molybdate inhibition of sulphate reduction in two-phase anaerobic digestion

www.elsevier.com/locate/procbio

Process Biochemistry 40 (2005) 2079–2089

Molybdate inhibition of sulphate reduction in two-phase

anaerobic digestion

M. Hasnain Isaa,*, G.K. Andersonb

aSchool of Civil Engineering, Engineering Campus, Universiti Sains Malaysia, 14300 Nibong Tebal, Penang, MalaysiabSchool of Civil Engineering and Geosciences, University of Newcastle upon Tyne, Newcastle upon Tyne NE1 7RU, UK

Received 22 November 2003; accepted 16 July 2004

Abstract

Several researchers have used molybdate (MoO42�) for the selective inhibition of sulphate reducing bacteria (SRB) in anaerobic digestion.

The feasibility of using MoO42� for the control of sulphate reduction in biological reactors, however, is not well established as some published

results have also indicated an adverse effect on methane producing bacteria (MPB). Two possible reasons may be attributed for this

observation, namely: (a) chronic inhibition of MPBs by MoO42� and (b) preferential acclimatisation of acidogenic bacteria to MoO4

2�. A

two-phase anaerobic digestion experiment was conducted to establish the reason for the cessation of methanogenesis as mentioned above. The

idea was to grow acidogenic and methanogenic bacteria in separate reactors so that the effect of MoO42� could be evaluated independently.

The experiment comprised a 4.55-l packed-bed acidogenic reactor and a 4.75-l suspended-biomass methanogenic reactor operated over a

period of more that 3 months. Glucose and Na2SO4 were used as the carbon and sulphate sources respectively. The concentration of MoO42�

used in the test was 2.5 mM. The study showed that MoO42� inhibited both sulphate reduction as well as methane production and caused a

change in VFA dominance from acetic to butyric acid. Acetic acid dominance in the reactor was resumed after the loading rate was decreased.

Whereas SRBs showed complete recovery from the MoO42� dose once it was omitted from the feed, MPBs did not recover. MoO4

2� was

bacteriostatic to SRBs and bacteriocidal to MPBs. Acidogenic bacteria were the first to acclimatise to MoO42�. MoO4

2� is not a suitable

selective inhibitor of SRBs in anaerobic reactors.

# 2004 Elsevier Ltd. All rights reserved.

Keywords: Two-phase anaerobic digestion; Sulphate reducing bacteria (SRB); Methane producing bacteria (MPB); Inhibition; Molybdate

1. Introduction

The reduction of sulphate to sulphide by sulphate

reducing bacteria (SRB) during anaerobic digestion of

wastewaters has a number of problems associated with it;

including the sharing of methanogenic substrate to result in

lower methane production, precipitation of trace metals such

as Fe, Ni and Co by S2� to cause nutrient deficiency to

methane producing bacteria (MPB), requirement for

expensive corrosion resistant tank lining, corrosion of

burners, odour, requirement of post-treatment due to

increased oxygen demand of treated effluents and toxicity

to methanogenic bacteria. Control of sulphate reduction

* Corresponding author. Tel.: +60 604 593 7788x6217;

fax: +60 604 594 1009.

E-mail address: [email protected] (M.H. Isa).

0032-9592/$ – see front matter # 2004 Elsevier Ltd. All rights reserved.

doi:10.1016/j.procbio.2004.07.025

(sulphide production) in anaerobic digestion processes,

therefore, presents an area of interest to many researchers.

Methods for dealing with hydrogen sulphide in anaerobic

digesters include pH adjustment, precipitation with iron salts

and off-gas scrubbing. Selective inhibition of sulphate

reducing bacteria (SRB), however, may present an ideal

solution as it would prevent sharing of methanogenic

substrate by SRBs and hence could provide an economic

solution.

Several researchers have used molybdate (MoO42�) for

the selective inhibition of SRBs [1–4] because in addition to

being an SRB inhibitor, it is also a nutrient for MPBs. The

feasibility of using molybdate for the control of sulphate

reduction in biological reactors, however, is not well

established as some published results have also indicated an

adverse effect on MPBs [5,6]. These reports have shown

resumption of SO42� reduction, inhibition of methanogen-

M.H. Isa, G.K. Anderson / Process Biochemistry 40 (2005) 2079–20892080

esis and accumulation of volatile fatty acids in the reactors.

The presence of acetic acid in the reactors shows that non-

availability of methanogenic substrate was not the under-

lying problem for the inhibition of CH4 production in these

studies. Two possible reasons may thus be attributed to this

observation.

(a) C
hronic inhibition of MPBs by MoO42�.
(b) P
referential acclimatisation of acidogenic bacteria to
MoO42�.

Case (a) results in an obvious response of non-utilisation of

acetic acid. In case (b), accumulation of acetic acid can be

explained in the light of upsetting the microbial species

balance which would otherwise cause acetate to be utilised

(converted to CH4) as soon as it was produced.

A two-phase anaerobic digestion experiment was con-

ducted to establish the reason for the cessation of

methanogenesis as discussed above. Acidogenic and metha-

nogenic bacteria were grown in separate reactors so that the

effect of MoO42� on them could be evaluated independently.

2. Materials and methods

The system used for this study (Fig. 1) [7] was part of a

compact package-unit developed by Armfield Ltd. for

laboratory biological wastewater treatment studies. It had

individual units mounted on a stainless steel base equipped

with an automatic monitoring and control board.

Fig. 1. Schematic diagram of tw

The two reactors were made of plexi-glass columns, each

approximately 360 mm high and 140 mm i.d. Each reactor

was maintained at 35 � 1 8C using jacket heating con-

trollers. Watson–Marlow peristaltic pumps were used for

feeding the reactors. Effluent was discharged via an inverted

siphon to prevent the escape of biogas and gas volumes were

measured by collecting the gas in Mariotte flasks using water

as the displacement liquid.

2.1. Acidogenic reactor

The acidogenic reactor was an upflow anaerobic packed

bed reactor. It was randomly packed with 130 non-porous

Etapak pal-rings confined within two metal grids spaced

245 mm apart. The lower grid was 40 mm above the base of

the reactor. The pal-rings were 27.5 mm high, 25.4 mm in

diameter and had a specific surface area of 100 m2/m3 with

95–97% void space. A centrally located glass tube was used

to feed the reactor. The liquid level in the reactor was

maintained at 35 mm above the packing medium to keep it

fully submerged and the headspace was 40 mm. The

effective volume of the reactor was 4.55 l.

2.2. Methanogenic reactor

The methanogenic reactor employed for the study was an

upflow anaerobic reactor (suspended biomass) with a

centrally located glass tube for feeding. Its effective volume

was 4.75 l. In order to maintain a higher hydraulic retention

time (HRT) in the reactor, a 250-ml storage vessel was

o-phase anaerobic reactors.

M.H. Isa, G.K. Anderson / Process Biochemistry 40 (2005) 2079–2089 2081

Table 1

Composition of synthetic wastewater for two-phase anaerobic reactors

Compound Concentration (mg/l)

Glucose (C6H12O6) COD 8000

Ammonium hydrogen carbonate (NH4HCO3) 2500

Potassium di-hydrogen phosphate (KH2PO4) 400

Sodium hydrogen carbonate (NaHCO3) 400

Trace metal solution A

Magnesium sulphate (MgSO4.7H2O) 5

Trace metal solution B

Ferric chloride (FeCl3) 5

Calcium chloride (CaCl2) 5

Potassium chloride (KCl) 5

Cobaltous chloride (CoCl2) 1

Nickel chloride (NiCl2) 1

placed between the acidogenic and methanogenic reactors.

The effluent from the acidogenic reactor was first collected

in the storage vessel to allow pumping of a desired volume

into the methanogenic reactor and wasting of the excess as

overflow.

2.3. Feed

Glucose was used as the organic-carbon source in this

experiment. The composition of the synthetic wastewater is

shown in Table 1. Sodium sulphate was used as the sulphate

source and a COD/SO42� ratio of 2.6 was maintained at

steady-state. Thereafter, sodium molybdate (2.5 mM) was

included in the feed as the inhibitor. The COD/SO42� ratio

of 2.6 was adopted because studies conducted by McCartney

and other researchers [8,9] indicate that this value lies in the

range where SRBs and MPBs compete with each other

strongly. More recently, Weijma et al. [10] too have reported

the lowering of the COD/SO42� ratio in their methanol-fed

thermophilic bioreactor to favour sulphate reduction over

methanogenesis.

The feed vessel was kept in a refrigerator, maintained at

4 8C, to prevent rapid degradation of glucose. Its contents

were continuously mixed with a magnetic stirrer.

The feeding schedule of the acidogenic reactor is shown

in Table 2. The volume of synthetic wastewater fed to the

reactor was an average 14 l/day throughout the period of the

study. During the last phase of the experiment (Table 2), the

strength of the wastewater was reduced to one-fourth

because of the inhibition that resulted from MoO42�

addition. Only half of the acidogenic reactor effluent was

Table 2

Feeding schedule of acidogenic reactor

Days COD (mg/l) SO42� (mg/l) MoO4

2� (mM)

0–11 8000 – 0

12–25 8000 2000 0

26–42 8000 3000 0

43–49 8000 3000 2.5

50–76 8000 3000 0

77–94 2000 750 0

pumped into the methanogenic reactor throughout the

experiment.

2.4. Analytical techniques

COD (soluble) and sulphide were analysed according to

Standard Methods [11]. GF/A grade microfibre filters were

used. VFAs were analysed using a flame ionisation detector

(FID) equipped gas chromatograph with autoinjector and

computing integrator. Gas composition was determined using

a gas chromatograph with computing integrator. SO42� was

analysed by ion chromatograph with computing integrator.

3. Results and discussion

3.1. Acidogenic reactor

3.1.1. Chemical oxygen demand (COD)

In spite of keeping the feed vessel in a refrigerator, the

COD of the feed was found to decrease during reactor

operation. To compensate for this effect, the average of the

initial and final values of COD was assumed to be the

concentration fed to the reactor over the day.

A considerable degree of methanogenesis was observed

in the acidogenic reactor prior to MoO42� addition. Since a

packed-bed reactor provides a high solids retention time

(SRT) and allows a pH gradient to exist within the thickness

of biofilms, methanogenic bacteria were able to grow in the

acidogenic reactor used in this study. Fig. 2 shows the

influent and effluent concentrations of soluble COD in the

reactor. Table 3 summarises the performance of the reactor.

Addition of MoO42� affected the COD reduction in the

reactor. COD removal reduced from 47 to 20%. Exclusion of

MoO42� from the feed had little effect and the COD

reduction was marginally improved to 23%. At the end of the

experiment it had improved to 36% after the COD load was

reduced to one-fourth of the original (day 77 onwards).

Hilton and Archer [5], have also reported a decline in

performance of an anaerobic filter due to MoO42� (10 mM).

Their reactor remained unsteady even after MoO42� was

omitted from the feed and its condition improved only at

reduced loading rate.

3.1.2. Sulphate (SO42�)

SO42� concentrations in the influent and effluent of the

reactor are shown in Fig. 3. Table 4 presents the reactor

performance over the period of study.

Inhibition of SO42� reduction by MoO4

2� was immediate

and almost complete. The resumption of SO42� reduction on

discontinuation of MoO42� addition shows that the effect of

MoO42� on SRBs is bacteriostatic and not bactericidal. The

efficiency of SO42� reduction increased sharply to 74%

when the feed was diluted by a factor of 4. This shows that

recovery of SRBs from MoO42� toxicity was far higher than

that of MPBs (compare with Table 3).


Fig. 2. Soluble COD concentration vs. time for acidogenic reactor.

3.1.3. Sulphide (S2�)

Fig. 4 shows the sulphide concentration in the effluent

from the acidogenic reactor. The complete disappearance of

S2� from the effluent verifies the inhibition potential of

MoO42� to SRBs. Beyond day 50, S2� started to reappear in

the effluent. At reduced loading rate, the S2� concentration

was observed to be comparable to the initial values (i.e. at

full load). This may be attributed to the high rate of CH4

production prior to MoO42� addition which resulted in

stripping of H2S and thus maintained a low S2� concentra-

tion in the reactor.

The results obtained in this study do not appear

to be affected by S2� toxicity as its concentration was

low (50 mg/l). Mizuno et al. [12] observed no inhibition

of sucrose degradation at free H2S concentration up to

99 mg/l.

3.1.4. Volatile fatty acid (VFA)

Fig. 5 shows the equivalent COD concentration of VFA

relative to the total COD of the reactor effluent. The degree

of acidification (based on effluent COD) prior to MoO42�

addition was high, about 81%. After addition of MoO42�

from day 43, acidification reduced to a minimum of 15% on

day 45. Acidogens, however, rapidly acclimatised to

MoO42� as indicated by the recovery of acidification of

Table 3

COD of acidogenic reactora

Number Flow (l/day) OLRb (kg COD/(m3 day) CODinf (mg/l) COD

1 14.16 23.75 7630 405

2 14.17 23.70 7610 608

3 14.25 23.61 7540 582

4 13.60 5.38 1800 115

a Values are average based upon days 35–43 for (1), 46–50 for (2), 62–76 fob Organic loading rate.

up to 44% on day 49 even though MoO42� was still being

added. After MoO42� addition was stopped on day 50, the

degree of acidification averaged 67% (days 62–76). It was

almost complete (99%, averaged over days 88–94) when the

loading rate was reduced to one-fourth.

The variation of individual volatile acids over the period

of study is shown in Fig. 6. In the presence of SO42�, the

concentration of volatile acids were generally in the order

acetic > butyric > propionic > valeric. The addition of

MoO42� changed the composition of VFA with a sharp

increase in butyric acid concentration and a rapid reduction

in acetic acid concentration. Butyric acid then became the

dominant volatile acid. Butyric rather than acetic acid tend

to be the main fermentation product when there is an

accumulation of H2 in the system [13]. Exclusion of

MoO42� from the feed led to a reduction in butyric acid and

an increase in acetic acid concentrations. These acids were

now, however, in comparable proportions (Fig. 6). Reduc-

tion of the loading rate re-established the dominance of

acetic acid and the reactor appeared to be no longer under the

effect of MoO42�.

3.1.5. Methane (CH4)

The volume of CH4 produced per day and the percentage

of CH4 in the digester gas are shown in Fig. 7. The addition

eff (mg/l) MoO42� (mM) COD removal (g/day) Efficiency (%)

0 0 50.69 47

0 2.5 21.68 20

0 0 24.51 23

0 0 8.84 36

r (3) and 88–94 for (4).


Fig. 3. Sulphate concentration vs. time for acidogenic reactor.

of MoO42� resulted in a reduction of both the quantity as

well as the percentage of CH4. No significant amount of CH4

was recorded even at the reduced load. This shows that COD

removal in this period was basically due to SO42� reduction

and not methanogenesis. In other words, SRBs were able to

recover better from the shock due to MoO42� and

acclimatise better to MoO42� than MPBs.

3.2. Methanogenic reactor

3.2.1. Chemical oxygen demand (COD)

Fig. 8 shows the variation of soluble COD in the

methanogenic reactor. A summary of the performance of the

reactor is given in Table 5.

Methanogenesis was strongly affected by MoO42�. The

efficiency of COD removal was reduced from 44 to 8% by

the addition of MoO42�. It did not recover despite exclusion

of MoO42� from the feed and reduction of the organic

loading rate.

3.2.2. Sulphate (SO42�)

SO42� concentrations in the reactor are shown in Fig. 9.

The performance of the reactor is shown in Table 6.

As is evident from Fig. 9, SRBs were coexisting with

MPBs in the reactor. The SO42� removal efficiency,

Table 4

SO42� of acidogenic reactora

Number Flow (l/day) SLRb (kg COD/m3 day) SO42�

inf (mg/l) SO42�

1 14.16 9.31 2990 1820

2 14.17 9.19 2950 2900

3 14.25 9.24 2950 2380

4 13.60 2.30 770 200

a Values are average based upon days 35–43 for (1), 46–50 for (2), 62–76 fob SO4

2� loading rate.

however, was generally lower than in the acidogenic reactor

(compare Tables 4 and 6) in different phases of the study.

The sharp increase in effluent SO42� concentration

(reduction in SO42� removal efficiency) shows strong

inhibition of SRBs by MoO42�. In this case as well, SO4

2�

reduction resumed after MoO42� was excluded from the

feed.

3.2.3. Sulphide (S2�)

Unlike the acidogenic reactor which received no S2�

from the influent, the influent of the methanogenic reactor

(i.e. effluent of the acidogenic reactor) contained S2� which

appeared as a result of SO42� reduction in the acidogenic

reactor. Thus it would be reasonable to expect S2� in the

effluent of the methanogenic reactor even if it were not

produced in it. Fig. 10 shows the influent and effluent S2�

concentration of the methanogenic reactor. Higher S2�

concentration in the effluent of the reactor than its influent

confirms the reduction of SO42� in the reactor. S2�

completely disappeared from the effluent on addition of

MoO42�, but reappeared when MoO4

2� addition was

discontinued. Though SO42� reduction was observed at

the reduced loading rate, Fig. 10 shows that the S2�

concentration in the effluent was not substantially higher

than that in the influent (as it was prior to MoO42� addition).

eff (mg/l) MoO42� (mM) SO4

2� removal (g/day) Efficiency (%)

0 16.57 39

2.5 0.71 2

0 8.12 19

0 7.75 74

r (3) and 88–94 for (4).


Fig. 4. Sulphide concentration vs. time for acidogenic reactor.

The reduced load together with the high SO42� removal

in the acidogenic reactor (Table 4) left only a very small

amount of SO42� in the influent of the methanogenic reactor

(200 mg/l). The efficiency of SO42� reduction in the

methanogenic reactor during this period was 15%. Bearing

in mind that stochiometrically, the S2� produced equals one-

third of the SO42� reduced, an estimate of S2� produced

yields 200 � 0.15 � (1/3) i.e. only 10 mg/l. Apart from

assimilation into cells and precipitation in the reactor, this

amount will be distributed between the liquid and gas phases

of the reactor contents and considering the possibility of loss

of S2� as H2S gas during transportation and analysis of

samples and the accuracy of the visual colour comparison

technique used for its determination, it is not unreasonable to

Fig. 5. VFA-COD relative to CO

expect little difference in the influent and effluent S2�

concentrations.

3.2.4. Volatile fatty acid (VFA)

The equivalent COD of VFA relative to the total COD of

the reactor effluent is shown in Fig. 11 while Table 7 shows

the degree of acidification in the methanogenic reactor. The

effect of MoO42� was similar to that observed in the case of

the acidification reactor i.e. from initial inhibition by

MoO42� to acclimatisation and recovery after MoO4

2� was

omitted from the feed. At reduced loading rate, acidification

was almost complete here as well (Fig. 11).

The degree of acidification calculated as 50% for the last

phase of the experiment is not realistic as the unacidified

D for acidogenic reactor.


Fig. 6. Individual VFA distribution in acidogenic reactor.

influent COD was only 20 mg/l which is merely about 2.0%

of the soluble COD. This is a very low concentration and

keeping in view the high concentration of the influent VFA

(>1100 mg/l), this low value is unlikely to cause a

significant measurable difference.

Fig. 12 shows the variation of individual volatile acids in

the effluent of the methanogenic reactor over the period of

study. Generally, the concentrations of volatile acids were in

the order acetic > butyric > propionic > valeric prior to

MoO42� addition (similar to that of the acidogenic reactor).

Addition of MoO42� caused butyric acid to dominate over

acetic acid. On exclusion of MoO42� from the feed, the

concentration of butyric acid decreased such that it was

comparable, but lower than the concentration of acetic acid.

Fig. 7. Methane production vs. t

Reduced loading rate resulted in a distinct dominance of

acetic acid again.

It may be recalled that in the last phase (i.e. reduced

loading rate) of the experiment, there was no observable

COD reduction (Table 5). It appeared that this reactor too

was behaving merely as an acidification reactor.

3.2.5. Methane (CH4)

Fig. 13 shows the daily CH4 production and percentage of

CH4 in the digester gas. Addition of MoO42� caused

cessation of CH4 production. Tables 5 and 7 show that before

MoO42� addition the reactor was able to treat an influent

VFA concentration of over 3200 mg/l with an overall soluble

COD removal efficiency of 44%. After exposure to MoO42�

ime for acidogenic reactor.


Fig. 8. Soluble COD concentrated vs. time for methanogenic reactor.

Table 5

COD of methanogenic reactora

Number Flow (l/day) OLRb (kg COD/(m3 day) CODinf (mg/l) CODeff (mg/l) MoO42� (mM) COD removal (g/day) Efficiency (%)

1 6.45 5.50 4050 2250 0 11.61 44

2 6.34 8.12 6080 5620 2.5 2.92 8

3 6.64 8.14 5820 5740 0 0.53 1

4 6.71 1.62 1150 1230 0 – –

a Values are average based upon days 35–43 for (1), 46–50 for (2), 62–76 for (3) and 88–94 for (4).b Organic loading rate.

the CH4 production, however, did not resume even though

MoO42� was omitted from the feed and the VFA

concentration was reduced to about 1100 mg/l (reduction

of loading rate). Methanogens were neither able to

acclimatise to MoO42� nor recover from its shock. It

Fig. 9. Sulphate concentration vs. ti

appears that the nature of toxicity of MoO42� to MPBs is

bactericidal rather than bacteriostatic.

Previous studies [3,4] show that MoO42� was effective as

a selective SRB inhibitor at concentrations of 3–5 mM in

batch experiments, while its feasibility in continuously fed

me for methanogenic reactor.


Table 6

SO42� of methanogenic reactora

Number Flow (l/day) SLRb (kg COD/m3 day) SO42�

inf (mg/l) SO42�

eff (mg/l) MoO42� (mM) SO4

2� removal (g/day) Efficiency (%)

1 6.45 2.47 1820 1300 0 3.35 29

2 6.34 3.87 2900 2710 2.5 1.20 7

3 6.64 3.33 2380 2070 0 2.06 13

4 6.71 0.28 200 170 0 0.20 15

a Values are average based upon days 35–43 for (1), 46–50 for (2), 62–76 for (3) and 88–94 for (4).b SO4

2� loading rate.

Fig. 10. Sulphide concentration vs. time for methanogenic reactor.

anaerobic reactors could not be established. The present

study shows specific inhibition of the methane producers due

to MoO42�. This is evident from the fact that after the

exclusion of MoO42� from the feed acetic acid production

recovered but methane production did not. Accumulation of

Fig. 11. VFA-COD relative to CO

VFAs (mainly acetic acid) after MoO42� addition has also

been reported by other researchers [5,6].

It appears that the reason behind the discrepancies in the

successful use of MoO42� as a selective SRB inhibitor

reported in literature is attributed to the way the experiments

D of methanogenic reactor.


Table 7

Acidification in methanogenic reactora

Number Influent (mg/l) Effluent (mg/l) MoO42� (mM) Acidification (%)

COD CODVFA Unacidified COD COD CODVFA Unacidified COD

1 4050 3270 780 2250 1830 420 0 46

2 6080 2300 3780 5620 2420 3200 2.5 15

3 5820 3920 1900 5740 4550 1190 0 37

4 1150 1130 20 1070 1060 10 0 50

a SO42� values are average based upon days 35–43 for (1), 46–50 for (2), 62–76 for (3) and 88–94 for (4).

Fig. 12. Individual VFA distribution in methanogenic reactor.

were conducted. Experiments showing its effectiveness were

generally conducted using serum bottle techniques (static,

one-time feed procedure) or for a short duration (a few hours

or days).

Fig. 13. Methane production f

It should be realised that none of these procedures

represent the true-life situation, which is characterised by

dynamic and long-term operation. These procedures at best

would only reflect the instantaneous response of the biomass

or methanogenic reactor.


and still leave sufficient room for uncertainties. For

example, because of a relatively delayed inhibition of

MPBs compared to SRBs, ample CH4 could have been

produced and collected in the head-space of a serum bottle to

provide a misleading conclusion. Also, a short-term

experiment will not reveal the acclimatisation capability

of bacteria as was demonstrated by SRBs in the present

study.

4. Conclusions

� SO42� reduction in the acidogenic reactor was higher than

was in the methanogenic reactor.

� M
oO42� induced inhibition of SO4
2� reduction was

immediate and complete.

� S
RBs showed complete recovery from the MoO42� once
it was omitted from the feed.

� S
howing their high potential for acclimatisation, acido-
gens acclimatised to MoO42� even with its continued

inclusion in the feed.

� V
FA dominance was in the order acetic > butyric >propionic > valeric prior to exposure to MoO4
2� and with

the addition of MoO42� butyric acid became dominant

due to build-up of H2 in the reactor. Acetic acid

dominance in the reactor was resumed after the loading

rate was reduced.

� M
oO42� inhibited methanogenesis. Neither the dis-
continuation of MoO42� addition in the feed nor the

reduction of loading rate helped to re-start CH4

production.

� M
PBs were neither able to acclimatise to MoO42� nor
recover from its shock.

� M
oO42� is bactericidal to MPBs and bacteriostatic to
SRBs.

� U
se of MoO42� as a selective inhibitor of SRBs in
anaerobic reactors is not feasible.

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molybdate inhibition of sulphate reduction in two-phase anaerobic digestion

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