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31 Critical Issues That Can Underpin the Drive for Sustainable Anaerobic Biorefinery

and high-pressure approaches can promote and accelerate the future use of ligno-

cellulosic feedstocks for biogas production.

31.4.2

Multistage AD Process

Studies to evaluate and improve different reactor configurations (e.g. single- or mul-

tistage reactors) optimize the AD performance concerning improved methane yield,

organic loading rate (OLR), and process stability [30]. For example, AD performed

at two separate conditions produces more methane from organic compounds than a

single-stage process. The so-called two-stage AD involves two separate reactors for

hydrolysis + acidogenesis and acetogenesis + methanogenesis. The main reason for

the two-stage AD is the prevention of inhibition of microorganisms at low pH. The

first stage usually operates at an acidic pH of around 5.5–6.5 and utilizes a relatively

short hydraulic retention time (HRT) of 10 days for acid fermentation. Thus, the first

stage allows a relatively high OLR with a low pH value due to fast acid accumula-

tion and organic compound degradation. The benefits of the first-stage reactor are

better pH control and stability, increased volatile solids (VS) reduction, and better

pathogen removal. The second stage is usually maintained at a pH between 6 and

8 and utilizes a longer HRT of 20–30 days to facilitate the growth of slow-growing

methanogens [31].

Considering that the optimal pH for hydrolysis is lower compared with the

optimal pH of acidogenesis, Zhang et al. constructed a three-stage AD system

in which hydrolysis, acidogenesis, and acetogenesis + methanogenesis operate

separately. The three reactors should be vertically connected, resulting in a small

footprint in such a way that materials flow from one reactor into the next reactor by

gravity [32]. Based on several studies, most two-stage AD systems reported a 10–20%

increase in biogas production compared with single-stage AD [30]. The constructed

three-stage AD system reached more than 50% increase in biogas production and

nearly triplicated VS removal compared with single-stage AD [32].

The concept of multistage AD systems improved since technologists built the first

setups 20 years ago. However, the costs of such systems are a significant drawback

for economical applications [30]. Furthermore, a multistage AD system is of much

higher technical complexity compared with a single-stage AD system, and robust

automatization and process control algorithms are necessary to develop an efficient

process in the long run.

31.4.3

Dynamics of Methanogenic Communities

Researchers focus not only on the classification of microbial communities at a

particular time spot but also on the community shift between different classes of

methanogens along with different conditions [33]. Although extensive mapping

of the metabolomics networks and their interdependence are difficult, several

methane-synthesizing pathways exist in methanogens [34]. The methanogenic

pathways that convert methanol, CO2, or acetate in anaerobic digesters are:

CH3COOH

acetotrophic methanogens

−−−−−−−−−−−−−−−−−−→CH4 + CO2

(31.2)