Directory UMM :Data Elmu:jurnal:E:Environmental Management and Health:Vol08.Issue1.1997:
Anaerobic treatment of tannery wastewater: toxic
ef fects of wastewater constituents and dosage of
ferric chloride
Elke Genschow
Pro f. Dr-Ing., TU Be rlin, Be rlin, Ge rmany
Werner Hegemann
Pro f. Dr-Ing., TU Be rlin, Be rlin, Ge rmany
Christian M aschke
Pro f. Dr-Ing., TU Be rlin, Be rlin, Ge rmany
Investigates anaerobic twostage treatment of tannery
wastewater. This results in a
mean chemical oxygen
demand (COD) removal of 60
per cent for tannery B and
more than 70 per cent for
tanneries A and C with CODo
= 5,710mg l –1 and mean
detention time held at 3.5
days. Gas production was
small, rating an estimated
75l kg–1 CODo with CODo =
5,710mg l –1 . Tests simultaneously the infl uence of quality
and quantity of wastewater on
COD removal and gas production (multiple regression).
Finds signifi cant inhibitory
effects were caused by chloride on gas volume and by
sulphate on COD removal.
Chromium showed no signifi cant effect. Dosage of ferric
chloride for removal of the
toxic sulphide effected a
decrease in gas volume and
had no effect on COD removal
(analysis of variance).
Enviro nme ntal Manage me nt
and He alth
8 / 1 [ 1997] 2 8 –3 8
© MCB Unive rsity Pre ss
[ ISSN 0956-6163]
[ 28 ]
Introduction
An a er obic tr ea tm en t of ta n n er y w a stew a ter
h a s n ot been in vestiga ted ver y well. In m ost
ta n n er ies th e Bea m h ou se is su bject to tr ea tm en t[1-3], sin ce th e qu a lity does n ot u n der go
m u ch va r ia tion a n d sin ce th e or ga n ic loa d is
ver y h igh . (Th e Bea m h ou se in clu des a ll
sta ges befor e ta n n in g.)
Bu t even wh en th e en tir e w a stew a ter is
tr ea ted, th e qu a lity differ s depen din g on
wh eth er th e ta n n er y is u sin g ch r om iu m [4,5]
or ve geta ble ta n n in g a gen ts[6,7].
Usin g a n a er obic tr ea tm en t of ch r om iu m
ta n n er y w a stew a ter, th e objective in th is
stu dy w a s to r edu ce th e or ga n ic loa d, th er eby
pr odu cin g on ly a sm a ll a m ou n t of slu dge.
Th er efor e, two sta ge a n a er obic tr ea tm en t w a s
ch osen . Th e fir st sta ge w ith less sen sitive
a cidifica tion ba cter ia ser ves a s a bu ffer in th e
ca se of con cen tr a tion fl u ctu a tion s a n d
in h ibitin g su bsta n ces[8,9]. Ta n n er y w a stew a ter con ta in s h igh con cen tr a tion s of su lph a te, su lph ide, ch lor ide, a n d ch r om iu m
wh ich a r e k n ow n a s poten tia l in h ibitin g
ch em ica ls. In th e a n a er obic pr ocess su lph a ter edu cin g ba cter ia pr odu ce su lph ide by r edu cin g su lph a te.Th e su lph ide especia lly w a s
su spected of bein g th e m ost in h ibitin g su bsta n ce in ta n n er y w a stew a ter. Th er e a r e sever a l m eth ods for r edu cin g su lph ide.
For exa m ple, FeCl 3 ca n be a dded in th e fi r st
sta ge to r edu ce su lph ide con cen tr a tion s[10,11]. Fer r ic ch lor ide sh owed best
r esu lts in com pa r ison w ith fer r ic h ydr oxide
a n d fer r u gin ou s slu dge, sin ce it is ea sy to a dd
a n d h igh ly effective. Stir r in g is n ot r equ ir ed
a n d on ly sm a ll qu a n tities a r e n eeded for th e
en tir e elim in a tion [9].
An oth er possibility to decr ea se th e con cen tr a tion of su lph ide is to in h ibit su lph a ter edu cin g-ba cter ia (SRB). Th e a ddition of
N a 2MoO 4 sh ou ld in h ibit SRB, bu t th e
m eth a n e pr odu ction w a s a lso r edu ced a s a
con sequ en ce[12].
Su lph ide r em ova l is possible by biologica l
oxida tion of su lph ide to su lph u r. Su lph a te
pr odu ction ca n be con tr olled by oxygen con cen tr a tion [10]. An oth er m eth od to r edu ce
su lph ide w a s in vestiga ted by str ippin g su lph ide a n d r ecir cu la tin g th e w a sh ed ga s[13].
In th is stu dy FeCl 3 w a s ch osen .
Sta tistica l a n a lysis su ch a s m u ltiple r e gr ession a n d a n a lysis of va r ia n ce wer e n ecessa r y
du e to th e ch a n gin g qu a lity of w a stew a ter
w ith va r ia tion s in con cen tr a tion , h igh sta n da r d devia tion a n d com plex com position .
Th e objective w a s to test th e in flu en ce of
th e qu a lity of w a stew a ter sim u lta n eou sly, th e
qu a n tity of feed flow, a n d th e or igin (ta n n er y
A, B a n d C) on ch em ica l oxygen dem a n d
(COD) r em ova l a n d ga s pr odu ction (m u ltiple
r e gr ession ). After th is oper a tion , th e exper im en ta l oper a tion s wer e tested for th eir in flu en ce on ga s pr odu ction a n d COD r em ova l
(a n a lysis of va r ia n ce). As oper a tion a l pa r a m eter s th e sim ila r ity of pilot pla n ts a n d th e
dosa ge of fer r ic ch lor ide, a m on g oth er s, wer e
exa m in ed.
Material and methods
Pilot plant
Th r ee iden tica l a n a er obic pilot pla n ts wer e
r u n in pa r a llel. In F igu r e 1 th e sch em e of th e
two-sta ge pilot pla n t is given . It con sists of a
stir r ed ta n k a n d a su spen ded fixed fi lm bed
r ea ctor in u pfl ow -oper a tion filled w ith r a ssch ig-r in gs (7.9 l). Th e fixed fi lm bed r ea ctor
w a s oper a ted a t a tem per a tu r e of 34 °C th a t
w a s th er m osta tica lly con tr olled. Th e feed
flow w a s su pplied discon tin u ou sly fou r tim es
in 24 h ou r s. Th e bioga s w a s collected in volu m e ca libr a ted ba gs. In or der to avoid th e loss
of bioga s, a siph on w a s pla ced between th e
m eth a n e r ea ctor a n d th e efflu en t ta n k . Deten tion tim e va r ied between 2.4 a n d 10.5 days
w ith a m ea n va lu e of 3.5 days in both sta ges
togeth er.
Wastewater
Th e w a stew a ter w a s ta k en fr om th r ee differ en t ch r om iu m ta n n er ies (A, B a n d C). It w a s
Elke Ge nsc ho w,
We rne r He ge mann and
Christian Masc hke
Anae ro bic tre atme nt o f
tanne ry waste wate r: to xic
e ffe c ts o f waste wate r
c o nstitue nts and do sage o f
fe rric c hlo ride
Figure 1
Sc he me o f the pilo t plant
Enviro nme ntal Manage me nt
and He alth
8 / 1 [1 9 9 7 ] 2 8 –3 8
ga th er ed on ce a week a s a r a n dom sa m ple
fr om th e efflu en t of th e equ a lizin g ta n k a n d
u sed a s feed for th e exper im en ta l pla n t. It w a s
stor ed for seven to 14 days in th e feedin g ta n k .
In a ll in vestiga tion s u n dilu ted ta n n er y w a stew a ter w a s u sed.
Analytical methods
Th e a n a lyses of COD, ch lor ide (Cl –), a n d
ch r om iu m (Cr tot ) wer e ca r r ied ou t a ccor din g
to DIN -sta n da r ds. An a lyses of su lph ide (S 2–)
a n d su lph a te (SO 42–) wer e per for m ed ph otom etr ica lly, w ith com posite sa m ples bein g
ta k en ea ch day for five days. Sa m ples of COD
wer e a cidified a n d stir r ed for h a lf a n h ou r to
decr ea se su lph ide.
Experimental operation
Fe Cl3 – do sage
FeCl 3 w a s a dded in qu a n tities of 1gl –1 a n d
2gl –1 in th e in fl u en t ta n k . Th e objective w a s to
elim in a te th e su lph ide a s FeS, both in th e
for m existen t in th e r aw w a stew a ter a n d in
th e for m pr odu ced th r ou gh th e a n a er obic
pr ocess.
PH value s
Th e va lu es of th e ta n n er y w a stew a ter va r ied
between 8 a n d 12. For biologica l tr ea tm en t it
w a s n ecessa r y to r e gu la te th e pH. Th e pH w a s
a dju sted to 5, 6 or 7, depen din g on th e exper im en t.
An a lysis by m u ltiple sta tistica l m eth ods
(gr a du a l a n a lysis of cova r ia n ce)
Th ese wer e:
• N om in a l va r ia ble: qu a lita tive va r ia ble su ch
a s ta n n er y (A, B, C).
• Qu a n tita tive va r ia ble m ea su r ed va lu e su ch
a s COD va lu es (m gl –1).
• Con fou n der : pa r a m eter th a t ca u ses u n in ten tion a l va r ia n ce of th e depen den t va r ia ble.
[ 29 ]
Elke Ge nsc ho w,
We rne r He ge mann and
Christian Masc hke
Anae ro bic tre atme nt o f
tanne ry waste wate r: to xic
e ffe c ts o f waste wate r
c o nstitue nts and do sage o f
fe rric c hlo ride
Enviro nme ntal Manage me nt
and He alth
8 / 1 [1 9 9 7 ] 2 8 –3 8
• Depen den t va lu es: Y ; in th is ca se ga s pr odu ction a n d COD differ en ces.
• In depen den t va lu es x 1...n ; in th is stu dy:
con fou n der.
• Sta tistica l r elia bility (S ): r elia bility for th e
va lidity of th e sign ifi ca n t con n ection (m u ltiple r e gr ession , a n a lysis of va r ia n ce).
• Du m m y va r ia ble: n ecessa r y for tr a n sfor m a tion of n om in a l va r ia bles.
• Re gr ession coefficien t: slope of th e estim a ted fu n ction .
• P r ediction va lu es: estim a ted va lu es
th r ou gh m u ltiple r e gr ession .
• Adju sted va lu es: va lu es a dju sted w ith sign ifica n t con fou n der s in m u ltiple r e gr ession .
• F-va lu e: for decision wh eth er a pa r a m eter
is sign ifica n t (m u ltiple r e gr ession a n d
a n a lysis of va r ia n ce).
• Tr ea tm en t: oper a tion a l pa r a m eter for
a n a lysis of va r ia n ce.
Gradual analysis o f c o varianc e
If it is n ecessa r y to test th e in fl u en ce of th e
con fou n der, too, a gr a du a l a n a lysis of cova r ia n ce ca n be u sed[14]. In th e fi r st step th e m u ltiple r e gr ession a n d a dju stm en t wer e con sider ed. In th e secon d step th e in fl u en ce of oper a tion a l pa r a m eter s w a s tested.
First step: multiple regression and
adjustment
In th e fir st step of th e sta tistica l a n a lysis a
sign ifica n t in flu en ce on th e depen den t va lu es
(COD differ en ces a n d ga s pr odu ction ) w a s
tested w ith th e possible con fou n der s. Th e
lim it for sta tistica l er r or (α) w a s 5 per cen t, so
th a t sta tistica l r elia bility (S ) (equ a tion (1))
w a s a t lea st 95 per cen t. Th e objective w a s to
select on ly th e im por ta n t con fou n der s.
(1)
S = (1 – α) * 100 per cen t
Th e or igin of th e w a stew a ter (th r ee ta n n er ies), feed flow, th e ch em ica l pa r a m eter s in th e
in flu en t su ch a s COD, su lph a te, su lph ide,
ch lor ide, a n d Cr tot wer e tested a s possible
con fou n der s. Th e gen er a l r esu lt of m u ltiple
r e gr ession is equ a tion (2):
(2)
Y * = g1 • X 1 + g2 • X 2 + … + gn • X n + a
wh er e:
Y * = pr edicted, depen den t va lu e;
g 1 = r e gr ession coefficien t of th e
con fou n der w ith j = 1…n ;
X j = m ea su r ed va lu e of th e con fou n der
w ith j = 1…n ;
a = con sta n t ter m .
Adjustme nt
Th e a dju stm en t of th e sign ifi ca n t con fou n der
is pa r a llelized by equ a tion (3).
Yˆ = (Y – Y *) + y–
(3)
wh er e:
[ 30 ]
Yˆ = a dju sted, depen den t va lu e;
Y = m ea su r ed, depen den t va lu e;
Y * = pr edicted, depen den t va lu e of a ll sign ifi ca n t con fou n der ;
y = m ea n va lu e.
Lin ea r r ela tion s wer e a scer ta in ed a n d th e
a dju sted m ea su r ed va lu es (COD differ en ces
or ga s pr odu ction ) wer e ca lcu la ted by m ea n s
of m u ltiple r e gr ession . For th is a dju stm en t,
a ll fa ctor s for wh ich a sign ifica n t con n ection
w ith th e depen den t cou ld be dem on str a ted,
wer e ta k en in to a ccou n t.
Second step: analysis of variance
In th e secon d step, th e sign ifica n t in flu en ce of
th e exper im en ta l oper a tion s wer e tested w ith
a dju sted, depen den t va lu es to ca lcu la te th e Fva lu e w ith equ a tion (4). E qu a tion (4) is iden tica l w ith th e cla ssic a n a lysis of cova r ia n ce.
Th e th r ee pilot pla n ts a n d FeCl3 dosa ge wer e
in vestiga ted a s pa r a m eter s for tr ea tm en t. Th e
equ a tion u sed for va r ia n ce tests th e sign ifi ca n t differ en ce:
n –h
R 2 yx ;1, 2, 3…n
F =
•
( 4)
h – 1 1 – R 2 yx ;1, 2, 3…n
wh er e:
F = obser ved F-va lu e;
n = sa m ple size;
h = n u m ber of du m m y va r ia bles;
R 2 = squ a r e of m u ltiple cor r ela tion coefficien t;
Yˆ = a dju sted va lu e.
Th e two steps togeth er cor r espon d to cla ssic
a n a lysis of cova r ia n ce. As lon g a s con fou n der
a n d tr ea tm en t a r e in depen den t a n d th e size of
th e sa m ple is big en ou gh , th e r esu lts of th e
two m eth ods a r e iden tica l. In th is in vestiga tion , both ter m s wer e fu lfilled beca u se th e
n u m ber of con fou n der s w a s sm a ll a n d th e
sa m ple size in th is lon g-ter m stu dy w a s qu ite
la r ge.
An a lysis of va r ia n ce w a s fu r th er ca r r ied
ou t w ith th e or igin a l m ea su r ed va lu es w ith ou t a dju stm en t. Both r esu lts cou ld th u s be
com pa r ed, wh ich m a de it possible to ch eck
wh eth er th e in flu en ce of a tr ea tm en t h a d
been obscu r ed by th e a dju stm en t of th e m ea su r ed va lu es to th e con fou n der. In ou r exper im en ts th is w a s n ever th e ca se.
Results and discussion
Statistical analysis procedures
In a lon g-ter m stu dy w a stew a ter s of th r ee
ta n n er ies wer e in vestiga ted a n d u sed for
a n a er obic tr ea tm en t. In Ta ble I th e ch em ica l
qu a lity of th e w a stew a ter is given for COD,
su lph ide, su lph a te, ch lor ide a n d ch r om iu m .
E a ch va lu e r epr esen ts a five-day com posite
Elke Ge nsc ho w,
We rne r He ge mann and
Christian Masc hke
Anae ro bic tre atme nt o f
tanne ry waste wate r: to xic
e ffe c ts o f waste wate r
c o nstitue nts and do sage o f
fe rric c hlo ride
Enviro nme ntal Manage me nt
and He alth
8 / 1 [1 9 9 7 ] 2 8 –3 8
sa m ple. Min im a l, m a xim a l a n d m ea n va lu es
a n d th e sta n da r d devia tion a r e given . A sta tistica l a n a lysis w a s n ecessa r y du e to th e h igh
sta n da r d devia tion of a ll pa r a m eter s.
Usin g th ese sta tistica l m eth ods th e follow in g qu estion s ca n be a n swer ed:
• Wh ich w a stew a ter con stitu en ts in fl u en ced
COD r em ova l a n d ga s pr odu ction ?
• Is COD r em ova l a n d ga s pr odu ction depen den t on th e or igin of th e ta n n er y w a stew a ter ?
• Wer e th e pilot pla n ts equ a l in COD r em ova l
a n d in ga s pr odu ction ?
• Is th er e a sign ifica n t positive in fl u en ce of
fer r ic ch lor ide dosa ge on COD r em ova l a n d
ga s pr odu ction ?
Th e cr iter ion u sed to eva lu a te th e volu m e of
ga s w a s th e volu m e of ga s pr odu ced per week ,
wh ich w a s u sed a s a depen den t va lu e (Y *).
Th e COD r em ova l w a s ca lcu la ted by deter m in in g COD differ en ces. With r e ga r d to th e
m ea su r ed va lu es, on ly th e differ en ces
between th e in flu en t a n d th e fir st sta ge a n d
between th e fir st a n d secon d sta ges wer e
ca lcu la ted. Th ese differ en ces wer e u sed a s
depen den t va lu es. Th is pr ocedu r e fu r th er
pr ovided in for m a tion a bou t r em ova l.
It sh ou ld be n oted th a t in th e ca se of COD
differ en ces, m ea su r ed va lu es of ta n n er ies A,
B, a n d C wer e ava ila ble, wh ile th e ga s volu m e
w a s m ea su r ed on ly in exper im en ts w ith
w a stew a ter of ta n n er ies B a n d C. Th e m ea su r ed va lu e r a n ge of in fl u en ce fa ctor s a lso
va r ied for th is r ea son .
Th e objective w a s to obta in in for m a tion
a bou t oper a tion a l pa r a m eter s su ch a s th e
sim ila r ity of pilot pla n ts a n d th e in flu en ce of
fer r ic ch lor ide on th e depen den t va lu es.
Th er efor e, it w a s n ecessa r y fi r st to cor r ect
th e in flu en ce of th e or igin m a th em a tica lly,
th e feed flow a n d th e qu a lity of w a stew a ter on
COD r em ova l a n d ga s pr odu ction . Th e pr ocedu r e w a s sepa r a ted in to th r ee steps.
Th e r esu lt of th e fir st step is a r e gr ession
fu n ction w ith a specific slope for ever y sign ifica n t con fou n der. In clu ded in th e fir st step is
th e a dju stm en t of th e depen den t va r ia ble.
Th e COD differ en ces a n d th e va lu es of ga s
l–1 )
COD (mg
Sulphide (mg l–1 )
Sulphate (mg l–1 )
Chloride (mg l–1 )
Chromium (mg l–1 )
Number
467 a
413 a
355 a
255 a
319 a
M ultiple regression of gas production
In Ta ble II th e sta tistica l r elia bility of th e
con fou n der s is given .Th e or igin of th e w a stew a ter (th r ee ta n n er ies), feed flow, th e ch em ica l pa r a m eter s in th e in fl u en t su ch a s su lph a te, su lph ide a n d Cr tot h a d n o sign ifica n t
in fl u en ce on ga s pr odu ction . Two
con fou n der s wer e fou n d th a t sign ifica n tly
in fl u en ced th e volu m e of ga s.
Th e r e gr ession fu n ction (5) in clu des a ll
sign ifica n t con fou n der s a n d is given below.
E a ch con fou n der h a s its ow n specific slope.
Y * = 0.001 X 1 – 0.667 X 2 + a
M inimum
861
0
67
0
0.4
M aximum
30,500
1,600
3,590
11,800
837
SD
M ean value
4,510
187
794
3,140
88
5,710
196
1,180
5,060
36
Note:
a does not c orrespond to the number of samples taken from tanneries A, B and C
(5)
wh er e:
Y * = estim a te of ga s pr odu ction (on e
week –1)
X 1 = COD (m g l –1) 861 < X 1 < 16,800;
–
X 1 = 4,750 S > 99,9 per cen t
X 2 = ch lor ide (m g l –1) 0.0 < X 2 < 12,800
X 2 = 5,040 S = 99.4 per cen t
a = con sta n t ter m a = 8.64
In equ a tion (5) th e r a n ge of COD a n d ch lor ide
in th e in fl u en t a r e given . Seven ty-fi ve per cen t
of th e va lu es for COD in th e in fl u en t a r e in
Table II
Statistic al re liability o f c o nfo unde rs o n gas
pro duc tio n and o n COD diffe re nc e s (multiple
re gre ssio n)
Confounders
Table I
Charac te ristic s o f tanne ry waste wate r
Parameter
wer e con ver ted in to a dju sted va lu es w ith th e
m ea n va lu e of ea ch sign ifica n t con fou n der in
m u ltiple r e gr ession .
In th e secon d step th e in flu en ce of th e oper a tion a l pa r a m eter s on th e depen den t va lu es
w a s tested (a n a lysis of va r ia n ce). Sta tistica l
r elia bility (S ) between 95 per cen t a n d 99 per
cen t is n a m ed sign ifica n t, wh er ea s sta tistica l
r elia bility > 99 per cen t is ter m ed h igh ly
sign ifica n t. Appr oxim a tely 250 va lu es of ea ch
pa r a m eter, ea ch va lu e cor r espon din g to a fi veday com posite sa m ple, wer e in clu ded in th e
ca lcu la tion a n d th er eby exa m in ed for th eir
in fl u en ce on COD differ en ces a n d ga s pr odu ction .
Statistical reliability
Gas production COD differences
(%)
(%)
Tannery (A, B, C)
Feed flow (1 week–1 )
COD
(mg l–1 ) a
Sulphide (mg l–1 ) a
Sulphate (mg l–1 ) a
Chloride (mg l–1 ) a
Chromium (mg l–1 ) a
ns
ns
>99.9
ns
ns
99.4
ns
95.3
ns
>99.9
ns
95.3
ns
ns
Notes:
in the influent
ns: no significanc e
a:
[ 31 ]
Elke Ge nsc ho w,
We rne r He ge mann and
Christian Masc hke
Anae ro bic tre atme nt o f
tanne ry waste wate r: to xic
e ffe c ts o f waste wate r
c o nstitue nts and do sage o f
fe rric c hlo ride
Enviro nme ntal Manage me nt
and He alth
8 / 1 [1 9 9 7 ] 2 8 –3 8
th e r a n ge between 900 a n d 9,000g l –1. N ea r ly
a ll va lu es (95 per cen t) a r e in th e r e gion
between 900 a n d 15,000m g l –1. Seven ty-five per
cen t of th e va lu es for ch lor ide wer e fou n d to
be between 0 a n d 8,000m g l –1 a n d 95 per cen t of
th e va lu es lay between 0 a n d 11,000m g l –1. As a
con sequ en ce COD va lu es h igh er th a n
15,000m g l –1 a n d ch lor ide va lu es > 11,000m g l –1
n eed n ot be con sider ed for pr a ctica l oper a tion .
With th e a bove equ a tion , th e estim a ted ga s
pr odu ction w ith m ea n feed flow (24.2 l) a n d
–
m ea n con cen tr a tion of COD (X 1) a n d ch lor ide
–
(X 2 ) ca n be deter m in ed. E stim a tes of ga s
pr odu ction lay between 120 a n d 50 l k g -1 COD .
o
It r ea ch ed a m ea n va lu e of a bou t 75 l k g -1COD
o
wh ich ca n be ju dged a s r a th er low.
COD ( X 1 )
Th e depen den ce of ga s volu m e (Y *) on COD
con cen tr a tion in th e in flu en t (X 1) w a s deter m in ed a s h avin g ver y h igh sta tistica l r elia bility (S > 99.9 per cen t). High er con cen tr a tion s
of COD cor r espon ded to a n in cr ea se in ga s
volu m e. E qu a tion (5) w a s deter m in ed w ith in
th e m ea su r in g r a n ge of th e con fou n der a n d is
va lid on ly for th is r a n ge. With m u ltiple
r e gr ession it w a s possible to r ea ssu r e th is
well-k n ow n fa ct.
Chloride ( X 2 )
By m ea n s of m u ltiple r e gr ession a con n ection
w ith h igh sta tistica l r elia bility (99.4 per cen t)
between ch lor ide con cen tr a tion (X 2 ) a n d ga s
pr odu ction w a s fou n d.
A gr a ph ic r epr esen ta tion w ith sim u lta n eou s
va r ia tion of th e two sign ifi ca n t con fou n der s
ca n n ot ser ve a s a cla r ifyin g illu str a tion . On ly
by k eepin g on e con fou n der con sta n t, ca n a
gr a ph ic estim a te fu n ction for th e oth er (0.0 <
X 2 < 12,800m g l –1 ch lor ide) pr odu ce in telligible gr a ph ic r esu lts. It is a dva n ta geou s to u se
th e m ea n va lu e (X 1 = 4,750 m g l –1) a s a con sta n t con fou n der, beca u se it is th e best estim a te. F igu r e 2 r epr esen ts th is set-u p.
Ch lor ide sh owed a n e ga tive in flu en ce on
ga s pr odu ction (Y *), so th a t on e ca n spea k of
a n in h ibitor y effect. N o su ch in flu en ce of
ch lor ide w a s fou n d on COD r em ova l.Th e
in h ibitor y effect of ch lor ide on m eth a n e ba cter ia ca u sin g lower ga s pr odu ction w a s fou n d
for con cen tr a tion s > 10g l –1 ch lor ide (t R = 9 d )
in syn th etic w a stew a ter [15]. Bu t a n in h ibitor y effect h a s a lso been detected for lower
con cen tr a tion s[16].
Wh y ga s pr odu ction is in h ibited wh ile COD
r em ova l is n ot a ffected in th is exper im en t
m ay be expla in ed a s follow s. On e possible
expla n a tion lies in th e in h ibitor y effect of
ch lor ide on m eth a n e ba cter ia , wh ich r edu ce
less COD a t h igh er con cen tr a tion s of ch lor ide
a n d th u s pr odu ce sm a ller a m ou n ts of bioga s[16], wh ile th e con cen tr a tion of ch lor ide
does n ot n e ga tively a ffect th e a ctivity of th e
su lph a te-r edu cin g ba cter ia . It is k n ow n th a t
som e su lph a te-r edu cin g ba cter ia r equ ir e
N a Cl con cen tr a tion s between 4 a n d 20 g l –1[17]
or between 0.5 a n d 5g l –1[9]. Th e a m ou n t of
con ver ted or ga n ic m a tter in su lph a te
Figure 2
Influe nc e o f c hlo ride o n gas vo lume (multiple re gre ssio n)
[ 32 ]
Elke Ge nsc ho w,
We rne r He ge mann and
Christian Masc hke
Anae ro bic tre atme nt o f
tanne ry waste wate r: to xic
e ffe c ts o f waste wate r
c o nstitue nts and do sage o f
fe rric c hlo ride
Enviro nme ntal Manage me nt
and He alth
8 / 1 [1 9 9 7 ] 2 8 –3 8
r edu ction (th eor etica lly 530m g COD for 1g
r edu ced su lph a te) a lm ost r ea ch ed 1g COD per
g r edu ced su lph a te in pr a ctica l exper im en ts
w ith m in e w a ter [18]. Given a u n ila ter a l
in h ibitor y effect of h igh ch lor ide con cen tr a tion s on m eth a n e ba cter ia , th is wou ld lea d to
r edu ced ga s pr odu ction wh ile n ot n ecessa r ily
ca u sin g a decr ea se in COD r em ova l efficien cy.
M ultiple regression of COD efficiency
In Ta ble II th e sta tistica l r elia bility of th e
con fou n der s con cer n in g COD differ en ces is
a lso given . Feed fl ow, th e ch em ica l pa r a m eter s in th e in flu en t su ch a s su lph ide, ch lor ide
a n d Cr tot h a d n o sign ifi ca n t in fl u en ce on COD
r em ova l. In con tr a st to th e fin din gs w ith ga s
volu m e, th r ee con fou n der s wer e fou n d wh ich
sign ifica n tly in flu en ced COD differ en ces a n d
COD r em ova l r espectively. As a r esu lt, a
r e gr ession fu n ction (equ a tion 6) in clu din g a ll
sign ifica n t con fou n der s (COD, su lph a te, a n d
ta n n er ies) ca n be given w ith th eir specifi c
slopes:
Y * = 0.427 X 1 – 0.294 X 2 – 412 X 3 + a
(6)
wh er e:
Y * = estim a tion for COD differ en ces in
m g l –1.
–
X 1 = COD (m g l –1) 861 < X 1 < 30,500 X 1 =
–1
5,710m g l S > 99.9 per cen t
–
X 2 = su lph a te (m g l –1) 162 < X 2 < 3,590 X 2
–1
= 1,180m g l S = 95.3 per cen t
X 3 = ta n n er y A a n d X 3 = 0
ta n n er y B X 3 = 1
a = con sta n t ter m , a = 5.60
E qu a tion (6) cou n ts for th e fir st a n d th e secon d sta ge so th a t a fa ctor of 2 is n ecessa r y in
or der to ca lcu la te COD r em ova l in both sta ges
togeth er. Th e sa m e pr ocedu r e a s for ch lor ide
w a s u sed to display th is r esu lt.
In equ a tion (6) th e r a n ge of COD a n d su lph a te in th e in fl u en t a r e given . Seven ty-five
per cen t of th e COD va lu es in th e in flu en t a r e
in th e r a n ge between 900 a n d 10,000g l –1.
N in ety-five per cen t of th e va lu es a r e in th e
r e gion between 900 a n d 15,000m g l –1. Seven tyfive per cen t of su lph a te va lu es r a n ged
between 200 a n d 1,500m g l –1 a n d n ea r ly a ll (95
per cen t) lay between 160 a n d 2,900m g l –1. As a
con sequ en ce, va lu es of COD h igh er th a n
15,000m g l –1 a n d va lu es of su lph a te >
2,900m g l –1 n eed n ot be con sider ed for pr a ctica l oper a tion .
COD ( X 1 )
A h igh ly sign ifi ca n t cor r ela tion (S > 99.9 per
cen t) w a s fou n d between COD con cen tr a tion s
in th e in flu en t (X 1) a n d COD differ en ces (Y *).
As for ga s volu m e, h igh er COD con cen tr a tion s
in th e in flu en t led to h igh er COD differ en ces
or h igh er COD r em ova l r espectively. Th is is a
well-k n ow n fa ct. In F igu r e 3 on ly th e
depen den ce of COD differ en ces on COD in th e
in fl u en t (X 1) is sh ow n , k eepin g con sta n t th e
–
m ea n va lu e of su lph a te (X 2 = 1,180 m g l –1) a n d
ta n n er ies A a n d C (X 3 = 0). (Th e u se of X 3 w ill
be expla in ed below.)
Wh en COD differ en ces for th e COD va lu es
in th e in fl u en t (X 1) a r e ca lcu la ted by m ea n s of
th e equ a tion (6) cited a bove, on e ca n a scer ta in th e r em ova l in per cen t. It lay between 70
a n d 80 per cen t for h igh er COD con cen tr a tion s in th e in flu en t (5,000-30,000m g l –1). With
lower COD va lu es in th e in fl u en t (X 1), h igh ly
r edu ced r em ova l w a s fou n d.
As a con sequ en ce, a n a er obic tr ea tm en t
sh ou ld be u sed on ly for h igh ly pollu ted ta n n er y w a stew a ter, beca u se th e r em ova l efficien cy for low COD con cen tr a tion s is u n sa tisfa ctor y.
Sulphate ( X 2 )
With a sta tistica l r elia bility of 95.3 per cen t,
su lph a te con cen tr a tion in th e in flu en t (X 2)
sh owed a n e ga tive in flu en ce on COD differ en ces (Y *). E qu a tion (6) sh ow s th is con n ection . Th e sa m e pr ocedu r e a s for COD (X 1) w a s
u sed for displayin g th e in flu en ce of su lph a te
on COD r em ova l. F igu r e 4 r epr esen ts th is setu p.
–
With m ea n va lu es of su lph a te (X 2 = 1,180m g
–
l –1) a n d COD (X 1 = 5,710m g l –1) in ta n n er y
w a stew a ter (A a n d C), a ppr oxim a tely 70 per
cen t of th e COD cou ld be elim in a ted. With
in cr ea sin g su lph a te con cen tr a tion s in th e
in fl u en t u p to a m a xim u m va lu e of 2,900m g l –1
su lph a te (95 per cen t of th e va lu es); COD
r em ova l w a s estim a ted to be a bou t 55 per
cen t.
Th e in h ibitor y effect of su lph a te on COD
r em ova l cou ld be expla in ed by th e fa ct th a t
m eth a n e ba cter ia con ver t less COD a t h igh er
levels of su lph a te con cen tr a tion , wh er ea s
su lph a te-r edu cin g ba cter ia th eor etica lly
sh ou ld con su m e m or e COD a t th e sa m e tim e.
With h igh su lph a te con cen tr a tion th er e is
a ppa r en tly n o com plete su lph a te r edu ction ,
so th a t th e r edu ced COD r em ova l m ay r esu lt
fr om th e lower r em ova l r a te of th e m eth a n e
ba cter ia a n d su lph a te-r edu cin g ba cter ia .
Tanneries ( X 3 )
Th e or igin of th e w a stew a ter (X 3) w a s fou n d
to in fl u en ce COD differ en ces sign ifica n tly.
Th ese fin din gs differ ed fr om th ose for volu m e
of ga s, for wh ich a n in flu en ce of th e or igin of
th e w a stew a ter cou ld n ot be fou n d. For th e
COD differ en ces (Y *), exper im en ts w ith
w a stew a ter fr om ta n n er ies A, B a n d C (X 3)
wer e con sider ed, wh ile on ly th e va lu es of
ta n n er ies B a n d C wer e in clu ded for deter m in in g th e volu m e of ga s.
Ta ble III depicts th e in flu en ce of th e r espective ta n n er y on COD differ en ces. Sin ce th e
[ 33 ]
Elke Ge nsc ho w,
We rne r He ge mann and
Christian Masc hke
Anae ro bic tre atme nt o f
tanne ry waste wate r: to xic
e ffe c ts o f waste wate r
c o nstitue nts and do sage o f
fe rric c hlo ride
Figure 3
Influe nc e o f COD in the influe nt o n COD re mo val (multiple re gre ssio n)
Enviro nme ntal Manage me nt
and He alth
8 / 1 [1 9 9 7 ] 2 8 –3 8
Figure 4
Influe nc e o f sulphate in the influe nt o n COD re mo val (multiple re gre ssio n)
[ 34 ]
Elke Ge nsc ho w,
We rne r He ge mann and
Christian Masc hke
Anae ro bic tre atme nt o f
tanne ry waste wate r: to xic
e ffe c ts o f waste wate r
c o nstitue nts and do sage o f
fe rric c hlo ride
Enviro nme ntal Manage me nt
and He alth
8 / 1 [1 9 9 7 ] 2 8 –3 8
Table III
Diffe re nc e s o f COD re mo val in the thre e
tanne rie s (multiple re gre ssio n)
Tanneries
Statistical reliability (%)
Y* : COD difference (mg l–1 )
COD removal (%) a
A
B
C
95.3
4,200
72
95.3
3,400
60
95.3
4,200
72
Note:
–
–
With COD X 1 = 5,710 mg/ l, sulphate X 2 = 1,180mg/ l
a
ta n n er ies r epr esen t n om in a l va r ia bles, m u ltiple r e gr ession r equ ir ed du m m y codin g, i. e., a
tr a n sfor m a tion in to qu a n tita tive en tities
w ith a va lu e of “0” or “1”. With th is oper a tion ,
ta n n er y B beca m e sign ifica n t, so th a t it w a s
a ttr ibu ted th e n u m er ica l va lu e “1” in equ a tion (6), wh ile ta n n er ies A a n d C wer e
a ccor ded a “0”. On th e ba sis of th is codin g
a n d th e m ea n va lu es for COD a n d su lph a te
con cen tr a tion s, COD differ en ces wer e
4,200m g l –1 for ta n n er ies A a n d C, bu t on ly
3,400m g l –1 for ta n n er y B.
Ma n y differ en t su bsta n ces a r e u sed in
lea th er pr odu ction , so th a t, even th ou gh
ch r om iu m -ta n n in g, lea th er -pr odu cin g fa ctor ies in pr in ciple follow th e Reu tlin ger
m odel[19], both th e qu a n tity a n d qu a lity of
th e w a stew a ter differ con sider a bly between
fa ctor ies[20]. It is th u s pla u sible th a t on e
sh ou ld fi n d differ en ces in COD r em ova l.
Th ese sign ifica n t differ en ces ca n be ta k en a s
a h in t th a t fu r th er su bsta n ces in ta n n er y
w a stew a ter h ave a n in flu en ce on th e a n a er obic pr ocess.
Analysis of variance
Pilo t plants
In th e in vestiga tion s, th r ee sim ila r pilot
pla n ts wer e oper a ted in pa r a llel w ith differ en t oper a tion a l pa r a m eter s. An a n a lysis of
va r ia n ce m a de it possible to test th e sim ila r ities in th e volu m e of ga s pr odu ced a n d in
COD r em ova l. A sign ifica n t differ en ce
between th e pilot pla n ts w a s fou n d for COD
r em ova l bu t n ot for ga s volu m e.
In F igu r e 5 th e COD differ en ces (r aw a n d
a dju sted va lu es) a r e for ea ch pilot pla n t. With
sta tistica l r elia bility given a t S = 98.2 per cen t
for r aw va lu es a n d a t S = 99.6 per cen t for
estim a ted va lu es pilot pla n ts va r ied sign ifi ca n tly. By a dju stin g th e m ea su r ed va lu es th e
va r ia n ce-a n a lytica l effects r ea ch ed a h igh er
de gr ee of tr a n sfer a bility to oth er ca ses.
An a lyses of va r ia n ce wer e ca r r ied ou t w ith
th e or igin a l m ea su r ed va lu es w ith ou t a dju stm en t, in or der to ch eck if th e in flu en ce of a
tr ea tm en t h a d been obscu r ed by th e a dju stm en t. However, n o su ch in flu en ce w a s fou n d.
Figure 5
Diffe re nc e o f COD re mo val in the thre e pilo t plants (analysis o f varianc e )
[ 35 ]
Elke Ge nsc ho w,
We rne r He ge mann and
Christian Masc hke
Anae ro bic tre atme nt o f
tanne ry waste wate r: to xic
e ffe c ts o f waste wate r
c o nstitue nts and do sage o f
fe rric c hlo ride
Enviro nme ntal Manage me nt
and He alth
8 / 1 [1 9 9 7 ] 2 8 –3 8
In or der to eva lu a te th ese differ en ces w ith
r e ga r d to r eleva n ce for pr a ctica l oper a tion
th e estim a ted COD r em ova l of th e th r ee pilot
pla n ts is given for th e m ea n va lu e of
COD = 5,710m g l –1. P ilot pla n t 1 r ea ch ed a
COD r em ova l of 59 per cen t, pilot pla n t 2
a ch ieved a COD r em ova l of 65 per cen t a n d
pilot pla n t 3 obta in ed th e h igh est COD elim in a tion w ith 69 per cen t for a dju sted va lu es.
In a n a er obic tr ea tm en t pla n ts differ en t
biocen oses of ba cter ia m ay esta blish th em selves depen din g on feed fl ow, deten tion tim e
of w a stew a ter, tem per a tu r e, a n d pH va lu e[21].
Table IV
Influe nc e o f fe rric c hlo ride o n gas vo lume (analysis o f varianc e )
Ferric chloride dosage (g l–1 )
Volume of the gas
Statistical reliability
Gas volume
Gas production
Gas volume
0
1
2
Adjusted values
(%)
(1 week–1 )
(1kg–1 CODo ) a
(%) b
99.9
14.6
127
100
99.9
9.7
84.3
66
99.9
8.7
75.7
60
0
1
Raw values
99.9
14.3
124
100
Notes:
a with mean value of COD = 4,750mg l–1 , feed flow = 24.2 1 week–1
o
b related to 0g l–1 = 100 per c ent
99.9
6.3
54.8
44
2
99.9
8.8
76.5
62
On ly a sign ifica n t differ en ce in COD r em ova l
w a s fou n d, wh er ea s n on e cou ld be deter m in ed in ga s pr odu ction a n d desu lph u r iza tion . COD r em ova l is m a in ly ca u sed by th r ee
pr ocesses: a cidifi ca tion , desu lph u r iza tion
a n d pr odu ction of m eth a n e. Su lph a te-r edu cin g ba cter ia a n d m eth a n e ba cter ia in th e
th r ee pilot pla n ts pr oba bly did n ot r edu ce
sign ifica n tly differ en t a m ou n ts of COD, bu t
th e su m of COD r em ova l in a ll pr ocesses
togeth er w a s sign ifica n t in th e pilot pla n ts.
Fe Cl3
Th e dosa ge of fer r ic ch lor ide w a s in ten ded to
elim in a te su lph ide a n d th u s to pr even t its
in h ibitor y effect. Wh ile th e dosa ge of FeCl 3
sh owed a sign ifica n t in fl u en ce on ga s pr odu ction , n o su ch in flu en ce w a s n oted on COD.
In or der to eva lu a te th e r esu lts of th e a n a lysis, sta tistica l r elia bility a s well a s th e over a ll
r eleva n ce of th e r esu lts w a s con sider ed (Ta ble
IV). For r aw va lu es a n d for a dju sted va lu es
th e sa m e sta tistica l r elia bility w a s fou n d (S =
99.9 per cen t).
A dosa ge of 2g l –1 fer r ic ch lor ide r esu lted in
a 40 per cen t r edu ction of ga s pr odu ction .
E ven h a lf th e a m ou n t of fer r ic ch lor ide
effected a 34 per cen t r edu ction , a r esu lt
wh ich m u st be con sider ed r eleva n t (F igu r e 6).
Figure 6
Influe nc e o f the do sage o f fe rric c hlo ride o n gas vo lume (analysis o f varianc e )
[ 36 ]
Elke Ge nsc ho w,
We rne r He ge mann and
Christian Masc hke
Anae ro bic tre atme nt o f
tanne ry waste wate r: to xic
e ffe c ts o f waste wate r
c o nstitue nts and do sage o f
fe rric c hlo ride
Enviro nme ntal Manage me nt
and He alth
8 / 1 [1 9 9 7 ] 2 8 –3 8
A pla u sible r ea son for th e decr ea se of ga s
pr odu ction th r ou gh fer r ic ch lor ide (r e ga r din g on ly th e r aw va lu es) lies in th e fu r th er
in cr ea se of ch lor ide con cen tr a tion (F igu r e 2),
sin ce SRB a r e less sen sitive to ch lor ide th a n
m eth a n e ba cter ia . However, in th e a dju sted
va lu es, th e in fl u en ce of ch lor ide w a s r em oved
by m u ltiple r e gr ession a n d a dju stm en t.
A possible expla n a tion for th e lower ga s
yield is th e pr ecipita tion of or ga n ic m a tter,
wh ich pr odu ces slu dge. Th is slu dge is a n
u n welcom e side-effect of th e a ddition of
FeCl 3. Ow in g to pr ecipita tion , th e su bsta n ce
is th en n o lon ger ava ila ble for a n a er obic
elim in a tion a n d ga s pr odu ction , bu t its
a bsen ce cou n ts a s COD r em ova l efficien cy. In
ba tch exper im en ts a n ota ble COD r edu ction
w a s obser ved wh en fer r ic ch lor ide w a s u sed
a s a pr ecipita tin g a gen t[9]. Sin ce a positive
effect of fer r ic ch lor ide dosa ge cou ld n ot be
ver ified, on e sh ou ld r efr a in fr om a ddin g it to
ta n n er y w a stew a ter.
Conclusions
In a n a er obic tr ea tm en t of ta n n er y w a stew a ter in h ibitor y effects of su lph a te a n d ch lor ide wer e fou n d. In or der to r ea ch better COD
efficien cy or a decr ea se in deten tion tim e,
ta n n er ies sh ou ld ch a n ge pr odu ction m eth ods.
In or der pa r tly to su bstitu te ch lor ide, on e ca n
u se fr esh or on ly cooled h ides in stea d of
sa lted on es. In Ger m a n y today som e ta n n er ies u se u p to 60 per cen t fr esh h ides[22] a n d
th u s disch a r ge r a th er low a m ou n ts of ch lor ide in th e w a stew a ter.
With r e ga r d to su lph a te, on ly a ch a n ge in
th e lea th er pr odu ction pr ocess itself wou ld
yield better r esu lts. In or der pa r tly to su bstitu te su lph u r ic a cid it is possible to u se CO 2
for delim in g. For th in h ides a tota l con ver sion is possible, for n or m a l h ides on ly a pa r tia l su bstitu tion of a m m on ia su lph a te is fea sible[22]. Th is com bin a tion effects a r edu ction
in a m m on ia a n d in su lph a te.
An a er obic tr ea tm en t of ta n n er y w a stew a ter
is su ita ble especia lly for h igh con cen tr a tion s
of or ga n ic com pou n ds. Th e COD r em ova l for
COD > 5,000m g l –1 lay a t 70-80 per cen t bu t
decr ea sed for lower COD in th e in flu en t.
Th e COD r em ova l in th e ta n n er ies A a n d C
w a s sign ifica n tly differ en t fr om ta n n er y B.
Th is m ea n s th a t even if on e k n ow s th e fa ctor s
th a t sign ifica n tly in flu en ce COD r em ova l
(COD o a n d con cen tr a tion of su lph a te) th e
COD r em ova l of a n u n k n ow n w a stew a ter ca n
differ fr om th e va lu e th a t is r epor ted h er e.
Th is m ea n s th a t befor e tr a n sfer r in g th ese
r esu lts to oth er tr ea tm en t pla n ts, pilot tests
a r e n ecessa r y.
References
1 Ba iley, D.G., Tu n ick , M.H., F r iedm a n , A.A.,
a n d Rest, G.B, “An a er obic tr ea tm en t of ta n n er y
w a ste”, Proceed in gs of th e 38th In d u str ia l
Wa ste Con feren ce, P u r du e Un iver sity, In dia n a ,
IN, 1984, pp. 673-82.
2 Tu n ick , M.H., Ba iley, D.G., Moor e, J .H. a n d
Cooper, J .E ., “An a er obic-a er obic tr ea tm en t of
ve geta ble ta n n in g w a ste”, J ou r n a l of th e A m er ica n L ea th er Ch em ica l A ssocia tion , Vol. 80,
1985, pp. 101-5.
3 You n g, K.S., F r iedm a n , A.A. a n d Ba iley, D.G.,
“P r etr ea tm en t of ta n n er y Bea m h ou se w a stew a ter u sin g a n a n a er obic fi lter : pr elim in a r y
r esu lts”, 12th M id -A tla n tic In d u str ia l Wa ste
Con feren ce Proceed in gs, Bu ch n ell Un iver sity,
Lew isbu r g, 1980, pp. 102-10.
4 Ba i, R.K. a n d Siva th a n u , S.R., “An a er obic
tr ea tm en t of ta n n er y efflu en ts. A possibility
for m eth a n e r ecover y a n d or ga n ic m a tter
r em ova l”, Poster Pa pers of th e 5th In ter n a tion a l S ym posiu m on A n a erob ic Digestion ,
Bologn a , 1988, pp. 661-5.
5 Ch en , S.J ., Li, C.T. a n d Sh ieh , W.K., “An a er obic
fl u idized bed tr ea tm en t of a ta n n er y w a stew a ter ”, Ch em ica l En gin eer in g R esea rch &
Design , Vol. 66 N o. 6, 1988, pp. 518-23.
6 Ar or a , H.C. a n d Ch a ttopa dh ya , S.N., “An a er obic con ta ct fi lter pr ocess: a su ita ble m eth od for
th e tr ea tm en t of ve geta ble ta n n in g efflu en ts”,
Wa ter Pollu tion Con trol, Vol. 79, 1980, pp. 501-6.
7 Yon g, H., “Th e a n a er obic tr ea tm en t of ta n n er y
w a stew a ter ”, Proceed in gs of th e 42n d In d u str ia l Wa ste Con feren ce, P u r du e Un iver sity,
In dia n a , IN, 1988, pp. 749-55.
8 Kr oiss, H. (E d.) A n a erob e A b w a sser rein igu n g –
Gru n d la gen u n d Großtech n isch e Er fa h ru n g
(A n a erob ic T rea tm en t of Wa stew a ter – B a sics
a n d Ex per ien ce w ith Fu ll-sca le T rea tm en t),
Vol. 73, Wien er Mitteilu n gen , Wa sser
Abw a sser Gew ä sser, Wien , 1988.
9 Ver in k , J ., S u lfa tred u k tion u n d S u lfi d elim in ieru n g b ei d er ein - u n d z w eistu fi gen a n a erob en
B eh a n d lu n g h och su lph a th a ltiger A b w ä sser
(S u lph a te R ed u ction a n d Elim in a tion of S u lph id e w ith On e a n d T w o S ta ge A n a erob ic
T rea tm en t Pla n ts Con ta in in g High L oa d of
S u lph a te), Ver öffen tlich u n gen des In stitu tes
fü r Siedlu n gsw a sser w ir tsch a ft u n d Abfa lltech n ik der Un iver sitä t Ha n n over, Vol. 70, 1988.
10 Bu ism a n , C.J .N. a n d Lettin ga , G., “Su lfide
r em ova l fr om a n a er obic w a ste tr ea tm en t
efflu en t of a pa per m ill”, Wa ter R esea rch ,
Vol. 24 N o. 3, 1990, pp. 313-9.
11 Ya dav, V.K. a n d Ar ch er, D.B., “Specific in h ibition of su lfa te-r edu cin g ba cter ia in m eth a n ogen ic co-cu ltu r e”, L etters in A pplied M icrob iolog y, Vol. 7, 1988, pp. 165-8.
12 Hilton , M.G. a n d Ar ch er, D.B., “An a er obic
digestion of a su lfa te-r ich m ola sses w a stew a ter : in h ibition of h ydr ogen su lfi de pr odu ction ”, B iotech n olog y a n d B ioen gin eer in g,
Vol. 31, 1988, pp. 885-8.
13 Sä r n er, E ., “Rem ova l of su lfa te a n d su lfite in a n
a n a er obic tr ick lin g (An tr ic) fi lter ”, Wa ter
S cien ce T ech n ology, Vol. 22 N o. 1/ 2, 1990,
pp. 395-404.
[ 37 ]
Elke Ge nsc ho w,
We rne r He ge mann and
Christian Masc hke
Anae ro bic tre atme nt o f
tanne ry waste wate r: to xic
e ffe c ts o f waste wate r
c o nstitue nts and do sage o f
fe rric c hlo ride
Enviro nme ntal Manage me nt
and He alth
8 / 1 [1 9 9 7 ] 2 8 –3 8
[ 38 ]
14 Gen sch ow, E ., “An a er obe Rein igu n g von
Ger ber eia bw ä sser n – Au swer tu n g m it
kom plexen sta tistisch en Meth oden ” (An a er obic tr ea tm en t of ta n n er y w a stew a ter – u tiliza tion w ith com plex statistica l m eth ods), Disser tation , Ber ich te zu r Siedlu n gsw a sser w ir tsch a ft
2, Ber lin .
15 J a ck son -Moss, C.A., Du n ca n , J .R. a n d Cooper,
D.R., “Th e effect of sodiu m ch lor ide on a n a er obic digestion ”, J ou r n a l of th e A m er ica n
L ea th er Ch em ica l A ssocia tion , Vol. 84 N o. 9,
1989, pp. 266-71.
16 McCa r ty, P.L. a n d McKin n ey, R.E ., J W PCF: 339415 (1969, in [9]).
17 Widdel, F. a n d P fen n ig, N., “Dissim ila tor y
su lfa te- or su lfu r -r edu cin g ba cter ia ”, in
B ergey’s M a n u a l, Section 7, 1981, pp. 663-79.
18 Ma r ee, J.P. a n d Str ydom , W.F., “Biologica l su lfa te r em ova l in a n u pflow pa cked bed r ea ctor ”,
Water R esearch , Vol. 19 No. 9, 1985, pp. 1101-6.
19 Feik es, L., Ök ologisch e Problem e d er
L ed er in d u str ie (Ecologica l Problem s of L ea th er
Prod u cin g In d u str y), in Her feld (E d.), B iblioth ek d es L ed ers, Um sch a u Ver la g, F r a n k fu r t,
1983.
20 Rü ffer, H. u n d Rosen w in k el, K.H., T a sch en bu ch
d er In d u str ieab w a sser rein igu n g (Pa per b a ck of
th e T rea tm en t of In d u str ia l Wa stew a ter ),
Olden bou r g Ver la g, Mü n ch en .
21 Hw a n g, K.Y. a n d Br a u er, H., “An a er obic w a stew a ter tr ea tm en t w ith bioga s pr odu ction in a
pu lsed bior ea ctor ”, B T F-B iotech n ology-Foru m ,
Vol. 4, 1987, pp. 118 -30.
22 Ren n er, G., “Abw ä sser a u s Ger ber eien u n d
Möglich k eiten der Bela stu n gsver m in der u n g”
(“Ta n n er y w a stew a ter a n d possibilities of
r edu cin g th e pollu tion loa d”), S ch r iften reih e
B iologisch e A b w a sser rein igu n g, N o. 5 (Proceed in gs B iologica l Wa stew a ter T rea tm en t), Tech n isch e Un iver sitä t, Ber lin , 1995.
ef fects of wastewater constituents and dosage of
ferric chloride
Elke Genschow
Pro f. Dr-Ing., TU Be rlin, Be rlin, Ge rmany
Werner Hegemann
Pro f. Dr-Ing., TU Be rlin, Be rlin, Ge rmany
Christian M aschke
Pro f. Dr-Ing., TU Be rlin, Be rlin, Ge rmany
Investigates anaerobic twostage treatment of tannery
wastewater. This results in a
mean chemical oxygen
demand (COD) removal of 60
per cent for tannery B and
more than 70 per cent for
tanneries A and C with CODo
= 5,710mg l –1 and mean
detention time held at 3.5
days. Gas production was
small, rating an estimated
75l kg–1 CODo with CODo =
5,710mg l –1 . Tests simultaneously the infl uence of quality
and quantity of wastewater on
COD removal and gas production (multiple regression).
Finds signifi cant inhibitory
effects were caused by chloride on gas volume and by
sulphate on COD removal.
Chromium showed no signifi cant effect. Dosage of ferric
chloride for removal of the
toxic sulphide effected a
decrease in gas volume and
had no effect on COD removal
(analysis of variance).
Enviro nme ntal Manage me nt
and He alth
8 / 1 [ 1997] 2 8 –3 8
© MCB Unive rsity Pre ss
[ ISSN 0956-6163]
[ 28 ]
Introduction
An a er obic tr ea tm en t of ta n n er y w a stew a ter
h a s n ot been in vestiga ted ver y well. In m ost
ta n n er ies th e Bea m h ou se is su bject to tr ea tm en t[1-3], sin ce th e qu a lity does n ot u n der go
m u ch va r ia tion a n d sin ce th e or ga n ic loa d is
ver y h igh . (Th e Bea m h ou se in clu des a ll
sta ges befor e ta n n in g.)
Bu t even wh en th e en tir e w a stew a ter is
tr ea ted, th e qu a lity differ s depen din g on
wh eth er th e ta n n er y is u sin g ch r om iu m [4,5]
or ve geta ble ta n n in g a gen ts[6,7].
Usin g a n a er obic tr ea tm en t of ch r om iu m
ta n n er y w a stew a ter, th e objective in th is
stu dy w a s to r edu ce th e or ga n ic loa d, th er eby
pr odu cin g on ly a sm a ll a m ou n t of slu dge.
Th er efor e, two sta ge a n a er obic tr ea tm en t w a s
ch osen . Th e fir st sta ge w ith less sen sitive
a cidifica tion ba cter ia ser ves a s a bu ffer in th e
ca se of con cen tr a tion fl u ctu a tion s a n d
in h ibitin g su bsta n ces[8,9]. Ta n n er y w a stew a ter con ta in s h igh con cen tr a tion s of su lph a te, su lph ide, ch lor ide, a n d ch r om iu m
wh ich a r e k n ow n a s poten tia l in h ibitin g
ch em ica ls. In th e a n a er obic pr ocess su lph a ter edu cin g ba cter ia pr odu ce su lph ide by r edu cin g su lph a te.Th e su lph ide especia lly w a s
su spected of bein g th e m ost in h ibitin g su bsta n ce in ta n n er y w a stew a ter. Th er e a r e sever a l m eth ods for r edu cin g su lph ide.
For exa m ple, FeCl 3 ca n be a dded in th e fi r st
sta ge to r edu ce su lph ide con cen tr a tion s[10,11]. Fer r ic ch lor ide sh owed best
r esu lts in com pa r ison w ith fer r ic h ydr oxide
a n d fer r u gin ou s slu dge, sin ce it is ea sy to a dd
a n d h igh ly effective. Stir r in g is n ot r equ ir ed
a n d on ly sm a ll qu a n tities a r e n eeded for th e
en tir e elim in a tion [9].
An oth er possibility to decr ea se th e con cen tr a tion of su lph ide is to in h ibit su lph a ter edu cin g-ba cter ia (SRB). Th e a ddition of
N a 2MoO 4 sh ou ld in h ibit SRB, bu t th e
m eth a n e pr odu ction w a s a lso r edu ced a s a
con sequ en ce[12].
Su lph ide r em ova l is possible by biologica l
oxida tion of su lph ide to su lph u r. Su lph a te
pr odu ction ca n be con tr olled by oxygen con cen tr a tion [10]. An oth er m eth od to r edu ce
su lph ide w a s in vestiga ted by str ippin g su lph ide a n d r ecir cu la tin g th e w a sh ed ga s[13].
In th is stu dy FeCl 3 w a s ch osen .
Sta tistica l a n a lysis su ch a s m u ltiple r e gr ession a n d a n a lysis of va r ia n ce wer e n ecessa r y
du e to th e ch a n gin g qu a lity of w a stew a ter
w ith va r ia tion s in con cen tr a tion , h igh sta n da r d devia tion a n d com plex com position .
Th e objective w a s to test th e in flu en ce of
th e qu a lity of w a stew a ter sim u lta n eou sly, th e
qu a n tity of feed flow, a n d th e or igin (ta n n er y
A, B a n d C) on ch em ica l oxygen dem a n d
(COD) r em ova l a n d ga s pr odu ction (m u ltiple
r e gr ession ). After th is oper a tion , th e exper im en ta l oper a tion s wer e tested for th eir in flu en ce on ga s pr odu ction a n d COD r em ova l
(a n a lysis of va r ia n ce). As oper a tion a l pa r a m eter s th e sim ila r ity of pilot pla n ts a n d th e
dosa ge of fer r ic ch lor ide, a m on g oth er s, wer e
exa m in ed.
Material and methods
Pilot plant
Th r ee iden tica l a n a er obic pilot pla n ts wer e
r u n in pa r a llel. In F igu r e 1 th e sch em e of th e
two-sta ge pilot pla n t is given . It con sists of a
stir r ed ta n k a n d a su spen ded fixed fi lm bed
r ea ctor in u pfl ow -oper a tion filled w ith r a ssch ig-r in gs (7.9 l). Th e fixed fi lm bed r ea ctor
w a s oper a ted a t a tem per a tu r e of 34 °C th a t
w a s th er m osta tica lly con tr olled. Th e feed
flow w a s su pplied discon tin u ou sly fou r tim es
in 24 h ou r s. Th e bioga s w a s collected in volu m e ca libr a ted ba gs. In or der to avoid th e loss
of bioga s, a siph on w a s pla ced between th e
m eth a n e r ea ctor a n d th e efflu en t ta n k . Deten tion tim e va r ied between 2.4 a n d 10.5 days
w ith a m ea n va lu e of 3.5 days in both sta ges
togeth er.
Wastewater
Th e w a stew a ter w a s ta k en fr om th r ee differ en t ch r om iu m ta n n er ies (A, B a n d C). It w a s
Elke Ge nsc ho w,
We rne r He ge mann and
Christian Masc hke
Anae ro bic tre atme nt o f
tanne ry waste wate r: to xic
e ffe c ts o f waste wate r
c o nstitue nts and do sage o f
fe rric c hlo ride
Figure 1
Sc he me o f the pilo t plant
Enviro nme ntal Manage me nt
and He alth
8 / 1 [1 9 9 7 ] 2 8 –3 8
ga th er ed on ce a week a s a r a n dom sa m ple
fr om th e efflu en t of th e equ a lizin g ta n k a n d
u sed a s feed for th e exper im en ta l pla n t. It w a s
stor ed for seven to 14 days in th e feedin g ta n k .
In a ll in vestiga tion s u n dilu ted ta n n er y w a stew a ter w a s u sed.
Analytical methods
Th e a n a lyses of COD, ch lor ide (Cl –), a n d
ch r om iu m (Cr tot ) wer e ca r r ied ou t a ccor din g
to DIN -sta n da r ds. An a lyses of su lph ide (S 2–)
a n d su lph a te (SO 42–) wer e per for m ed ph otom etr ica lly, w ith com posite sa m ples bein g
ta k en ea ch day for five days. Sa m ples of COD
wer e a cidified a n d stir r ed for h a lf a n h ou r to
decr ea se su lph ide.
Experimental operation
Fe Cl3 – do sage
FeCl 3 w a s a dded in qu a n tities of 1gl –1 a n d
2gl –1 in th e in fl u en t ta n k . Th e objective w a s to
elim in a te th e su lph ide a s FeS, both in th e
for m existen t in th e r aw w a stew a ter a n d in
th e for m pr odu ced th r ou gh th e a n a er obic
pr ocess.
PH value s
Th e va lu es of th e ta n n er y w a stew a ter va r ied
between 8 a n d 12. For biologica l tr ea tm en t it
w a s n ecessa r y to r e gu la te th e pH. Th e pH w a s
a dju sted to 5, 6 or 7, depen din g on th e exper im en t.
An a lysis by m u ltiple sta tistica l m eth ods
(gr a du a l a n a lysis of cova r ia n ce)
Th ese wer e:
• N om in a l va r ia ble: qu a lita tive va r ia ble su ch
a s ta n n er y (A, B, C).
• Qu a n tita tive va r ia ble m ea su r ed va lu e su ch
a s COD va lu es (m gl –1).
• Con fou n der : pa r a m eter th a t ca u ses u n in ten tion a l va r ia n ce of th e depen den t va r ia ble.
[ 29 ]
Elke Ge nsc ho w,
We rne r He ge mann and
Christian Masc hke
Anae ro bic tre atme nt o f
tanne ry waste wate r: to xic
e ffe c ts o f waste wate r
c o nstitue nts and do sage o f
fe rric c hlo ride
Enviro nme ntal Manage me nt
and He alth
8 / 1 [1 9 9 7 ] 2 8 –3 8
• Depen den t va lu es: Y ; in th is ca se ga s pr odu ction a n d COD differ en ces.
• In depen den t va lu es x 1...n ; in th is stu dy:
con fou n der.
• Sta tistica l r elia bility (S ): r elia bility for th e
va lidity of th e sign ifi ca n t con n ection (m u ltiple r e gr ession , a n a lysis of va r ia n ce).
• Du m m y va r ia ble: n ecessa r y for tr a n sfor m a tion of n om in a l va r ia bles.
• Re gr ession coefficien t: slope of th e estim a ted fu n ction .
• P r ediction va lu es: estim a ted va lu es
th r ou gh m u ltiple r e gr ession .
• Adju sted va lu es: va lu es a dju sted w ith sign ifica n t con fou n der s in m u ltiple r e gr ession .
• F-va lu e: for decision wh eth er a pa r a m eter
is sign ifica n t (m u ltiple r e gr ession a n d
a n a lysis of va r ia n ce).
• Tr ea tm en t: oper a tion a l pa r a m eter for
a n a lysis of va r ia n ce.
Gradual analysis o f c o varianc e
If it is n ecessa r y to test th e in fl u en ce of th e
con fou n der, too, a gr a du a l a n a lysis of cova r ia n ce ca n be u sed[14]. In th e fi r st step th e m u ltiple r e gr ession a n d a dju stm en t wer e con sider ed. In th e secon d step th e in fl u en ce of oper a tion a l pa r a m eter s w a s tested.
First step: multiple regression and
adjustment
In th e fir st step of th e sta tistica l a n a lysis a
sign ifica n t in flu en ce on th e depen den t va lu es
(COD differ en ces a n d ga s pr odu ction ) w a s
tested w ith th e possible con fou n der s. Th e
lim it for sta tistica l er r or (α) w a s 5 per cen t, so
th a t sta tistica l r elia bility (S ) (equ a tion (1))
w a s a t lea st 95 per cen t. Th e objective w a s to
select on ly th e im por ta n t con fou n der s.
(1)
S = (1 – α) * 100 per cen t
Th e or igin of th e w a stew a ter (th r ee ta n n er ies), feed flow, th e ch em ica l pa r a m eter s in th e
in flu en t su ch a s COD, su lph a te, su lph ide,
ch lor ide, a n d Cr tot wer e tested a s possible
con fou n der s. Th e gen er a l r esu lt of m u ltiple
r e gr ession is equ a tion (2):
(2)
Y * = g1 • X 1 + g2 • X 2 + … + gn • X n + a
wh er e:
Y * = pr edicted, depen den t va lu e;
g 1 = r e gr ession coefficien t of th e
con fou n der w ith j = 1…n ;
X j = m ea su r ed va lu e of th e con fou n der
w ith j = 1…n ;
a = con sta n t ter m .
Adjustme nt
Th e a dju stm en t of th e sign ifi ca n t con fou n der
is pa r a llelized by equ a tion (3).
Yˆ = (Y – Y *) + y–
(3)
wh er e:
[ 30 ]
Yˆ = a dju sted, depen den t va lu e;
Y = m ea su r ed, depen den t va lu e;
Y * = pr edicted, depen den t va lu e of a ll sign ifi ca n t con fou n der ;
y = m ea n va lu e.
Lin ea r r ela tion s wer e a scer ta in ed a n d th e
a dju sted m ea su r ed va lu es (COD differ en ces
or ga s pr odu ction ) wer e ca lcu la ted by m ea n s
of m u ltiple r e gr ession . For th is a dju stm en t,
a ll fa ctor s for wh ich a sign ifica n t con n ection
w ith th e depen den t cou ld be dem on str a ted,
wer e ta k en in to a ccou n t.
Second step: analysis of variance
In th e secon d step, th e sign ifica n t in flu en ce of
th e exper im en ta l oper a tion s wer e tested w ith
a dju sted, depen den t va lu es to ca lcu la te th e Fva lu e w ith equ a tion (4). E qu a tion (4) is iden tica l w ith th e cla ssic a n a lysis of cova r ia n ce.
Th e th r ee pilot pla n ts a n d FeCl3 dosa ge wer e
in vestiga ted a s pa r a m eter s for tr ea tm en t. Th e
equ a tion u sed for va r ia n ce tests th e sign ifi ca n t differ en ce:
n –h
R 2 yx ;1, 2, 3…n
F =
•
( 4)
h – 1 1 – R 2 yx ;1, 2, 3…n
wh er e:
F = obser ved F-va lu e;
n = sa m ple size;
h = n u m ber of du m m y va r ia bles;
R 2 = squ a r e of m u ltiple cor r ela tion coefficien t;
Yˆ = a dju sted va lu e.
Th e two steps togeth er cor r espon d to cla ssic
a n a lysis of cova r ia n ce. As lon g a s con fou n der
a n d tr ea tm en t a r e in depen den t a n d th e size of
th e sa m ple is big en ou gh , th e r esu lts of th e
two m eth ods a r e iden tica l. In th is in vestiga tion , both ter m s wer e fu lfilled beca u se th e
n u m ber of con fou n der s w a s sm a ll a n d th e
sa m ple size in th is lon g-ter m stu dy w a s qu ite
la r ge.
An a lysis of va r ia n ce w a s fu r th er ca r r ied
ou t w ith th e or igin a l m ea su r ed va lu es w ith ou t a dju stm en t. Both r esu lts cou ld th u s be
com pa r ed, wh ich m a de it possible to ch eck
wh eth er th e in flu en ce of a tr ea tm en t h a d
been obscu r ed by th e a dju stm en t of th e m ea su r ed va lu es to th e con fou n der. In ou r exper im en ts th is w a s n ever th e ca se.
Results and discussion
Statistical analysis procedures
In a lon g-ter m stu dy w a stew a ter s of th r ee
ta n n er ies wer e in vestiga ted a n d u sed for
a n a er obic tr ea tm en t. In Ta ble I th e ch em ica l
qu a lity of th e w a stew a ter is given for COD,
su lph ide, su lph a te, ch lor ide a n d ch r om iu m .
E a ch va lu e r epr esen ts a five-day com posite
Elke Ge nsc ho w,
We rne r He ge mann and
Christian Masc hke
Anae ro bic tre atme nt o f
tanne ry waste wate r: to xic
e ffe c ts o f waste wate r
c o nstitue nts and do sage o f
fe rric c hlo ride
Enviro nme ntal Manage me nt
and He alth
8 / 1 [1 9 9 7 ] 2 8 –3 8
sa m ple. Min im a l, m a xim a l a n d m ea n va lu es
a n d th e sta n da r d devia tion a r e given . A sta tistica l a n a lysis w a s n ecessa r y du e to th e h igh
sta n da r d devia tion of a ll pa r a m eter s.
Usin g th ese sta tistica l m eth ods th e follow in g qu estion s ca n be a n swer ed:
• Wh ich w a stew a ter con stitu en ts in fl u en ced
COD r em ova l a n d ga s pr odu ction ?
• Is COD r em ova l a n d ga s pr odu ction depen den t on th e or igin of th e ta n n er y w a stew a ter ?
• Wer e th e pilot pla n ts equ a l in COD r em ova l
a n d in ga s pr odu ction ?
• Is th er e a sign ifica n t positive in fl u en ce of
fer r ic ch lor ide dosa ge on COD r em ova l a n d
ga s pr odu ction ?
Th e cr iter ion u sed to eva lu a te th e volu m e of
ga s w a s th e volu m e of ga s pr odu ced per week ,
wh ich w a s u sed a s a depen den t va lu e (Y *).
Th e COD r em ova l w a s ca lcu la ted by deter m in in g COD differ en ces. With r e ga r d to th e
m ea su r ed va lu es, on ly th e differ en ces
between th e in flu en t a n d th e fir st sta ge a n d
between th e fir st a n d secon d sta ges wer e
ca lcu la ted. Th ese differ en ces wer e u sed a s
depen den t va lu es. Th is pr ocedu r e fu r th er
pr ovided in for m a tion a bou t r em ova l.
It sh ou ld be n oted th a t in th e ca se of COD
differ en ces, m ea su r ed va lu es of ta n n er ies A,
B, a n d C wer e ava ila ble, wh ile th e ga s volu m e
w a s m ea su r ed on ly in exper im en ts w ith
w a stew a ter of ta n n er ies B a n d C. Th e m ea su r ed va lu e r a n ge of in fl u en ce fa ctor s a lso
va r ied for th is r ea son .
Th e objective w a s to obta in in for m a tion
a bou t oper a tion a l pa r a m eter s su ch a s th e
sim ila r ity of pilot pla n ts a n d th e in flu en ce of
fer r ic ch lor ide on th e depen den t va lu es.
Th er efor e, it w a s n ecessa r y fi r st to cor r ect
th e in flu en ce of th e or igin m a th em a tica lly,
th e feed flow a n d th e qu a lity of w a stew a ter on
COD r em ova l a n d ga s pr odu ction . Th e pr ocedu r e w a s sepa r a ted in to th r ee steps.
Th e r esu lt of th e fir st step is a r e gr ession
fu n ction w ith a specific slope for ever y sign ifica n t con fou n der. In clu ded in th e fir st step is
th e a dju stm en t of th e depen den t va r ia ble.
Th e COD differ en ces a n d th e va lu es of ga s
l–1 )
COD (mg
Sulphide (mg l–1 )
Sulphate (mg l–1 )
Chloride (mg l–1 )
Chromium (mg l–1 )
Number
467 a
413 a
355 a
255 a
319 a
M ultiple regression of gas production
In Ta ble II th e sta tistica l r elia bility of th e
con fou n der s is given .Th e or igin of th e w a stew a ter (th r ee ta n n er ies), feed flow, th e ch em ica l pa r a m eter s in th e in fl u en t su ch a s su lph a te, su lph ide a n d Cr tot h a d n o sign ifica n t
in fl u en ce on ga s pr odu ction . Two
con fou n der s wer e fou n d th a t sign ifica n tly
in fl u en ced th e volu m e of ga s.
Th e r e gr ession fu n ction (5) in clu des a ll
sign ifica n t con fou n der s a n d is given below.
E a ch con fou n der h a s its ow n specific slope.
Y * = 0.001 X 1 – 0.667 X 2 + a
M inimum
861
0
67
0
0.4
M aximum
30,500
1,600
3,590
11,800
837
SD
M ean value
4,510
187
794
3,140
88
5,710
196
1,180
5,060
36
Note:
a does not c orrespond to the number of samples taken from tanneries A, B and C
(5)
wh er e:
Y * = estim a te of ga s pr odu ction (on e
week –1)
X 1 = COD (m g l –1) 861 < X 1 < 16,800;
–
X 1 = 4,750 S > 99,9 per cen t
X 2 = ch lor ide (m g l –1) 0.0 < X 2 < 12,800
X 2 = 5,040 S = 99.4 per cen t
a = con sta n t ter m a = 8.64
In equ a tion (5) th e r a n ge of COD a n d ch lor ide
in th e in fl u en t a r e given . Seven ty-fi ve per cen t
of th e va lu es for COD in th e in fl u en t a r e in
Table II
Statistic al re liability o f c o nfo unde rs o n gas
pro duc tio n and o n COD diffe re nc e s (multiple
re gre ssio n)
Confounders
Table I
Charac te ristic s o f tanne ry waste wate r
Parameter
wer e con ver ted in to a dju sted va lu es w ith th e
m ea n va lu e of ea ch sign ifica n t con fou n der in
m u ltiple r e gr ession .
In th e secon d step th e in flu en ce of th e oper a tion a l pa r a m eter s on th e depen den t va lu es
w a s tested (a n a lysis of va r ia n ce). Sta tistica l
r elia bility (S ) between 95 per cen t a n d 99 per
cen t is n a m ed sign ifica n t, wh er ea s sta tistica l
r elia bility > 99 per cen t is ter m ed h igh ly
sign ifica n t. Appr oxim a tely 250 va lu es of ea ch
pa r a m eter, ea ch va lu e cor r espon din g to a fi veday com posite sa m ple, wer e in clu ded in th e
ca lcu la tion a n d th er eby exa m in ed for th eir
in fl u en ce on COD differ en ces a n d ga s pr odu ction .
Statistical reliability
Gas production COD differences
(%)
(%)
Tannery (A, B, C)
Feed flow (1 week–1 )
COD
(mg l–1 ) a
Sulphide (mg l–1 ) a
Sulphate (mg l–1 ) a
Chloride (mg l–1 ) a
Chromium (mg l–1 ) a
ns
ns
>99.9
ns
ns
99.4
ns
95.3
ns
>99.9
ns
95.3
ns
ns
Notes:
in the influent
ns: no significanc e
a:
[ 31 ]
Elke Ge nsc ho w,
We rne r He ge mann and
Christian Masc hke
Anae ro bic tre atme nt o f
tanne ry waste wate r: to xic
e ffe c ts o f waste wate r
c o nstitue nts and do sage o f
fe rric c hlo ride
Enviro nme ntal Manage me nt
and He alth
8 / 1 [1 9 9 7 ] 2 8 –3 8
th e r a n ge between 900 a n d 9,000g l –1. N ea r ly
a ll va lu es (95 per cen t) a r e in th e r e gion
between 900 a n d 15,000m g l –1. Seven ty-five per
cen t of th e va lu es for ch lor ide wer e fou n d to
be between 0 a n d 8,000m g l –1 a n d 95 per cen t of
th e va lu es lay between 0 a n d 11,000m g l –1. As a
con sequ en ce COD va lu es h igh er th a n
15,000m g l –1 a n d ch lor ide va lu es > 11,000m g l –1
n eed n ot be con sider ed for pr a ctica l oper a tion .
With th e a bove equ a tion , th e estim a ted ga s
pr odu ction w ith m ea n feed flow (24.2 l) a n d
–
m ea n con cen tr a tion of COD (X 1) a n d ch lor ide
–
(X 2 ) ca n be deter m in ed. E stim a tes of ga s
pr odu ction lay between 120 a n d 50 l k g -1 COD .
o
It r ea ch ed a m ea n va lu e of a bou t 75 l k g -1COD
o
wh ich ca n be ju dged a s r a th er low.
COD ( X 1 )
Th e depen den ce of ga s volu m e (Y *) on COD
con cen tr a tion in th e in flu en t (X 1) w a s deter m in ed a s h avin g ver y h igh sta tistica l r elia bility (S > 99.9 per cen t). High er con cen tr a tion s
of COD cor r espon ded to a n in cr ea se in ga s
volu m e. E qu a tion (5) w a s deter m in ed w ith in
th e m ea su r in g r a n ge of th e con fou n der a n d is
va lid on ly for th is r a n ge. With m u ltiple
r e gr ession it w a s possible to r ea ssu r e th is
well-k n ow n fa ct.
Chloride ( X 2 )
By m ea n s of m u ltiple r e gr ession a con n ection
w ith h igh sta tistica l r elia bility (99.4 per cen t)
between ch lor ide con cen tr a tion (X 2 ) a n d ga s
pr odu ction w a s fou n d.
A gr a ph ic r epr esen ta tion w ith sim u lta n eou s
va r ia tion of th e two sign ifi ca n t con fou n der s
ca n n ot ser ve a s a cla r ifyin g illu str a tion . On ly
by k eepin g on e con fou n der con sta n t, ca n a
gr a ph ic estim a te fu n ction for th e oth er (0.0 <
X 2 < 12,800m g l –1 ch lor ide) pr odu ce in telligible gr a ph ic r esu lts. It is a dva n ta geou s to u se
th e m ea n va lu e (X 1 = 4,750 m g l –1) a s a con sta n t con fou n der, beca u se it is th e best estim a te. F igu r e 2 r epr esen ts th is set-u p.
Ch lor ide sh owed a n e ga tive in flu en ce on
ga s pr odu ction (Y *), so th a t on e ca n spea k of
a n in h ibitor y effect. N o su ch in flu en ce of
ch lor ide w a s fou n d on COD r em ova l.Th e
in h ibitor y effect of ch lor ide on m eth a n e ba cter ia ca u sin g lower ga s pr odu ction w a s fou n d
for con cen tr a tion s > 10g l –1 ch lor ide (t R = 9 d )
in syn th etic w a stew a ter [15]. Bu t a n in h ibitor y effect h a s a lso been detected for lower
con cen tr a tion s[16].
Wh y ga s pr odu ction is in h ibited wh ile COD
r em ova l is n ot a ffected in th is exper im en t
m ay be expla in ed a s follow s. On e possible
expla n a tion lies in th e in h ibitor y effect of
ch lor ide on m eth a n e ba cter ia , wh ich r edu ce
less COD a t h igh er con cen tr a tion s of ch lor ide
a n d th u s pr odu ce sm a ller a m ou n ts of bioga s[16], wh ile th e con cen tr a tion of ch lor ide
does n ot n e ga tively a ffect th e a ctivity of th e
su lph a te-r edu cin g ba cter ia . It is k n ow n th a t
som e su lph a te-r edu cin g ba cter ia r equ ir e
N a Cl con cen tr a tion s between 4 a n d 20 g l –1[17]
or between 0.5 a n d 5g l –1[9]. Th e a m ou n t of
con ver ted or ga n ic m a tter in su lph a te
Figure 2
Influe nc e o f c hlo ride o n gas vo lume (multiple re gre ssio n)
[ 32 ]
Elke Ge nsc ho w,
We rne r He ge mann and
Christian Masc hke
Anae ro bic tre atme nt o f
tanne ry waste wate r: to xic
e ffe c ts o f waste wate r
c o nstitue nts and do sage o f
fe rric c hlo ride
Enviro nme ntal Manage me nt
and He alth
8 / 1 [1 9 9 7 ] 2 8 –3 8
r edu ction (th eor etica lly 530m g COD for 1g
r edu ced su lph a te) a lm ost r ea ch ed 1g COD per
g r edu ced su lph a te in pr a ctica l exper im en ts
w ith m in e w a ter [18]. Given a u n ila ter a l
in h ibitor y effect of h igh ch lor ide con cen tr a tion s on m eth a n e ba cter ia , th is wou ld lea d to
r edu ced ga s pr odu ction wh ile n ot n ecessa r ily
ca u sin g a decr ea se in COD r em ova l efficien cy.
M ultiple regression of COD efficiency
In Ta ble II th e sta tistica l r elia bility of th e
con fou n der s con cer n in g COD differ en ces is
a lso given . Feed fl ow, th e ch em ica l pa r a m eter s in th e in flu en t su ch a s su lph ide, ch lor ide
a n d Cr tot h a d n o sign ifi ca n t in fl u en ce on COD
r em ova l. In con tr a st to th e fin din gs w ith ga s
volu m e, th r ee con fou n der s wer e fou n d wh ich
sign ifica n tly in flu en ced COD differ en ces a n d
COD r em ova l r espectively. As a r esu lt, a
r e gr ession fu n ction (equ a tion 6) in clu din g a ll
sign ifica n t con fou n der s (COD, su lph a te, a n d
ta n n er ies) ca n be given w ith th eir specifi c
slopes:
Y * = 0.427 X 1 – 0.294 X 2 – 412 X 3 + a
(6)
wh er e:
Y * = estim a tion for COD differ en ces in
m g l –1.
–
X 1 = COD (m g l –1) 861 < X 1 < 30,500 X 1 =
–1
5,710m g l S > 99.9 per cen t
–
X 2 = su lph a te (m g l –1) 162 < X 2 < 3,590 X 2
–1
= 1,180m g l S = 95.3 per cen t
X 3 = ta n n er y A a n d X 3 = 0
ta n n er y B X 3 = 1
a = con sta n t ter m , a = 5.60
E qu a tion (6) cou n ts for th e fir st a n d th e secon d sta ge so th a t a fa ctor of 2 is n ecessa r y in
or der to ca lcu la te COD r em ova l in both sta ges
togeth er. Th e sa m e pr ocedu r e a s for ch lor ide
w a s u sed to display th is r esu lt.
In equ a tion (6) th e r a n ge of COD a n d su lph a te in th e in fl u en t a r e given . Seven ty-five
per cen t of th e COD va lu es in th e in flu en t a r e
in th e r a n ge between 900 a n d 10,000g l –1.
N in ety-five per cen t of th e va lu es a r e in th e
r e gion between 900 a n d 15,000m g l –1. Seven tyfive per cen t of su lph a te va lu es r a n ged
between 200 a n d 1,500m g l –1 a n d n ea r ly a ll (95
per cen t) lay between 160 a n d 2,900m g l –1. As a
con sequ en ce, va lu es of COD h igh er th a n
15,000m g l –1 a n d va lu es of su lph a te >
2,900m g l –1 n eed n ot be con sider ed for pr a ctica l oper a tion .
COD ( X 1 )
A h igh ly sign ifi ca n t cor r ela tion (S > 99.9 per
cen t) w a s fou n d between COD con cen tr a tion s
in th e in flu en t (X 1) a n d COD differ en ces (Y *).
As for ga s volu m e, h igh er COD con cen tr a tion s
in th e in flu en t led to h igh er COD differ en ces
or h igh er COD r em ova l r espectively. Th is is a
well-k n ow n fa ct. In F igu r e 3 on ly th e
depen den ce of COD differ en ces on COD in th e
in fl u en t (X 1) is sh ow n , k eepin g con sta n t th e
–
m ea n va lu e of su lph a te (X 2 = 1,180 m g l –1) a n d
ta n n er ies A a n d C (X 3 = 0). (Th e u se of X 3 w ill
be expla in ed below.)
Wh en COD differ en ces for th e COD va lu es
in th e in fl u en t (X 1) a r e ca lcu la ted by m ea n s of
th e equ a tion (6) cited a bove, on e ca n a scer ta in th e r em ova l in per cen t. It lay between 70
a n d 80 per cen t for h igh er COD con cen tr a tion s in th e in flu en t (5,000-30,000m g l –1). With
lower COD va lu es in th e in fl u en t (X 1), h igh ly
r edu ced r em ova l w a s fou n d.
As a con sequ en ce, a n a er obic tr ea tm en t
sh ou ld be u sed on ly for h igh ly pollu ted ta n n er y w a stew a ter, beca u se th e r em ova l efficien cy for low COD con cen tr a tion s is u n sa tisfa ctor y.
Sulphate ( X 2 )
With a sta tistica l r elia bility of 95.3 per cen t,
su lph a te con cen tr a tion in th e in flu en t (X 2)
sh owed a n e ga tive in flu en ce on COD differ en ces (Y *). E qu a tion (6) sh ow s th is con n ection . Th e sa m e pr ocedu r e a s for COD (X 1) w a s
u sed for displayin g th e in flu en ce of su lph a te
on COD r em ova l. F igu r e 4 r epr esen ts th is setu p.
–
With m ea n va lu es of su lph a te (X 2 = 1,180m g
–
l –1) a n d COD (X 1 = 5,710m g l –1) in ta n n er y
w a stew a ter (A a n d C), a ppr oxim a tely 70 per
cen t of th e COD cou ld be elim in a ted. With
in cr ea sin g su lph a te con cen tr a tion s in th e
in fl u en t u p to a m a xim u m va lu e of 2,900m g l –1
su lph a te (95 per cen t of th e va lu es); COD
r em ova l w a s estim a ted to be a bou t 55 per
cen t.
Th e in h ibitor y effect of su lph a te on COD
r em ova l cou ld be expla in ed by th e fa ct th a t
m eth a n e ba cter ia con ver t less COD a t h igh er
levels of su lph a te con cen tr a tion , wh er ea s
su lph a te-r edu cin g ba cter ia th eor etica lly
sh ou ld con su m e m or e COD a t th e sa m e tim e.
With h igh su lph a te con cen tr a tion th er e is
a ppa r en tly n o com plete su lph a te r edu ction ,
so th a t th e r edu ced COD r em ova l m ay r esu lt
fr om th e lower r em ova l r a te of th e m eth a n e
ba cter ia a n d su lph a te-r edu cin g ba cter ia .
Tanneries ( X 3 )
Th e or igin of th e w a stew a ter (X 3) w a s fou n d
to in fl u en ce COD differ en ces sign ifica n tly.
Th ese fin din gs differ ed fr om th ose for volu m e
of ga s, for wh ich a n in flu en ce of th e or igin of
th e w a stew a ter cou ld n ot be fou n d. For th e
COD differ en ces (Y *), exper im en ts w ith
w a stew a ter fr om ta n n er ies A, B a n d C (X 3)
wer e con sider ed, wh ile on ly th e va lu es of
ta n n er ies B a n d C wer e in clu ded for deter m in in g th e volu m e of ga s.
Ta ble III depicts th e in flu en ce of th e r espective ta n n er y on COD differ en ces. Sin ce th e
[ 33 ]
Elke Ge nsc ho w,
We rne r He ge mann and
Christian Masc hke
Anae ro bic tre atme nt o f
tanne ry waste wate r: to xic
e ffe c ts o f waste wate r
c o nstitue nts and do sage o f
fe rric c hlo ride
Figure 3
Influe nc e o f COD in the influe nt o n COD re mo val (multiple re gre ssio n)
Enviro nme ntal Manage me nt
and He alth
8 / 1 [1 9 9 7 ] 2 8 –3 8
Figure 4
Influe nc e o f sulphate in the influe nt o n COD re mo val (multiple re gre ssio n)
[ 34 ]
Elke Ge nsc ho w,
We rne r He ge mann and
Christian Masc hke
Anae ro bic tre atme nt o f
tanne ry waste wate r: to xic
e ffe c ts o f waste wate r
c o nstitue nts and do sage o f
fe rric c hlo ride
Enviro nme ntal Manage me nt
and He alth
8 / 1 [1 9 9 7 ] 2 8 –3 8
Table III
Diffe re nc e s o f COD re mo val in the thre e
tanne rie s (multiple re gre ssio n)
Tanneries
Statistical reliability (%)
Y* : COD difference (mg l–1 )
COD removal (%) a
A
B
C
95.3
4,200
72
95.3
3,400
60
95.3
4,200
72
Note:
–
–
With COD X 1 = 5,710 mg/ l, sulphate X 2 = 1,180mg/ l
a
ta n n er ies r epr esen t n om in a l va r ia bles, m u ltiple r e gr ession r equ ir ed du m m y codin g, i. e., a
tr a n sfor m a tion in to qu a n tita tive en tities
w ith a va lu e of “0” or “1”. With th is oper a tion ,
ta n n er y B beca m e sign ifica n t, so th a t it w a s
a ttr ibu ted th e n u m er ica l va lu e “1” in equ a tion (6), wh ile ta n n er ies A a n d C wer e
a ccor ded a “0”. On th e ba sis of th is codin g
a n d th e m ea n va lu es for COD a n d su lph a te
con cen tr a tion s, COD differ en ces wer e
4,200m g l –1 for ta n n er ies A a n d C, bu t on ly
3,400m g l –1 for ta n n er y B.
Ma n y differ en t su bsta n ces a r e u sed in
lea th er pr odu ction , so th a t, even th ou gh
ch r om iu m -ta n n in g, lea th er -pr odu cin g fa ctor ies in pr in ciple follow th e Reu tlin ger
m odel[19], both th e qu a n tity a n d qu a lity of
th e w a stew a ter differ con sider a bly between
fa ctor ies[20]. It is th u s pla u sible th a t on e
sh ou ld fi n d differ en ces in COD r em ova l.
Th ese sign ifica n t differ en ces ca n be ta k en a s
a h in t th a t fu r th er su bsta n ces in ta n n er y
w a stew a ter h ave a n in flu en ce on th e a n a er obic pr ocess.
Analysis of variance
Pilo t plants
In th e in vestiga tion s, th r ee sim ila r pilot
pla n ts wer e oper a ted in pa r a llel w ith differ en t oper a tion a l pa r a m eter s. An a n a lysis of
va r ia n ce m a de it possible to test th e sim ila r ities in th e volu m e of ga s pr odu ced a n d in
COD r em ova l. A sign ifica n t differ en ce
between th e pilot pla n ts w a s fou n d for COD
r em ova l bu t n ot for ga s volu m e.
In F igu r e 5 th e COD differ en ces (r aw a n d
a dju sted va lu es) a r e for ea ch pilot pla n t. With
sta tistica l r elia bility given a t S = 98.2 per cen t
for r aw va lu es a n d a t S = 99.6 per cen t for
estim a ted va lu es pilot pla n ts va r ied sign ifi ca n tly. By a dju stin g th e m ea su r ed va lu es th e
va r ia n ce-a n a lytica l effects r ea ch ed a h igh er
de gr ee of tr a n sfer a bility to oth er ca ses.
An a lyses of va r ia n ce wer e ca r r ied ou t w ith
th e or igin a l m ea su r ed va lu es w ith ou t a dju stm en t, in or der to ch eck if th e in flu en ce of a
tr ea tm en t h a d been obscu r ed by th e a dju stm en t. However, n o su ch in flu en ce w a s fou n d.
Figure 5
Diffe re nc e o f COD re mo val in the thre e pilo t plants (analysis o f varianc e )
[ 35 ]
Elke Ge nsc ho w,
We rne r He ge mann and
Christian Masc hke
Anae ro bic tre atme nt o f
tanne ry waste wate r: to xic
e ffe c ts o f waste wate r
c o nstitue nts and do sage o f
fe rric c hlo ride
Enviro nme ntal Manage me nt
and He alth
8 / 1 [1 9 9 7 ] 2 8 –3 8
In or der to eva lu a te th ese differ en ces w ith
r e ga r d to r eleva n ce for pr a ctica l oper a tion
th e estim a ted COD r em ova l of th e th r ee pilot
pla n ts is given for th e m ea n va lu e of
COD = 5,710m g l –1. P ilot pla n t 1 r ea ch ed a
COD r em ova l of 59 per cen t, pilot pla n t 2
a ch ieved a COD r em ova l of 65 per cen t a n d
pilot pla n t 3 obta in ed th e h igh est COD elim in a tion w ith 69 per cen t for a dju sted va lu es.
In a n a er obic tr ea tm en t pla n ts differ en t
biocen oses of ba cter ia m ay esta blish th em selves depen din g on feed fl ow, deten tion tim e
of w a stew a ter, tem per a tu r e, a n d pH va lu e[21].
Table IV
Influe nc e o f fe rric c hlo ride o n gas vo lume (analysis o f varianc e )
Ferric chloride dosage (g l–1 )
Volume of the gas
Statistical reliability
Gas volume
Gas production
Gas volume
0
1
2
Adjusted values
(%)
(1 week–1 )
(1kg–1 CODo ) a
(%) b
99.9
14.6
127
100
99.9
9.7
84.3
66
99.9
8.7
75.7
60
0
1
Raw values
99.9
14.3
124
100
Notes:
a with mean value of COD = 4,750mg l–1 , feed flow = 24.2 1 week–1
o
b related to 0g l–1 = 100 per c ent
99.9
6.3
54.8
44
2
99.9
8.8
76.5
62
On ly a sign ifica n t differ en ce in COD r em ova l
w a s fou n d, wh er ea s n on e cou ld be deter m in ed in ga s pr odu ction a n d desu lph u r iza tion . COD r em ova l is m a in ly ca u sed by th r ee
pr ocesses: a cidifi ca tion , desu lph u r iza tion
a n d pr odu ction of m eth a n e. Su lph a te-r edu cin g ba cter ia a n d m eth a n e ba cter ia in th e
th r ee pilot pla n ts pr oba bly did n ot r edu ce
sign ifica n tly differ en t a m ou n ts of COD, bu t
th e su m of COD r em ova l in a ll pr ocesses
togeth er w a s sign ifica n t in th e pilot pla n ts.
Fe Cl3
Th e dosa ge of fer r ic ch lor ide w a s in ten ded to
elim in a te su lph ide a n d th u s to pr even t its
in h ibitor y effect. Wh ile th e dosa ge of FeCl 3
sh owed a sign ifica n t in fl u en ce on ga s pr odu ction , n o su ch in flu en ce w a s n oted on COD.
In or der to eva lu a te th e r esu lts of th e a n a lysis, sta tistica l r elia bility a s well a s th e over a ll
r eleva n ce of th e r esu lts w a s con sider ed (Ta ble
IV). For r aw va lu es a n d for a dju sted va lu es
th e sa m e sta tistica l r elia bility w a s fou n d (S =
99.9 per cen t).
A dosa ge of 2g l –1 fer r ic ch lor ide r esu lted in
a 40 per cen t r edu ction of ga s pr odu ction .
E ven h a lf th e a m ou n t of fer r ic ch lor ide
effected a 34 per cen t r edu ction , a r esu lt
wh ich m u st be con sider ed r eleva n t (F igu r e 6).
Figure 6
Influe nc e o f the do sage o f fe rric c hlo ride o n gas vo lume (analysis o f varianc e )
[ 36 ]
Elke Ge nsc ho w,
We rne r He ge mann and
Christian Masc hke
Anae ro bic tre atme nt o f
tanne ry waste wate r: to xic
e ffe c ts o f waste wate r
c o nstitue nts and do sage o f
fe rric c hlo ride
Enviro nme ntal Manage me nt
and He alth
8 / 1 [1 9 9 7 ] 2 8 –3 8
A pla u sible r ea son for th e decr ea se of ga s
pr odu ction th r ou gh fer r ic ch lor ide (r e ga r din g on ly th e r aw va lu es) lies in th e fu r th er
in cr ea se of ch lor ide con cen tr a tion (F igu r e 2),
sin ce SRB a r e less sen sitive to ch lor ide th a n
m eth a n e ba cter ia . However, in th e a dju sted
va lu es, th e in fl u en ce of ch lor ide w a s r em oved
by m u ltiple r e gr ession a n d a dju stm en t.
A possible expla n a tion for th e lower ga s
yield is th e pr ecipita tion of or ga n ic m a tter,
wh ich pr odu ces slu dge. Th is slu dge is a n
u n welcom e side-effect of th e a ddition of
FeCl 3. Ow in g to pr ecipita tion , th e su bsta n ce
is th en n o lon ger ava ila ble for a n a er obic
elim in a tion a n d ga s pr odu ction , bu t its
a bsen ce cou n ts a s COD r em ova l efficien cy. In
ba tch exper im en ts a n ota ble COD r edu ction
w a s obser ved wh en fer r ic ch lor ide w a s u sed
a s a pr ecipita tin g a gen t[9]. Sin ce a positive
effect of fer r ic ch lor ide dosa ge cou ld n ot be
ver ified, on e sh ou ld r efr a in fr om a ddin g it to
ta n n er y w a stew a ter.
Conclusions
In a n a er obic tr ea tm en t of ta n n er y w a stew a ter in h ibitor y effects of su lph a te a n d ch lor ide wer e fou n d. In or der to r ea ch better COD
efficien cy or a decr ea se in deten tion tim e,
ta n n er ies sh ou ld ch a n ge pr odu ction m eth ods.
In or der pa r tly to su bstitu te ch lor ide, on e ca n
u se fr esh or on ly cooled h ides in stea d of
sa lted on es. In Ger m a n y today som e ta n n er ies u se u p to 60 per cen t fr esh h ides[22] a n d
th u s disch a r ge r a th er low a m ou n ts of ch lor ide in th e w a stew a ter.
With r e ga r d to su lph a te, on ly a ch a n ge in
th e lea th er pr odu ction pr ocess itself wou ld
yield better r esu lts. In or der pa r tly to su bstitu te su lph u r ic a cid it is possible to u se CO 2
for delim in g. For th in h ides a tota l con ver sion is possible, for n or m a l h ides on ly a pa r tia l su bstitu tion of a m m on ia su lph a te is fea sible[22]. Th is com bin a tion effects a r edu ction
in a m m on ia a n d in su lph a te.
An a er obic tr ea tm en t of ta n n er y w a stew a ter
is su ita ble especia lly for h igh con cen tr a tion s
of or ga n ic com pou n ds. Th e COD r em ova l for
COD > 5,000m g l –1 lay a t 70-80 per cen t bu t
decr ea sed for lower COD in th e in flu en t.
Th e COD r em ova l in th e ta n n er ies A a n d C
w a s sign ifica n tly differ en t fr om ta n n er y B.
Th is m ea n s th a t even if on e k n ow s th e fa ctor s
th a t sign ifica n tly in flu en ce COD r em ova l
(COD o a n d con cen tr a tion of su lph a te) th e
COD r em ova l of a n u n k n ow n w a stew a ter ca n
differ fr om th e va lu e th a t is r epor ted h er e.
Th is m ea n s th a t befor e tr a n sfer r in g th ese
r esu lts to oth er tr ea tm en t pla n ts, pilot tests
a r e n ecessa r y.
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Anae ro bic tre atme nt o f
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e ffe c ts o f waste wate r
c o nstitue nts and do sage o f
fe rric c hlo ride
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8 / 1 [1 9 9 7 ] 2 8 –3 8
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