1.3 Digester Designs

13

digestion, foaming, and lesser yield of methane. OLR is further related to HRT,

which implies the time taken by the digester for maximum gasification of the

feedstocks. Shorter RT is preferable to avoid accumulation of fatty acids and toxins

but way less than shorter RT can lead to microbial washout. Minimum one day RT

is enough for stable buildup of fermentation bacteria especially for protein and

nonfiber carbohydrates-based feedstocks; cellulose and hemicelluloses may require

two to three days to establish the process, while fat-based feedstock may require

longer RT of five days.

Complete gasification of waste can be achieved in a digester by increasing RT to

35 days (in case of batch digestion); the process is influenced by temperature: higher

the temperature, shorter the RT, and RT of more than 35 days is required for psy-

chrophilic temperature. Longer RT leads to improvement in quality of biogas in

terms of methane concentration, shorter RT may generally exhibit 70% methane con-

tent while the percentage of methane tends to increase with longer RT. Total solid

(TS) of more than 30% is not preferred for AcD as it leads to the problem of mix-

ing concentrated pockets of temperature and pH burst in a continuously operated

digesters depends on feedstock composition. The volatile solid (VS), which is a part

of TS, is generally preferred in a range of 60–90% for efficient biogas production and

for optimum microbial growth.

Pretreatment of feedstock is essential to minimize the natural flora on the surface

of substrate as it will hinder the role of potential consortium developed for the

purpose that is already active inside the digesters.

1.3

Digester Designs

The earliest digesters were simple in design with a digestion chamber, an inlet for

feedstocks, and two outlets, one for spent slurry and one for biogas. The appropriate

modeling of anaerobic digesters is imperative for biogas production. Digesters are

designed with the view of maintaining strict anaerobic conditions and for collection

and retrieval of biogas. The digesters can be operated in batch or continuous

phase. Anaerobic biogas digester such as the one used in WWTP is distinct as it is

continuously fed with heterogeneous liquid wastes, microbes agglomerate to form

the granules (sludge) that set in to form a layer/blanket with a constant upflow

hydraulic regime [15]. WWTPs around the world have opted for upflow anaerobic

sludge blanket (UASB) digester for anaerobic treatment, which has been found to

be cost-effective and emphasizes the role of microbial granules (solid phase) that

knit into a group of specialized agglomerated bacterial biofilm [16].

Expanded granular sludge beds (EGSBs) are a modified version and next-

generation biogas digesters with enhanced flow rate of liquid waste that could result

in mixing of sludge particles establishing contact with nutrient for the purpose of

breakdown. Further efforts have been taken to make thin, lighter-weight biofilm

of uniform thickness (granular sludge) for better fluidization and at lower energy

expenses in the form of inverse fluidized bed reactors (IFBR), which would reduce

HRT at a higher OLR that was initially carried out for distillery effluent [17].