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1 Anaerobic Co-digestion as a Smart Approach for Enhanced Biogas Production

mesophilic and thermophilic digesters, for example, at 55 ^∘C, hydrogenotrophs are

found to dominate and if properly supplemented by syntrophic acetate-oxidizing

bacteria [14] could even lead to sustainable biogas production in complete absence

of acetoclastic methanogens.

1.2.5.4

Volatile Fatty Acids

Efficient monitoring of digesters can also be carried out by constant evaluation of

VFA content of the digesters. Though VFA accumulation above 2000 mg/l leads

to digester failures, still it should be kept in mind that the same VFA gets finally

converted to methane, in fact carbon atom of VFA is the principal source for

methane production. The answer lies in the nature of VFA that accumulates in the

digesters; most preferred form of VFA is acetic acid as it is the essential substrate

for methanogens.

Fatty acid oxidizing bacteria breakdown LCFA to acetic acid, and these bacteria

are inherently resistant to the toxic effects of accumulated LCFA. It has been noted

that microbial load of fatty acid oxidizing bacteria fluctuates within the digesters

directly influencing LCFA conversion rate, and their total absence in digesters leads

to digester failures. Fatty acids oxidizing bacteria have been identified to be either

producer of hydrogen (obligate hydrogen-producing acetogens [OHPAs]) or hydro-

gen consumer (homoacetogens) but certainly lead to the formation of acetic acid.

Not all VFA contributes to methane, certain volatile acids have a deleterious effect

on the overall process especially propionic acid, and its accumulation decreases the

pH to an extent of inhibiting the growth of methanogens, leading to fall in biogas

production.

1.2.5.5

Ammonia

High protein content-based feedstocks on AD can trigger an alkaline shock with

accumulation of ammonia or ammonium ions, at about pH 8.0 the drastic reduction

in microbial activity can be noted and with pH reaching 8.5 can completely deacti-

vate methanogens thereby completely stopping methane production. The problem

can be circumvented by balancing C/N ratio of the feedstock; immediate actions

would be to reduce loading rate and further diluting the digester content. This cor-

rective action can quickly adjust the pH to optimum range, it is imperative that the

microbial consortia play a significant role in AcD.

Both ammonia and VFA thus play a crucial role and are intricately related to pH

fluctuations; a VFA/ammonia ratio of 0.1 is preferred for a balanced sustainable

digesters and increase to 0.5 indicates that the digesters could fail and further rise

can completely stop biogas production.

1.2.5.6

Organic Loading Rate

Continuously operated digesters require balanced input of feedstock, (feed-

stocks/organic) loading rate (OLR) refers to the rate at which the feedstocks are

fed into the digesters. OLR depends on the waste composition and is directly

correlated to microbial growth rate, substrate conversion rate and evaluated by

the rate of methane production. Excess OLR can dilute the microbial load, reduce