p rocess . Pro . 10 th In t . Conf.a s a p p l i e d t o t h e C S A S P a l l o w s t h e p r e d i c t i o n o f t h e 1 . A . W . P . R . , o r o n to . In Prog. War. Technol . Vol. 12,d y n a m i c b e h a v i o u r o f t h e p r o c e s s u n d e r c y cl i c l o a d - 1 98 0.Ekam a G. A. & Ma rais G. V. R. (1979) Let ter to the Edi-i n g c o n d i t i o n s. T h i s m o d e l i s v e r y c u m b e r s o m e a n d to r. W a t e r S .A . 5, 57-60.can o n ly be u t i l i z ed in the fo rm of a com pu te r p ro - Guje r W. & J enk ins D . (1975a ) T he con tac t s t ab il i za tiongram . ac t ivated s ludge process---oxygen ut i l iza t ion, s ludge pro-duction and efficiency. W a t e r R e s . 9, 553-560.Des ig n eng in ee rs p re fe r s imp le des ign cha r t s an d Guje r W. and J enk ins D . (1975b) A n i t r if i ca tion mode l fo rmo de l s . E x i s t ing mo de l s and the in i t i a l de s ign p ro - the con tac t s t ab i li za t ion ac t iva ted s ludge p rocess . W a t e rc e d u r e p r e s e n t e d i n t h i s p a p e r s a t i s f y t h i s p r e f e r e n c e . Res . 9, 561-566.H o w e v e r , d y n a m i c s o l u t i o n s o f t h e p r o c e s s b e h a v i o u r M a r a i s G . V . R . & E k a m a G . A . ( 19 7 6) T h e a c t iv a t edcan no t be ob ta in ed by s im pl i s t i c mo de l s , com pu te r s ludge p rocess . Pa r t 1 - -S teady s t a te behav iour . W a t e rS .A. 2, 163-200.mo de l s have to b e em ploy ed , i f s uch s o lu t ion s a re Ohron D. M. & Jenk ins D . (1972) T he mechan is m andrequ i red . T h e us e o f a s impl i s t i c m od e l fo r the p r e - design o f the con tac t s t ab i li za t ion ac t iva ted s ludge p ro -l i m i n a r y d e t e r m i n a t i o n o f t h e i n d e p e n d e n t p r o c e s s c es s. Adv. Wat . Po l lu t . Res . 6, 353-362.

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