1.2 Anaerobic Co-digestion (AcD)

9

with other VFAs leading to the formation of animal fat) [10] while active digesters

exhibit higher load of acetoclastic methanogens in comparison to hydrogenotrophs.

Even within digesters the microbial population may change, which can be

attributed to the complex metabolic processes leading to accumulation of various

intermediates that continuously influence the dynamics of microbial population.

Hence, there is need for inoculum development, which involves acclimatizing a set

of microbes to the digester environment; this could be done by pooling in a set of

potential dominant anaerobes isolated from successfully running digesters to form

a working consortium. Such microbial consortium had proven to give higher yield

of biogas and better degradation of biological waste [11].

Consortium development is mostly targeted on methanogens as they are

found to be the sole reason for biogas digester failure. The consortium has to

be tested under lab-scale digesters for their efficiency before implementing in

larger-scale biogas digesters. Care should be taken while developing consortium

to select potential strains capable of withstanding digester environment fluctua-

tions in pH and temperature, resistance to inhibitors, nutritionally diverse, and

can syntrophically coexist. Potential strains of methanogens have been mostly

identified to be hydrogenotrophic methanogens, acetoclastic and methylotrophic

methanogens. The most abundant species among hydrogenotrophic methanogens

are Methanobacterium, an hydrogen foraging methanogen that is known to dom-

inate rumen intestinal environment while its role in a typical biogas digester is

overshadowed by acetate utilizing methanogens (Methanosaeta, Methanosarcina,

and Methanospirillum) that represent nearly 75% of the methane produced in

digesters, still hydrogenotrophs are crucial for interspecies hydrogen transfer

between syntrophic bacteria that could help diminish the concentrations of fatty

acids in digesters [1], especially propionic acid as its presence can upset digester

performance.

As mentioned earlier, there are four groups of bacteria in a synergetic action in

digesters, each group of bacteria have their own physiological requirements and

show varying degree of growth efficiency and wide range of sensitivity to environ-

mental parameters. Acidogenic bacteria are among the fastest-growing organisms,

generally leading to quick accumulation of acid end products. While acetogenic bac-

teria and methanogens are slow-growing organisms, to further complicate the mat-

ter, the methanogens are found to be very sensitive to changes in environmental

parameters, which is detrimental for sustained biomethanation. Hence, inoculum

is a critical parameter for determining the efficiency of anaerobic digesters. There is

still diverse population of microbes that could not be cultivated and assessed from

AD, and hence, any potential microbial consortium that is developed in laboratory

should be considered as an supplementary feed and cannot by itself regarded as sole

group of organisms that could digest waste in a digester [12].

1.2.5

Real-Time Monitoring of AcD

Real-time monitoring is essential for sustainable biogas production, will help us

to continuously evaluate the digester performance, and help us to take immediate