7 Application and Function of Synthetic Polymeric Flocculents in Wastewater Treatment Downloaded by UNIV OF CINCINNATI on May 23, 2016 | http://pubs.acs.org Publication Date: May 5, 1986 | doi: 10.1021/ba-1986-0213.ch007
P. A. Rey and R. G . Varsanik Calgon Corporation, Pittsburgh, PA 15230
The performance of a polymeric flocculent in wastewater systems is influenced by several factors that can be grouped into the categories of system variables, polymer-related parameters, and operational factors. System variables encompass factors such as particle charge, particle size, and solids composition. Polymer -related parameters include flocculent chemistries and structural characteristics that can affect polymer performance. Operational factors cover the importance of proper sludge conditioning and polymer makeup procedures. The specific advantages and disadvantages of a polymer's physical form are discussed. Municipal wastewater treatment can be broken into several stages. The purpose of each treatment stage and the reason for the polymer application are discussed. Polymers are used to reduce the suspended solids and biological oxygen demand loadings and are especially effective in operations at or near design capacity.
THE APPLICATION OF SYNTHETIC FLOCCULENTS
for solid-liquid separ a t i o n s is n o w w i d e l y p r a c t i c e d ; e c o n o m i c a n d e n v i r o n m e n t a l r e s t r i c tions p r o v i d e the greatest i m p e t u s t o w a r d a c c e p t a n c e . I n the 1970s, F e d e r a l s t r e a m d i s c h a r g e s t a n d a r d s (2) r e q u i r e d m u n i c i p a l w a s t e w a t e r plants to i m p r o v e their suspended solids a n d b i o l o g i c a l o x y gen d e m a n d ( B O D ) removals. T h e most e c o n o m i c a l m e t h o d of u p g r a d i n g w a s b y i m p r o v i n g the s o l i d - l i q u i d s e p a r a t i o n processes w i t h c h e m i cals. P o l y e l e c t r o l y t e s , i n m a n y cases, are u s e d to r e p l a c e i n o r g a n i c c o a g u l a n t s s u c h as l i m e , a l u m , a n d F e C l 3 , b u t o c c a s i o n a l l y p o l y e l e c t r o lytes are u s e d as a s u p p l e m e n t . A w i d e r a n g e o f f l o c c u l e n t s t a i l o r e d to satisfy the u n i q u e r e q u i r e m e n t s o f e a c h p l a n t exists. A l t h o u g h r e l a t i v e l y f e w m o n o m e r s that are c o m m e r c i a l l y e c o n o m i c a l exist, p r e s e n t t e c h n o l o g y c a n p r o d u c e f l o c c u l e n t s w i t h a w i d e r a n g e o f c h a r g e densities a n d m o l e c u l a r w e i g h t s , w h i c h are a v a i l a b l e i n d r y , l i q u i d , or e m u l s i o n f o r m . 0065-2393/86/0213-0113$08.75/0 © 1986 American Chemical Society Glass; Water-Soluble Polymers Advances in Chemistry; American Chemical Society: Washington, DC, 1986.
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C u r r e n t l y , f l o e e u l a t i o n r e m a i n s m o r e a n art t h a n a s c i e n c e a n d s e l e c t i o n o f t h e m o s t c o s t - e f f e c t i v e p o l y m e r f o r a p a r t i c u l a r a p p l i c a t i o n is d e t e r m i n e d b y laboratory evaluations f o l l o w e d b y plant trial.
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Factors That Influence Polymer Performance F o r e x a m i n a t i o n o f the p e r f o r m a n c e o f a p a r t i c u l a r p o l y e l e c t r o l y t e , the m o s t i m p o r t a n t v a r i a b l e s m u s t b e i d e n t i f i e d . F r o m the l a r g e a m o u n t o f l i t e r a t u r e (2-7) a v a i l a b l e o n v a r i o u s aspects o f f l o e e u l a t i o n , the k e y p a r a m e t e r s a f f e c t i n g s e t t l i n g a n d d e w a t e r i n g c a n b e g r o u p e d i n t o the categories o f s y s t e m v a r i a b l e s , p o l y m e r - r e l a t e d p a r a m e t e r s , a n d o p e r a tional factors.
PARTICLE CHARGE.
System Variables. P a r t i c l e c h a r g e has o n e o f the greatest e f f e c t s o n c o l l o i d a l s t a b i l i t y . U n d e r n a t u r a l c o n d i t i o n s , m o s t colloidal solids f o u n d i n m u n i c i p a l wastewater carry a negative charge. In w a t e r , the p a r t i c l e c h a r g e is c o u n t e r b a l a n c e d b y a n i o n i c a t m o s p h e r e that d e v e l o p s a r o u n d the s o l i d , a n d this p r o p e r t y is k n o w n as the e l e c t r i c a l d o u b l e l a y e r (8). T h e c h a r g e a s s o c i a t e d w i t h a p a r t i c l e c a n b e i n f e r r e d b y the m e a s u r e d £ p o t e n t i a l . C o l l o i d a l s t a b i l i t y c a n b e a t t r i b u t e d to the r e p u l s i v e f o r c e s that arise w h e n t w o s i m i l a r l y c h a r g e d p a r t i cles a p p r o a c h a n d t h e i r d o u b l e l a y e r s i n t e r a c t . D i s s o l v e d s o l i d s , o r g a n i c s s u c h as h u m i c a c i d s o r p o l y s a c c h a r i d e s , a n d p H h a v e a s i g n i f i c a n t e f f e c t o n this d o u b l e l a y e r . F i g u r e 1 (9) i l l u s t r a t e s the e f f e c t o f p H a n d o f t w o c o m m o n d i v a l e n t ions o n the £ p o t e n t i a l o f c l a y p a r t i c l e s . A h i g h c o n c e n tration of cations or adsorption of oppositely charged polyelectrolytes causes a c o m p r e s s i o n o f the e l e c t r i c a l d o u b l e l a y e r a n d enhances i n t e r p a r t i c l e c o n t a c t (10-12). C e r t a i n i o n s , s u c h as C a , are also i n s t r u m e n t a l i n p r o m o t i n g a d s o r p t i o n o f p o l y m e r s (13, 14). T h e n a t u r e a n d m a g n i t u d e of the particle charge d e t e r m i n e the a m o u n t a n d t y p e of flocculent n e e d e d to satisfy the c h a r g e d e m a n d o f the s o l i d . 2 +
PARTICLE SIZE.
P a r t i c l e s i z e is o f t e n r e c o g n i z e d as the m o s t i m p o r tant f a c t o r i n f l u e n c i n g s e t t l i n g a n d d e w a t e r i n g . F i n e p a r t i c l e s cause t u r b i d i t y o r c a n b l i n d f i l t e r m e d i a w h e n n o t e f f e c t i v e l y t i e d u p i n the s o l i d l i q u i d s e p a r a t i o n p r o c e s s (15). C o l l o i d a l s o l i d s c o n s u m e m o r e t h a n t h e i r f a i r share o f p o l y m e r , b a s e d o n t h e i r w e i g h t p e r c e n t o f s o l i d s c o n c e n t r a t i o n . T h i s f e a t u r e is d u e to t h e i n c r e a s e d s u r f a c e a r e a to mass r a t i o that a c c o m p a n i e s the d e c r e a s e i n p a r t i c l e s i z e . F l o c c u l e n t c o n s u m p t i o n is d i r e c t l y r e l a t e d t o s u r f a c e a r e a (16-19). I n g e n e r a l , this f a c t is the r e a s o n that d i g e s t e d s l u d g e , w h i c h has a s m a l l e r m e a n p a r t i c l e s i z e , has a h i g h e r p o l y m e r d e m a n d than undigested sludge.
SOLIDS CONCENTRATION.
P o l y m e r c o n s u m p t i o n is r e l a t e d to the n u m b e r o f p a r t i c l e s o l i d s p r e s e n t . A h i g h e r solids c o n c e n t r a t i o n i m p r o v e s p a r t i c l e i n t e r a c t i o n a n d , i n g e n e r a l , results i n b e t t e r p e r f o r m a n c e at a l o w e r p o l y m e r d o s a g e o n a p o u n d p e r d r y t o n basis.
Glass; Water-Soluble Polymers Advances in Chemistry; American Chemical Society: Washington, DC, 1986.
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+20
C O M P O S I T I O N O F S O L I D S . T h e c o m p o s i t i o n of wastewater w i l l dep e n d o n m a n y f a c t o r s . C o n t a m i n a n t s s u c h as p r o t e i n s , c a r b o h y d r a t e s , o i l s , c l a y s , t r a c e m e t a l s , c o l o r b o d i e s , a n d d i g e s t e d m a t t e r are c o m m o n constituents i n w a s t e w a t e r . I n d u s t r i a l sources c o n t r i b u t e a w i d e v a r i e t y o f i n o r g a n i c a n d s y n t h e t i c o r g a n i c c o m p o u n d s to the s y s t e m . K n o w l e d g e o f these m a t e r i a l s p r i o r to d e s i g n o f the t r e a t m e n t p l a n t c a n p r e v e n t m a n y h e a d a c h e s . F o r e x a m p l e , c o p p e r ions i n excess o f 1 0 0 m g / L are t o x i c to the b i o l o g i c a l s y s t e m i n s l u d g e digesters ( 2 0 ) . C y a n i d e s a n d p h e n o l s m a y also b e p r e s e n t i n i n d u s t r i a l wastes. I n m a n y cases, p r e t r e a t m e n t at the i n d u s t r i a l site is r e q u i r e d b e f o r e the wastes enter the m u n i c i p a l system. G r o u n d w a t e r infiltration a n d storm sewer runoff can h a v e a p r o n o u n c e d e f f e c t o n the s t r e n g t h o f the s e w a g e . F o r i n s t a n c e , i n a c o m b i n e d s e w e r s y s t e m , t h e s t r e n g t h o f the s e w a g e (e.g., B O D a n d s u s p e n d e d solids) d u r i n g a s t o r m is g e n e r a l l y m u c h l o w e r t h a n n o r m a l d u e to the d i l u t i n g a c t i o n o f the r u n o f f . F a c t o r s s u c h as those m e n t i o n e d c o n t r i b u t e to the u n i q u e n e s s o f the s e w a g e .
Glass; Water-Soluble Polymers Advances in Chemistry; American Chemical Society: Washington, DC, 1986.
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N u m e r o u s p u b l i c a t i o n s (15, 21-23) are a v a i l a b l e r e l a t i n g s l u d g e c h a r a c t e r i s t i c s to d e w a t e r a b i l i t y . A s a g e n e r a l r u l e , h i g h l y m e t a b o l i z e d s o l i d s , s u c h as m a t e r i a l f r o m the a c t i v a t e d s l u d g e p r o c e s s , r e d u c e the d e w a t e r i n g e f f i c i e n c y as i l l u s t r a t e d i n F i g u r e 2 (24). M o s t m u n i c i p a l waste treatment plants dewater a b l e n d of p r i m a r y a n d waste-activated sludge. Waste-activated sludges generally require treatment w i t h h i g h l y charged cationic polymers. Polymer-Related Parameters. C o m m e r c i a l s y n t h e t i c p o l y m e r s are g e n e r a l l y d e f i n e d as either n o n i o n i c , c a t i o n i c , o r a n i o n i c (see T a b l e I ) . T h e t y p e o f i o n o g e n i c g r o u p o n the p o l y m e r is i n s t r u m e n t a l i n d e t e r m i n i n g the e f f e c t i v e n e s s o f a f l o c c u l e n t , w h i c h d e p e n d s o n its a b i l i t y to attach to a particle surface. A d s o r p t i o n m a y occur b y electrostatic attraction, c h e m i c a l b o n d i n g , h y d r o g e n b o n d i n g , h y d r o p h o b i c association, c o m p l e x f o r m a t i o n , or a n u m b e r of other means (25-28). T h e energy of b o n d i n g w i l l d e p e n d o n the m a g n i t u d e a n d n u m b e r o f i n t e r a c t i o n s b e t w e e n the c h a r g e g r o u p s o f the p o l y m e r a n d the p a r t i c l e s u r f a c e . C h a r g e d p o l y m e r s have an extended conformation i n solution because of intramolecular repulsion of their functional groups. T h i s extended conformation m a y be of some advantage for b r i d g i n g between particles, b u t e l e c t r o s t a t i c r e p u l s i o n m a y h i n d e r a d s o r p t i o n i f the m o l e c u l e c o n -
0
10
20
30
100
90
80
70
40 50 60 70 % Waste Activated Sludge 60
50
40
30
80
90
100
20
10
0
% Raw Sludge Figure 2. Relationship between dewatering rate and sludge composition [adapted from Morgan et al. (24)]. Key: polymer A; and •, polymer B.
Glass; Water-Soluble Polymers Advances in Chemistry; American Chemical Society: Washington, DC, 1986.
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Polymeric Flocculents in Wastewater Treatment 117
tains too great a n u m b e r o f c h a r g e g r o u p s s i m i l a r to those o n the s o l i d s . A l t e r n a t e l y , s t r o n g a d s o r p t i o n m a y cause the p o l y m e r to f l a t t e n out o n the p a r t i c l e s u r f a c e ; the result is v e r y l i t t l e b r i d g i n g . T h e r e f o r e , d i f f e r e n t flocculent chemistries a n d structural characteristics can have a significant effect on flocculent performance.
SIGN AND MAGNITUDE OF CHARGE.
Anionic and Nonionic Polymers. A g o o d m a n y o f the s y n t h e t i c p o l y e l e c t r o l y t e s u s e d b y i n d u s t r y as f l o c c u lents a r e b a s e d o n p o l y ( a c r y l a m i d e ) . P o l y (ethylene o x i d e ) has s o m e l i m i t e d a p p l i c a t i o n i n the t r e a t m e n t of m i n i n g t a i l i n g s (29). T h e m o s t c o m m o n f u n c t i o n a l g r o u p o n a n a n i o n i c p o l y m e r is a c a r b o x y l a t e i o n ( - C O O ) . A m o n g p o l y m e r f l o c c u l e n t s , a n i o n i c p o l y m e r s t e n d to f o r m the m o s t v i s c o u s s o l u t i o n s b e c a u s e o f c h a i n e x t e n s i o n — t h e r e s u l t o f i n t r a m o l e c u l a r repulsion of their i o n i z e d functional groups. P o l y m e r charges r a n g e f r o m n o n i o n i c (-2 2 2 > 2 2 2 22 0
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