detoxification of tannery waste liquors with an electrolysis system

8/3/2019 Detoxification of Tannery Waste Liquors With an Electrolysis System

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E L S E V I E R

En v i ro n me n t a l Po l l u t i o n , Vol . 97• No . 1 2 , pp . 147 152 , 1997,~ 1997 E ls ev ie r S c ience L td

A l l r i g h t s r e s e r v e d . P r i n t e d i n G r e a t B r i t a i n

P I I : S 0 2 6 9 - 7 4 9 1 ( 9 7 ) 0 0 0 6 2 - 6 0 2 6 9- 7 49 1 /9 7 $ 1 7 . 0 0 + 0 . 0 0

D E T O X I F I C A T I O N O F T A N N E R Y W A S T E L I Q U O R S W I T H A N

E L E C T R O L Y S I S S Y S T E M

Apostolos G. Vlyssidesa* and Cleanthes J. Israilidesb

aNational Technical University of Athens, Laboratory of O rganic and C hemical Technology, 9, Heroon Polytechniou Street,Zographou 157 00, Greece

blnstitute o f Technology of Agricultural Products, National Agricultural Research Foundation, 1, So./: Venizelou Street,Lycovrissi 141 23, Greece

(Received 14 Feb ruary 1997; accepted 23 Apr i l 1997)

Abstract

T his paper descr ibes an e lec t rochemica l t r ea tment and

d e t o x if i ca t i o n o f t a n n e ry w a s t e l iq u o r s ( T W L ) . I n t h i s

technique , T W L was pas sed through an e lec t ro ly t i c ce l l

us ing a T i /P t anode and a s ta in les s s tee l30 4 ca thode . O wing

to the s t rong ox id i z ing po ten t ia l o f the chemica ls produce d

( ch lor ine , oxygen , hydroxy l rad ica l s and o ther ox idants )

the organic and inorganic pollutants (ammonia, sul f ides and

chrom ium) were we t ox id i z ed to carbon d iox ide , n it rogen

ox ides and su l fur d iox ide . I n addi t ion , ch romium was prec i -

p i t a t e d a s C r 2 ( S 0 4 ) 3 . Exper iments were run in a ba tch ,

laboratory-scale , pi lot-plant , and the resul ts are reported

here in . A f te r 30 min a nd 3 h o f e lec tro lys i s a t 0 . 26 A cm - 2 ,

4 5 ° C a n d p H 9 , t o ta l c h e m i c a l o x y g e n d e m a n d ( C O D ) w a s

r e d u c e d b y 5 2 a n d 8 3 % a n d b i o c h e m i c a l o x y g e n d e m a n d( B O D s ) w a s r ed u c e d b y 3 5 a n d 6 6 % , r e sp e c ti v el y . A d d i -

t ionally , to ta l suspended so l ids ( T S S) were r educed by 8 .6

a n d 2 6 % , t o t a l p h e n o l ic c o m p o u n d s w e r e r e d u c ed b y 9 5 . 6

and 99 .4%. Ammonia , su l f ides and so lub le chromium were

reduced by 1 00% in both cases , while the mea n anod e eff i-

c ie n cy w a s 8 1 g h -1 A - l m - 2 a n d 1 . 9 g h - 1 A - l m - e . A l so ,

t h e m e a n e n e r g y c o n su m p t i o n w a s 4 . 8 k w h k g - 1 o f C O D

r e d u ce d a n d 2 0 0 k w h k g - 1 o f C O D r e d u c ed f o r 0 . 5 a n d

3 h, respect ively . These resul ts s trong ly indicate that this

e l e c tr o l yt i c m e t h o d o f t o t a l o x id a t io n o f T W L c a n n o t b e c os t

e f f ec t ive for w ide use. H owever , i t can be used as an e f fec t i -

vepre t rea tment s tage fo r de tox i f i ca t ion o f the w as tewater ,

owing to grea t e f f ic iency espec ia l ly w i th r espec t to C OD andtox ic i t y ( phenol ic s ) r educ t ion . © 1997 E lsev ier Sc ience L td

K e y w o r d s : T a n n e r y w a s t e l i q u o r s , e l e c t ro c h e m i c a l t r e a t -

m e n t , o x i d a t i o n o f p h e n o ls , c h r o m i u m ( I l l ) r e m o v a l .

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

E n v i r o n m e n t a l p o l l u t i o n o w i n g t o t a n n e r y w a s t e l iq u o r s

( T W L ) i s a n i m p o r t a n t p r o b l e m a n d c a u s e s g r e a t c o n -

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

a t e s o n t h e i r r e s o u r c e s . T h e T W L a r e a m o n g t h e m o s t

* To whom c o r r e s ponde nc e s hou l d b e a dd re s se d . F a x : 3 017723163.

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

T a n n e ri e s a r e p r o d u c in g a b o u t 7 - 1 5 m 3 o f T W L p e r

t o n n e o f r o u g h l e a th e r . A n a v e r a g e c o m p o s i t i o n o fT W L i s g i v e n o n T a b l e 1 ( E S C A P , 1 98 2).

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

t h e s e w a s t e s a re t a n n i n s ( p o l y p h e n o l i c c o m p o u n d s )

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

h i g h ly t o xi c p o l l u ta n t s . A m o n g t h e i n o r g a n i c c o m p o u n d s ,

w h i c h a r e p r e s e n t i n T W L i n h i g h c o n c e n t r a t i o n s , .a re

a m m o n i a ( S z p r y o k o w i c z et al . , 1 9 9 5 ; N a u m c z y k et al . ,

1 9 9 6 ) , s u l f i d e s a n d c h r o m i u m ( S y k e s a n d C o r n i n g ,

1 9 8 7 ) t h a t a r e s t r o n g i n h i b i t i o n f a c t o r s f o r a n y k i n d o f

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

a c c e p t e d m e t h o d f o r a n e f fe c ti v e t r e a t m e n t o f T W L a t

w o r l d w i d e l e ve l. A t h o r o u g h r e v i e w o f t a n n e r y w a s t e -w a t e r t r e a t m e n t w a s g i v e n b y T s o t s o s ( 1 9 8 6 ).

E x i s ti n g t e c h n o l o g i e s f o r t h e t r e a t m e n t o f T W L

i n c lu d e t h e us e o f o z o n e g a s, o z o n e a n d U V , a n d U V a n d

h y d r o g e n p e r o x id e . I n s p it e h o w e v e r , o f i m p r o v e m e n t s

r e s u l ti n g i n l es s e x p e n s i v e o z o n e p r o d u c t i o n , t h e c o s t o f

o z o n e g e n e r a t i o n i s st il l o n e o f t h e m a i n d r a w b a c k s w h e n

i t i s c o m p a r e d w i t h o t h e r t e c h n o l o g ie s • T h e m o s t w i d e l y

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

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

n o t c o n s i d e r e d c o s t e f fe c t iv e i n s m a l l a n d m e d i u m s iz e

i n d u s t r ie s l i ke m o s t o f t h o s e w h i c h o c c u r i n G r e e c e . I n

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

o r a n a e r o b i c , e v e n t h o u g h t h e y le a d t o a s i g n i fi c a n tr e m o v a l o f c h e m i c al o x y g e n d e m a n d ( C O D ) a n d b i o-

c h e m i c a l o x y g e n d e m a n d ( B O D ) , s ti ll le a v e u n a c c e p t a b l e

l e v e l s f o r d i r e c t d i s c h a r g e . M o r e o v e r , f u l l n i t r i f i c a t i o n

c a n n o t b e e s t a b l i s h e d ( G r e n s c h o w a n d H e g e m a n n ,

1 99 3). O x i d a t i o n t e c h n o l o g i e s f o r th e t r e a t m e n t o f

i n d u s t r i a l w a s t e w a t e r s h a v e b e e n a n a t t r a c t i v e a l t e r n a -

t i v e t o b i o l o g i c a l m e t h o d s , e s p e c i a l l y w h e r e b i o r e c a l c i -

t r a n t a n d t o x i c w a s t e s a r e to b e t r e a t e d ( M a n t z a v i n o s e t

al. , 1 9 96 ). I n t h e c a s e o f T W L , e l e c t r o c h e m i c a l t r e a t m e n t

h a s b e e n r e p o r t e d a s a f e a s i b l e p r o c e s s ( S z p r y o k o w i c z

a n d Z i l li o G r a n d e , 1 9 94 ). H o w e v e r , s o m e r e s e a r c h e r s a r e

i n f a v o u r o f a c o m b i n e d o x i d a t i v e a n d b i o l o g ic a l t re a t -

m e n t f o r t a n n e r y w a s t e w a te r s ( S z p r y o k o w i c z a n d Z i ll io

G r a n d e , 1 9 9 4 , 1 9 9 5 ; J o c h i m s e n et al . , 1 9 9 6 ; K a u l a n d

S z p r y o k o w i c z , 1 99 6; N a u m c z y k et al . , 1996).

147



14 8 A . G . V l y s s i d e s , C . J . I s r a i l id e s

Table 1. Mean composition o f tannery waste l iquors (TWL )

Cha r a c t e r is t ic s Ra nge ( m g l i t r e - 1 )

BOD5 210-4300CO D 18 0-27 000To ta l suspended sol ids 925-36 000To t a l c h r om i u m 3 - 3 50Sulfides 1-500

Chlor ides 1500-28 000To t a l phe no l i c c om p ou nds 0 .4 - 100A m m on i u m n i t r oge n 17 -3 8 0Kje hda h l n i t r oge n 90 - 63 0Fats and oi ls 49-620pH 1 - 13

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

p r o d u c t i o n o f C 1 0 3 - t h a t is a s t a b le c h l o r a t e a n i o n .

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

t h e e l e c t r o l ys i s p r o c e s s p r o d u c i n g f r e e c h l o r in e , w h i l e i n

h i g h p H t h e c h l o r i d e s a r e o x i d i s e d p r o d u c i n g c h l o r a t e s .

O n l y i n m o d e r a t e a l k a l i n e t o n e u t r a l c o n d i t i o n s , t h e

i n i ti a l c o n c e n t r a t i o n s o f c h l o r i d e s r e m a i n s t a b le ( V l y s -

s i de s e t a l . , 1 9 9 6 ) d u r i n g t h e c o n t i n u o u s p r o d u c t i o n o ff r e e h y d r o x y l r a d i c a l s.

T a b l e 2 s h o w s t h e e l e c t ro c h e m i c a l a n d c h e m i c a l r e a c -

t i o n s ( I s r a i l i d e s e t a l . , 1 9 9 7 ) t h a t c a n t a k e p l a c e i n a

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

l y s i s c e l l u s i n g T i / P t a n o d e .

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

t h e t r e a t m e n t o f T W L , b a s e d o n t h e p r in c i p le o f o x id a -

t iv e d e g r a d a t i o n o f a ll it s o rg a n i c c o m p o u n d s i n a n

e l e c t r o l y s i s s y s t e m .

The o r e t i c a l a ppr o a c h

T h e t i m e o f o x i d a t io n d e p e n d s u p o n t h e s t a bi li ty a n d

c o n c e n t r a ti o n o f c o m p o u n d s ; c o n c e n t r at i o n o f N a C I

u s e d ; t e m p e r a t u r e ; p H o f t h e s o l u t io n ; t i m e o f r e c i rc u -

l a t io n ; s iz e o f t h e a n o d e ; a n d c u r r e n t a n d v o l t a g e

a p p l i e d ( V l y s s i d e s e t a l . , 1996) .

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

t h e e l e c t r o ly s i s o f a b r i n e s o l u t i o n , a r e c o m p l i c a t e d a n d

n o t e n t i r e ly k n o w n . F o r t h e t i m e b e i n g o n l y a s s um p t i o n s

c a n b e m a d e , b a s e d o n t h e p r o d u c t s t h a t c a n b e d e t e r-

m i n e d ( C 12 , C IO 2 , 0 3 , O H , O , C 1 O H , H 2 0 2 , 0 2 , H 2 ,

C O 2 ). T h e ra d i c al s O H , O a n d C 1 O H h a v e a v e r y s h o r t

l if e o w i n g t o t h e i r h i g h o x i d a t i o n p o t e n t i a l a n d t h e y a r ee i t h e r d e c o m p o s e d t o o t h e r o x i d a n t s ( C 12 , 0 2 , C 1 0 2 , 0 3 ,

a n d H 2 0 2 ) o r t h e y o x i d i s e o r g a n i c c o m p o u n d s ( d i r e c t

o x i d a t i o n ) . T h e p r i m a r y ( C 12 , 0 2 ) a n d s e c o n d a r y ( C 1 0 2 ,

0 3 , a n d H 2 0 2 ) o x i d a n t s t h a t a r e p r o d u c e d f r o m t h e

d e s t r u c t i o n o f r a d i c a l s h a v e q u i t e a l o n g l if e a n d a r e d i f -

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

t h e o x i d a t i o n p r o c e s s ( i n d i r e c t o x i d a t i o n ) .

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

i s d e p e n d e n t o n t h e c a t a l y t ic a c t iv i t y o f t h e a n o d e ; o n

t h e d i f f u si o n r a te s o f o r g a n i c c o m p o u n d s i n th e a c t i v e

p o i n t s o f t h e a n o d e ; a n d o n t h e a p p l ie d c u r r e n t d e n s i ty

( A n t r o p o v , 1 9 7 7 ; P r e n t i c e , 1 9 9 1 ) . T h e i n d i r e c t e l e c t r o -

o x i d a t i o n r a t e i s d e p e n d e n t o n t h e d i f f us i o n r a t e o f se c -o n d a r y o x i d a n t s i n t o t h e s o l u t io n , t e m p e r a t u r e a n d t h e

p H . A n e f f e c ti v e p o l l u t a n t d e g r a d a t i o n i s b a s e d o n t h e

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

o x i d a n t s a r e n o t a b l e t o c o m p l e t e l y c o n v e r t a ll o r g a n i c s

i n t o w a t e r a n d c a r b o n d i o xi d e .

F r o m p r e v i o u s i n v e s t i g a t i o n s ( W i l k e t a l . , 198 7 ;

V l y s s i d e s e t a l . , 1 9 9 6 ) , i n a c i d s o l u t i o n s , o x y g e n , f r e e

c h l o r in e a n d m a y b e s o m e a m o u n t s o f o z o n e a n d

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

b y p r o d u c t s o f t h e d i re c t o x i d a t i o n p r o c e ss . I n m o d e r -

a t e a l k a l i n e s o l u t i o n s a c y c l e o f c h l o r i d e - c h l o r i n e - -

h y p o c h l o r i t e - -c h l o r i d e t a k e s p l a c e , w h i c h p r o d u c e s

O C I - , o x y g e n a n d s o m e a m o u n t s o f h y d ro g e n p e r o x id e

a n d m a y b e o z o n e . I n s t r o n g a l k a l i n e s o l u t i o n s t h e c y c l e

M A T E R I A L S A N D M E T H O D S

The la bo r a to r y p i l o t p lan t

T h e e x p e r i m e n t a l p l a n t c o n s i s t e d ( F i g . 1 ) o f t h e e l e c t ro -

l y ti c ce ll , t h e r e c i r c u l a t io n s y s t em , t h e p H - c o r r e c t i o n

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

T h e e l e c t r o l y t i c c e l l

T h e e l e c t r o ly t i c c e ll c o n s is t e d o f t h e c a t h o d e , w h i c h w a s

a s t a i n le s s s t e el 3 0 4 c y l i n d e r o f 2 0 c m d i a m e t e r , a n d o f

t h e a n o d e , w h i c h w a s l o c a t e d i n t h e c e n t r e o f th e c y l in -

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

4 8 c m i n l e n g t h a n d 2 . 5 4 c m i n d i a m e t e r , c o v e r e d b y p l a -

t i n u m a l l o y f o il a p p r o x i m a t e l y 0 . 2 5 m m t h i ck . T h e e l e c -

t r o d e s w e r e o p e r a t e d a t 1 5 v o l t s D . C . a n d 1 00 a m p e r e s .

T h e r e c i r c u l a t i o n s y s t e m

T h e r e c i r c u l a t i o n s y s t e m i n c l u d e d a v e s s e l o f 1 5 l i t r e s ,w h i c h c o n t a i n e d t h e w a s t e w a t e r t o b e t r e a t e d , a n d a

Table 2. Electrochemical reactions

A n o d ePr im ary e lec trochemical reac t ions

n 2 0 + M [ ] + C l - ~ M [CI OH- ] + H + + 2e -H20 + M [ ] ~ M [ O H - ] + H + + e -

Seco ndary e lec t rochemical reac t ion6 OC 1- + 3 H 2 0 - 6 e - ~ 3 / 2 0 2 + 6 H ÷ + 4 C 1 - + 2 C 1 0 3 -

C a t h o d ePr im ary e lec t rochemical reac t ions2 H 3 0 + + 2 e - ~ H2 + 2 H2 02 H2 0 + 2e -- -- ~ H2 + 2 O H -

Seco ndary e lec t rochemical reac t ionO C1- + H 2 0 + 2 e - ~ C l ~ + 2 O H -

Ox i da t i on r e a c t ionsI n a c l o s e d a node a r e a

R + M[OH-]- --~ M [] + RO + H + + e-R + M [ C I O H ] ~ M [ ] + R O + H + + C I - + e -

H 2 0 + M [ O H - ] - - - * M [ ] + 0 2 + 3 H + + 3 e -H2 0 + M[C1OH-] + C1- ~ C12 + M [] + 02 + 3 H ÷ + 4 e -

A wa y f r o m e l e c t rode sR + 0 2 + 2H ÷ ~ R O + H 2 0 - 2 e -

R + C 1 2- -* R C I + C I - - e -R + OH- -- -~ RO + H ÷ + 2 e -

R C1 + O H - ~ R O + C 1- + H + + e -

Equi l ib r ium reac t ionsC12 + 2 0 H - ~ H20 + O C I - + C I -



Detoxification o f tannery waste liquors 14 9

v E

cool ing water

ca thodeSS-304

F

C

+ anode Pt /Ti

cool ing jacket

surge vessel

p p S ~ - - - - - - - ~ - ' \

I - N a OH -1

sampling

Fig . 1 . Expe r imenta l l ab ora to ry pi lo t p lant . C = e lec t ro ly t i c ce ll ; E = e lec tromag net ic va lve; P = rec ircu la t ing pump ; TI CR = tem-pera ture indica tor cont rol l e r recorder ; pH IR = p H indica tor recorder ; RIR = redo x indica tor recorder ; PP = per is tal ti c pump .

c e n t r i f u g a l p u m p ( P ) o f 4 0 l it re m i n - I f lo w r a t e , w h i c h

c o n t i n u o u s l y r e c i r c u l a t e d t h e r e a c t o r c o n t e n t s i n t o t h e

e l e c t r o l y t i c c e ll .

The p H correct ion system

T h e p H c o r r e c t i o n s y s te m c o n s is t e d o f a p H i n d i c a to r -

r e c o r d e r ( p H I R ) w h i c h c o n t i n u o u s l y m e a s u r e s t h e p H

c o n t e n t o f th e s u r g e v e s se l a n d w i t h t h e h e l p o f t h e

d o s i n g p u m p s P P - 1 a n d P P - 2 w h i c h s u p p ly H C L a n d

N a O H , r e s p e c t iv e l y , t h e p H i s k e p t c o n s t a n t a t 9 .0 .

Cooling system

T h e c o o l i n g s y s t e m i n c l u d e d a t e m p e r a t u r e i n d i c a t o rc o n t r o l l e r r e c o r d e r ( T I C R ) , a n e l e c t r i c v a l v e f o r t h e

w a t e r a n d a c o o l i n g w a t e r j a c k e t w h i c h w a s l o c a t e d i n

t h e r e a c t o r . T h e c y l i n d e r w a ll a n d t h e c o o l i n g j a c k e t

c o n s t i t u t e d t h e c a t h o d e . W h e n e v e r t h e t e m p e r a t u r e o f

t h e w a t e r e x c e e d e d 4 5 ° C , c o o l i n g w a t e r w a s c i r c u l a t e d

i n t h e j a c k e t , u n t i l t h e t e m p e r a t u r e r e t u r n e d t o t h e

d e s i r e d v a l u e .

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

c o n t i n u o u s l y r e c o r d e d b y a r e d o x i n d i c a t o r - r e c o r d e r

( R I R ) . T h e s a m p l e s f o r a n a l y s is w e r e d r a w n f r o m t h e

r e c i r c u l a t io n r e a c t o r .

Table 3 . C om pos i t ion of tannery waste l iquor (TWL ) used inex p er i m en t s

Charac ter i s t i cs Value (mg l i t re - I )

p H 4BOD5 1720C O D 8 5 4 0Tota l suspended sol ids 6200To tal dissolved sol ids 22 400To t a l c h r om i u m 67Sulfides 385Ch lorides 18 500To t a l phe no l i c c om p ou nds 91A m m on i u m n i t roge n 28 8Kje hda h l n i t r oge n 520

Alkal in i ty a s C a C O 3 1770Fats and oi ls 280

M a t e r i a l

T h e c o m p o s i t i o n o f th e T W L t h a t w a s u s ed i n all

e x p e r i m e n t s is s h o w n i n T a b l e 3 .

M e t h o d o l o g y

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

p H a n d t e m p e r a t u r e r e m a i n e d s t a b l e a t 9 . 0 a n d 4 5 ° C ,

r e s p e c ti v e l y, d u r i n g t h e e x p e r i m e n t .

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

r u n , a n d t h e r e s u l t s p r e s e n t e d i n t h i s w o r k a r e t h e a v e r -

a g e o f th e f o u r s e p a r a t e m e a s u r e m e n t s .

E a c h e x p e r i m e n t w a s o f b a t c h o p e r a t i o n a n d i t s d u r a -

t i o n w a s 3 h . E v e r y 5 r a in s a m p l e s w e r e t a k e n f r o m t h er e a c t o r . S o l u b le C O D , B O D s , t o t a l s u s p e n d e d s o l id s

( T S S ) , su lf id e s , a m m o n i a , t o t a l K j e i d a h l n i t r o g e n ( T K N ) ,

t o t a l s o l u b l e c h r o m i u m , p H , r e d o x , c h l o r i d e s , c h l o r i n e

a n d t o t a l o x i d a n t s w e r e m e a s u r e d . A d d i t io n a l l y , s a m p l e s

w e r e t a k e n f o r t o t a l p h e n o l ic c o m p o u n d d e t e r m in a t i o n .

T h e C O D , B O D s , T S S , T K N , a m m o n i a , s u lf id e s a n d

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

S t a n d a r d M e t h o d s f o r th e E x a m i n a t i o n o f W a t e r a n d

W a s t e w a t e r ( A P H A - A W W A - W P C F , 1 989). T h e

d e t e r m i n a t i o n o f o z o n e a n d t h e t o ta l o x i d a n t s e x c e p t fo r

c h l o r i n e w a s c a r r ie d o u t a c c o r d i n g t o W i lk ( 1 98 9 ). T h e

90008000

7000

6000

E 5 0 0 0

4000

8 3 000

2000

1000

0

time, minutes

Fig . 2 . Reduct ion of to ta l suspended sol ids ( - -F- I - - ) , COD( - - 0 - - ) a n d B O D s ( - - ~ - - ) d u r in g th e ele ct ro ly si s tr e a tm e n t .



150 A. G. Vlyssides, C. J . Israi l ides

10090

~¢ 80e.,= 70~. 60

o 40o 30

~ 10

t ime, m i n u t e s

F i g . 3 . Conc e n t r a t i on o f t o t a l phe no l i c c om pou nds du r i ngelec t ro lysi s t rea tmen t as a fun ct ion o f time.

40 0

35 0

30O

o 250

r 200

~o 150o

10 o

50

0 - - It - ~ .~ . - 7 .~ .~ .7 . .7 .~ .7 .7 . ' -117 .7 .7 .~2 . . . . . .

t ime, m i n u t e s

Fig. 5 . Su l fides ( - - I -1 - - ) and to ta l so lub le chrom ium ( - - @ -- )reduct ion dur ing e lec t ro lysi s t rea tm ent .

t o t a l p h e n o l i c c o m p o u n d s w e r e m e a s u r e d b y t h e F o l i n -

C i o c a l t e u m e t h o d ( S l i n k a r d a n d S i n g l e t o n , 1 9 77 ) .

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

a P e r k i n E l m e r ( m o d e l 2 3 8 0 ) a n a l y s e r .

R E S U L T S A N D D I S C U S S I O N

A s s h o w n i n F i g . 2 t h e T S S w e r e r e d u c e d b y o n l y

2 6 % ( f r o m 6 2 0 0 to 4 5 6 0 m g l i t r e - ~ ) d u ri n g t h e 3 h o f

e l e c tr o l y si s . T h i s i s a n i n d i c a t i o n o f t h e v e r y s l o w r a t e

o f h y d r o l y s i s , a t l e a s t d u r i n g t h e f i r st h o u r s o f e l e c-

t r o l y s i s .

O n t h e o t h e r h a n d , t h e r e w a s a g r e a t d e c r e a s e o f

B O D 5 ( F i g . 2 ) , f r o m 1 7 2 0 t o 5 9 0 m g l i t r e - l ( 6 5 % d u r i n g

t h e 3 h o f e l e c t r o l y s i s ) .

T h e C O D w a s r e d u c e d e v e n f u r t h e r f r o m 8 5 4 0 t o1 3 70 m g l i tr e - I , a s s h o w n i n F i g . 2 , a r e d u c t i o n o f a b o u t

8 4 % d u r i n g t h e 3 h o f e le c tr o ly s is . T h e C O D / B O D r a t io

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

m e n t s w a s i m p r o v e d c o n s i d e r a bl y a n d d r o p p e d f r o m 5

t o 2 .3 .

P h e n o l i c c o m p o u n d s w e r e d e s t r o y e d v e r y q ui c k ly , a s

s h o w n i n F ig . 3 . T h e y w e r e r e d u c e d b y 9 5 . 5 % d u r i n g

t h e f i rs t 3 0 m i n o f t h e e l e c tr o l y si s . T h e r e d u c t i o n o f

a m m o n i u m n i t r o g e n w a s a l so v e r y fa s t. A s s h o w n i n

F i g . 4 , i t w a s r e d u c e d b y 9 9 % d u r i n g t h e f i rs t 6 0 m i n o f

t h e e l e c t r o l y s is .

600 - -

5OO

400

j 3OO

8 20O

100

0 . . . . . . . . . _ ,,. ~ . . .L. JL __.. . ._ : : ;.~. .~t._L

~O o o

time, minutes

F i g . 4 . T o t a l K j e l d a h l n i t r o g e n ( - - D - - ) , a m m o n i a ( - - Q - - )r e du c t i on a nd n i t r a t e s ( - - A - - ) p r odu c t i on du r i ng e le c tr o ly s ist r e a t m e n t .

T h e T K N w a s a l m o s t c o m p l e t e ly d e g r a d e d ( 9 8 %

r e d u c t i o n ) d u r i n g t h e 3 h o f e l e c t r o l y s i s ( F i g . 4 ) . T h e

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

o b s e r v e d w h i ch s t a r t ed t o b e p r o d u c e d a f t er 3 0 m i n o f

e l e c tr o l y si s , w h e n t h e a m m o n i u m n i t r o g e n w a s d e p l e t e da n d r e a c h e d i ts h i g h e s t c o n c e n t r a t i o n (2 1 m g l i t r e - ~ ) a t

1 20 m i n . T h e n i t w a s g r a d u a l l y r e d u c e d t o 1 1 m g l i tr e -

a t t h e e n d o f t h e 3 h o f e l e c t r o l y s i s .

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

( F i g . 5 ) w h i c h w e r e r e d u c e d f r o m 3 8 5 m g l i t r e - 1 t o n i l ,

d u r i n g t h e f i r s t 25 m i n o f t h e e l e c t r o l y s i s .

I n t h e e n v i r o n m e n t , c h r o m i u m i s f o u n d i n t h e f o r m

o f a t r iv a l e n t a n d a m u c h m o r e t o x i c s ix - v a l e n t a t o m .

I n t h e T W L c h r o m i u m i s f o u n d i n t h e t r i v a l e n t f o r m .

I t s s a l t s a nd ox i de s ( Cr C13 , Cr2(SO4)3, C r 2 0 3 ) a r e

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

c o n t r a s t w i t h t h e s i x - v a le n t c h r o m i u m s a lt s a n d o x i d e sw h i c h a r e w a t e r s o l u b l e . D u r i n g t h e s t r o n g o x i d a t i v e

c o n d i t i o n s o f e l e c tr o l y s is th e t r i v a l e n t c h r o m i u m w a s

c o n v e r t e d t o s i x - v a l e n t . A s a r e s u l t , i t r e m a i n s i n s o l u -

t i o n w h i l e t h e t o x i c i t y o f t h e s o l u t i o n i n c r e a se s .

T h e r e w a s a n i m p r e s s i v e d e c r e a s e i n th e c o n c e n t r a t i o n

o f s o l u b le c h r o m i u m w h i c h w a s n u l l if ie d d u r i n g t h e f i rs t

1 0 - 1 5 r a i n o f e l e c t ro l y si s . T h e r e d u c t i o n o f c h r o m i u m

c o u l d p r o b a b l y b e c o n n e c t e d w i t h t h e p a r a l l e l r e d u c t i o n

o f s u l fi d e s a c c o r d i n g t o t h e f o l l o w i n g r e a c t i o n s ( J o r g e n -

s e n a n d J o h n s e n , 1 9 89 ):

200001800016000

- - 1400012000100008000

8

400020OO

0 + -H -H- ÷ H- -~ ~ ~ ~ + ~- - '~c - t I I t I I I ~ -H -+ + -~- + + q -+ + -

time, minutes

F ig . 6 . Ch l o ri de s ( - - I 7 - - ) , t o t a l c h lo r i ne ( - - @ - - ) a nd o t he roxidants ( - - /k - - ) changes dur ing e lec t ro lys i s t rea tment .



Detoxi ficat ion of tannery waste liquors 151

90

80

< 70

60

,~ 50

m 4 0©o 30

~ 20

10

time, minutes

Fig. 7. An od e efficiency dur ing electrolysis t rea tme nt .

25 0

..~ 200O

o 100

50

o

time, minutes

Fig . 8 . Ene rgy cons ump t ion dur ing e lec tro lys is t rea tm ent .

S- + 02 --~ S02 + e-

S02 + 1120 --~ H2S03

3H2S03 + 2H2Cr04 --~ Cr2(S04)3 + 51120

T h e a b o v e c o n s i d e r a t i o n s m a y b e c o n f i rm e d b y :

( a ) r e p e t i t i o n o f t h e e x p e r i m e n t u n d e r t h e s a m e c o n -

d i t i o n s w i t h o u t t h e p r e s e n c e o f s ul fi d es w h i c h

w e r e r e m o v e d b y n i t r o g e n s t r i p p i n g . O n l y v e r y

l i t t l e c h r o m i u m w a s r e m o v e d ( 1 5 % ) a n d t h e

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

f o r m C r ( V I ) ;

( b ) a n a l y s is o f t h e p r e c i p i t a n t s h o w e d t h a t c h r o m i u mw a s p r e s e n t as Cr2(504)3;

( c ) a f t e r 1 h o f e l e c t r o l y s is s o l u b l e C r ( V I ) h a s s t a r t e d

t o a p p e a r ( F i g . 5 ) r e a c h i n g 5 m g i i tr e - t a f t e r 3 h

o f e l e c tr o l y si s . T h e r e s o l u b i l i s a t io n o f c h r o m i u m

w a s p r o b a b l y d u e t o t h e o x i d a t i o n o f p r e c ip i t a n t s

a n d t o a n i n a b i l i t y t o r e d u c e C r ( V I ) t o C r ( I I I )

d u e t o t h e a b s e n c e o f su l fi d e s.

P r o d u c t i o n o f o x i d a n t s

F i g u r e 6 s h o w s t h e c o n c e n t r a t i o n s o f c h l o r i d e i o n s a n dt o t a l c h l o r i n e , a s w e ll a s o f o t h e r o x i d a n t s a s a w h o l e .

T h e c o n c e n t r a t i o n o f c h l o r id e s w a s r e d u c e d i n t h e fi rs t

1 0 m i n f r o m 1 8 5 4 0 m g li t r e - I t o 1 2 3 0 0 m g l i t r e - 1 a n d

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

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

i n g t h e f ir s t 1 0 r a i n f r o m z e r o t o 6 3 0 0 m g l i t r e - I a n d

r e m a i n e d s t a b l e t h e r e a f t e r . T h i s p r o v e s t h e e x i s t e n c e o f

t h e t h e o r e t i c a l c y c l e , c h l o r i d e s - c h l o r i n e - c h l o r i d e s . T h e

o t h e r o x i d a n t s , e x c e p t f o r c h l o r i n e , i n c r e a s e d r a p i d l y i n

t h e f ir s t 1 0 r a i n f r o m z e r o t o 5 8 4 0 m g l i t r e - 1 a n d t h e n

c o n t i n u e d t o i n c r e a s e w i t h a r a t e o f 6 .8 m g l i t r e - I r a i n - ~ .

T h e p r e s e n c e o f 0 3 a n d H 2 0 : w a s a ls o d e te c t e d.

A n o d e e f f i c i e n c y

F i g u r e 7 s h o w s t h e a n o d e e f f ic i e n cy o f t h e e l e c t r o d e

m e a s u r e d i n K g C O D r e m o v e d p e r h o u r p e r m 2 o f

a n o d e s u r f a c e a n d p e r a m p e r e a p p l i e d ( k g C O D ~ h - I

m - Z A - I ) . W i t h t h e s t a r t o f t h e e l e ct r o ly s i s t h e e ff i-

c i e n c y w a s 8 1 k g C O D r h - 1 m - 2 A - I w h i c h i s c o n s i d -

e r e d v e r y s a t i s f a c t o r y ( C o m n i n e l l i s , 1 9 9 2 ) , b u t i n t h e

n e x t 1 5 m i n i t w a s r e d u c e d to 6 . 8 k g C O O t h -1

m - 2 A - 1 . T h e n i t c o n t i n u e d t o b e r e d u c e d r e a c h i n g

1 . 8 k g C O D ~ h J m - 2 A - 1 a t t h e e n d o f t h e 3 h . T h e s e

r e s u l t s c a n n o t b e c o n s i d e r e d s a t i s f a c t o r y , a f t e r t h e

1 5 m i n o f e l e c t r o l y s i s. T h i s i s c l e a r l y s e e n i n F i g . 8 ,w h e r e t h e d e m a n d s i n en e r g y c o n s u m p t i o n a r e s h o w n .

T h e g r a d u a l d e c r e a s e o f e l e c t r o d e e f fi c ie n c y i n d i c a t e s

t h e i n c r e a s in g d i f fi c u lt y fo r o x i d a t i o n o f r e s i d u a l

o r g a n i c s i n t h e w a s t e .

E n e r g y c o n s u m p t i o n

F i g u r e 8 s h o w s t h e e n e r g y d e m a n d s v e r s u s ti m e i n th e

e l e c t r o l y s i s s y s t e m . D u r i n g t h e f ir s t 1 0 m i n o f e l e c t r o l y -

s i s t h e e n e r g y c o n s u m p t i o n w a s f a i r l y l o w ( f r o m 4 . 8 t o

1 1 .0 K w h p e r k g C O D r ) b u t t h e n i n c r e a s e d a b r u p t l y t o

5 8 K w h p e r k g C O D r . L a t e r , a l t h o u g h a t a s lo w e r r a t e,

t h e e n e r g y c o n s u m p t i o n c o n t i n u e d t o i n c re a s e r ea c h i n g

2 0 0 K w h p e r k g C O D r r e n d e r i n g u n f a v o r a b l e t h e ap p l i-c a t i o n o f t h e e l e c t r o ly t i c o x i d a t i o n f o r t h e c o m p l e t e

t r ea t m e n t o f T W L .

Ta ble 4 . Eff i c iency o f e l ec tro lys i s for the f ir st 3 0 m in o f e l ec tro lys is

Du r a t i on o f So l u b le CO D To t a l phe no l i ce lect ro lysi s reduc t ion com pou nds reduct ion(min) (%) (% )

Am mo nium Sul fides Soluble chrom iumni t r oge n r e du c t ion r e du c t i on r e du c t ion

( % ) ( % ) ( % )

C O D / B O Drat io

5 20.0 27.4l0 38.7 40.615 47.0 67.020 48.73 80.2

25 50.4 93.430 52.0 95.6

19.4 28 .5 85.0 4 .9627.0 63.1 98.5 4.0034.7 84.4 100 4.0042.0 98 .7 - - 4 .00

49.6 100 - - 3 .8464.5 100 - - 3 .70



152 A. G . V l y s s i de s , C . J . I s r a i l i de s

C O N C L U S I O N S

T h e e l e c t r o l y t ic o x i d a t i o n a s a n i n t e g r a t e d s o l u t i o n f o r

t h e t r e a t m e n t o f T W L d o e s n o t s e e m t o b e fe a si b le d u e

t o h i g h e n e r g y d e m a n d s . H o w e v e r , t h e a p p l ic a t i o n o f

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

t o 3 0 m i n s e e m s t o b e v e r y e ff ic i en t , m a i n l y f o r t h e

d e t o x i fi c a ti o n o f T W L .I n t h e f i r s t 3 0 m i n o f e l e c t r o ly s i s th e f o l l o w i n g c o u l d

b e a c h i e v e d ( T a b l e 4 ) :

1 . r e d u c t i o n o f p h e n o l i c c o m p o u n d s b y 9 5 % ;

2 . r e d u c t i o n o f a m m o n i u m n i t r o g e n by 6 4 .5 % ;

3 . c o m p l e t e o x i d a t i o n o f s ul fi d es ;

4 . c o m p l e t e s e d im e n t a t i o n o f c h r o m i u m ;

5 . r e d u c t i o n o f C O D b y 5 2 % ;

6 . i m p r o v e m e n t o f b i o d e g r a d a b i l it y i n d e x ( C O D /

B O D ) f r o m 4 . 9 6 t o 3 .7 .

T h e p r e t r e a t m e n t o f T W L i s f a v o u r e d b y t h e f a c t t h a t

i t c o n t a i n s h i g h a m o u n t s o f N a C I : 0 . 2 5 - 4 . 6 % .

R E F E R E N C E S

A P H A - A W W A - W P C F ( 1 9 8 9 ) Standard M ethods for theExaminat ion of Water and Wastewater , 17th edn. Amer icanPu b l i c He a l t h A s s oc i a ti on , Wa s h i ng t on , D C.

A n t r opov , L . ( 1977) Theoretical Electrochemistry, 1st edn.Mi r , Mos c ow.

Comninell is , C. (1992) Th e e lec t rochemical t rea tme nt ofwastewater , gw a 1 1 , 7 9 2 - 7 9 7 .

ESCA P ( 198 2) Report on Industrial Pollution Control Guide-lines Part VII." Tanning Industry. Uni t e d N a t i ons , Ba ngk ok .

G r e ns c how , E . a nd H e ge m a nn , W. ( 1993) A na e r ob i c t r e a t m e n to f t a nn e r y wa s t e wa t e r , gw f W as s e r Abw as se r 134, 262-268.I s ra i l ides , C. J . , Vlyss ides , A . G. , Mourafe t i , V . N . and Kar -

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detoxification of tannery waste liquors with an electrolysis system

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