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Water ges.

Vol. 16. pp. 655 to 662. 1982 0043-1354/82/050655-08503.00/0

Printed in Great Britain.All rights reserved Copyright 1982 Pergamon Press Ltd

E F F E C T O F M A G N E S I U M A N D C A L C I U M P R E C I P I T A T I O N

O N C O A G U L A T I O N F L O C C U L A T I O N W I T H LI M E

J. LEENTVAAR and M. REBHUN2

t Ministry of Transport and Public Works, Delta Department, Environmental Research Section,

P.O. Box 439, Middelburg, The Netherlands and

-'Division of Environmental and Water Resources Engineering, Technion-lsrael Institute of Technology,

Haifa, Israel

( R e c e i t e d J u l y

1981)

Abstraet--Coagulation-flocculation studies were carried out to identify and quantify the organic matter

(TOC) fraction removed by Mg (OH)2 precipitation and the fraction removed due to CaCO~ precipi-

tation in l ime treatment of wastewater. Most experiments were carried out with domestic sewage and

some with a tannic acid solution simulating coagulable organics in wastewater. The experiments showed

a clear relation between the amount of TOC removed and the amount of Mg (OH)2 formed. This

relation between TOC removal and Mg(OH)2 precipitation can be expressed in terms of an adsorption

isotherm. The coagulation-flocculation tests of sewage with lime as a coagulant indicated that about

26Jh of the removable TOC in sewage is eliminated by Mg(OH)2.

I N T R O D U C T I O N

Chemical treatment is applied in advanced treatment

and renovation systems and lime is often used as a

coagulant. Several authors have indicated the role of

magnesium hydroxide as a coagulant aid in lime

treatment. Flentje (1927)reported an increase in clari-

fication efficiency when excess lime was used to pre-

cipitate the magnesium in the water. Lecompte (1966)

patented a process for using magnesium carbonate as

a coagulant. Black & Christman (1961)showed that

Mg 2+ ions are adsorbed on the surface of the CaCO3

particles at pH 9.8, thereby lowering or reversing the

potentials and corresponding mobility values. Van

Vuuren

e t a l .

(1967) described the role of magnesium

hydroxide in lime softening and found that the struc-

ture of the Mg(OH)2 floc was most suitable to adsorb

CaCOa, which normally tends to remain in suspen-

sion. They concluded that (1) the characteristically

high magnesium content of their sewage effluent

(approx. 100 mg 1-t Ms) due to the dolomitic origin

of their water supply appeared to be an important

factor in lime flocculation/flotation and that (2) it

seems that the use of dolomitic lime as a coagulant

might have inherent advantages for this purpose.

Folkman & Wachs (1973) reported on experiments on

algae removal from oxidation pond effluents by lime

treatment and showed that magnesium hydroxide

precipitation was responsible for flocculation of the

algae.

Significant magnesium hydroxide precipitation can

be expected at pH values greater than 10.5, due to its

solubility constant . Such pH values are achieved at

high lime treatment. However, low lime treatment is

sometimes used, re sulting in final pH values below

10.5 with little or no magnesium precipitation, cal-

cium carbonate and phosphate being the main pre-

cipitation products. Significant reduction in sus-

pended solids, extinction (light absorbing matter) and

TOC can be obtained by low lime treatment, how-

ever, much better clarity and lower TOC's are

obtained at high lime treatment. Apparently, both cal-

cium carbonate and magnesium hydroxide play a role

in coagulation-f locculation with lime. Both calcium

carbonate and magnesium hydroxide precipitated in

lime treatment effect removal of suspended solids and

TOC, magnesium hydroxide being able to flocculate

and remove TOC fractions not affected by calcium

carbonate alone.

The aim of this study was: (a) to elucidate the role

of magnesium precipitation in TOC removal and

reduction of extinction; (b) to quantify the TOC frac-

tion that can be removed by calcium carbonate pre-

cipitation (low lime treatment) and the TOC fraction

that can be removed only by coagulation-flocculation

with magnesium hydroxide; and (c) to find a quanti-

tative relationship between the amou nt of magnesium

hydroxide and TOC removed.

P R O G R M O F S T U D I E S

TO meet the research objectives, the experimental

program included:

1. Treatment at lime doses and final pH's where

only calcium carbonate precipitates.

2. Treatment at initial magnesium concentrations,

lime doses and final pH's where various amounts of

magnesium hydroxide would precipitate in addition

to calcium carbonate. Addition of magnesium chlor-

ide to wastewater containing little magnesium was

planned accordingly. To find quantitative corre-

lations, various amounts of magnesium hydroxide

were precipitated. This was accomplished in two dif-

655

8/11/2019 Effect of Magnesium and Calcium Precipitation on Coagulation

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(156 J l. lE h[ \ ~,AR lin d ~'|. Rt Hlll.:x

Table 1. Typ ica l c ha rac te r i s t i c s o f the ray , sewage

B O I ) ~

T O C

C O D

Suspended so l ids

Ext inc t ion

pH

Alka l in i ty

Ca lc ium

M a g n e s i u m

Spec . conduc t iv i ty

o r t h o - P

T o t a l - P

Kje ldah l -N

(mg 1Oct

~mg ~CI

(mg ~O21

(rag ~)

( m e q " I H C O 3 )

( m g l - l C a a ' )

( m g l l

M g 2 l

(uS cm- ~)

m g l - I P )

m g

I - ~ P )

( m g l ~ N )

2(X)

I

70

5 X)

370

1.4

7.3

7.O0

34.5

7.O

I 1 X )

23

9 0

f e r e nt w a y s, e i t h e r b y ( a ) o p t i o n a l a d d i t i o n o f M g t o

r e a c h a c o n s t a n t h i g h i n i t i a l m a g n e s i u m c o n c e n -

t r a t i o n a n d v a r y i n g t h e l im e d o s e a n d f i n al p H , o r b y

( b) w o r k i n g a t a c o n s t a n t f i na l h i g h p H a n d v a r y i n g

t h e i n i t i a l m a g n e s i u m c o n c e n t r a t i o n .

B o t h m e t h o d s w c r c u s e d a n d a ~ i d e r a n g e o l p r e -

c i p i t a t e d m a g n e s i u m h y d r o x i d e w a s o b t a i n e d a n d

c o r r e l a t e d w i th t h e r e d u c t i o n m T O C a n d e x t i n ct i o n .

T h e p o s s i b l e e ff e ct o f f i n a l p H a s a s e p a r a t e ~ a r i a b l e

( n o t o n l ~ t h r o u g h i t s e f fe c t o n t h e a m o u n t o f m a g n e -

s i u m h y d r o x i d e } c o t, l d b e t a k e n i n t o a c c o u n t . T a n n i c

a c i d s o l u t i o n w a s s e l ec t e d t o r e p r e s e n t o r g a n i c s t h a t

a r e r e m o v e d b y c o a g u l a t i o n - f l o c c u l a t i o n a n d t o i n v es -

t i g a t e t h e m e c h a n i s m a n d s t o i c h i o m e t r y o f t h e p r o -

c e s s i n a c o n t r o l l e d s y s t e m .

E X P E R I MENTS

T h e e x p e r i m e n t s w e r e c a r r i e d o u t w i t h r a w u n s e t t l e d

sewage f rom the v i l l age o f Bennekom, Ne ther lands , ma in ly

of domes t ic o r ig in . Th is was tewate r i s o f medium s t reng th ,

a s C O D w a s a b o u t 5 0 0 m g l - ~ 0 2 , B O D 5 w a s a b o u t

2 0 0 m g l ~ 0 2 a n d T O C w a s a b o u t 1 7 0 m g 1 - 1 C . T h e

i n i t i a l a v e r a g e m a g n e s i u m c o n c e n t r a t i o n o f t h e s e w a g e

a m o u n t e d t o 7 m g I- ~ M g ( T a b l e 1 ). In e x p e r i m e n t s

w h e r e

p H w a s v ar i ed . 8 0 m g l ~ M g w a s a d d e d t o t h e se w ag e.

L ime was used as a coagu lan t to ob ta in d i f fe ren t pH

o

14e

9 o

8 s

8 0

TS

zo

X.

. . . . ~ F L U t N X

= 8 7 m Q I M Q

0 ~ 7 m Q I - I M 0

I 1 1 I v

tO 10 S I~ I1S 12

L

g

8

10

8

O6

O4

02

o

~ .

8 7 m Q I - ~ MQ

o

2..Z

r i r l

i o i o 5 s 2

D H

F i g . I . W a s t e w a t e r f l o c c u l a te d a t d i ff e r en t p H v a l u e s w i t h l i m e a s c o a g u l a n t a n d w i t h 8 7 a n d 7 m g I -

M g a s i n i ti a l m a g n e s i u m c o n c e n t r a t i o n .

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Effec t o f Mg and Ca p rec ip i t a t ion on coagu la t ion - f loccu la t ion wi th l ime 657

r_

l o

20

t h e o r e h c a l

~ 7

m g

I t

M g

\

\

\

~ /

l

p H

. . t h o o r e t l C O i

~

M g

\

\

i

1o l O S 11 11s I~

~---

p

Fig . 2 . Was tewate r f loccu la ted a t d i f fe ren t pH va lues . The inh ib i t ion o f the Mg(OH)2 p rec ip i t a t ion i s

i l l u s tr a t e d b y t h e t h e o r e t ic a l l in e a n d t h e m e a s u r e d m a g n e s i u m c o n c e n t r a t i o n w i t h a h i g h a n d a l o w

in i t i a l conce n t ra t io n o f Mg 2 * .

va lues. In exper im ents a t con s tan t f ina l pH var ious

a m o u n t s o f m a g n e s i u m c h l o r i d e w e r e a d d e d t o o b t a i n i n -

i t ia l c o n c e n t r a t i o n s r a n g i n g f r o m 5 t o 4 4 m g I - 1 M g . L i m e

was dosed to a f ina l pH of 11 .5 .

T h e e x p e r i m e n t s w i t h s i m u l a t e d o r g a n i c s w e r e c a r r i e d

ou t wi th a so lu t ion o f t ann ic ac id in d i s t i l l ed wa te r a t

con cen t r a t ion s o f 40 , 80 and 100 mg 1- t and wi th four d i f-

fe ren t in i t i a l magnes ium ch lo r ide concen t ra t ions : 12 .1 .

22 .1 , 32. I , 42 .1 mg l - 1 Mg. Tw o l i t r e samples were coagu la -

ted- f loccu la ted a t d i f fe ren t pH ' s (8 .8 , 9 .5 , 10 .5 and 11 .5 ) .

N a O H w a s u s e d t o r a i s e t h e p H t o t h e d e s i r e d v a l u e . T h e

ba tch coagu la t ion- f loccu la t ion t e s t s were done in rec tangu-

la r 21 . p lex ig lass t anks wi th a c ross - sec t iona l a rea o f

10 x 10cm 2 . The fo l lowing exper imenta l p rocedu res were

used :

1 . Mix ing w i th tu rb ine type s t i r r e r a t 660 rev ra in - t ,

which ga ve a m ean G va lue o f 680 s - 1 .

2 . A d d i t i o n o f c o a g u l a n t a n d a d j u s t m e n t o f p H o v e r a

60 s pe r io d a t 660 rev ra in - 1 ( rap id mix).

3 . F locc u la t ion fo r 30 min wi th a p rope l le r type s t i r r e r a t

9 0 r e v r a in - 1 ( G = 4 1 s - t ) .

4 . Sed ime nta t ion fo r 30 min .

T h e G v a l u e w a s m e a s u r e d w i t h a p i l o t s ca l e d y n a m o m e t e r

as desc r ibed by Leen tv aar an d Ywem a (1980) . Af te r sed i -

m e n t a t i o n , a s a m p l e o f a b o u t 1 00 m l w a s w i t h d r a w n a t h a l f

h e i g h t o f t h e t a n k a n d t h e f o l l o w in g p a r a m e t e r s w e r e d e te r -

mine d : Ex t inc t ion a t 620 nm w ave leng th in a cu ve t te (d ia -

m e t e r = 2 c m ) , T O C , M g 2 a n d C a 2 c o n c e n t r a t i o n s . I n

c a s e o f ta n n i c a c i d s o l u t i o n , T O C a n d M g 2 c o n c e n t r a t i o n

were de te rmined .

R E S U L T S

I n th e e x p e r i m e n t s c a r r i e d o u t w i th B e n n e k o m s e w -

a g e w i t h l im e a s a c o a g u l a n t a n d a n i n i ti a l m a g n e -

s i u m c o n c e n t r a t i o n i n t h e w a s t e w a t e r o f 7 o r 8 0 m g

I - 1 M g , t h e r e d u c t i o n o f e x t i n c t i o n (E ) a n d T O C

w e r e m e a s u r e d a t d if f e re n t p H v a l u e s o b t a i n e d b y d i f-

f e r e n t li m e d o s e s a s i l l u s t r a t e d i n F i g . 1. T h e s e e x p e r i -

m e n t s s h o w e d t h a t f o r t h e s a m e e f f e c t a h i g h e r p H

w a s r e q u i r e d i n c a s e o f a l o w i n i t ia l M g c o n c e n t r a t i o n

i n t h e se w a g e a s c o m p a r e d t o a h i g h M g c o n c e n -

t r a t i o n .

T h i s i s d u e t o t h e d if f e re n t a m o u n t s o f p r e c i p i t a t e d

m a g n e s i u m h y d r o x i d e - - t h e c o m p o u n d r e s p o n s ib l e

f o r r e d u c i n g t h e c o ll o i d a l T O C f r a c ti o n a n d e x t i n c -

t io n . T h e p r e c i p i t a t i o n o f M g ( O H ) 2 w a s f o u n d t o b e

i n h i b i t e d i n w a s t e w a t e r , a s c a n b e s e e n i n F i g . 2 w h e r e

r e s id u a l M g c a l c u l a t e d f r o m t h e s o l u b il i t y c o n s t a n t i s

c o m p a r e d w i t h M g r e s i d u a l s a c t u a l l y o b t a i n e d i n th e

e x p e r i m e n t s .

I t s h o u l d b e n o t e d t h a t a l a r g e f ra c t i o n o f T O C i s

r e m o v e d a n d a h i g h r e d u c t i o n i n e x t i n c t i o n i s

o b t a i n e d b e l o w t h e i n s o l u b i l i z a ti o n p H o f M g ( O H ) 2 .

T h i s i s d u e t o p l a i n s e t t l i n g o f t h e s e t t l e a b l e f r a c t i o n s

a s w e ll a s t o s e t t l i n g o f d i s p e r s e d m a t t e r a i d e d b y t h e

p r e c i p i t a t i o n o f C a C O 3 . T h e r e d u c t i o n o f T O C f r o m

1 4 8 to 8 5 m g 1 - z s h o w n i n F i g . 1 i s d u e t o t h i s m e c h -

a n i s m . A n y f u r t h e r r e d u c t i o n i n T O C b e l o w

8 5 m g 1 - t c o u l d b e a c h i e v e d o n l y t h r o u g h M g ( O H ) ,

p r e c i p i t a t i o n .

A n o t h e r s e ri e s o f c o a g u l a t i o n - f l o c c u l a t io n e x p e r i-

m e n t s w i t h li m e as a c o a g u l a n t w e r e c a r r i e d o u t a t a

c o n s t a n t p H , h i g h e r t h a n 1 0.5 a n d a v a r i a b l e a d d i t i o n

o f m a g n e s i u m c h l o r i d e t o t h e s e w a g e i n o r d e r t o

8/11/2019 Effect of Magnesium and Calcium Precipitation on Coagulation

4/8

65S I I . I I n . I V A A R n n d M R I I I I I ( " ,.

T o

o

I

i o ~ 1 o j l o * 1 o

~ - / ~ M g m g l -

Fig. 3. Sewage flocculated at pH 11.5 with lime with differ-

ent initial Mg concentrations.

i

:-

is

I i .

to

" . ' ,

i . \

i o , s ; , ,

a A M g m g i -

Fig. 5. Tannic acid solution flocculated at pH 10.5 and pH

11.5 wi th differen t initial Mg and tannic acid

concentrations.

obtain various amounts of magnesium precipitates

(AMg), so as to be able to correlate TOC and extinc-

tion reduction to the precipitated amount of

Mg(OH):. By these experiments the stoichiometric,

4 J 8 I N F L U E N T

>

nt

O~

nl

w

O2

O

, o / o ~ o , o

~ ~ M g m g l

Fig. 4. Sewage flocculated at pit 11.5 with lime with differ-

ent initial Mg concentrations.

quantitative relationships between the amount of

TOC (z~TOC) or extinction [A extinction) removed

and the amount of magnesium hydroxide formed

could be evaluated. An illustration of results of the

coagulation-floccula tion experiments with lime as a

coagulant at variable initial Mg concentrations and

fixed pH of 11.5 is given in Fig. 3 and Fig. 4 where

TOC and E are plotted vs AMg.

It can also be seen in these figures that a large

reduction in TO C (from 146 to 90 mg 1- t) and extinc-

tion (from 0.438 to 0.07) was obta ined without any

significant Mg precipitation. But reductions below

these values could be accomplished only by precipi-

tation of Mg(OH)2 and residual concentrations of

TOC decreased with increase of AMg.

Results of flocculation experiments with tannic acid

at various Mg conce ntrat ions at pH 10.5 and 11.5 are

shown in Fig. 5. In this case removal is due only to

precipitated Mg(OH)2 since no other components

except Mg and tannic acid were present. Experiments

at pH 8.8 and 9.5, which are below Mg insolubiliza-

tion, showed no tannic acid removals, except at very

high Mg concentra tions, where some removal was

perceptible, possibly by Mg-tannate formation.

DISCUSSION

Many research workers reported that the residual

concen trati on of TOC and extinction decreased as the

amou nt of magnesium removed during lime coagula-

tion-floceu lation increased (Minton & Carlson, 1973;

Mennel e t a l . 1974; van Vuuren e t a l . 1967).

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Effect of Mg and Ca precipitation on coagulation-flocculation with lime 659

Co

ca

U

T N

c j

A C w q

~ Mg

Fig. 6. Illustration of the method used to determine the amo unt o f TO C removed by calcium car bon ate

(ACc,), the amo unt removed by M g(OH)z p recipitation (AC~s) and th e non-removable fraction of TO C

(CNR).

I t h a s b e e n o b s e r v e d ( B e r g

e t a l .

1970), wi th both

raw was tewa te r and s econda ry e f f luen t , t ha t c l a r i f i c a -

t i o n o c c u r r e d a t a n i n s o l u b i l i z a ti o n p H o f o n l y 9. 5 i n

h i g h h a r d n e s s / h i g h a l k a l i n i t y w a s t e w a t e r . I n c o n t r a s t

t h e t r e a t m e n t o f l o w h a r d n e s s / l o w a l k a l i n i t y w a s t e -

wa te r s ha s requ i red a pH o f 11 -11 .5 fo r s a t i s fac to ry

c l a r i fi c a t io n a n d i n m a n y i n s t a n c es a c o a g u l a n t a i d

s u c h a s f e r ri c c h l o r i d e o r a n o r g a n i c p o l y m e r h a s b e e n

requ i re d (W uhrm an , 1968).

T h e r e s u lt s o f t h e w o r k d e s c r i b e d i n t h e p r e s e n t

p a p e r a r e i n a g r e e m e n t w i t h a n d e x p l a i n t h e v a r i o u s

f i n d in g s c it e d a b o v e . I n l i m e t r e a t m e n t o f w a s t e w a t e r

bo th C aC O 3 an d M g(OH)2 a re e f fec t ive in the f loccu -

l a t i o n - c l a ri f i c a t io n a n d r e m o v a l p r o c e s s e s :

1. T h e p r e c i p i t a t e d C a C O 3 a c t s b y t h e " s w e e p

c o a g u l a t i o n " m e c h a n i s m a f fe c ti n g t h e r e m o v a l o f s u s -

p e n d e d s o l i d s a n d e a s il y c o a g u l a b l e l a r g e c o l lo i d a l

ma te r i a l . I t a l s o a ids in se t t l i ng a s a " we igh t ing agen t "

by inc rea s ing the dens i ty o f the s e t t l i ng aggrega te s .

H o w e v e r , s t a b l e a n d o r g a n i c c o l l o i d s c a n n o t b e

a f fe c te d b y c a l c i u m c a r b o n a t e . S i g n if i c an t a m o u n t s o f

C a C O 3 p r e c i p i t a t i o n c a n b e o b t a i n e d i n h i g h

a l k a l i n i t y ( b i c a r b o n a t e ) w a t e r s.

2 . T h e p r e c i p i t a t i n g M g ( O H ) 2 a c t s b y a n a d s o r p t i v e

c o a g u l a t i v e m e c h a n i s m . I t s s t ru c t u r e w h i c h p r o v i d e s a

l a rge ads o rp t ive s u r face a rea and i t s pos i t ive e l ec t ro -

s t a t i c s u r face cha rge enab le s i t t o a c t a s a power fu l

and e f f i c ien t coag u lan t a l s o on s t ab le o rga n ic co l lo ids

and g ive h igh deg ree s o f c l a r i f i c a t ion .

S i g n i fi c a n t a m o u n t s o f M g ( O H ) 2 c a n b e e x p e c t e d a t

h i g h l i m e d o s e s g i v i n g h i g h p H ' s a n d i n w a t e r s c o n -

t a in ing s u f f i c i en t M g . In o rde r to s epa ra te quan t i t a t -

w.a 16,5 K

i ve ly the e ffec t o f M g(OH)2 fo rm a t ion f rom the e f fect

o f C a C O 3 p r e c i p i t a t i o n t h e f ra c t io n o f T O C a n d

e x t i n c t i o n r e m o v e d b y c a l c i u m c a r b o n a t e a l o n e

( A C e , ) h a s t o b e e s t i m a t e d . T h i s c a n b e d o n e b y t w o

m e t h o d s :

1. B a s e d o n t h e p l o t o f T O C o r e x t i n c t i o n v s p H ,

ex t r apo la t io n to a lowe r pH ( s ee F ig . 1 ), whe re the re

i s n o M g ( O H ) 2 p r e c i p i t a t i o n (b e l o w p H 1 0 . 0 a c c o r d -

ing to Fig . 2) .

2 . B a s e d o n a p l o t o f T O C o r e x t i n c t i o n v s A M g ,

e x t r a p o l a t i o n t o A M g = 0 ( w h i c h m e a n s n o M g ( O H ) z

prec ip i t a t ion ) .

T h i s s e c o n d m e t h o d w a s s e l e c t e d t o d e t e r m i n e t h e

a m o u n t o f T O C o r e x t in c t io n r e m o v e d b y c a l c iu m

ca rbona te a lone . Th i s me thod i s i l l u s t ra t ed in F ig . 6 .

T h e e x p e r i m e n t s w i t h s e w a g e s h o w e d t h a t o n a v e r a g e

4 5% o f T O C a n d 8 5 ~ o f e x t in c t io n w a s r e m o v e d w i t h

t h e a i d o f C a C O 3 .

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( s o lu b l e ) T O C o r e x t i n c t i o n (C N R ) b y b o t h c a l c i u m

c a r b o n a t e a n d m a g n e s iu m h y d r o x i d e w a s d o n e b y d e -

t e r m i n a t i o n o f th e a s y m p t o t i c v a l u e o f th e T O C o r

e x t i n c t i o n to t h e c u r v e o f T O C o r e x t i n c t i o n v e r su s

A M g a t h ig h A M g , w h i c h i s e q u i v a l e n t t o a l a r g e

a m o u n t o f M g ( O H ) 2 p r e c i p i t a te d . T h i s a l s o i s i ll u s -

t r a t e d i n F i g. 6 . O n a v e r a g e th e n o n - r e m o v a b l e p o r -

t i o n o f T O C o r e x t i n c t i o n b y l i m e c o a g u l a t i o n - f l o c c u -

l a t i o n o f s e w a g e a m o u n t e d t o 3 9 a n d 8 ~ r e s p ec t iv e l y .

T h e e v a l u a t i o n o f th e a d s o r p t i o n c a p a c i t y o f

M g ( O H ) 2 w a s b a s e d o n t h e a s s u m p t i o n t h a t m a g n e -

s i u m h y d r o x i d e r e m o v e s o n l y m a g n e s i u m r e m o v a b l e

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T O C o f r aw s e w a g e

T o t a l r e s i d u a l T O C a f t e r c o a g u l a t i o n - f l o c c u -

l a t i o n a n d s e t t l i n g

R e s i d u a l T O C a f te r l im e t r e a t m e n t a t h i g h

p H a t A M g i s e q u a l t o 0

N o n - r e m o v a b l e r e si d ua l T O C = a s y m p t o t ic

v a l u e o f C R a t h i g h A M g

R e s i du a l M g r e m o v a b l e T O C = C R C,,R

A m o u n t o f T O C r e m o v e d b y C a ( ' O 3 p r ec ip i -

t a t i on a l one = Co Cc . ,

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M g ( O H h - f l o c = C ~ .,.- C,

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r e s i d u a l m a g n e s i u m c o n c e n t r a t i o n .

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v a l u e s h i g h e r t h a n 1 0 . 5 t h e a m o u n t a d s o r b e d (A C s~ g)

p e r u n i t a d s o r b e n t M g ( O H ) 2 ( e x p r e s s e d a s A M g l w a s

c a l c u l a t e d a t c o n s t a n t p H . T h i s v a l u e A C ~ g . . A M g i s

r e l at e d t o t h e c o n c e n t r a t i o n o f a d s o r b a t e r e m a i n i n g i n

s o l u t io n ( C ~ . A n e x a m p l e o f t h is c a l c u l a ti o n is g i v e n

i n Ta b l e 2 .

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s h o w t h a t i n c as e o f s e w a g e t h e m a x i m a l a d s o r p t i o n

c a p a c i t y r a n g e s f r o m 1 .5 t o 3 .0 m g T O C m g - ~ M g ,

i n d e p e n d e n t o f w a s t e w a t e r c o m p o s i t i o n , T O C r a n g -

i n g f ro m 1 4 0 t o 2 2 0 m g 1 . ~ C a n d i n d e p e n d e n t o f p H .

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m o n t h s . T h e r e m o v a l o f o r g a n i c s u b s t a n c e s b y a d -

s o r p t i o n b y M g ( O H ) 2 s h o w s a b e h a v i o u r s i m i l a r t o

a n a d s o r p t i o n i s o t h e r m a n d f it s i n a F r e u n d l i c h e q u a -

t i o n w i t h a c o r r e l a t i o n c o e f f i c ie n t ( r) r a n g i n g f r o m

( I. 81 t o 0 .91 . Th i s va l ue r i s s i gn i f i c a n t w i t h a s i gn i t i -

c a nc e l e ve l o f 0 .01 .

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l a t i o n o n M g ( O H ) 2 a l s o s h o w s a b e h a v i o u r s i m i l a r t o

a F r e u n d l i c h a d s o r p t i o n i s o t h e r m a t p H 1 0 .5 a n d p H

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0 .9 2 re s p e c t i v e ly ) . T h e r e m o v a l o f t a n n i c a c i d a t p H

8 . 8 a n d 9 . 5 w h e r e n o M g I O H ) 2 p r e c i p i t a t e s s h o w s n o

s i m i l ar i ty w i t h a F r e u n d l i c h a d s o r p t i o n i s o t h e r m

( r = 0 . 51 a nd 0 . 28 r e s pe c t i ve l y ) .

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M g ( O H h a t p H g r e a t e r th a n 1 0.5 s h o w a m a x i m a l

a d s o r p t io n c a p a c i t y of a b o u t 13 m g T O C m g 1 M g

a s i l l u s tr a t e d i n F i g . 1 0. T h e a d s o r p t i o n c a p a c i t y f o r

t a n n i c a c i d i s a b o u t f o u r t i m e s h i g h e r t h a n t h a t f o r

o r g a n i c s i n s e w a g e . T h i s i s p o s s i b l y d u e t o t h e d i f f e r -

e n t n a t u r e o f t h e s p e ci fi c c o m p o u n d s i n v o l v e d , as w e l l

a s t o t h e fa c t t h a t i n s e w a g e t r e a t m e n t p a r t o f t h e

M g ( O H ) 2 i s u s e d f o r f l o c c u l a t i o n o f A C c t .

C O N C I . U S I O N

T h e s t u d y h a s sh o w n t h a t t w o m a j o r m e c h a n i s m s

a r e e f f e c t i v e i n l i m e c o a g u l a t i o n - f l o c c u l a t i o n o f s e w -

a g e :

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2 . A d s o r p t i o n - c o a g u l a t i o n o f st a b le o r g a n i c c o l l o i d s

b 2, M g ( O H I 2 . T h e l a st m e c h a n i s m p r o v i d e s h i g h c l a r -

i ty ef f luents .

T h e e x p e r i m e n t s w i th s e w a g e s h o w e d t h a t o n a n

a v e r a g e :

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m o v e d b y C a C O 3 f o r m a t i o n ;

a n a d d i t i o n a l 1 6% o f T O C a n d 7 0 o f e x t i n c t i o n o f

t h e ra w s e w a g e c o u l d b e r e m o v e d b y M g ( O H } 2 p r e -

c i p i t a t i o n ;

t h e n o n - r e m o v a b l e f r a c ti o n a m o u n t e d t o a b o u t

3 9 % o f T O C a n d 8 %,; o f e x t i n c t i o n .

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a n d M g ( O H h p r e c i p i t a t io n e x p re s s e d a s a n a d s o r p -

t i o n i s o t h e r m s h o w e d a b e h a v i o u r s i m i l a r t o t h a t o f

a n a d s o r p t i o n p r o c e ss . T h e m a x i m a l a d s o r p t i o n c a -

p a c i ty v a r i e d b e t w e e n 1 .5 --3 .0 m g T O C m g ~ M g f o r

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t e d v s t h e r e s id u a l m a g n e s i u m r e m o v a b l e T O C i n c a s e o f a

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t h e r m t h i s d o e s n o t p r o v e t h a t a d s o r p t i o n i s t h e o n l y

p r o c e s s i n v o l v e d . I n p r a c t i c e c o a g u l a t i o n - f l o c c u l a t i o n

w i t h l i m e s h o u l d g i v e b e t t er r e m o v a l s w i t h a h i g h

i n it i a l m a g n e s i u m c o n t e n t i n t h e w a te r . I n c a s e o f a

l o w m a g n e s i u m c o n t e n t o f t h e w a t e r t r e a tm e n t w i t h

l i m e w h i c h c o n t a i n s m a g n e s i u m i s t o b e p r ef e rr e d .

A e k n o w l e d . q e m e n t s T h i s r e s e a r c h w a s c a r ri e d o u t w h i l e J .

l . e e n t v a a r w a s a s t a f f m e m b e r a t t h e D e p a r t m e n t o f W a t e r

I ' ot l u t+ t m ( ' o n t r o l . A g r i c u l t m ' a l U n i ' , e r s it , , . W a g e m u g e n

a n d M . R e h h u n w a s it v i s it i n g p r o f e s s o r a t t h e , , a m e

d el '~ ar tm e i' kt . A c k n o w l e d g e m e n t i s d u e t o M r 1 S . . I

Yv .e ma an d Mr t : A .. l . ang er e i s f l~ r t he i r t e chn ic a l axszs t -

; t I ' 1 C .

R E F [ R I '~N 'ES

Ber g I . L .. Br um me r C. A . & W i l l i am s W. T . 11970) S ing le

s t a g e l i m e c l a r i f i c a ti o n . 1 4 ) a . W a s t e s E n g n g 7, 3 42.

B l a c k A . P . & C h r i s t m a n R . F . [ 1 9 6 1 ) E l e c t r o p h o r e t i c

s i t , d i e s o f s l u d g e p a r t i c l e s p r o d u c e d i n l i m e - s o d a s o f t e n -

ing . J . Am. W a r . W k s ,,l.s.~. 53 , 7 37 747.

F l e n tj e M . E . (1 9 27 ) C a l c i u m a n d m a g n e s i u m h y d r a t e s . J .

.4m. Wa r. 14"k.s .,ls.s. 17, 25 3 260.

F o l k m a n Y . & W a c h s A . M . ( 1 97 3 ) R e m o v a l o f a l g a e f r o m

s l a b l i s a t i o n p o n d s e f f lu e n t b y l im e t r e a t m e n t . W a t e r R e s .

7, 419- 435.

L e e n t v a a r J . & Y w e m a T . S . J . ( 1 9 8 0 ) S o m e d i m e n s i o n l e s s

p a r a m e t e r s o f i m p e ll e r p o w e r i n c o a g u l a t i o n - f l o c c u l a t io n

p r o c e s s e s . I,V a t e r R e s . 14, 135 . 140.

L e c o m p t e A . R . (1 9 66 ) W a t e r r e c l a m a t i o n b y e x c e s s li m e

t r e a t m e n t o f e f f lu e n t . T A P P I 4 9 , 1 21 .. 12 4.

M c n n e l M . . M e r r i l l D . T . & J o r d e n R . M . ( 1 9 74 ) T r e a t m e n t

o f p r i m a r y e f f l u e n t b y l i m e p r e c i p i t a t i o n a n d d i s s o l v e d

a i r f l o t a t i on . J . Wa r . Po l l u t . Co n t ro l Fed . 4 6 , 2471 2485.

M i n i o n G . R . & C a r l s o n D . A . ( 19 7 3) P r i m a r y s l u d g e s

p r o d u c e d b y t h e a d d i t i o n o f l i m e to r a w w a s t e w a t e r.

W a t e r R e s . 7, 1821 1847.

V u u r e n L . R . J . v a n , S t a n d e r G . J . , H e n z e n M . R . , M e l t i n g

I :'. G . J . & Ble r k S . H . V . van ( 1967) Ad va nc ed p u r i f i ca -

l i o n o f s e w a g e w o r k s e f f lu e n t u s i n g a c o m b i n e d s y s t e m

o f l im e s o f t e n i n g a n d f l o t a t io n . W a t e r R e s . I , 4 6 3 . 4 7 4 .

W u h r m a n K . ( 1 96 8 ) O b j e ct i v e s. t e c h n o lo g y a n d r e s ul t s o f

n i t r o g e n a n d p h o s p h o r u s r e m o v a l p r o c e s s e s . I n

A d v a n c e s i n W a t e r Q u a l i t y I m p r o l e m e n t pp . 21 48 . U n i -

ve r s i t y o f T e x a s P r e s s , A u s t in .