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War. Res. Vo l. 25, No. 12, pp. 1549-1554, 1991 0043-1354/91 $3.00 + 0.00Printed in Great Britain. All rights reserved Copy right © 1991 Pergamo n Press pie

D E G R A D A T I O N O F AC E T IC A N D P R O P I O N I C A C ID S I N

T H E M E T H A N E F E R M E N T A T I O N

A. J . M AWSON, R . L . EARLE* a n d V. F . LARSEN'["

D e p a r t m e n t o f B i o t e c h n o lo g y , M a s s e y U n i v e rs i ty , P a l m e r s to n N o r t h , N e w Z e a l a n d

First received Ma rch 1990; accepted in revised for m M ay 1991)

A b s t r a c t - - A se r ie s o f ba t c h d iges t i on expe r im en t s w as pe r fo rm e d to e va lua t e t he e f fec t o f e leva t ed ace t icac id concen t r a t i ons on t he degrada t i on o f p rop ion i c ac id and v i ce ve r sa . F l a sk d iges t e r s w ere i ncuba t eda t 37°C w i th ace t i c and p rop ion i c ac ids added a t up t o 2000 an d 1500 m g 1 ~ , re spect ive ly . I t w as fou ndtha t i nc reas ing t he co ncen t r a t i on o f e i t he r ac id f rom low l eve ls r educed t he r a t e o f i t s u t i l i za t i on and t h a ti nc reas ing t he ace t i c ac id con cen t r a t i on f rom 1000 t o 2000 m g 1 -~ s ign i f i can tl y i nh ib i t ed deg rada t i on o fp rop io n i c ac id added a t 500 m g I-~. G oo d ag reem en t w as ob t a ine d f rom f i t t i ng t he l og i st i c equ a t i on t ot h e m e a s u r e d d a t a a n d b e t w e e n d u p l ic a t e r u n s p e r f o r m e d w i t h in t h e e x p e r i m e n ts . T h e o b s e r v e d i n h i b i t i o nof p rop io na t e de grad a t i on b y ace t a t e conf i rm s seve ra l ea r l i e r r epor t s o f p rod uc t i nh ib i t i on i n t he l i t e r a tu re

and em phas i zes t he im por t ance o f con t ro l l i ng ac id l eve l s i n m a in t a in ing sa t i s f ac to ry d iges t e r ope ra t i on .

Key words- -acet ic ac id , p rop ion i c ac id , V F A , anae rob i c d iges t i on , m e thane f e rm en ta t i on , i nh ib i t i on

NOMENCLATURE

A ci = i n i t i a l ace ti c ac id conc en t r a t i on (m g 1-1

A c t o r , i = i n i t i a l po t en t i a l ace t i c ac id p oo l (m g 1 - ~P r~ = i n i t i a l p rop ion i c ac id con cen t r a t i on (m g 1 - ~

U V F A = un- ion i zed vo l a t i l e f a t t y ac idsV F A = vo l a t i l e f a t t y ac ids

INTRODUCTION

A c e t i c a n d p r o p i o n i c a c i d s a r e th e m a j o r v o l a t il e f a tt y

a c id s ( V F A s ) p r e s e n t d u r i n g a n a e r o b i c d i g e s t io n a n d

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

d i g e s t e r p e r f o r m a n c e . L o w a c i d l e ve l s i n d i c a t e s t a b l e

o p e r a t i o n w h i l e h i g h a c i d c o n c e n t r a t i o n s a r e i n v a ri -

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

B l o o d g o o d , 1 9 63 ; H i l l et a l . , 1 9 8 7 ) . P r o p i o n i c a c i d

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

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

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

n o t e d ( P o h l a n d a n d B l o o d g o o d , 1 96 3; A n d r e w s a n d

P e a r s o n , 1 9 6 5; H o b s o n et a l . , 1 9 7 4 ; A s i n a r i d i s a n

M a r z a n o et a l . , 1 9 8 1 ; S p e e c e , 1 9 8 3 ) .

D e g r a d a t i o n o f th e s e a c c u m u l a t e d a c i d s is e s s e n ti a l

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

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

t o t h e p o s s i b il i t y o f p r o d u c t i n h i b i t io n o f p r o p i o n a t e

d e g r a d a t i o n b y a ce t at e ( K a s p a r a n d W u h r m a n n ,

1 97 8; B o o n e a n d B r y a n t , 1 9 8 0; Z e h n d e r a n d K o c h ,

1 9 83 ; B o o n e a n d X u n , 1 9 87 ; T h o l o z a n et a l . , 1988 ;

G o r r i s et a l . , 1 9 89 ) . A s n o s y s t e m a t i c s t u d y o f t h e

p h e n o m e n o n h a s b e e n r e p o r t e d a se ri es o f b a t c h

* M e m b e r o f t h e G o v e r n i n g B o a r d .

t P r e s e n t a d d r e s s : D e p a r t m e n t o f C h e m i c a l E n g i n e e r i n g ,U n ive r s i t y o f S t r a thc lyde , G lasgow G 1 IX J , S co t l and .

e x p e r i m e n t s w a s p e r f o r m e d t o p r o v i d e m o r e i n f o r -

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

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

METHODS AND MATERIALS

Equipment

T he expe r im en t s w ere pe r fo rm ed us ing ba t ch f l a sk d i -ges t e r s w i th a w o rk ing vo lum e o f 250 m l . T hese com pr i sedf i a t -bo t t om ed 500 m l bo i l i ng f l asks s t oppe red w i th r ub be rbung s and f i t t ed w i th s am pl ing and gas ou t l e t po r t s . T he gasp r o d u c e d w a s v e n t e d t o t h e a t m o s p h e r e t h r o u g h a w a t e rt r ap . T he f l a sks w ere m ixed by hand a t l ea s t once pe r dayand a l so w i th in 1 h o f s am pl ing .

Inoculum source and preparation

Eff luent f rom a complete ly-mixed 5.01. digester fed asem i - syn the t i c m ed ium w as co l l ec ted unde r a N 2-a tm os -phe re and used a s t he s eed m a te r i a l . T he d iges t e r w asb a t c h - f e d e v e r y s e c o n d d a y a n d o p e r a t e d w i t h a n a v e r a g ev o l u m e t r ic l o a d i n g r a t e o f 0 . 5 k g C O D m - 3 d a y - j . T h em ed ium w as t ha t o f H ansson (1979) m od i f i ed by t he

add i t i on o f r e sazur in a t 0 . 0001% (w /v ) and com pr i sedg lucose , yeas t ex t r ac t , s a l t s and v i t am ins . A s t he V F Ac o n c e n t r a t i o n w a s v e r y l o w ( t o t al V F A ~ 2 0 m g l - t ) a n ds t ab l e i n t h i s e f f l uen t (M aw son , 1986) , f u r t he r t r ea tm en tp r io r t o use w as unneces sa ry .

Culture conditions and preparation

T w o se t s o f runs w ere pe r fo rm ed . In t he f ir s t, p rop iona t ew as added a t a con s t an t i n i t i a l level (P r~ ) o f 500 m g 1 - ~ andthe conc en t r a t i o n o f ace t a t e added (A c~ ) w as va r i ed be tw een0 an d 2000 m g 1 - ~. I n t h e s econd se t o f runs t he p roc edurew as r eve r sed : ace t a t e w as adde d a t 2000 m g 1 ~ an d p rop i -ona t e w as ad ded i n t he r ange 0 -1500 m g 1 ~ . I n i t i a l ac idconcen t r a t i ons a r e show n in T ab l e 1 .

S t anda rd so lu t i ons o f ace t i c and p rop ion i c ac ids w erep repa red con t a in ing e i t he r 10 o r 100 g 1 ~ ac id . A n in oeu lumof 50 m l w as ad ded t o a bou t 150 m l o f f r e sh ly d is t i ll edw a t e r ; t h e a p p r o p r i a t e v o l u m e s o f s u b s t r a t e s w e r e a d d e da n d t h e d i g e st e r w as m a d e u p t o v o l u m e w i t h m o r e w a t e r .T h e r u n s w e r e p e r f o r m e d i n r a n d o m o r d e r a n d c o m m e n c e don 3 sepa ra t e days ove r a I w eek pe r iod . T he sub s t r a t e w a s

1549

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1550 A .J . MAWSONet al.

Table 1. Initial aci d concentrations and fitte d rate co efficients (b) for acidutilization

Measu red initial Rate coefficientacid concentration for utilization of

Cond ition Aq Pr~ A c t o r ~ Run Ac Pr

Pr ~ 500

Ac ~ 2000

ND* 490 400 1 0.86 1.12530 520 950 2 0.64 0.80540 520 960 3 0.65 0.82

1060 480 1450 4 0.61 0.842110 480 2500 5 0.42 0.472190 510 2600 9 0.41 0.45

2240 ND 2240 6 0.35 - -t2170 ND 2170 7 0.31 -- t2220 220 2400 8 0.35 0.642110 480 2500 5 0.42 0.472190 510 2600 9 0.41 0.452140 1490 3350 10 0.14 --~1960 1510 3190 11 0.13 --~

*Below the limit o f detection, tlnsufficient data for analysis. :~Lineardegradationof a cids observed (see text).

n e u t r a li z e d w i t h a n e q u i m o l a r m i x t u r e o f 2 N N a O H a n dK O H t o r e d u c e p o t e n t i a l i n h i b i t io n a r is i n g f r o m a d d i t i o n o fthe ca t i ons (K u ge lm an an d C h in , 1971) . T he f l a sk d iges t e r sw ere f l u shed w i th 20% C O 2 i n N 2 (pas sed ove r hea t edcoppe r a t 320° C to r em ove t r aces o f 02 ) and i ncuba t ed a t37 + I"C .

Sampling and analytical procedures

T he d iges t e r l i quor w as sam pled im m edia t e ly a f t e r p r ep -a ra t i on and t hen eve ry second day . A 5 m l s am ple w asr e m o v e d a n d t h e V F A c o n c e n t r a t i o n d e te r m i n e d b y g a sc h r o m a t o g r a p h y f o l lo w i n g t h e p r o c e d u r e o f B a n f i e ld et al.( 19 7 8) . A S h i m a d z u G C - 5 A g a s c h r o m a t o g r a p h ( S h i m a d z uS e i sakusho L td , K yo to , J ap an) f i t t ed w i th a f l am e ion i za t i onde t ec to r op e ra t ed a t 185°C w as used . T he 2m x 3 m m i .d .g l as s c o l u m n w a s p a c k e d w i t h 1 0 % w / v F F A P o n C h r o m o -

so rb G . T he co lum n t em p era tu re w as 145°C and t he i n j ec to rpor t t em pera tu re w as 185° C . T he n i t rogen f l ow ra t e w as80 m l r a in -h a nd t he a i r a nd hydro gen f l ow ra t e s w ere 900an d 55 ml m in - ~, respect ively. Diges ter ef f luent sam plesw ere cen t r i fuged a t 2700g fo r 10m in and fo rm ic ac id(99 . 9% ) w as added t o t he su pe rna t an t l i quor (1 vo l fo r -m a te : 10 vo l supe rna t an t ) . T h e sam ple vo lum e in j ec t ed w as

# l . T he pH o f each sam ple w as a lso no t ed .

Analysis of results

T he da t a w ere ana lysed us ing t he non- l i nea r r eg re s s ionrou t i ne o f G E N S T A T (V er s ion 4 .04B ; L aw es A gr i cu l t u r a lT rus t 1980) run on a P r im e 750 com pute r . T he m ode lse l ec t ed t o desc r ibe subs t r a t e u t i l i za t i on w as t he s im plelog i st i c equ a t i on (D eW i t t , 1943 ; E dw a rds a nd W i lke 1968):

s = K/ 1 + e a+bt) (1)

w here

s = subs t r a t e conce n t r a t i on (g 1 - t )t = t ime (day)

K, a , b = constants .

T h e t e r m K / I + C ) i s an e s t im a te o f t he i n i t i a l subs t r a t econ cen tra t ion (g I t ) an d b i s a ra te coeff ic ient for su bst ra t eu t i l i za t i on (day l ) .

RESULTS

E f f e c t o f a c e t i c a c i d o n u t i l i za t i o n o f p r o p i o n i c a c i d

P r o p i o n a t e w a s a d d e d a t 5 0 0 m g 1 - 1 i n r u n s 1 - 5

a n d r u n 9 , w i t h r u n s 2 a n d 3 a n d r u n s 5 a n d 9

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

p r o p i o n a t e c o n c e n t r a t i o n s f o r r u n s 1, 2 , 4 a n d 5 a re

s h o w n i n F i g . 1 a n d t h e f i t t e d r a t e c o e f fi c i e n t s ( b ) f o r

a l l t h e r e l e v a n t r u n s a r e l i s t e d i n t h e u p p e r p a r t o f

T a b l e 1. N o a c e t a t e w a s a d d e d i n r u n 1 a n d p r o p i -o n a t e d e g r a d a t i o n p r o c e e d e d a t a fa s t r a t e

( b = l . 1 2 d a y - 1 ) w i th a 9 0 % r e d u c t i o n i n Pr~

a c h i e v e d w i t h i n 6 d a y s . A q w a s i n c r e a s e d i n e a c h

s u b s e q u e n t r u n a n d i t i s c l e a r f r o m F i g . 1 t h a t t h i s

i n h i b i t e d p r o p i o n a t e d e g r a d a t i o n . T h e t i m e r eq u i r e d

t o a c h i e v e 9 0 % u t i l i z a t io n i n c r e a s e d t o 7 d a y s i n r u n

2 ( A c i = 5 3 0 m g l - ~ ) , t o 8 d a y s i n r u n 4

( A c ~ = 1 0 6 0 m g l - l ) a n d t o 11 d a y s f or r u n 5 ,

( A c i = 2 1 1 0 m g l - ~ ) . F o r r u n s 2 , 3 a n d 4 t h e r a te

c o e f fi c ie n t s f o r p r o p i o n a t e u t i l i z a t io n w e r e s i m i l a r a t

0 . 8 - 0 . 8 4 d a y - 1 , b u t t h i s w a s l o w e r e d s i g n i f i c a n t l y in

r u n s 5 a n d 9 t o 0 .4 5 - 0 . 4 7 d a y 1.D a t a f o r th e d u p l i c a t e r u n s a r e s h o w n i n F ig . 2 a n d

t h e o b s e r v e d a n d p r e d i c t e d d a t a f o r r u n s 2 a n d 5 a r e

p l o t t e d i n F i g . 3 . T h e s e c o n f i r m t h a t t h e f i t o f t h e

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

a c c e p ta b l e , a l t h o u g h t h e p r o p i o n a t e c o n c e n t r a t i o n i n

r u n 9 w a s a lw a y s a p p r o x . 5 0 m g l - ~ h i g h e r t h a n t h a t

i n r u n 5 . A c c e p t a b l e a g r e e m e n t w a s a l s o o b t a i n e d i n

a l l o t h e r c a s e s ( M a w s o n , 1 9 8 6) .

E f f e c t o f p r o p io n i c a c i d o n u t i l i za t i o n o f a c e t i c a c i d

A c e t a t e w a s a d d e d a t a p p r o x . 2 0 0 0 m g 1 -l i n r u n s

5 - 1 1 w h i l e P r i w a s v a r i e d b e t w e e n l e ss t h a n 1 0 m l - i( r u n s 6 a n d 7 , p r o p i o n a t e n o t d e t e c te d ) a n d

1 5 0 0 m g l - I ( r u n s 1 0 a n d 1 1) . T h e r a t e c o e f fi c ie n t s

a p p e a r i n th e l o w e r p a r t o f T a b l e 1 . T h e s e w e r e

c a l c u la t e d f r o m t h e d e g r a d a t i o n o f a c e t a te a n d t h e

c o n t r i b u t i o n t o t h e a c e t a t e p o o l f r o m t h e u t i l i z a t i o n

o f p r o p i o n a t e a s s u m i n g t h i s w a s g i v en b y ( B r y a n t ,

1979) :

C H 3 C H 2 C O O - + 3 H 2 0

= C H 3 C O O - - + H C O 7 + H + + 3 H 2 .

T h e i n i t a l v a l u e o f t h i s p o t e n t i a l a c e t a t e p o o l (A cto r,i)

i s a l s o s h o w n i n T a b l e I .

F o r r u n s 5 - 9 ( P r < 6 0 0 m g l - l ) t h e r a t e c o e f f i c i e n ts

f o r a c e t a t e u t i l i z a t i o n w e r e s i m i l a r a n d t y p i c a ll y i n t h e

r a n g e 0 . 3 5 - 0 .4 1 d a y - 1, a l t h o u g h a s l ig h t ly l o w e r

v a l u e o f 0. 3 1 d a y - 1 w a s e s t i m a t e d f o r r u n 7 . A Cto r, f o r

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V FA de g r ada t i o n i n anae r o b i c d i g e s t io n 1 55 1

6 0 0

• 5 0 0-.,.

.o 40 0 o . ,,

s , -

e . -

~ - 3 0 0 \ \ \ ~ . . .I . I " ' ° ' , ,

, , % ,

'. . %, .2 0 0= \ , o " . , ~0 ~ %% % .% . ' - °~ \ . % ,,.

0 b ' ' .\ % , % .% . - , .

Zi %.,, ~ .1 0 0

t

2 4 6 8 I 0 1 2

T im e s i n c e i n o c u l a t i o n ( d a y s )

Fig . 1 . Propionate ut i l i zat ion in s e lec ted runs wi th pro piona te ad ded at 500 mg I -L Init ial ace tate

c o nc e n t r a t i ons o f : < 1 0 m g l - ~ (r un 1 , ( Z )) , 5 3 0 m g l - l ( r un 2, A) , 1 0 6 0 m g l ' ( run 4 , [ ] ) , 2 1 1 0 m g l - l

(run 5, *).

t h e s e r u n s r a n g e d f r o m 2 1 7 0 t o 2 6 0 0 m g 1 ~. H o w e v e r

w h e n t h e t ot a l a c i d c o n c e n t r a t i o n w a s i n c r e a s e d to

o v e r 3 0 0 0 m g 1 ~ ( r u n s 1 0 a n d 1 1 ) th e r a t e p a r a m e t e r

w a s r e d u c e d s i g n i f ic a n t l y t o a p p r o x . 0 . 1 3 d a y ~ a n d

a c e t a t e w a s n o t c o m p l e t e l y d e g r a d e d e v e n a f t e r a n

e x t e n d e d f e r m e n t a t i o n ( M a w s o n , 1 9 86 ).

Effect o f increased propionate concentration on propi-

on te degradation

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

s i m i l a r A q i s c o n s i d e r e d a s l ig h t d r o p i n t h e r at e

6 0 0

5 O 0t ~E

o

- 4 0 0k .

e -

3 0 0

0

2 0 00

' g .0S , -

1 0 0

c o e f f i c i e n t f r o m 0 . 6 4 d a y - 1 ( r u n 8 , P r i = 2 2 0 m g 1 - 1 )

t o 0 . 4 5 d a y - ~ i n r u n 9 ( P r i = 5 1 0 m g 1 ~ ) i s o b s e r v e d .

I n r u n s 1 0 a n d 1 1 h o w e v e r , p r o p i o n a t e d e g r a d a t i o n

w a s m u c h r e d u c e d a n d w a s l in e a r w i t h t im e . A b o u t

4 0 o f t h e i n it i al p r o p i o n a t e r e m a i n e d u n d e g r a d e d

a f te r 2 4 - 2 8 d a y s ( M a w s o n , 1 9 8 6) .

Effect of increased acetate concentr t ion on cet te

degradation

R a t e c o e f f i c ie n t s f o r a c e ta t e d e g r a d a t i o n i n t h o s e

r u n s w i th c o n s t a n t P q c a n b e c o m p a r e d i n T a b l e 1 .

i~ . ~ " ' ~ . : : .. . . . . . o . .

° " ° " , : ' : ~ : , , , ° " ° . , , ° ° . °

""? '&. "o

o ) : : . . . . . . . . . ° . . . . ." ,° , " ° , ° " ° ' ° . ~ . " ' , , °

" , ,> , . . . . . . . . .. " a .• , " , ~

' ° % , " O

:;:: ,-2 2 1 ,,., . . . . . . . . . .. . . . . . . .% ° %

; " A%

" ° ° ' ~ ) , . i ' ° ' ° , , ° .

i : : : : 2 k

" ° , , % , " ' , , , , .

• ~ t ° . . °

° " ' , , , ° " ' " r l ," ' , ° ° , , ° ° ° ° , ° . . , , °

w .

. . . . . . . . : : : ~ . .I I

2 4 6 8 1 0 1 2 1

T im e s i n c e i n o c u l a t i o n ( d a y s )

Fig . 2 . Pr o p io na t e u t i l i z a t i o n in dup l i c a t e runs w i t h pr o p io na t e adde d a t 5 0 0 m g l - ' . I n i ti a l ac et a te

concen trat ions o f : 530 mg 1 - ' ( run 2 , C) ) , 540 mg 1 ~ (run 3 , A ) , 2110 mg I ~ (run 5 , * ) , 2190 mg 1 - ' ( run

9 , [ ] ) .

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1552 A .J . MAWSON t a l .

6 0 0

e . - -

I . $ , . ; ' - . - I : . .- . ,, . , , . ~ , . _ , : ~ . .

° ' ; ' ~ . h . . . ,

i 4 0 0 : : :: ': : :: : -: : . , , , ' . : : . ~ : . . .

=u 3 0 0 ~ A ' ' : % ' ' ' ' ' '~ ,

I I )

' ~ 2 0 0 ::.,~. ".:-~o ,.n.:::::::::::::::::.~l

m _ 1 0 0 A

=: : : : :22.m ...' , . : : : : : .~ .• ~ . . . ' - - - . . . . . .

I I I I . . . . . . . . T , . ' ~

2 4 6 8 1 0 1 2 1 4T im e s i n c e i n o c u l a t i o n ( d a y s )

Fig. 3. Observed and predicted pro pion ate concentrations for selected runs. Run 2, observed (. . . A .. .) ;run 2 , predicted ( . . . O . . . ; K = 0 .54 , a =- 3 .1 5 , b = 0 .80) ; run 5 , observed . . . , . . . ) ; run 5 , predicted

( . . .Fq . . . ; K = 0.49 , a = -3 .19 , b =0 .47 ) .

A s A C t o t , i i n c r e a s e d f r o m 4 0 0 m g 1 -1 (r u n 1 ) t o a b o u t

2500 mg -1 ( runs 5 an d 9 ) the r a te coef f ic ien t f o r

a c e t a t e u t i l i z a t i o n w a s r e d u c e d f r o m 0 .8 6 t o a b o u t

0 .4 d a y - 1 .

DISCUSSION AND CONCLUSIONS

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

c o n s i d e r a t i o n o f t h e r e s u lt s o f s e v e ra l C S T R d i g e s t i o n

e x p e r i m e n t s w i t h t h e s e m i - s y n t h e ti c m e d i u m ( M a w -

s o n , 1 9 8 6 ) . T h e p r o p i o n a t e c o n c e n t r a t i o n u s u a l l y

v a r i e d w i t h i n a n a r r o w r a n g e a r o u n d 5 00 m g 1-1 a n d

H a n s s o n ( 19 7 9) a l s o n o t e d a s i m i l a r c o n c e n t r a t i o n i n

h i s w o r k w i t h t h e s a m e m e d i u m . T h e c o n c e n t r a t i o n

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

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

w h e n t h e l e v e l d i d n o t e x c e e d 5 0 0 m g l - l , w h i l e

2 0 0 0 m g 1 -1 w a s c h a r a c t e r i s t i c o f t h e c o n c e n t r a t i o n

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

c e e d e d a b o u t 1 0 0 0- 1 50 0 m g 1 - 1 t h e r e w a s s o m e

e v i d e n ce p r o p i o n a t e d e g r a d a t i o n w a s r e t a r d e d (M a w -

son, 1986) .

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

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

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

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

i n c r e a s i n g A c i to 2 0 0 0 m g 1 -1 s i g n i f i c a n t ly r e d u c e d t h e

u t i l i z a t i o n o f p r o p i o n a t e a d d e d a t 5 00 m g 1 -1 . O v e r a l l

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

a d d e d a t 2 0 0 0 m g 1 -1 w a s a p p r o x i m a t e l y h a l f o f t h a t

w h e n a c e t a t e w a s p r e s e n t a t a b o u t 5 00 m g i - 1 o rl o w e r .

A s A q w a s i n c r e a s e d t o 2 0 00 m g 1 -1 w i t h p r o p i -

o n a t e a d d e d a t a c o n s t a n t l e v e l , a p r o g r e s s i v e re -

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

o b s e r v e d . S i m i l a r l y , i n c r e a s i n g P r i w h e n a c e t a t e w a s

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

p r o p i o n a t e d e g r a d a t i o n a n d e v e n tu a l ly d e g r a d a t i o n

o f b o t h a c i d s w a s s e v e r e l y r e t a r d e d a t t h e h i g h e s t

p r o p i o n a t e c o n c e n t r a t i o n . T h e s e i n h i b i t i o n e f f e c t s

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

0 . 1 - 0 . 3 p H u n i t s a c r o s s a l l d i g e s t e r s , o r t o t h e s a l t s

a d d e d ( K u g e l m a n a n d C h i n , 1 9 7 1 ; K a s p a r a n d

W u h r m a n n , 1 97 8; B o o n e a n d X u n , 1 98 7) .

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

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

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

v a t i o n s i n t h e l i t e r a t u r e .

P r o d u c t i n h i b i t i o n o f p r o p i o n a t e d e g r a d a t i o n

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

( K a s p a r a n d W u h r m a n n , 1 97 8) a n d a m i x e d a c i d

f e e d ( G o r r i s e t a L , 1 9 8 9 ) , i n p r o p i o n a t e e n r i c h m e n t

c u l t u r e s ( Z e h n d e r a n d K o c h , 1 9 8 3 ; B o o n e a n d

X u n , 1 9 8 7; T h o l o z a n e t a l . , 1988) and in def ined

c o - c u l t u r e s ( B o o n e a n d B r y a n t , 1 98 0) . I n m o s t

c a s e s m o d e r a t e i n h i b i t i o n w a s r e p o r t e d a t a c e t i c

a c i d c o n c e n t r a t i o n s o f 1 5 - 30 m M ( 9 0 0- 1 8 00 m g l - 1 )

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

r a n g e 1 0 - 2 5 m M ( 7 4 0 - 1 8 5 0 m g l - ~ ) ( B o on e a n d

B r y a n t , 1 9 8 0 ; Z e h n d e r a n d K o c h , 1 9 8 3 ; B o o n e a n d

X u n , 1 9 8 7 ; T h o l o z a n e t a L , 1 9 8 8 ) . K a s p a r a n d

W u h r m a n n ( 19 78 ) d i d n o t o b s e r v e p r o d u c t i n h i b i t i o n

a t 8 m M a c e t a t e ( 4 8 0 m g 1 - 1 ) b u t t h e e f fe c t w a s

p r o n o u n c e d a t 8 0 m M ( 48 0 0 m g 1 -1 ). T h e s e d a t a

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

h e r e a n d a l s o t h a t o f H i l l e t a l . ( 1 9 8 7 ) w h o r e p o r t e d

t h a t g r e a t e r t h a n 5 0 % o f f a i li n g a n i m a l - m a n u r e

d i g e s t e r s h a d a c e t a t e c o n c e n t r a t i o n s i n e x c e s s o f

8 00 m g 1-1 a n d a p r o p i o n a t e t o a c e t a t e r a t i o g r e a t e r

than 1 .4 .

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VFA degradation in anaerobic digestion 1553

Gorris e t a l . (1989) observed marked inhib itio n of

propio nic acid degradation (40-80 ) at levels of

abou t 200-500 mg 1 ~ acetate. This toxic concen-

tration appears very low and may have been due to

the reactor chosen (fluidized bed compared to sus-

pended growth systems) or the fact that the data were

obtained during start-up of the digesters when the

population may have been more sensitive to acetic

acid.

The toxicity of high acid concentrat ions, regardless

of the actual species present, has been document ed in

many studies and possible mechanisms for this have

been summarized in a recent review (Bajpai and

Iann otti , 1988). The inhi bit ion of methanogenesis has

been attributed to the action of un-ionized acids

(UVFAs) so both the pH and the total acid concen-

tration are impo rtant in determining the observed

effect. Kroeker e t a l . (1979) reported inhibition when

the UVF A concentration was in the range

30 -6 0m gl -~, while Duar te and Ander son (1982)

observed 50 inhibiti on of methane formation when

the U VFA co ncentr at ion exceeded 10 mg 1-~ in acetic

and glucose-fed digesters. In this work, approx. 50

inhibiti on of both acetic and pr opionate degradation

was observed in batch digesters with initial concen-

tra tio ns of 2000 mg 1- ~ acetate and 500 mg 1- ~ propi-

onate. At pH7.0, the corresponding UVFA

concentration is ab out 14m gl -~ (Kroeker e t a l . ,

1979) so in this respect the results are in general

agreement with these studies.

The culture used in this experiment appeared more

sensitive to the acetic acid concentr ation than others

reported. Substrate in hibi tion for acetic acid has been

noted at concentrations greater than 100mM

(6000 mg 1-l) (Yang and Okos, 1987; Clarens e t a l . ,

1988) while satisfactory performance is usually ob-

served for acetate concen trati ons of 50 mM or less

(Hob son and Shaw, 1976; Huser e t a l . , 1982; Valcke

and Verstraete, 1983; Yang and Okos, 1987). Propi-

onic acid appears more toxic to methano genic ecosys-

tems and significant substrate inhib ition has been

observed following additions of 1000mgl -l or

greater (McCar ty and Brousseau, 1963; Andrews,

1968; Hobso n and Shaw, 1976) although Jarrell and

Saulnier (1987) observed no inhibition of pure

methanog enic cultures at 70 g 1-1 propionate.

The results of this study are significant for digester

operation and control. Digester failure is character-

ized by an increase in the VFA conce ntrat ions and

there is now considerable evidence which shows this

can be detrimental to degradation of both acetic and

propio nic acids. Recovery of a digester from a period

of retarded operation will necessarily require ade-

quate time for the accumulated acids to be degraded

at the lower rate. Additionally, as the un-ionized acid

concentration appears to play an important role in

the reduction of methanogenic activity, the provision

of adequate alkalinity to buffer the pH is important

to minimize the effect of an increase in acid concen-

tration. Further difficulties may also arise when di-

gesters are poorly provided with nutrients as VFAs,

and particularly acetate, can accumulate rapidly

under these conditions (Speece and McCarty, 1964;

Speece, 1983; Mosey and Fernan des, 1988). In such

situations propionate degradation may be signifi-

cantly retarded and the time required for digester

recovery could be prolonged.

R E F E R E N C E S

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