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2-
3-
phosphate (H 3PO 4/H 2PO 4 /HPO 4 /PO 4 , pKa 2.1, 7.2, and 12.3) are also influence the pH
balance in the anaerobic digester (Boe, 2006).

2.4.4.3 Organic loading rate

The organic loading rate is an important operational parameter for continuously biogas
production in continuous processes, to avoid overfeeding which could potentially inhibit or
overload the process and lead to decrease in biodegradability and process failure. Organic
loading rate is amount of organic matter fed per unit volume of digester capacity per unit time. A
study from Saraphirom and Reungsang (2010) showed that the substrate removal efficiency was
found to decrease when OLR was increased and also the microbial community composition in
the digester was changed caused by the variation of OLRs.

2.4.4.4 Hydraulic retention time

Hydraulic retention time (HRT) is one of the most important operating parameter for
dimensioning the biogas digester. The average time interval during the substrate is kept inside
the digester is known as HRT. In fact, the retention time should be sufficiently long to ensure
that the quantities of microorganisms removed in the effluent are not higher than the quantities of
reproduced microorganisms. A study from Zhang et al. (2006) showed that there are some
advantages and some disadvantages of using a short HRT are comprehensive reduction of
diversity of microbial population associated with an elimination of propionate production
without affecting the existence of dominant species, lower both substrate consumption and
removal efficiency, and washout of microorganisms which was the presumably reason for the
observed decrease in hydrogen yield. Moreover, the HRT could be used as a tool to select
microbial populations whose growth rates are able to catch up with the mechanical dilution
created by continuous volumetric flow.

2.4.3.5 Toxic/inhibiting compounds

VFA is the main intermediate compounds such as ethanol, butanol, acetate, propionate,
butyrate, valerate, and lactate, produced during acidogenesis in anaerobic digestion process.
Generally, the VFA inhibition is due to their accumulation inside the digester and consequently a