31.4 Technological Trends and Challenges in the Anaerobic Biorefinery

481

4H2 + CO2

hydrogenotrophic methanogens

−−−−−−−−−−−−−−−−−−−−−→CH4 + 2H2O

(31.3)

4 CH3OH

methylotrophic methanogens

−−−−−−−−−−−−−−−−−−−−→3 CH4 + CO2 + 2 H2O

(31.4)

CH3OH + H2

methylotrophic methanogens

−−−−−−−−−−−−−−−−−−−−→CH4 + H2O

(31.5)

The reason for the shift of acetate consumers to hydrogen consumers is not known.

Many studies suggest that a syntrophic relationship between acetate-consuming and

hydrogen-consuming methanogens is present. Although acetoclastic Methanosaeta

species are the most dominant organisms for biogas production, the stability of

the reactor correlates better to the presence of hydrogenotrophic methanogens

[35]. Quantitative real-time PCR experiments showed that a shift in the archaeal

communities is associated with changes in the chemical composition of the reactor

[36]. Delbes et al. monitored the metabolome dynamics of several archaeal species

throughout an anaerobic digester crisis period [37]. They detected a high activity

and a substantiation of acetoclastic methanogens in the digesters, which followed

the acetate degradation pattern.

The resistance of methanogenic species in suboptimal conditions is an indicator

of efficient operation. Goux et al. reported that in a failed AD reactor a negative

correlation between Methanosaeta sp. and total volatile fatty acids (VFAs) con-

tent in the reactors exists [38]. They proposed the addition of hydrogenotrophic

Methanoculleus sp. to restore the performance of the anaerobic digester. This species

tolerates acidosis and promotes process recovery. Adaptation of Methanoculleus

sp. has also been examined in a membrane reactor treating swine manure and a

hybrid bioreactor exposed to OLR changes [39]. Studies conducted elsewhere found

that Methanosarcina sp. dominated methanogenic populations in two-stage anaer-

obic digestion to alleviate VFAs accumulation [40]. The second reactor showed

an elevated number of methanogens making up a likely scenario for enhanced

methane production. Methanosarcinaceae consume the remaining intermediates

from hydrolysis. They might improve their growth and resistance in digesters with

a lower pH [41, 42]. The acclimation procedure increased the Methanosarcina

population helping to reduce acetate and ammonia loads [41, 43].

The temperature of the digester is an important parameter for the methanogen

activity, and the feasibility of hydrogenotrophic methanogens dominance was exam-

ined in thermophilic conditions [44]. Methanothermobacter thermautotrophicus can

form >90% of the methanogenic community in a hyperthermophilic digester [45].

Another study treating synthetic wastewater and glucose to examine the effects

of high temperature (65–80 ^∘C) on the methanogenic distribution and the AD

efficiency showed a similar behavior of methanogens in the upper-temperature

levels [46]. Tuana et al. applied biological approaches (denaturing gradient gel

electrophoresis (DGGE), clone library, and pyrosequencing technique) for the

identification of archaeal sequences in a thermophilic digester that belonged to the

order Methanobacteriales instead of Methanomicrobiales in previous studies [47].

Psychrophilic anaerobic digestion (<20 ^∘C) has also been studied to determine the

dynamic of methanogenic species under low temperature and revealed that there