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Anaerobic digestion of various feedstocks for second-generation biofuel production 165

6.4.3 pH

The digestion process is recalcitrant due to variations in levels of pH. Microorganism
growth is also affected by acidic conditions, which results in lower methane yield.
Compared to methanogenesis organisms, acidogenesis microorganisms are relatively
less sensitive to pH. The optimal value is pH 5.5 and 6.5 for hydrolysis and
acidogenesis, respectively [33]. The pH value of the substrate can be adjusted by
adding a solution of sodium bicarbonate, sodium carbonate, ammonium hydroxide,
potassium hydroxide, and sodium hydroxide. Deublen et al. investigated the optimum
pH value in the range of 6.7–7.5 for the growth of methanogenic bacteria. Jay Chang
et al. states that instead of methane production, hydrogen production occurs when the
pH value falls below 6. Shambo et al. [42] said that maximum biogas yield is reported
when using cow dung as a feed with the pH value of 7 (pH value indicates a healthy
environment for the microorganisms in the digester). Sambo et al. studied three dif-
ferent parameters. 4,7,9 a better biogas yield was obtained when the cow dung is used
as the raw material with a pH of 7 follow-ups of 9 and 4. Bougrier et al. examined
seven different pH parameter from (5 to 8), using spoiled milk as a feedstock, in which
pH 7 produced a better yield than other pH parameters [43]. Bien et al. [21] investi-
gated bioethanol waste as a feedstock produced better biogas yield with a pH param-
eter value of 7 compared with pH of 8 and 6.

6.4.4 Retention period

The period for which organic material remains inside the digester is called retention
time. It is divided into two types, solid retention time and hydraulic retention time.
The time associated with microorganisms present in the digester denotes solid reten-
tion time. Hydraulic retention time (HRT), better known as hydraulic residence time,
is a measure of the average time that feedstock remains in an anaerobic digester unit
[44]. The biogas yield will be higher if the HRT of the feedstock in the digester is
longer. For a given feedstock feeding rate, a more prolonged HRT will correspond
to a more substantial digester and therefore more investment. In practice, there is
an optimum HRT that is a compromise between gas production and digester volume
[45]. In a typical two-stage mesophilic digestion, HRT varies between 15 and 40 days
while for single-stage thermophilic digestion, residence times usually are faster and
take around 14 days. HRT can significantly affect the growth and metabolic activities
of microorganisms, therefore affecting the production rate of methane. Short HRT
favors the establishment of the acidogenic phase and prevents the establishment of
methanogens. The short HRT also leads to the washout of the population of anaerobic
microorganisms. Therefore, it becomes a challenge to maintain a high density of
anaerobic microorganisms in the digester. Using long HRT can increase the quality
of biogas as well as save the microorganism population [46]. Sharma et al. [47] inves-
tigated maximum biogas yield produced when seaweed is used as a feedstock with
six different solid concentration from 5 to 30g/L. Bragugha et al. [48] states that
maximum biogas yield is produced as 20% total solid followed by 15.25% and
10% total solid content of Jatropha seed cake feedstock.