Transformation of Lower Fatty Acids by Acetogenic Bacteria

In order to study the formation of acetate from butyrate and propionate by acetogenic bacteria, it is necessary to reduce the partial pressure of H2 down to pH2 = 10-3 bar (butyrate) or between 10-4 bar and 2 ■ 10-6 bar (propionate; Fig. 8.4). It is useful to investigate the growth of Synthrophobacter wolinis on n-butyrate (Eq. 8.2) or the growth of Synthrophomonas wolfeii on propionate (Eq. 8.3) in mixed culture with methanogenic bacteria, as they reduce the pH2 (Eq. 8.5) and the acetate concentration (Eq. 8.4). The removal of acetate is absolutely necessary to prevent product inhibition. Two results of butyrate transformation to acetate in experiments are presented in Table 8.2. The yield coefficient YX/S = 0.0094 ... 0.022 = 0.014 g MLSS (g COD)-1 and the maximum specific growth rate pmax = 0.17 ... 0.39 = 0.28 d-1 are considerably lower than those of acidogenic bacteria: the pmax is lower by a factor of about 100 and Y°/S by a factor of about 15. From these values, the following specific maximal substrate removal rates are obtained for butyrate:

rs/X = Pmax/YXs = 0.28/0.014 = 20 g COD (g MLSS ■ d)-1

178 | 8 Anaerobic Degradation of Organics Table 8.2 Fermentation, p-oxidation and methanization - stoichiometric and kinetic coefficients.

Bacterial substrate Ref. Reactor T

(pH) (°C) YV Hmaxb) Ksc> KSHd) KiHe) kd"1

Fermentation of glucose to lower fatty acids by acidogenics

Ghosh and

Pohland

(1974)

Chemostat

35

0.162

30.1

24.1

1.06

Pohland

(1978)

Chemostat

35

0.31

64.8

2583

1.56

Hanaki et al. (1985)

Chemostat

35

0.212

10.9

73.4

4.13

p-Oxidation of ra-Butyrate

Lawrence and McCarty (1969)

Chemostat (pH 7.0)

35

0.0094

0.39

48.3

0.027

Chang (1982)

Batch

35

0.022

0.17

47.0

0.005

p-Oxidation of propionate to acetate and H2 by acetogenics

Andrews (1969)

Lawrence and McCarty (1969)

Chemostat (pH 7.0)

Chemostat (pH 7.0)

38 35

0.05

0.4 0.4

0.36

60

Gujer and Zehnder (1983)

Chemostat (pH 7.0)

33

0.025

0.155

248

Methanization from H2

Zehnder and Wuhrmann

Chemostat (pH 7.0)

37 33

0.043

1.06 1.4

556[f]

0.009

Bryer (1985)

55

0.038

1.4

37.5

0.1

Methanization from acetate

Graef and Andrews (1973)

Chemostat (pH 7.0)

38

0.057

0.4

2.13

42.7

Carr and O'Donell (1977)

35

0.027

0.11

1.15

35.6

0.006

Bolle et al. (1986)

Chemostat (pH 7.0)

35

0.037

0.038

2.1

16.9

0.0036

a) g MLSS (g COD)-1, b) d-1, c) mg L-1 COD, d) mg L-1 COD, e) mg L-1 COD, f ^mol L-1.

a) g MLSS (g COD)-1, b) d-1, c) mg L-1 COD, d) mg L-1 COD, e) mg L-1 COD, f ^mol L-1.

and for propionate:

if the results of three studies are considered (Table 8.2). The maximum specific removal rate of propionate is lower than that of butyrate nearly by a factor of 7. But in reality, the reaction in Eq. (8.2) is very sensitive to the following influences:

• As mentioned in Section 8.13, pH2 must be reduced by methanogenic bacteria down to 10-4 bar.

• The conversion of propionate to acetate can be inhibited by higher concentrations of non-ionized propionate SHPr. As shown by several authors, the influence of S HPr on the specific growth rate can be described by Haldane kinetics (Andrews 1969; Attal et al. 1988; Fukuzaki et al. 1990):

SHPr

KSH + SHPr+SHPr/KiH

• In addition to SHPr, the concentration of non-ionized acetate SHAC influences p by non-competitive inhibition (Mosey 1983; Denac 1986; Fukuzaki et al. 1990; Kus 1993):

SHPr KiH

KSH+ SHPr+SHPr/KiH KiH + SHAc

Therefore, the formation of methane from H2 and acetate should not be disturbed. Otherwise, propionate and butyrate cannot be biodegraded.

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