32.7 Factors Influencing Biogas Production

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trace elements to the microbial activity. Supplementation of trace elements has

demonstrated promising effect on methanogenic population [5].

32.7.9

Environment/Alkalinity

A favorable environment for the microbiome in a digester can be maintained

by proper mixing, preventing overloading, excess water, and temperature in the

digester. The optimal volatile acid (VA)/Alkalinity (ALK) relationship ratio is

0.4, thereby resulting in good buffering capacity. The digester becomes sour when

volatile acids increase and alkalinity decreases. Shift in VA/ALK ratio from 0.5 to 0.8

is associated with drop in pH (less than 6.7). Under this condition, the percentage of

CO2 increases with the inability of the digester gas to burn. Addition of bicarbonates

to the digester is recommended to increase the alkalinity. The optimal AD results in

25–35% CO2 and 65–70% methane by volume [11].

The process stability depends largely on the reactor’s ability to resist a change in

pH. This is commonly known as its buffering capacity measured as alkalinity. Buffers

are essential in the reactors. During the digestion process, the methane workers also

produce some buffering material, such as bicarbonates, carbonate, and ammonia,

which goes into the solution. The amount of buffer produced at this stage is usually

enough to balance the acid produced by the acid workers so that the pH will remain

at a constant level. Reactors need to add a caustic material such as lime, soda ash,

caustic soda, or agricultural ammonia to raise the alkalinity. The most important

buffer system in anaerobic wastewater treatment is the bicarbonate buffer system.

Another important buffer system is ammonia: the presence of ammonia (NH3) shifts

the pH to higher values, which will decrease the toxicity of H2S and VFA, but the

toxicity of ammonia increases with increasing pH. Therefore, a low concentration

is beneficial (as nutrient and as buffer), but high concentration can be harmful [6].

The environment must be kept within extremely narrow ranges. The optimum con-

ditions are (Table 32.2):

Anaerobic conditions

No oxygen (air)

Temperature

36 ± 2 ^∘C

PH

7.0–7.2

No toxic material

VFA/Alkalinity

At least 0.50

32.7.10

Toxicity

The sensitivity for toxic compounds by anaerobic bacteria is always mentioned as the

major drawback of anaerobic. The toxic compounds can be classified as –inorganic,

natural organic, man-made organic (-antibiotic), and antibiotic toxins. Ammonium,

sulfur, and high salt concentration are some of the examples of inorganic toxins. Nat-

ural organic compounds cause toxicity to AD microbiota by polarity and hydrogen