Achieving enhanced filter backwashing with combined air scour and sub-fluidising water at pilot and operational scale

@ANCEDFILTERBACKWASHING Achieving Enhanced Filter Backwashing with Combined Air Scour and Sub-fluidising Water at Pilot and Operational Scale M.J. Chipps, M.J. Bauer and R.G. Bayley ° Thames Water Utilities Ltd, Group Research & Development, Spencer House, Manor Farm Road, Reading RG2 0JN, UK *Thames Water Utilities Ltd, Operational Science, Wraysbury Reservoir, Coppermill Lane, Staines, Middlesex, UK Presented at the Fittech Europa 93 Conference in Karlsruhe, Germany on 19 October 1993

The paper presents evidence to demonstrate thai filter backwash sequences should include a combined air and water phase to maintain the long-term performance of rapid gravity filters when treating lowland river water without the use of coagulants or pre-chlorination. Techniques for assessing filter backwash effectiveness in maintaining clean media included laboratory analysis for retained organic and inorganic material, scanning electron microscopy and examination of trends in clean bed head-loss data. Backwashing with air scour followed by a water wash, without a combined air and water stage, tailed to keep the filter media clean. It led Io increased starting head-losses, fluctuating filtrate quality and ultimately the appearance of cracks in the bed. Backwashing with combined air and water at a sub-fluidising rate ensured consistent filtrate quality and maintained low clean bed head-losses. Data were obtained from pilot plants and fullscale works (200--790 MIIday range) treating London's reservoir stored water prior to further biological filtration by slow sand filters. Air scour rates investigated were from 20 to 50 m/hr free air and were combined with water at 7 - 1 2 m/hr. These rates were successful for several different media types, including standard (0.7 mm) and coarse (1.2 mm) filter sand and dual-media (anthracite/sand) filters. Backwash launder position and configuration were evaluated at large scale and found to be critical to successful retention of media during combined air and water washes. The results confirm, for water containing living organisms, theoretical and laboratory findings of other workers using predominantly inert suspensions in clean water. The degree of filter maturation attributed to biofilm and inorganic coating of media was affected by backwashing conditions. The impacl of filter maturation on filfrate quality is discussed.

Advances in theoretical understanding

Introduction The importance of cleaning rapid gravity filters properly The c l e a n i n g of g r a n u l a r m e d i a f i l t e r s u s e d in t h e w a t e r i n d u s t r y h a s l o n g b e e n a p o o r l y u n d e r s t o o d p r o c e s s . In l ) o t a b l e w a t e r t r e a t m e n t , r a p i d g r a v i D ~ f i l t e r s (RGFs) h a v e b e e n u s e d a s a final p o l i s h i n g s t a g e a R e r c o a g u l a t i o n a n d s e t t i m e n l a l i o n , or as a r o u g h i n g s t a g e , for p r e l i m i n a r y p a r t i c l e r e m o v a l l)rior to sh)w s a n d filtration. T h e s e f i l t e r s r ~ q u i r e c l e a n i n g e i t h e r w h e n t h e s o l i d s c a p t u r e d in t h e f i l t e r he(t r e s t r i c t t h e flow to t h e p o i n t t h a i t h e a v a i l a b l e d r i v i n g h e a d is dilly u t i l i s e d , or w h e n l h e f i h r a t e q u a l i t y starl.s to d e t e r i o r a t e . C l e a n i n g r a p i d g r a v i t y filters h a s b e e n by b a c M v a s h i n g , to lift t h e c a p t u r e d p a r t i c l e s from t h e bed in t h e ( l i r e c l i o n from w h i c h t h e y c a m e , o v e r a w e i r or l a u n d e r to s u b s e q u e n l w a s h w a t e r t r e a t m e n t s t a g e s . E u r o p e a n p r a c t i c e h a s b e e n t([ b a c k w a s h filters by a i r s c o u r i n g to l o o s e n p a r t i c l e s , a n d a n u p l l o w w a t e r r i n s e to (:lear t h e d i r t y w a t e r . In t h e US it. h a s b e e n m o r e u s u a l t o w a s h t h e filters w i t h a l l u i d i s i n g r a t e o1" w a t e r , g i v i n g a r o u n d 20% bed e x p a n s i o n , folh)wed by a s u r l a e e s c o u r or m e c h a n i c a l raking. [1] F a i l u r e to (:lean RGFs r e s u l t s in h i g h e r s t a r t i n g b e d hea(l-h)sses, so t h e r e is less a v a i l a b l e h e a d for s o l i d s c a p t u r e in e a c h filter cycle, h e n c e g i v i n g s h o r t e r r u n t i m e s . As t h e m e d i a a c c u m u l a t e (tirt t h e g r a i n s a g g r e g a t e t o g e t h e r ; ' m u d b a l l s ' m a y he found, a n d c r a c k s m a y a p p e a r on t h e s u r f a c e of t h e bed. T h i s c a u s e s p o o r e r - q u a l i t y f i l t r a t e as t h e w a t e r flows a t h i g h r a t e s t h r o u g h l i m i t e d -- b u t c o a r s e p a t h w a y s . Old m a t e r i a l , p a r t i a l l y h ) o s e n e d by t h e b a c k w a s h , m a y also p a s s t h r o u g h t h e filter. The f i l t e r b e d m a y h a v e c l e a n a n d d i r t y patches, which cause high localised filtration rates and bacMvash r a t e s in s o m e p a r t s of t h e bed ( ' j e t t i n g ' ) a n d h)w r a t e s t h r o u g h o t h e r p a r t s . Ill

Filtration & Separation

January 1995

R e c e n t a d v a n c e s in u n d e r s t a n d i n g w h a t o c c u r s d u r i n g 1)ackwashin~ a n d o p t i m i s a t i o n h a v e c o m e li'om t h e w o r k ol A m i r t h a r a j a h a n d c o w o r k c r s (a r e v i e w of the,)relical, l a b o r a t m ) ' a n d | ) l a n t - s c a h ' w o r k is pr<,vided in R e l b r e n c e 2). They d e s c r i b e d bray c o m b i n i n g ail s c o u r w i t h w a t e r a l s u b f l u i d i s i n g r a t e s in u p l l o w l)ro(lu('(M a c(m(tition ( t e r m e ( I ' c o ] l a p s e - t m l s i n g ' ) w h i c h p r o d u c e d o l ) t i m u n ) c o n d i t i o n s li)r r e m ( w a l of d e p o s i l s li'om f i l l e r g r a i n s . W a t e r Ul)fl(m, llow r a t e s w e r e ( l e l i n e d in t e r m s ()r m i n i m u m f l u i d i s a l i o n veh)cit~ (~'~,/). T h i s is d e f i n e d as t h e p o i n t w h e r e h e a d - h ) s s s t o p s r i s i n g a n d t)eeornes c o n s t a n t , w h i l e u p l ] o w l h r ( ) u g h a g r a n u l a r bet] i n c r e a s e s . III The v a l u e uf I'%¢ v a r i e s w i l h m e d i a anti w a t e r l e m t ) e r a t u r e . L i n e a r r e g r e s s i o n s w e r e l)ro(hwe(I u s i n g l h e r a t e of w a t e r its a p e r c e n t a g e of l;,,/ a g a i n s t t a l e or a i r w h e n c o l l a p s e t)ulsing w a s o b s e r v e t l for s a n d a n d d u a l - m e d i a filters. L o w e r a i r r a t e s r e q u i r e ( l h i g h e r w a t e r rates, anti vice versa. A r e c o m m e n d e d r e g i o n l o t d e s i g n w a s l ) r o p o s e d w i t h air" r a h ' s set t)etween 3(! a n d 1:~5 m hr, a n d w a t e r r a t e s b e t w e e n 40 a n d 6[)",, ()r 1;,,/ w h e n a i r r a t e s welx' h)w anti 2,5- 45", of ~;,,1 at h i g h a i r r a t e s . Visual ( ) b s e r v a t i o n s of c t ) l l a p s e - p u l s i n g h a v e been m a d e a t l a b o r a t t ) t y s c a l e by F i t z p a t r i c k I:q u s i n g e n d o s c o p e s fitted i n t o a s m a l l filter, a n d h i g h - s p e e d v i d e o re('ortiing. T h e s e o h s e t w a l h m s were

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@ANCEDFILTERBACKWASHING not slated whether this water was pre-chh)rinated, but this would be u s u a l p r a c t i c e n n w o r k s of t h i s t y p e in tile l!K. This paper presents observations nn backwashing filters t r e a t i n g w a l e r c o n t a i n i n g a d i v e r s i t y u f living o r g a n i s m s , w i t h o u t chh)rination or coagulation anti sedimentation pretreatment, ~Qlel'e tile m e d i u n ; i n e v i t a b l y h e c o m e s c o a t e d w i t h a bit)film. Frills with separate air and water and combined air/water at rates 11/ l h e r(,gion in w h i c h c n l l a p s e t m l s i n g ( ) t o u r s a r e r e p n r l . e d a t p i l o t :,R(I t)hlnt st'ale.

S a m p l e s o f s u p e r n a t a n t w e r e f i l t e r e d t h r o u g h g l a s s - f i b r e filter d i s c s , u n e d r i e d a n d p r e w e i g h e d l o t 'silt'. T h e P O C t e s t is a s i l v e r c a t a ] y s e d a c i d - d i ( ! h r o m a t e o x i d a t i u n o f all o r g a n i c m a t e r i a l m tile sample. C a l e u l a t i n n o f P O C a n d silt in t h e d e p t h ()l" t h e b e d w a s m a d e hy t r a p e z o i d a l i n t e g r a t i o n o f t h e s a m p l e v a l u e s to a s t a n d a r d b e d d e p t h o f 6 5 0 ram. C l e a n s a n d w a s d e f i n e d hy T h a m e s W a t e r O p e r a t i o n a l S e i e n c e a s c o n t a i n i n g < 2 5 0 g IX)C/In ~ b e d a r e a a n d < 1 5 0 0 g ' s i l t ' / m 2.

Rapid gravity filtration in London

Full-scale plant studies Walton water treatment works

Ill l,nn(hut, tltc p r i m a r y filters t ) r e f i l t e r s h ) r e d reserv()ir w a t e r , a b s t racle(l l r n m t h e River T h a m e s a n d R i v e r Lea, p r i o r It) s l o w s a n d lilt ral iou. Tl/ere is n o p r o c h h ) r i n a t i o n o f t h e w a t e r b e c a u s e u f t h e - l o w san(I l i l t r a t i o n s t a g e . T h e p r i n c i p l e o f s l o w s a n d f i l t r a t i o n is t o ( n c t m r a g e a i)ioh)gically a c t i v e S c h m u t z d e c k e ( l i t e r a l l y ' d i r t l a y e r ' ) ill w h i c h p h y s i c a l a n d ( h e r o i c a l l i l l r a t i o n f o r c e s c o m b i n e w i t h ba('teria a n t i p r o t o z o a t o r e m o v e v i r u s e s , p a t h ( ) g e n i c t ) a c t e r i a , i n ' o l o z o a n oo(%'sls, z ( ) n p l a n k t o n a n d i ) h y l o p l a r l k t o n . T h a t t h e r a p i d ~ r a v i t y priuta~5, l i l t e r s a l s o act p a r t l y in a b i o h ) g i c a l w a y h a s b e e n .een I)y m e a s u r i n g a m m o n i a r e ( h ] c t i o n s a c r n s s t h e fillers. T h e h)w f i l t r a t i o n r a t e s t h r o u g h s l o w s a n d f i h e r s ( t y p i c a l l y (L2 0.3 m h r ) c a u s e t h e d i r l tn be l a r g e l y c o n f i n e d t o t h e t u p 2 5 ran/ o f t h e hod. T h i s l a y e r is r e m u v e d w h e n t h e filler r e a c h e s l u a x J m u m I t c a d l()ss, a n d c l e a n e d at a r e m ( ) t e c l e a n i n g p l a n t . T h e ( ' h a l l e n g e is t o k e e p t h e R G F s s u f f i c i e n t l y c l e a n t n g a i n t h e I)enefits of partially bi()lugica[ly enhanced filtration, while in'evenling the detcrioratiun of lhe media c()ndition leading tn t h e l ) r o l ) l e m s d e s c r i b e d a b o v e . T h e b a c k w a s h p r o c e s s a n d filt.er slru('tUl'CS s h u u l d t)e d e s i g n e d t o p r e v e n t n t e d i a loss, a n d w h e r e ( l u l l m e d i a a r e use(l, tn r e t a i n t h e s t r a t i l i e d ule(lia strtl(!lure. ( ) p l i II/isal.ion 0 f b a ( : k w a s h i n g w a s i d e n t i fled by R a c h w a l cl al. [41 a s a n i m t ) o r t a n l r e q u i r e m e n t in a c h i e v i n g h i g h e r p r o d u c t i v i t i e s l h r o n g h I , o n d ( ) n ' s e x i s t i n g s l o w s a n d lilter t r e a l n / e n l w o r k s .

Experimental The experimental work was can'ied out on rapid gravity primary rfllers at t w o n f L n n d u n ' s s l n w s a n d l i l t e r w o r k s . T r i a l s at. p i l o t p l a n t , ~cale t o o k p l a c e a t A s h l b r d C o m m o n , a n d f u l l - s c a l e i n v e s t i g a t i o n s l o o k i)lace a t W a l t o n a n d ( ' u p p e r m i l l s w a t e r t r e a t m e n t w o r k s , I,)Ilowing filter r e f l n ' h i s h m e n l . The Ashiord experintents included both sand anti dual media tillers, t he l a t t e r w i t h p r o o z o n a t i u n a n d i r o n ( t i t ) s u l p h a t e d o s i n g p r i o r to lilt r a t i o n ( c n n t a c t lilt r a t i o n ) . C o m b i n e d a i r / w a t e r w a s h e s h a v e b e e n o l ) e r a t e d fin" t h r e e y e a r s . As g r e a t e r h e a d - l o s s e x i s t s a c r o s s t h e s a n d l a y e r u f d u a l - m e d i a filters t h a n a c r o s s t h e a n t h r a c i t e layer, w a s h i n g r a t e s w e r e s e l e c t e d w h i c h w o u h t m a i n t a i n l.he s a n d in a c l e a n c o n d i t i o n . E v a l u a t i n g tile s u c c e s s o f b a c k w a s h i n g w a s by t w o m e a u s . First, I,y n t e a s u r i n g s a n t t ) l e s o f b a c k w a s h e d filter s a n d f o r a t t a c h e d d i r t , and comparing with empirically derived standards (discussed below): and secondly, by using filter head-h)ss data after b a c k w a s h i n g ( ' c l e a n ' he(t h e a d - l o s s ) .

Method of media analysis Sampling T h e s a n ( t in f u l l - s c a l e l i l t e r s w a s s a n ] p i e d a f t e r t h e filter h a d b e e n I , a c k w a s h e d , ( l r a i u e d a n d l o c k e d o u t o f service. T h e p i l o t f i l t e r s t ( , q u i r e d h a c k w a s h i n g a n d d i s m a n t l i n g . T h e d u a l - m e d i a filters a l s o ~ e q u i r e d t h e a n t h r a c i t e l a y e r t o b e c a r e f u l l y r e m o v e d , in u r d e r to u x p o s e t h e s a n d . A n y a n t h r a c i t e r e m a i n i n g in t h e s a n d w a s ( a r e f u l l y r e m o v e d b y h a n d in t h e l a b o r a t n r y . T h e c o m l i t i u n o f t h e filter t)ed w a s n o t e d , t h e n s a n d s a m p l e s ~ e r e t a k e n f r o m t h e t o p 2 5 m m a n d fi'om 1 0 0 - 1 2 5 r a m , 2 5 0 3 0 0 Illlll a n t i 5 0 0 - 601) m m i n t o tile b e d , e x c e p t w h e n t h e s a n d l a y e r s w e r e o n l y 34(I rant d e e p . E a c h s a l n p l e w a s a b o u t 0.5 I in v()lume. T h e s a m p l e s w e r e a n a l y s e d o n t h e s a m e d a y o r s t o r e d in a freezer. T h e laborat<)~5' nleth()(ls fi)r m e d i a a n a l y s i s w e r e n o t s u i t a b l e f u r u s e w i t h a n t h r a c i t e b e c a u s e n f its f r i a b l e , c a r b o n b a s e d s t r u c t u r e . [n t h e f u l l : s c a l e filter t h e s a m p l e s w e r e a c o m p o s i t e t ? o m t h r e e s e [ ) a r a l e e x c a v a t inns. A n a l y s i s of particulate organic carbon and 'silt' Tile m e d i a a n a l y s i s c o n s i s t e d o f m e a s u r i n g p a r t i c u l a t e o r g a n i c ('art)on (IX)(;) a n d d ~ ' w e i g h t s o l i d s ( r e l ~ r r e d t o a s 'silt.', f o r c o n v e n i e n c e ) in t h e s u p e r n a t a n t p r o d u c e d hy v i g o r o u s l y s h a k i n g k n o w n v o l u m e s u f s a n d a n d w a t e r fl)r l m i n u t e in a s t u p p e r e d cylinder.

56

E x p e r i m e n t s a t W a l t n n w e r e c a r r i e d o u t in 1 9 7 7 o n t h r e e fldl s c a l e lilters. T w o w e r e o p e r a t e d w i t h n e w l i l t e r s a n d , a n d o n e w i t h o h l filter s a n d . O n e u f t h e r e s a n d e d f i l t e r s a n d tile old filter w e r e washed with air scour at approximately 40 m/hr followed with w a t e r a t a p p r o x i m a t e l y 3 0 m / h r f u r 12 [ n i n u t e s . T h e u t h e r c l e a n s a n d filter w a s w a s h e d w i t h 12 m i n u t e s o f c o m b i n e d a i r w a t e r f l o w i n g tn w a s t e . T h e w a t e r r a t e lbr t h e c o m h i n e d a i r / w a t c r w a s approximately 111 m . , h r . T h e w a s h o u t w e i r w a s u n i y a R~w c e n t i m e t r e s a b n v e t h e filter m e d i a , l i n t a s it w a s a c r o s s t h e s h o r t s i d e o f a r e e t a n g u l a r filter, m e d i a loss w a s l i m i w d .

Coppermills water treatment works Refurbishment of the priiuaw RGFs at Coppermills took place ow,r 1 9 8 9 - - 9 0 . T h i s i n v o l v e d r e m o v i n g all m e d i a t o i n s p e c t f l o o r nnzzles, l a y i n g n e w g r a v e l s , p l a c i n g 6 0 0 m m o f n e w 1 4 / 2 5 filter s a n d , r a i s i n g t h e w a s h o u t weir, a n d i n s t a l l i n g t h r e e c r o s s - l a u n d e r s t u e n h a n c e r e m o v a l o f dirt, f r o m tile filler. T h e l a u n d e r s w e r e f i t t e d w i t h baffles, w h i c h e n a b l e d t h e u s e o f c o m b i n e d a i r / w a t e r w a s h a s the water discharged tn waste, but av()ided the loss of sand. N e w m e d i a w e r e i n s t a l l e d in R G F s 2 a n d 3 in e a r l y 1989, a n d R G F s 4 a n d 5 h a d n e w m e d i a i n s t a l l e d n i n e m o n t h s later. F o r e x p e r m m n t a l p u r p o s e s filters 3 a n d 4 w e r e g i v e n a l 0 m i n u t e c o m b i n e d a i r / w a t e r w a s h , w i t h w a t e r p a s s i n g o v e r t h e b a f f l e s to w a s t e f o r 5 m i n u t e s . As a contr(fl, f i l t e r s 2 a n d 5 w e r e g i v e n a b a c k w a s h w i t h a short. (2 m i n u t e ) c o m b i m ~ d p h a s e ; o n l y t h e f i n a l r i n s e p a s s e d a v e r t h e weir. T h i s r a t e o f w a t e r w a s so h)w t h a t l o o s e n e d m a t e r i a l t e n d e d tu s e t t l e b a c k i n t o t h e f i l t e r i n s t e a d ()f passing over the outlet. Although this gave the appearance that c l e a n b a c k w a s h w a t e r w a s s t a r t i n g t() p a s s o u t o f t h e b e d , a n i m m e d i a t e s e c u n d w a s h s h o w e d t h a t c o n s i d e r a b l e q u a n t i t i e s uf m a t e r i a l w e r e still in t h e b e d .

Backwash rates and timings T h e n o v e l b a c k w a s h p r o c e d u r e a t C u p p e r m i l l s Ior t i l t e r s 3 a n d 4 w a s t o d r a i n d o w n t o 150 m m a b n v e t h e bed, 3 0 s n f a i r a t 2 2 m ' h r , 10 r a i n i t ' a i r w i t h w a t e r at 12 m / h r , w i t h t h i s d i s c h a r g i n g o v e r t h e w e i r f o r 5 rain, a n d ;I r a i n u f w a t e r o n l y a t 17 m / h r ( m a x i m u m r a t e available), discharging tn waste. F i l t e r s 2 a n d 5 w e r e g i v e n t h e o r i g i n a l w a s h w h i c h c o n s i s t e d uf d r a i n - d a w n , 1.5 r a i n o f a i r a t 22 m / h r , 2 r a i n u f c o m b i n e d a i r / w a t e r a t 12 m / h r , t h e n 8 r a i n o f w a t e r at 17 m / h r , i n c l u d i n g 5 m i n o v e r t h e weir.

Experiments with washwater launder baffles G i v e n tile h)w w a t e r r a t e s a v a i l a b l e a t C o p p e r m i l l s , effective w a s i l i n g r e q u i r e ( l c o m b i n e d a i r w a t e r to o p e r a t e ()vet t h e weir, b u t s a n d l o s s w o u l d h a v e b e e n u n a c c e p t a b l e . S i n c e t h e aeti()n u f t h e a i r nluves the sand throughout the water vohune abnve the bed, c o n s i d e r a b l e v o l u m e s o f s a n d w o u l d h e lost u n l e s s a C()ITecI[y designed baffle was installed. The design of the baffles was tested ona2m × 2msquaretest tank, 3m high, designed as a replica of a s m a l l s e c t i o n o f t h e C u p p e r m i l l s filters. T h i s t e s t t a n k w a s s u b s e q m m t l y u s e d t o d e v e h ) p a n o v e l l a u n ( t e r d e s i g n In m i n i m i s e a n t h r a c i t e h)ss in new" d u a l - n t e d i a f i l t e r s f o r t h e A s h l o r d C o m l m m water treatment works. The baffle comprised two sections. The h)wer baffle diverted air a n d w a t e r a w a y l~om t h e l a u n d e r b o t t o m a n d t h e t o p b a f f l e tn p r e v e n t s h o ~ l . - c i r c u i t i n g . T h e u p p e r baffle d i v m l , e d t h e a i r a w a y f r o m t h e l a u n d e r ' s sides, so t h a t t h e w a t e r a p p r o a c h i n g t h e l a u n d e r was not turbulem, and sand grains settled dnwn out uf the flow stream. The bottom uf this baffle was sh)tted parallel to the l a u n d e r , t o a l h ) w t h e s a n d t o r e t u r n t:n t h e bed. T h e d e s i g n w a s t e s t e d , a n d e x t r a p o l a t i o n s s u g g e s t e d t h a t less t h a n O. 1% o f t h e s a n d w o u l d be h)st p e r a n n u m w i t h 6 m i n u t e s p e r (lay o f c o m b i n e d a i r / w a t e r w a s h i n g . E x t r a p o l a t i o n s o f l o s s e s in t e s t s w i t h o u t t h e b a f f l e s s u g g e s t e d t h a t all t h e s a n d w u n l d be washed out over a (me-year period. J a n u a r y 1995

Filtration & Separation

@ANCEDFILTERBACKWASHING Pilot plant experiments Pilot p l a n t filtration t r i a l s were carried out at Ash l brd C o m m o n v~ater t r e a t m e n t works over four years. The p i l o t p l a n t c o n s i s t e d of six e x p e r i m e n t a l filters, each 4.7 m tall x 584 m m i n t e r n a l d iameter. The filters h a d a p l e n u m c h a m b e r a n d false-floor ,'~mstruction with 0.2 m m slit nozzles a t a d e n s i t y of 5 0 / m e. The m e d i a s t u d i e d were (effective sizes, uniformi t y coefficients ~md British Sieve Sizes from s u p p l i e r s ' d a t a ) : ', 0.5-- 1.0 m m r o u n d e d sand, British Sieve Sizes 16/3(}, effective size 0 . 5 4 - 0.71 a m , u n i f o r m i t y coefficient < 1.4 ] 0.6 1.18 mm r o u n d e d sand, British Sieve Sizes 14/25, eflective size 0 . 6 3 - - 0 . 8 5 a m , uniformity coefficient < 1.4 { } 0.85-- 1.7 m m r o u n d e d sand, British Sieve Sizes 10/18, effective size 0.9-- 1.18 mm, uniformity coefficient < 1.4

[ ~ 1.18--2.8 m m r o u n d e d sand, British Sieve Sizes 6/14, effective size 1.25

1.7 a m , u n i f o r m i t y coefficient < 1.7

i_ 1.2--2.5 m m a n g u l a r a n t h r a c i t e , uniformity coefficient < 1.5

effective size

1.3 mm,

1.7 --2.5 m m a n g u l a r a n t h r a c i t e , uniformity coefficient -= l . l .

effective size

1.8 mm,

":'he t ' ; , , / v a l u e s for t h e sand a n d a n t h r a c i t e s were estimate(] u s i n g a ]hddisation test apparatus. U n d e r s t a n d i n g filtration was the key aim of t h e investigations. An i m p o r t a n t p a r t of the t a s k was to o p t i m i s e the b a c k w a s h i n g , to •vtain the m e d i u m and m a i n t a i n it in a clean c o n d i t i o n so t h a t t h e :iltration e x p e r i m e n t s could be accurately interpret e d. E x p e r i m e n t s were c o n d u c t e d to select m e d i a types a nd d e p t h s md chemical dosing to opt]raise removal of particles, especially dgae, from e u t r o p h i c reservoir water. Stored w a t e r t e m p e r a t u r e s ..aried s e a s o n a l l y between 6:C and 23'C. Peak t u r b i d ] t i e s were 8 NTU. Chlorophyll a p e a k e d at 30 m g / m :). S a m p l e s of b a c k w a s h e d filter s a n d were analysed fi)r a t t a c h e d m a t e r i a l 'dirt', and c o m p u t e r - l o g g e d d a t a of filter h e a d - l o s s were ,,xamined by averaging t h e first four head-loss r e a d i n g s after i)ackwashing, t a k e n a t 15 m i n u t e intervals.

~iltration experimental conditions I'he c o l u m n s were o p e r a t e d at filtration r a t e s from 8 to 25 m/ hr. I'he filters were initially o p e r a t e d with rising w a t e r level in t h e Iilters c o m p e n s a t i n g for h e a d - l o s s increases. Backw a s h took pl a c e ,~ hen a s u s p e n d e d float-switch was tilted. This was l a t e r ('.hanged to ~:~shing on reaching a p r e s e t differential p r e s s u r e value. This w a s :t~ first 1.2 m total h e a d q o s s across t h e m e d i a and nozzles, t h e n m e r e a s e d to 1.8 m, and t h e n t.o 2.5 m. D e m a n d lor b a c k w a s h e s d e p e n d e d on the media, flow rate, c h e m i c a l dosing a nd e s pe c i a l l y ,m the a m o u n t and t a x a of algae p r e s e n t in t h e water: filter run h,ngths varied from a few hours to several days. Some of the d u a l - m e d i a e x p e r i m e n t s received w a t e r after preozonation a n d / o r c h e m i c a l t r e a t m e n t with iron (IIl) s u l p h a t e . The stnred w a t e r was filtered w i t h o u t p r e - t r e a t m e n t by t he s a nd-onl y ~()ntrol filter. The s t o r e d w a t e r was never p r e - c h l o r i n a t e d .

w a t e r to s e t t l e b a c k into t h e e x p a n d e d bed, t h e d ir ty w a t e r o u t l e t valves were not o p e n e d unt i l 90 s after t h e h i g h - r a t e w a t e r started. This b a c k w a s h us e d 2 -- 2.5% of t he p r o d u c t i o n of a filter o p e r a t i n g a t 10 m / h r w i t h a 24-hour run length. The low-rate w a t e r r a t e s us e d in t h e s t u d i e s were b etw een 30 a n d 75% of V,,:, d e p e n d i n g on media, t e m p e r a t u r e a n d low-rate w a t e r flow. There w a s no attempt, t o vary the ra t e as V,,,f c h a n g e d with w a t e r t e m p e r a t u r e . Because c o m b i n e d a i r / w a t e r c a us e d c o n s i d e r a b l e m i x i n g of th e s a n d a n d a n t h r a c i t e , e x p e r i m e n t s were carried ou t to d e t e r m i n e a fluid]sing w a t e r ra t e to achieve fast a nd effective d u a l - m e d i a s e p a r a t i o n . S e p a r a t i o n was e x a m i n e d in a clear-walled filter, an d defined subjectively ms w h e n t he layers a p p e a r e d to be a l m o s t t ot a l l y distinct. The r a t e s a nd t i m i n g s us e d on the p i l o t p l a n t a c hi e ve d s a t i s f a e t o .ry s e p a r a t i o n .

Results Walton trials The Walton t r i a l s were c o n d u c t e d after l a b o r a t o r y t e s t s i n d i c a t e d t h e s u p e r i o r i t y of c o m b i n e d a i r / w a t e r washing. Figure 1 s h o w s t h a t s a m p l e s of di rt y m e d i a re c orde d POC v a l u e s be t w e en 250 an d 3000 g / m , t h a t clean s a nd w i t h s e p a r a t e air a n d w a t e r w a s h i n g r a p i d l y i n c r e a s e d in POC content, a nd after four m o n t h s s t a r t e d to s h o w t h e wide f l u c t u a t i o n s in POC c o n t e n t a s s o c i a t e d with a dirty, bed. The clean sand, w a s h e d with c o m b i n e d a i r / w a t e r h a d a consistentl~¢ ) 2 / low I OC a ve ra ge of 122 g/in", a n d a s t a n d a r d d e v i a t i o n of 68 g: m=', , 2 so t h a t 95% of v a l u e s were < 258 gl m . As a c o n s e q u e n c e th e t a r g e t for a well w a s h e d filter bed w a s set a t 250 g / m 2 by T h a m e s Water O p e r a t i o n a l Science. Dirty beds showed wide f u c t u a t i o n s in IK)C v a l u e s between s a mpl e s . This w a s be c a us e t h e di rt t e n d e d to a c c u m u l a t e in pa t c he s , so t h a t t he restllt d e p e n d e d on w h e t h e r th e s a m p l e s h it (:lean or di rt y pa t c he s .

Coppermills trials F ol l ow i ng r e f u r b i s h m e n t of t h e filters w i t h new m e d i a a n d c o m b i n e d a i r / w a t e r ba c kw a s hi ng, s a n d s a m p l e s a n d clean bed h e a d - l o s s m e a s u r e m e n t s were m a d e to check on th e b a c k w a s h i n g efl~,ctiveness. P robl e ms w i t h t he c o m p u t e r - c o n t r o l l e d b a c k w a s h valves a nd t i m i n g s were e x p e r i e n c e d , so filters 3 an d 4 were given carefully c ont rol l e d a nd m o n i t o r e d w a s h e s over a period from mid1990 to mid-1991. Figure 2 s how s a g r a d u a l b u i l d u p of POC in m e d i a s a m p l e s such t h a t w i t h i n 4 - - 8 m o n t h s of o p e r a t i o n t he filter m e d i a in all four be ds were dirty. The d a t a s u g g e s t pos s i bl e s e a s o n a l trends, w ith th e be ds b e c o m i n g d i r t i e r in t h e s umme r, a nd t h e w i n t e r seeing a fall in d i r t content. This w oul d be c o n s i s t e n t w i t h t h e p r e s e n c e of b l o o m s of large f i l a m e n t o u s a l ga e in t h e s umme r, w hi c h t h e b a c k w a s h w a s ineffective at removing, since t he m a x i m u m w a s h r ate w a s 17 m / h r . I d e n t i c a l t r e n d s were seen with silt data. In t h e winter, r e d u c t i o n s in POC a nd silt were seen on all beds. The d a t a for beds 3 a n d 4 from mi d-1990 to early' 1991 s h o w t h a t it w a s p o s s i b l e to r e t u r n s a n d t h a t had been allowed to b eco m e

Backwash protocol Filter o p e r a t i o n a n d b a c k w a s h t i m i n g s were c o n t r o l l e d hy a , ' o m p u t e r o p e r a t i n g relays and electrically a c t u a t e d valves. Backwash flow r a t e s were set by m e a s u r i n g rise r a t e for t h e low-rate water, and set to give (by visual i n s p e c t i o n ) a total bed e x p a n s i o n (,f 2 0 - 30% for h i g h - r a t e water. Air was provided at a p p r o x i m a t e l y 13 m / h r .

Separate air and water wash In t h e first y e a r t h e w a s h c o n s i s t e d of d r a i n i n g down to a p p r o x i m a t e l y 320 m m above t h e m e d i a surface. This was followed by 3 rain of air scour followed by a fluid]sing wash for 5 rain. C o m b i n e d air/water w a s h In s u b s e q u e n t y e a r s the w a s h was c h a n g e d to a s h o r t e r air-only p h a s e (15 s) a n d t h e i n s e r t i o n of a c o m b i n e d air" a n d low-rate w a t e r phase, l a s t i n g 5 rain. The low-rate w a t e r p i p e w o r k a nd valves were i n s t a l l e d in February 1990. This was set to a f e w r a t e of 7.2 m/hr. All low-rate flows were r e s e t in February 1992 to 9.6 m/ hr. The wash was c o m p l e t e d by 7.5 min of a h i g h - r a t e fluid]sing rinse. For final rinse a n d bed s t r a t i f i c a t i o n the p i l o t p l a n t t r i a l s used 40 -- 50 m / h r for d u a l - m e d i a w i t h 14/25 sand and 35 m / h r for dualm e d i a with 16/30 sand. For s i n g l e - m e d i a 14/25 sand, 4 5 - - 6 5 m / h r achieved 2 0-- 30% bed e x p a n s i o n . In order to remove a i r from t he i)lenum c h a m b e r a n d filter bed, and allow m e d i a g r a i n s in t he dirty

Filtration & Separation

January 1995

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Figure 1. Sand cleanliness: Results for particulate organic carbon (POC) from the rapid gravity primary filters at the Walton water treatment works. [ ] New sand, combined air/water wash New sand, separate air and water washes A Old sand, separate air and water washes The horizontal rule shows the target of 250 g POC/m 2

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Figure 2. Sand cleanliness: Results Ior POC from the primary tilters at the Coppermills water treatment works. [ ] Bed 2 0 Bed 3 /~ Bed 4 x Bed 5 The horizontat rule shows the target of 250 g POC/m 2 ('clean')

m o d e r a t e l y dirty to a clean condition, by using 8 min of c o m b i n e d a i r / w a t e r as the main wash step. The fall in levels from a p e a k in mid-1990 in beds 2 a nd 5 may be due to th e 2 min c o m b i n e d a i r / r i s i n g w a t e r b a c k w a s h i n g phase. This would have been loosening some long-term a t t a c h e d ma t e ri a l , which ]night have been removed d u r i n g the wash, but it is also p o s s i b l e t h a t t h i s m a t e r i a l may have p a s s e d out of t h e filter in t he filtrate. C h a n g e s in w a t e r t e m p e r a t u r e may have h a d an effect, by r e d u c i n g microbial activity on t h e filter g r a i n s and i n c r e a s i n g t he a m o u n t of m a t e r i a l carried out of the bed as a r e s u l t of t he i n c r e a s e d viscosity of t h e water. Nevertheless, IX)C v a l u e s (lid not fall to w i t h i n t h e 250 g / m e target. A second s e a s o n a l p e a k w a s m e a s u r e d in all beds in s u m m e r 1991; this was a s c r i b e d to p r o b l e m s with t he c o m p u t e r c o n t r o l l i n g t h e w a s h sequence. S t a r t i n g h e a d - l o s s e s for filter runs, m e a s u r e d in late 1990, sh owed filter 2 averaging 1.0 m, filter 3 a v e r a g i n g 0.35 m, filter 4 a v e r a g i n g 0.45 m and filter 5 averaging 0.6 m. Filter run l e n g t h s were adversely affected by high s t a r t i n g head-losses.

Ashford Common pilot plant experiments Filter media analysis Tables 1 and 2 p r e s e n t r e s u l t s for POC and silt from s a nd s a mpl e s . The v a l u e s p r e s e n t e d have been n o r m a l i s e d to a sand layer d e p t h of 650 ram, to p e r m i t c o m p a r i s o n with the T h a m e s Wa t e r O p e r a t i o n a l Science section s t a n d a r d s of less t h a n 250 g / m e POC a nd 1590 g/ m e silt. The t i m e s shown are the n u m b e r of m o n t h s t h e m e d i a had been in service. Table 1 sh ows t h a t the s e p a r a t e air scour a n d w a t e r w a s h w a s not c a p a b l e of keeping the sand clean. The s a n d u n d e r t h e s h a l l o w a n t h r a c i t e layer in column 3 failed t h e POC test, b u t was w i t hi n t he silt limit. The s h a l l o w layer of s a n d in c o l u m n s 5 a nd 6 I ~ n g u n d e r n e a t h 660 mm of a n t h r a c i t e p a s s e d both tar ge t s , a l t h o u g h only narrowly in the ease of POC. The silt v a l u e s for c o l u m n s 5 a nd 6 were q uite different, w h e r e a s the POC v a l u e s were v i r t u a l l y identical. A l t h o u g h it is p o s s i b l e t h a t difli~rential d i r t a c c u m u l a t i o n w a s due to iron d osi ng i n h i b i t i n g t h e growth or the a t t a c h m e n t of t h e biomass, t h e p r o b a b l e e x p l a n a t i o n lies in the m i s s i n g samples. Given t h a t t h e d i r t t e n d s to a c c u m u l a t e from t h e t op of t he bed down, t h e v a l u e s for c o l u m n 5 using t h e top a n d 2 5 0 - - 3 0 0 m m s a m p l e s with zero s u b s t i t u t e d for t h e m i s s i n g 11}0--150 mm s a m p l e p r o b a b l y r e p r e s e n t an u n d e r e s t i m a t e of t h e t r u e d i r t values. C o l u m n 6, w i t h t h e top and 100-- 150 m m samples , was p r o b a b l y still an u n d e r e s t i m a t e , but much closer to t h e real figure. There was c o n s i d e r a b l e a c c u m u l a t i o n of d i r t in th e t op 100 m m of s a n d in c o l u m n 4. The colour of t h i s s a n d was no longer golden b ut d a r k brown, and it could be formed into bails, b e c a u s e of t he a m o u n t of clay present. The fluidising wash had the d i s a d v a n t a g e t h a t it was k e e p i n g the s a m e p a r t of the bed always in c o n t a c t w i t h th e i n c o m i n g dirt. The d i r t is more evenly d i s t r i b u t e d t h r o u g h t he 58

January 1995

Filtration & Separation

@ANCEDFILTERBACKWASHING bed w h e r e f l u i d i s a t i o n d o e s n o t t a k e place. By t h e s a m e a r g u m e n t , the a n t h r a c i t e s h o u l d t a k e u p m u c h of t h e d i n ; however, t h e s a n d layer u n d e r n e a t h t h e a n t h r a c i t e w a s t b u n d to b e lktiling or c l o s e to liiiling t h e t a r g e t s . In T a b l e 2 t h e r e s u l t s d o n o t s h o w t h a t t h e s a n d w a s k e p t p e r l ~ c t l y clean, but t h e y d o s h o w t h a t t h e w a s h w h i c h i n c l u d e d c o m h i n e d a i r / w a t e r w a s l a r g e l y a b l e to k e e p t h e s a n d w i t h i n t h e s p e c i f i e d POC a n d s i l t t a r g e t s . The li~ilure in c o h m m l w a s d u e to l i m e s c a l e d e p o s i t s p r e v e n t i n g p r o p e r o p e r a t i o n of a valve, so t h a t I he l o w - r a t e w a t e r w a s o n l y 1.1 m / h r .

Influence of chemical dosing ¢ : n m p a r i s o n of t h e s a n d P 0 C a n d silt v a l u e s in T a b l e 2 Ior c o l u m n s 5 a n d 6 w i t h 660 m m 1 4 / 2 5 s a n d u n d e r 340 m m a n t h r a e i t e , w h e n , ' o l u m n 5 w a s iron d n s e d a n d c o l u m n 6 n o t d o s e d , s u g g e s t s t h a t i he b u i l d u p of d i r t w a s m u c h s l o w e r u n d e r t h e c o n d i t i o n s of iron dosing. In c n n t r a s t w i t h c o l u n m 6 w h i c h h a d o n e b a d result, t h e ( o h u n n 5 s a m p l e s w e r e nme.h c l e a n e r . T h i s p o i n t s to t h e iron d o s i n g h a v i n g a n i n h i b i t n D ' effect on e i t h e r b i o l o g i c a l g r o w l h or m t a e h m e n t . C o n t a c t f i l t r a t i o n u s i n g iron i m p r o v e d t h e flit.rate qualiW, so iron d o s i n g m u s t h a v e b r o u g h t m o t e m a t e r i a l in c o n t a c t w i t h t h e filter g r a i n s , b u t b e i n g b o u n d tip in t h e floe it w a s p r e s u m a b l y m o r e e a s i l y remnw~d by t h e c o m b i n e d a i r a n d w a t e r ~ash. The r e s u l t s fi'om c o l u m n 5, in T a b l e 1, w h e n w a s h i n g w a s u n s u c c c s s f n l , d o n o t s h o w t h a t t h e u s e of iron i n h i b i t e d t h e b i o l o g i c a l m a t u r a t i o n of m e d i a , u n d e r c o n d i t i o n s w h e n g r o w t h w o u l d he e x p e c t e d to b e prolific, h u t f i l t r a t e r e s u l t s s u g g e s t e d it m i g h l h a v e s l o w e d d o w n t h e r a t e of m a t u r a t i o n . W h e n t h e iron w a s l u r n e d off in s u m m e r 1989, c o m p a r a t i v e l y p o o r f i l t r a t e s w e r e p r o d u c e d , w h e n c o n s i d e r e d a g a i n s t t h e o t h e r five filters• It appeared that column 5 was only achieving the turbidity removals ~hat t h e o t h e r filters h a d m a n a g e d s o m e w e e k s p r e v i o u s l y , i.e. iron h a d p r e v e n t e d the filter from a c h i e v i n g m a t u r i W . Clean' bed head-loss examination: Separate air and water wash F i g u r e s 3 9 s h o w b a c k - t o - s e r v i c e ( ' c l e a n ' ) bed h e a d - l o s s e s a c r n s s ii~e m e d i a a n d t h e f i l t e r nozzles. Flow r a t e s are s h o w n a t 1 / l ( I t h t r u e v a l u e lbr clarity. F i g u r e 3 p r e s e n t s d a t a fi'om a 1.(I in d e e p s a n d I]lter w a s h e d w i t h s e p a r a t e a i r a n d w a t e r ( c o l u m n 4). In F i g u r e 3 t h e s t a r t i n g h e a d d o s s s h o w s a f l u c t u a t i n g r i s i n g i e n d (wet t i m whnh, year. INrtJy t h i s is e x p l a i n e d by t h e flow r a t e m ' r e a s i n g from 6 m / h r a t the, s t a r t to 8 m / h r in m i d - M a r c h , a n d u p ,, l 0 m / h r in l a t e ,hme. Flows w e r e held a t t h i s for t h e r e s t of t h e ,ear. II is a l s o t r u e t h a t a s r i m s s h o r t e n e d , a s a r e s u l t of b o t h t h e : w ( , u m u l a t i o n of d i r t in t h e bed a n d s e a s o n a l b l o o m s nf filter i q o c k i n g a l g a e , (:lean bed h e a d - l o s s c a l c u l a t e d as a n a v e r a g e o v e r : he first h o u r w o u l d he higher. ~ ' a t e r t e m p e r a t u r e is a n o t h e r factor. i h)wever, t h e s t a r t i n g h e a d loss nf 1 m a t t h e e n d of t h e p e r i u d is a high figure, a n d c o n f i r m s t h e s a n d c l e a n l i n e s s d a t a t h a t t h e '~a(:k~vash w a s not. effective. In Fignre 4 h i g h h e a d - l n s s e s (:an be s e e n for a bed of 660 m m 1 2 5 s a n d u n d e r 200 m m a n t h r a c i t e ; F i g u r e 5 s h o w s a less a c u t e pks(, in an i r o n - d o s e d hed of 340 m m 1 6 / 3 0 s a n d u n d e r 661) m m a n t h r a c i t e ; a n d F i g u r e 6 s h o w s t h a t t h e s a m e m e d i a w i t h o u t iron d(~sing h a d a s t e e p e r r a t e of rise. T h e s e F i g u r e s s u g g e s t t h a l t h e u s e ,,I a n t h r a c i t e d o e s not l m w e n t t h e m e d i a from b e c o m i n g dirD,, a n d ;~11h o u g h t h e a n t h r a c i t e cannot, be d i r e c t l y t e s t e d l o t s i l t a n d POC, t h e s a n d u n d e r n e a t h w a s f o u n d to be l~tiling or c l o s e In f a i l i n g t h e t n'gets (see T a b l e 1). Both t h e POC anti s i l t f i n d i n g s a n d t h e h e a d - l o s s d a t a R)r , o m m n 5 m a y i n d i c a t e t h a t t h e u s e of iron in c o n t a c t f i l t r a t i o n m o d e l e s s e n e d t h e d e g r e e to w h i e h t h e m e d i a b e c a m e dirty. C~ean' bed head-loss examination: C o m b i n e d air~water wash ~-igure 7 s h o w s r e s u l t s from a f i l t e r c o n t a i n i n g 0.9 m of 1 4 / 2 5 s a n d , ' , , l u m n 4). S o m e of t h i s s a n d (0.6 m ) w a s i n s t a l l e d in N o v e m b e r It.90, a n d it. w a s t o p p e d tip to 0.9 m w i t h c l e a n s a n d in O c t o b e r IvH. This sand was washed with the extra combined air/water 'aash. II can be s e e n in F i g u r e 7 t h a t t h e (:lean bed h e a d - l o s s for this s a n d s l a y e d r e l a t i v e l y c o n s t a n t over 16 m o n t h s , b e c a u s e o f t h e h o l l e r wash. The flnw r a t e s in t h e s e r e s u l t s w e r e s l i g h t l y h i g h e r t h a n in l h e p r e v i o u s g r a p h . F i g u r e s 8 a n d 9 s h o w g r a p h s of h e a d - l o s s for d u a l - m e d i a filters c o m p r i s i n g 600 m m 1 4 / 2 5 s a n d u n d e r 600 m m a n t h r a c i t e , given c o m b i n e d a i r / w a t e r w a s h i n g a n d o p e r a t e d in c o n t a c t f i l t r a t i o n m o d e w i t h ir, m ( l l l ) d o s i n g anti p r e - o z o n a t i o n . The h e a d - l o s s e s ,showed gnod long-terin consisten(y, with son)e fluctuations due to Filtration & Separation

January 1995

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Figure 3. Sand filter clean bed head-loss and flow rate data, for separate air and water wash. Head-loss m

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Figure 4. Dual-media filter clean bed head-loss and flow rate data, for separate air and water wash (bed 3). [ ] Head-loss, m

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Figure 5. Dual-media filter clean bed head-loss and flow rate data, for separate air and water wash (bed 5). Head-loss, m

+ Flow rate, .(m/hr)/tO 59

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Figure 6. Dual-media filter clean bed head-loss and flow rate data, for separate air and water wash (bed 6). [ ] Head-boss, m + Flow rate, (m/hr)/10

Figure 7. Sand filter clean bed head-loss and flow rate data, for the combined air/water wash. [ ] Head-loss, m + Flow rate (m/hr)/10

flow rate, temperature and probably some dirt accumulation, but not the high head-losses seen with separate air and water. The slight rises may indicate that there was further room for improving backwashing. The fact that these results confirm the results for the sand-only filter which did not receive chemically pretreated water suggests that the use of chemicals was less important than the c o r r e c t w a s h in k e e p i n g t h e m e d i a in g o o d c o n d i t i o n . In F i g u r e 9, a t t h e s t a r t o f t h e e x p e r i m e n t , t h e r a p i d r i s e in clean bed head-loss, seen at the highest flow rates, may be indicative that a dirt layer formed quickly, coating the media, which was not removed by the wash used. This might be said to show that the media 'matured'.

The improvement in f i l t e r p e r f o r m a n c e e a n n o t b e w h o l b a s c r i b e d t o i n f l u e n c e s s u c h a s b i o f i l m d e v e l o p m e n t , b e c a u s e th,, effect of the other raw water parameters - such as temperatur,, a n d c o n c e n t r a t i o n o f p a r t i c l e s n o t m e a s u r e d by t u r b i d i t y -- c a n n o ~ be eliminated. For example, the filter performanee deteriorated b r i e f l y in s u m m e r 1 9 9 1 a n d w i n t e r 1 9 9 2 / 3 . P o s s i b l y t h e b i o f i h n i~ a e t i v e in r e m o v i n g m a t e r i a l m e a s u r e d a s t u r b i d i t y , in w h i c h cas,, f a l l i n g w a t e r t e m p e r a t u r e s w o u l d r e s u l t in h ) w e r r a t e s o f m i c r o b i a l a c t i v i t y . In p a r t s o f t h e s u m m e r a n d river t h e w i n t e r p o o r e r r e m o v a l s c o u l d h a v e b e e n d u e t o t h e a b s e n c e o f a l g a e in t h e w a t e r , a s s u m i n g t h a t a l g a l c e l l s d e p o s i t e d o n f i l t e r g r a i n s c h a n g e t h e filter m e d i a g e o m e t ~ in a w a y w h i c h e n h a n c e s t h e d e p o s i t i o n o f f i n e r particles. A c o n t r o l f i l t e r r e c e i v i n g p r e - c h l o r i n a t e d water" m i g h t have' h e l p e d t o e l i m i n a t e t h e i n f l u e n c e o f t h e b i o f i l m . H o w e v e r , in th,. authors" experience, studies of algal removal after ozonation hay,, shown that pre-oxidation can also enhance filtration.

Maturation F i g u r e 10 s h o w s all t h e t u r b i d i t y r e m o v a l d a t a f o r t h e t h r e e c h a n g e s o f m e d i a in t h e 1 4 / 2 5 s a n d - o n l y filter (0 - n o r e m o v a l , 1 - 100% r e m o v a l ) . F h ) w r a t e s a r e s h o w n in ( m / h r ) / 10, f o r e a s e o f s c a l i n g , a n d r a w w a t e r t u r b i d i t y is s h o w n a t 1 / 1 0 t h o f t h e r e a l v a l u e for the same reason. The vertical lines denote when media were c h a n g e d . T h e c h a n g e in O c t o b e r 1 9 9 1 w a s t h e a d d i t i o n o f 3 0 0 m m of new media to an established 600 mm bed. Baclm,ashing included c o m b i n e d a i r / w a t e r a f t e r m i d - J a n u a r y , 1990. W i t h e a c h m e d i a c h a n g e it c a n b e s e e n t h a t t u r b i d i t y r e m o v a l w a s l o w for n e w s a n d , a n d g r a d u a l l y i m p r o v e d w i t h t i m e . F h ) w r a t e did not seem to be an influence, nor did raw water turbidity. There a p p e a r e d t o b e little d i f f e r e n c e in t h e b e s t r e m o v a l s o b t a i n e d b y dirty sand and mature sand.

O b s e r v a t i o n s of m e d i a d u r i n g column dismantling it w a s n o t i c e d p e r i o d i c a l l y t h a t a g g r e g a t i o n s o1 m a t t e r ( m u d b a l l s ? in t h e 1 0 - 50 m m d i a m e t e r r a n g e c o u l d b e s e e n o n t h e t o p o f s a n d or a n t h r a c i t e filters. T h e s e w e r e g r e y s o f t l u m p s w h i c h c o n t a i n e d fibrous and mud-like material, sometimes lormed around an o r g a n i c c o r e s u c h a s a p i e c e o f a fish o r a q u a t i c invertebrate, T h e y s e e m e d t o a p p e a r in t h e s u m m e r , a n d d i s a p p e a r o v e r th,, w i n t e r . T h e y c o u l d c o v e r u p t o a n e s t i m a t e d 25% o f t h e f i l t e r s u r f a e , , o n b o t h s a n d a n d d u a l m e d i a filters, a n d wet-(' lk)und t o p e n e t r a t , ,

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Figure 9. Dual-media filter clean bed head-loss and flow rate data, for combined air/water wash (bed 6). [] Head-loss, rn + Flow rate, (m/hr)/lO January 1995

Filfration & Separation

ANCED FILTERBACKWASHING into th e a n t h r a c i t e and as tar as the sand layer. There w a s no evidence of discs having grown to a size t h a t c a u s e d t h e m to p e n e t r a t e into t h e body of t h e sand, and become c l a s s i c a l filter nmdballs. They were not i m m e d i a t e l y b a c k w a s h e d o ut of t he filter, b ut as they were t e m p o r a r y it was a s s u m e d t h a t th e y d e c o m p o s e d ( ,ver time. Also n o t i c e d w a s t h e a p p e a r a n c e (m t h e s u r f a c e (if a haekwashed and d r a i n e d filter bed of s m a l l deposits, a p p r o x i mately 50 m m 2 in size, some white a n d powdery, s ome blue a nd sol(, an d when iron was dosed they were a bright orange -brow n soft material. It is not clear if t h e s e formed in the bed, or were shed from t h e feed water pipework.

Scanning electron microscopy studies S a m p l e s of m e d i a from clean, m a t u r e and dirty filters were e x a m i n e d usin g a s c a n n i n g electron m i c r o s c o p e (SEM). The r e s u l t i n g p i c t u r e s r e v e a l e d c o m p l e x a r r a n g e m e n t s of biofihn p a t c h e s on s a n d grains, p r o b a b l y c o n s i s t i n g of b a c t e r i a a nd fungi a n d e x t r a c e l l u l a r p o l y s a c c h a r i d e s . How t h e o r i g i n a l biofihn a p p e a r e d before the drying involved in p r e p a r i n g t he m e d i a for lhe SEM can only be guessed at. The difference between clean, m a t u r e and d i t t y m e d i a is p r e s u m a b l y in t h e amount, of biofilm ~',werage over t h e filter grains, and the e x t e n t of b r i d g i n g be t w e e n ,~rains.

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0,3 0.2 O,l 0 Dec-88

Jul-89

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Figure 10. All 14/25 sand filter flow rate and turbidity removal data, with raw water turbidity. [ ] Turbidity removal ~ Media changes + Flow rate, (m/hr)/lO -Raw turbidity/lO

Summary

Maturation of filter media In London r a p i d gravity filters provide a r o u g h i n g filtration of reservoir-stored w a t e r prior to slow s a n d filtration; p r e - e h l o r i n a I ion is n ot used, and c o n s e q u e n t l y biofilms can dev e l op ra pi dl y on ~he filter media, b i n d i n g organic and i n o r g a n i c m a t t e r onto t he ~rains. This m a t e r i a l can build up to an e x t e n t which seriously i m p a i r s th e functioning of the filter. The a c c u m u l a t i o n of b i o m a s s ~,ii the grain s was of benefit, as t h e filter a p p e a r e d to ' m a t u r e ' over I he first few m o n t h s of operation. Folh)wing i n s t a l l a t i o n of clean m e d i a it was not i c e d t h a t filtration performance, in t e r m s of t u r b i d i t y removals, i m p r o v e d from 40% to a r o u n d 70%. S c a n n i n g electron m i c r o g r a p h s from fullscale filters showed t h a t dirty sand b e c a m e colonised by b a c t e r i a an d fungi, pro viding evidence t h a t m a t u r a t i o n s h o u l d be a s c ri be d ~. the growth of b a c t e r i a and fungi on the filter grains. This is s o m e w h a t ak in to t h e 'ripening' r e p o r t e d in i n d i v i d u a l filter runs, w h e r e an i m p r o v e m e n t in filtrate q u a l i t y occurs over t he first p a r t ,ff a filter t u n after backwashing. This is due to collected p a r t i c l e s acting as b e t t e r collectors for o t h e r p a r t i c l e s t h a n t h e original m e d i a grains. [~1 In th e case of the A~shford e x p e r i m e n t s , b e c a u s e m e d i a were ~enerally c h a n g e d in t h e winter, the m a t u r a t i o n pe ri od u s u a l l y coincided with t h e s p r i n g t u r b i d i t y p e a k s and the p r e s e n c e of filterp e n e t r a t i n g algae. It is p o s s i b l e t h a t m a t u r a t i o n was observed a h m g with s o m e t h i n g else c o n c e r n i n g t h e ease with which c e rt a i n types or c o n c e n t r a t i o n s of p a r t i c l e s were removed from t he raw water, i.e. t h e s p r i n g t i m e f i l t e r - p e n e t r a t i n g algae were p r e s e n t when th e filter m e d i a were new. The difference between a m a t u r e and a dirty filter is subjective; a m a t u r e filter is one t h a t p r o d u c e s a good q u a l i t y filtrate a nd meets r u n - l e n g t h targets, w h e r e a s a di~17 filter h a s p r o b l e m s with Ill(rate q u a l i t y and run lengths. The use of SEM or r e g u l a r m o n i t o r i n g of m e d i a cleanliness, (:lean bed h e a d loss a nd filtrate q u a l i t y may p r o d u c e w o r k a b l e d e f i n i t i o n s of ' m a t u r e ' filter sand. W i t h o u t p r o p e r filter washing, m a t u r i t y was a s t a ge in t he process of getting dirty. C o m b i n e d a i r / w a t e r w a s h i n g a p p e a r e d to keep the beds in a b e t t e r condition, i.e. some m a t u r a t i o n w a s (*bserved in filtrate quality. The clean bed head-los s a p p e a r e d to rise, then v i r t u a l l y level off. Since a l i m i t e d level of d i r t is i n e v i t a b l e (and p r o b a b l y useful), the q u e s t i o n arises w h e t h e r w a s h i n g can be c o n t r o l l e d to m a n a g e the m a t u r i t y of t h e filter. For e x a m p l e , when run l e n g t h s are short, resulting from the p r e s e n c e of filter-blocking algae, a highly efficient wash would be used to m a x i m i s e the a v a i l a b l e head, w h e r e a s in th e winter, when t u r b i d i t y l o a d i n g s are high, t h e be ds ,',told be w a s h e d so t h a t m a t u r a t i o n was encouraged, in o r d e r to ,ptiInise the filtrate quality. The benefit of a few poor w a s h e s on filtrate q u a l i t y was seen in late 1991. When t h e t u r b i d i t y removal p e r f o r m a n c e of c o a r s e r 10/ 18 sand in filter 3 s u d d e n l y b e c a m e a b o u t 20% b e t t e r t h a n 14/25 s a n d in c o l u m n 4, the cause was found to be a failed valve, which m e a n t t h a t t h e filter was not being w a s h e d properly. A c o n s e q u e n c e Filtration & Separation

January 1995

of d i r t b u i l d i n g up on t h e gra i ns was i m p r o v e d t u r b i d i t y removals.

Backwashing Air s c our a nd fiuidising b a c k w a s h w a t e r us e d s e p a r a t e l y c a n n o t p r e v e n t t h e filter b e d s froitl b e c o m i n g dirty. The p r o b l e m s e x p e r i e n c e d with poorly w a s h e d filters o p e r a t i n g in London are s i m i l a r to t h o s e r e p o r t e d in t he literature, i.e. high s t a r l i n g headl o s s e s a nd s h o r t runs, i n c o n s i s t e n t f i l t r a t e q u a l i t y an d th e d e v e l o p m e n t of j e t t i ng, mudba l l s , c ra c ks and m e d i a aggregation. C o m b i n e d air/'water w a s h i n g m u s t be used to keep biologically active filters from d e v e l o p i n g e xc e s s i ve coatings. Sand s a m p l e s w i t h i n T h a m e s Water's POC a n d silt t a r g e t s c a n n o t be c o n s i d e r e d to be s p o t l e s s l y clean, but rather, t h e a m o u n t of m a t e r i a l p r e s e n t ma y benefit filter performance, and does not c o n t r i b u t e to filter problems. With a single d e n s e medium, such as sand, t h e c o m b i n e d a i r / w a t e r wash can be used w hi l e t he w a t e r p a s s e s to w a s t e if a baffle d e s i g n e d to p r e v e n t m e d i a loss is used. This e l i m i n a t e s th e r e q u i r e m e n t for h i g h - r a t e fluidising water, a nd h as th e benefit of not re s t ra t i fyi ng t he filter. Some e l e m e n t of final w a t e r only w a s h i n g s h o u l d be used to e l i m i n a t e a i r bubbl e s from th e m e d i a a nd u n d e r d r a i n s . The d u r a t i o n of t he c o m b i n e d a i r/ ' w a t e r p h a s e was i m p o r t a n t . The 2 m i n u t e s c o m b i n e d a i r / w a t e r w a s h on beds 2 and 5 at C o p p e r m i l l s was too s h o r t to be effective, b u t 10 m i n u t e s was successful. It would a p p e a r from t h e Ashford r e s u l t s t h a t at l e a s t 5 m i n u t e s was ne c e s s a ry there. Timings of b a c k w a s h i n g have not been opt i mi s e d. It is p o s s i b l e t h a t t i m i n g s of fl ui d is in g rinse w a t e r couht be placed u n d e r l~edback from a dirty w a t e r t u r b i d i m e t e r . C o m b i n e d a i r / w a t e r w a s h e s l a s t i n g 20 m i n u t e s were used in r ecen t t r i a l s a t Walton. The r e s u l t i n g poor filtrate qua l i ty h as shown t h a t Otis w a s excessive. In t he case w he re m e d i a have be c ome dirty, it was p o s s i b l e to recover from t h i s p o s i t i o n if c o m b i n e d air a nd w a t e r were u s ed at t h e c orre c t r a t e s lor a sufficient time. C o m b i n e d a i r / w a t e r w a s h i n g can be us e d successfully in a dualm e d i a a p p l i c a t i o n . The a n t h r a c i t e may delay the a c c u m u l a t i o n of d i r t on t h e u n d e r l y i n g sand, or lessen t he significance in t e r m s of filter bed head-loss, b u t it was shown not to p r e v e n t high levels of d i r t from forming. The use of p r e - n z o n a t i o n or iron ( t i l t d o s in g in c o n t a c t filtration mode (lid not i n h i b i t m a t u r a t i o n . The r a t e s of combim~d a i r a nd w a t e r which successfully k ep t m e d i a w i t h i n c l e a n l i n e s s t a r g e t s were between 7.2, an d 12 m / h r (or 14,,"25 sand, i.e. 38 63% of ~';,f ( m e a s u r e d as 19 m / h r at 1 4 C). Air r a t e s used were from 22 to 43 m / h r . These r a t e s were in 1.he range at which c o l l a p s e - p u l s i n g t a k e s place. A w a t e r r ate of 1.1 m / hr was not snffieient to ke e p t h e m e d i a clean. The m e d i a were never e nt i re l y free from biofilm growth. The need to stratil}" d u a l - m e d i a a nd remove t h e air from t h e m e d i a a n d u n d e r d r a i n s , a nd allow t he m e d i a to s e t t l e to the e x p a n d e d bed p o s i t i o n before w a t e r discharges, m e a n s t h a t control of w a t e r levels d u r i n g t he w a s h and t h e design of filter shells, 61

@ANCEDFILTERBACKWASHING inclnding t h e s h a p e and p o s i t i o n of the w a s h o u t weirs m u s t be carefully c a l c u l a t e d in order to retain m e d i a in a s u i t a b l e eondition, and thereb y achieve p r o c e s s reliability. l'roblenls which s h o u l d be controlled in more rigorous s t u d i e s of b a c k w a s h i n g included the s t r a t i f i c a t i o n of t h e media. After t he first two ['luidising w a s h e s of a bed, the top 25 m m was removed in case any tines were present. It is not known w h e t h e r t h i s w a s sufficient, or w h e t h e r finer g r a i n s t a k e a longer t i m e to e me rge on Ibe surface -- and if they dn, w h e t h e r this influences t he clean bed head-loss. There w a s also s o m e loss of m e d i a in w a s h i n g , p a r t i c u l a r l y a n t h r a c i t e , which m u s t be carefully avoided in thllscale plants. This m i g h t have had the effect of removi ng t he l i g h t e s t an d possibly d i r t i e s t m a t e r i a l from d u a l - m e d i a filters.

Final note On p o l i s h i n g filters, in a w o r k s w h e r e elarifiers w ork well, b a c k w a s h i n g has only had to remove small fioc p a l l i c l e s . Generally [.his has been in an e n v i r o n m e n t where p r e - c h l o r i n a t i o n h a s ke pt m i c r o b i o l o g i c a l a c t i v i t y to a m i n i m u m . The i m p l i c a t i o n s of reducing chlorine doses tu control t r i h a l o m e t h a n e s (THMs) in w a t e r work s which previously used high doses of p r e - c h l o r i n a t i o n and m a i n t a i n e d a residual t h r o u g h p o l i s h i n g filters have yet to be felt. One c o n s e q u e n c e of e l i m i n a t i n g or reducing the chh)rine dose may be i n c r e a s e d biological growth on t h e filters, e x p o s i n g w e a k n e s s e s in baekwashing.

Acknowledgments The a u t h o r s w oul d like to t h a n k Mr J. Sexton, D ir ecto r of E n v i r o n m e n t & Science, T h a m e s Water Utilities Ltd for p e r m i s s i o r to p u b l i s h t h i s paper. T h a n k s also go to c ol l e ag u es in Tham~,,,, Water O p e r a t i o n a l Science ( p a r t i c u l a r l y Mrs K. A s to n ) , Operations. E n g i n e e r i n g a nd Group R&D for t h e i r a s s i s t a n c e in m a k i n g th is p a p e r possible. The views e x p r e s s e d are t hos e of t h e authors, and nol ne c e s s a ri l y t h o s e of T h a m e s Water.

References 1 Cleasby, J.L.: 'Filtration', i'm Pontius, F.W. (Ed.): 'Water quality ard treatment, 4th edn.' (American Wal.er Works Association/McGraw Hill, Ne~ York, 199()). 2 Amirtharajah, A., MeNelly, N., Page, G. and Meleod, J.: 'Optimum backwash of dual media filters and GAC filter-adsori)ers with air sen~lr' American Water Works Association Research Foundation, Denver, Colorad[~. USA, 1991. 3 Fitzpatrick, C.S.B.: 'l)etaehment of deposits by fluid shear during filt~'r baekwashing', Wt~ter Supply, 1990, 8, Jonkoping, pp 177 - 182{. 4 Rachwal, A.J., Bauer, M.J. and West, J.T.: 'Advanced techniques f()r upgrading large scale slow sand fillers', in." Graham, N.J.D. (Ed.): 'Slow sail d filtration' (Ellis Horwood, Chiehester, 1988). 5 O'Melia, C.R. and All, W.: 'The role of retained particles in deel) bed filtration', Prog. Water Te(hnol., 1978, 10, pp. 167- 182.

Separations Technology Associates

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January 1 9 9 5

Filtration & Separation

TRACTS OFREFEREEDPAPERS Achieving enhanced filter backwashing with combined air scour and subaluidising water al pilot and operational scale Erzielung einer besseren FilterrQcksp/Hung durch eine Kombination van Luftspfilung und subfluidisierendem Wasser im h a l b i n d u s t r i e l l e n M a f l s t a b u n d im n o r m a l e n B e t r i e b s m a B s t a b I ' o , M . J . Chipp.~, M . J . B a u e r u n d R . G . Br~:qley t ,let~Tag~ e*wMfelC wet)el Oasm Condoner Slauoammen get4)e.ctmtte #asset rot oWt we~eten t ~ , ., ~'1} ; K:nld~ffl~J,~wassel Oh.q~O~n Ftt,sal/ yon Koagutat~onsmqlo~, oa~r VorcLlol: ;o0 aultecr,~/'Je:l',a~eo ;.! Me:h'J(~r, :¢I; A~saevldn3 or.~ W,~k~a.~ke,', *ioi, Fr~et-H,J:.'ks~PJOO ~ * de~ Re,D~ha#ueg .~mDo~ot #;9.n,*d4.~ unltd~]len 0,~! I a t x ) t a t ! o ! ~ n u l / a : b c k b ~ ! o~aq¢$~.ho i;,'~($aeratq;Intr~ct~ S!OI~P.~OWtt' ',l.'Je,I X'M:')l~l'u1~kt~*.~op;~ !lr~ ~e UtffPIIUCBI:tI~ ~01, T ~ | / e l ; ~ I~uckvoth..~e: "~ll~t;oF*.tCP'~'q BOt.f '.t.:k~I) #,~"q m~t I t,ll~4:U~b~:g .~.qO!O!VO.~ Wa,~...PL~#Ung. ohne kcm~rveil¢% i dtt dnO W~s~rM~lum *~n~ ~.'tl illF ~ltl/u¢.n'chet.o ~ern O~ F,ffe~ff,cd,Po ~dilO¢* /:* haIfe,i D,e~¢ i n f f ~ verwsachte "~':,t~h~,'.f$:l:¢Kbt /;i;$fe. schwa.,~ano ~. F,tha!:ll,8,:al unO - ~t~e.'~V,c.h - ~as A!almter, vet, R,sser, ,n, vel P*/;k,~[JUl~,ltl; t~'l f':'K*: K',;ffICtttdl~;P H:t' I I*/I d:13 Wa.~t;tw be* S~J~//Le;¢.~,s~lJ'K~,ta.ee$010~0 h;I .~.t~O ''ch.%/I';bf'q[$C [':;l;at'Jui#:;al II:hq ~t,t*l;{]e D,.ckver;uMe '..~ :e,~'/; tL~l ('~ Wlll.q~.~, i~a!ell Vt')tl ,'tz;:',t:;~l;*c'~(!r' At)';KP~ ,~)W*O H:t' VO;/,P' Ft'l~ot/ .x'e~N~l~JOt.Air,get, /';'l t~tt'.Ct; ~JI;. ~/0" ~J~ ~ 0

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~;~I ~ COt'O'I;Ot'LO'J t~l~va~l~J Oe ~vseule ;':t110~CIde ia matu~atmn Ou t~tm SUt fa OuaMe du t~ral (8 l ~ g S . I0 h:l~ 2 l~CS 5 rots I

de lavamiento

al rev(~s en f i l t r o s c o n u n a c o m b i n a c i o n d e f r e g a r c o n a i r e y a g u a s u b f l u i d i z a n t e , a e s c a l a s p i l a t e y o p e r a c i o n a l p o t M . J . (.THpp.% M..I. I - t a u e r y t¢.G. B a q l e y • II;;;',.d. e:.nfO.vffd;~,O,~.K,~Q:R' OL':%*o.~ffo:#qua .~f~.i,ol~.oa$~t(,;3vltglo~ac ill tflh,~ (h~ ~Jl,o.~.'~'0¢~1tq~'.tU,t da!as OP platIfas pe;cm r d~ l a ~ a s a g~an escala tgama 200- 790 MLDI fratando agua guard~de de los ;,i h&,H*:h' .I*/P t ~ u a vn .;¢):n~);lt~eOCt'I)~lO .'?.*dOILY'!ei let~o~,l~llc, d la ~lg~ ~ l:~tos ta[J~o$ b~J~o On~a/SeS ~ I eeOlos, aofe$ ~ rna$ tgltac~)n tJ~ologtca gn Idlros ffeS~8".tOS de atotla. $8 ~ eMU~qldO .I,f~l;IC I'SlO~ ';ttto,~ .,e emo~.a,i p lta aqua de pos Oe sas f;e~ra$ Dale& ~R use de co~Ju~anles o ta,'~nes ~ h e , a t cop e,re desOo 20 haste 50 trdt)rda ~ t~e. en comOmacKm COn ague a 7 - 1 2 n~ho~a. : , CIt)e'ti,,I.O'l !!'celia.~. lie .ip,(~.~t I j e~g?;/~,a Oe; layallNf~Dlo ~l I ~ OTI fe~lO~ #ate ma~|or¢l tr,edr:J$ "~g,q~ ,*'K*~.~'I' a~3h~&~~Ooralo?lo ~ matc,t,~s (vgar~a> e ,n~'gamcas tc~n~ta$, m,c~o~eoom ek:ct:on~a Ql.~a.~a l1 2 mini r meo~o debar fanl~aofa;a~ena) Se nan evalu~o s ~ y conhgutaOon del iava.'nk~r~o al ., ,,:'! ~ t~,sO~ de l,.,;~,ncl=~ ev; delos pa~a pOI~Oa de a#!#a d~. ca~/,l con ea~l ttmp,a ope~ac:onal ~eve$ a qtar, .~ala. q~e se hat~ c~t¢o a la ~ffrade tffenc~n d~ranlo Mvam~lrl*o ~ n a d o LOS ..,,z::e.,,n at tP*PS Con , r e Imganfo con .It~ lavatr~'nlo s~g¢¢.nfe de a.qua .~.1ura etap~ comb, naOa de ~ESd~aOOSCOP ~ ¢ a QGe CDt~l~nO alqar~n~s WVOSc~lh~man fas leco~el~ac~ones ~ olios autO!aS, .~a ao ;¢qiJa a !*r~ C a r d vat,ate. O~ r,;l/;L,/o V a I uff:trx;, a ~a apal:c,~n Ce gt~l;t.~ en ~a cape rk~ IWrO , . am,*'r:;z ,l: t( ~e.~OCt' de;: cuml)~I~iCK)t. Oe alto y a~,la ,I ~a/:ln ~zb!tdd'~/ar,:e. ase~;u/a C,1'J'J~d,c~w~anfe • tP,,::r., , 'ncll;l~.~ ~).~ l)~eil.dn- ¢~! a~bta , ~ ?a~la de ague COt, e:ll~.~ #lr*p.,.~ Se hen c~r~Pq:Jtdo

]~ ~

~laer~s !tan O e r , ~ $ a ~ a s , ¢ t ~ s w..,Uom,naofamerffe ;eact,vas en a~aa ~ [:l QfaOo de maOole/ des I,~ro. ai,Du'do a caoas L~")/g~in~case ~m.tpanK:as. ~e effluyo CaP el mode ~W lavamaento ai areas Se msouM el m ~ t o de. mead:el ~ ' ; I,e:o a la ca~e~ de !~¢aOo (8 ~a:ls. 10 t..m,s. E la~:.. 5 tufa)

/~"]

Improved technique Ior river water flocculation Verbesserte Methode zur FluBwasserflockung t o n M . S . tf~lmee
Re~e~a~~M~ ~~MIai~#~r~he~ ~

Amelioration

M~`

ResP~.~lplg von SchwebeMollen ~ow~ekutze~e Yetwetizeff Oel~lkl ,~ ~Jl.16 a ~ . ~ r~.)

d e la t e c h n i q u e d e f l o c u l a t i o n d ' e a u d e rivi(~re

t m r M . N . t l r l m e e d , T J . M ' t * h a m r t w d et A . A . S a p r ~ : .~a~kunlK~qde~t~PstÈl`~*s~(~danskP.seai~dP~£v~ae~n~-~tP~UC~u~det~Cr~J~che$ expet:w,emakts O b l O ~ Or~ t ~ l ~ e q ~ ~ tloculaleut a lUL~OShekcotOa,,~x condu~ a une me#/TJfe ~ ;a,s (~s ann~.~. ;xu;t her,m~ d am~..~o~et ;e des~v ~.. i c¢.:mnm~e des m P l ~ cen~l,~s te e l ~ m ~ o n Oes sohdes en susDens~n avec un g;a~en~ de vqesse l)lus ~ el ~ l ~ de ~ ~ ~~ ! d ~ avig~C~.~-~b h*L~C(~u~ eht ~M~$ ~ W r ~ ~f~F~tL.~.*r*| ~ a ~ ~ a~!~ ~$ ~ ; ~ l ~ $ C ~ l ! compa~es au lm lesl class,que et. Ya~temenl O'eau. f6 p~Jjs. 161e~s 22 r6ffs.*

T e c n i c a m e j o r a d a p a r a la f l o c u l a c i o n d e a g u a d e r i o p
el OWRal afttelotldo O~ ~or.dosc u & o t ~ s , co~ ~ a ~ a m~s bai& de v e ~ d a o ~ feel~oo de delencebn ~ s ~ f o . ~ m ~ s Ct~;' L', ttalarPJottlo de a~f~l con la Dfu-'Vo~a !afte coft~c~)Pel. (~ ~lg$. IB fm~s. 22 ¢els )

~

Model to simulate the structure and performance of c e l l u l a r p o l y m e r i c m e m b r a n e s : Structure, flux and filtration characteristics M o d e l l z u r S i m u l a t i o n d e r S t r u k t u r u n d L e i s t u n g v a n z e l l e n t 6 r m i g e n p o l y m e r e n M e m b r a n e n : S t r u k t u r - , FluB- u n d F i l l r a t i o n s m e r k m a l e ran N,M. Jackso*l t~, , ~ R(~cct.,uRv riP..;Vofrk3~l.~_so.s;tw$Ctl~n F~.? Id,d Druckablar, Ix,, lz)lVn~.re. Memrdaot~ rtut HM~ "#obet davon a u s g e g a t ~ wtrd dab de, Potem~no~ z r f ~ r ~ ~mt s ~ . 7 m s c ~ o ~ l~tet~.r,c h ~

u.ir~. ~.l:G~¢,dl~ V~h #Ifn~sses i~*tdet; t~l~ o,o PoJeeg:Qlfe u,~d A~b:,~t.l:~le m ROIhYCIll q e / O ~ l ~.l l ~l (~' :PIPII)OZO~IIS//IIk.%J{ r ad.~t Ac,/~ 0J~lbl ~ Sff;skh# Oetall,Qet ~vn~a:~: t~l m;lllLven,t~. ~blch ~t,qe>,~lar,enff.ktO~d~op~" ISFM; uod BddeP~a:t~lh;pgsmefhooo:~ H.us!an(P,ch ;cwoldee; . l i d m '~:'t:.~tIll e'net O*.,talf ~():.QDM~OI?to/err.eli Sl~eklut YP~t e~ .~l! (J¢.~Han:l d.L~ e ~ " r.at~e~e~trach!urKj .

~.~'¢'J*.~; I .bf%M O ~ ~.~ll~ ~Y~UI/[ 0~}$0 S;t~JklUlP.qlb SdRUhOIeP..I)q~ VOtl)P*.~Jt~Y~de: Po,ene~get.~:haflon J: gl;hP ura~l~pl~l~ltrnul;(] II~1 De}!O:l (119ddtCh RdOat~lt~.va~H;I' ~;M {:'ROl1.1kleDd~03 f:lmltlOd ~l~deR

Un mod(~le p o u r s i m u l e r la s t r u c t u r e el les p e r f o r m a n c e s

des membranes

' `~!~`de~;:~qa;~n~l~u~r~u~edepe~ss,~qp~u~u¢~smem~a~v~D~ym~tr~uesapai!~t~:~Ha~

. ..~ .~.e ,)P D,en~t en consxtetaPon ouc ~ a~meem O ~ oo;es et l'epal.vaeu~ Oe la r r e m ~ a ~ (~ ~ , ~ ~ct.i,e ,,~'L~ne c~!k,~a~e l "elude o~ ta shuclum de le~s m ~ , b , ar¢~. a ere ellcclw#e p~t rne,:toscop.e . Itui;~|l;c a ~lt:lya :/~ IS~M/et 8 I*agla#~fJI (I e.'~a~egt av ~ tJl~ Mtuc~{lte iq~.t~, d..is~ c c ~ 1 ~ 9 , d L,~ • quftf;nd~valtellvlsR~¢.tOOlteCt~tches~dlJsa~Jol:N'.dees UntT~)OP)B~/eoerlell,q.JeallOis~rt~et;s:on$ ~qa,,,l,q~e e~J ul~$e pt;t,r ~ t n d ~ ce.s Mluct~es ~ ~.Sptetl~l~ps O~Sp:cw,eto~ ~!s ~X~leSsam 9n boq c*o ;*v(~'k's m ~ a t s rie "aoa;rseur o'~ages e~ave~ ~s pt~e'Jmec¢ograp~.,r~ S; M I a vat:al~on du liu~ Oe ~:mc~ar~e ~ p ~ e a ~ c la pe~e d~ ct~a~geust p~PrMeen c ~ n t ~ an~ un ec~urerr~'~ v ~ u x ~ ~ ~ - fl~ ~)q.,l~lle$ ~*P(~ZtL~~.lnl ~'*~041~Ifial~& fJPP~o01e.~:)r/ISat~O~Ob'll~at!oil !lee.~ ~Ot~" c r ~ l l q u e U ~

Vo,Polsage un'3 dep e~pe~memelien Et,jeb~s~n ~t bo~ ~ unfetsucNen Me~ltxanen a u ~ z e c h . , ~ e Ubem~s!,mmung et~el~ #omen Des stochaM,sctte MOOOfi d~enl /tit Voftlefsa~ des TraPspO~I$ vo~ i atexaulsoh~an',mungeP Dg.:Ft~Mtang.O~uckl~ffal~n unff WUtde !weeks Vort~ ¢l~ge e~nes Obetflachenkuche~s ~no Ors Vot~mgens yap Patf~=m ,n ~*e Memtxan enh~¢kML /:a ~ mo0#ch, d~e Bedeueung des Paq,l(ei~drchm~.&se,s l u t d ~ PorrngtoP~r, om~utung und die PostluDe Oat Vo[scllm~tung ~r. dee MemO~an vort;otl,jlaQep Gfal~cth~ B I ~ F W~rO~P n~l Fo~Gm~lDOl~Oetn vetgb,$~n Um Ftel~kClt m ~ M e t ~ L ~ Wect~sP;w,.'ku,~g~Y. /w,sonet; Paa~kai-Parl~kel- und Patlixel.Pomt~-Kothr~oneo ~u gemnr*e~, sowot~ m,,etr, a¢ aS auch un de~ Obertlache o ~ ,.mte~s~.~.~ Men,bear,an Da,ube~ t~aus kann Oas I,to~e~ auch au! andefe FoJmen Zel~lcrmigef ~ ) t . q ~ S~¢u~t,~en 'dbOtUa~en watson (8 $,1.24 ebb. 9 tel)

cellulaires

polymeriques:

S t r u c t u r e , f l u x et c a r a c t e r i s t i q u e s

de filtration

mce!;e~fecancotga~ceentmkesp~ed~ct,onsfneonqueseHes~ats~auxa~u~ membtanes d t ~ Le ~ sWchasleOue osI .jfd~s~Dou~ j~,,J~e une f~raf~n sous ~ m s ~ ~ ~ de suspems,o~s tta latex C.,x:~a ~ c ~ po,~ pledge up pillea~ en s~dace et la o~v)M~al~on de ~J~lt?¢l;~ d~ns la memb~arm If eel poss,?,tede o t ~ e r~oonapoe du Cmmmre des DarXcu~e~pout ~ ch~x Gbu,amojle~tsl)otes~soq;Bflb~r)c~s,ltl'eoctas~ll!t~la~8~ lestma~ms~afJV~uessom com.pateo$ aux pnolo,'n~crogtaph~.s ~Oul eonna/!~e las InlcractPons st~r~/t~as des calks!one ~rl~cufe; pa~l¢l~e el pamcuket pore a f ,n!br~eur el a la surlace des ,P~J~)tanes #tude.e~ De ~tus ~e mode~ eat .v~sceooblc o'~fte Otendu a O'aut~es lo~m~s Oe sble:h~e cequla~ras~ e u ~ (81~g$, 2 4 1 ~ . g ml¢~

M o d e l o p a r a s i m u l a r la e s t r u c t u r a y r e n d i m i e n t o

de membranas celulares polimericas: caracteristicas de estructura, flujo y filtracion p o t A;M. ,lru:ks.~z ~cm~ son c~nOw:os Se /~a haMdo ~ acl.~to~J chirr tos pwn~st¢os leoncos r los m~atedos ~;,dte,'l.Po~..v#~. I¢~le el~ cberJa .~HamenR., el M,'n~no (~ Dotex y es,o('~'Jca do ~r~r~,btal~a. y fd#a ox~.~,~entales :on/as n~r~'anas escog~l.¢ S~ eml~/ea el moo'elo oata I~ora~¢,ar el Itar~ooae de :~):;".,rdeJ;, Id os~luct,.#a ,~tema arid;at Se na k;gVddO una comDren~n ~q~ la PStl~t'J'a de laPS ~ , ~ e , q s r o r ~ ltlez en t#ftsc~n o~tecta. So 'e/la ~sarrot~gO f~lta D t O ~ K ' S r ~ Iotla s u l ~ l l r ~1 , I(.!nb:ands con fPc..~..~s de mr.'tO,SCO~a e~et/o.¢a 9scane~ tMFE) ~ Woce~) de ~ a ~ n e s y COP fat pom.,ttac,on Oe ~arl*cutms e~ la rrmm~ana FBposd~e~'edeca' ;a tt~oor/anCal d e ' o~dlmettode par/~'u/~ n la :,, :r~'l~ea mtema romp!eta es cla~o q'Je se Oce/a:nc#J~rmas c o ~ a c ~ n de ;a mar~4 ",:P;ulalD o t a l ! c a caasax:acr'~ des tamale de p~'os y el ~ de obaaa:~n en la mem~ana Se cotr~oatan mmgeoes g M h c u * I elT~o;eado U~ ~ It~¢ff)Qt~S'Oelalq ~ l ~ G ~l~la stmu;~l ~Jab e~tl,cRda~ PrOqOS~lC.~O~ laS cnn ! o t ~ c r o ~ a l ~ s oa~a conoce~ las ~tetaceooes e~er;c~s e~tte patPculu, Y choq~,s entre ~lrbcu~as y ; ,, p:,~ouaes de ,'as ,x)rcs es~a,~de ~',.re,do col, dales o~e.l~os ~ ana,~s ~ c t ; de Iolo,,~:mg~at:a ooms Oenlro r so~e la supea,cm de las rr~mo~ana~ Arlemas. el nxm~o l)ueOe emylearse con o~¢as t.fk ~ Se p*ede=e la ~anac*c¢~~ llu~Ode una membrane lemp,a con ca~a de. p ~ e ~ , p ~ ::o~s~e'a~; de flelll*C!£PaS Celd~ge~ ~l~Ota~S (8 p~OS 24 liftS. 9 t ~ I I , t w.¢fcNt~pto:e'fPFttn')~t)# (J~'~lt~g~"d/~ lip COt~dfleP~ CJ~I~(J?ooO~.~UOS..$¢tU¢;/*~r~.J[;¢;UOJO~IMtdC,.~(]e IO3 54

January

1995

Filtration

& Separation