1.2 Anaerobic Co-digestion (AcD)

11

preferred for sustainable biomethanation; and any variation in pH can drastically

cut down methane production. Fluctuations in pH are one of the biggest problems

associated with AD and mostly shift toward lower pH, which is directly attributed

to accumulation of VFAs. Sometimes pH may shift toward alkalinity contributed

by accumulation of ammonia. This pH problem is due to microbial metabolism,

especially by higher growth activity of acid-producing bacteria, compounded by the

absence of buffering agents. Simultaneous degradation of proteins can lead to forma-

tion of ammonia that could help in balancing of pH in a digester averting shift toward

acidic range. As mentioned earlier, too much of protein degradation in digesters

can lead to excessive ammonia shifting pH toward 8.0 that shuts down microbial

activity. The pH fluctuations should be seriously dealt with and a delay could per-

manently alter the microbial population of the digesters and sometimes cause irre-

versible damage to digester performance. Either way the methanogens are said to

very sensitive to pH change and the problem can be overcome by neutralizing the

pH with an alkali or a weak acid, but could turn to costlier affair to invest on alkali

treatment, which is not generally recommended. A robust and an efficient micro-

bial population of VFA converters are essential, while few digesters have adopted

for dual digesters/two-stage digestion for circumventing the pH problem.

1.2.5.2

Carbon–Nitrogen Content

It is essential to know the total carbon (TC) and nitrogen (N) content of the feed-

stock while the optimum C/N ratio for AD should preferably be in a range of 20–30.

And increase in the value signifies the problem of nitrogen shortage leading to lesser

load of microbes and process of AD getting delayed while lower ratio could imply

higher microbial growth but the biogas could abruptly stop due to problems asso-

ciated with by-products of protein degradation significantly changing the digester

balance toward inactivity. The AcD thus plays a crucial role as we can finely balance

the carbon–nitrogen ratio for optimum biogas production.

Anaerobic digesters can work in a wide range of temperature; however, it been

noted that temperatures below 20 ^∘C can affect the efficiency of digesters by consid-

erably slowing down the process; still in natural habitats, methanogenesis is found to

happen significantly at low temperatures and over a period of time has contributed

to global warming [13].

1.2.5.3

Temperature

Eightfold reductions in COD can be observed with mesophilic and thermophilic

digestion at hydraulic retention time (HRT) of 35 days, while digesters at lower

temperature are stable for a longer period of time more than 45 days [12]. Digesters

around the globe are mostly operated in mesophilic conditions with recommended

temperatures of around 35 ^∘C, while faster digestion is generally reported at

thermophilic temperatures of 55 ^∘C but that comes with an inherent need of heat

exchangers for temperature maintenance that can either shoot up or drastically fall

reflecting microbial metabolism. Here biogas can be self-employed for heating the

digesters, and thus it could be self-sustained process without much investment.

It has been noted that the microbial population dynamics vary greatly between