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this work was carried out using anaerobic dark co-fermentation of SLS and POME under
thermophilic temperature to enhance the efficiency of biogas formation.

2.4.3 Operating conditions

2.4.3.1 Temperature

A wide range of temperature is possible for anaerobic dark fermentation, divided into
three temperature is comprehensive psychrophilic (< 20°C), mesophilic (25-40°C), thermophilic
(45-60°C), and extreme-thermophilic conditions (> 60°C). Temperature is considered as one of
the most important factors affecting on biogas production because it has direct effect on
physical-chemical properties of all components in the digester and also affects thermodynamic
and kinetic of the biological processes. There are several advantages of increasing temperature
including increase chemical and biological reaction rate, shorter hydraulic retention time (HRT)
in a continuous process, improve diffusivity of soluble substrate, decrease liquid viscosity,
increase death rate of pathogenic bacteria. On the other hand, the increasing temperature in turn
will also increase the concentration of free-ammonia (NH 3) which is inhibitory to
microorganisms.

2.4.3.2 pH and buffers

pH value has play a vital role on enzyme activity in microorganisms because each group
of microorganisms has different optimal pH range. Furthermore, enzyme is active in a specific
pH range and also has maximum activity under the optimal pH. Several researchers reported
most of the hydrogen producing bacteria is fast growing and prefers slightly acidic pH range
from 5-6 with an optimal pH of 5.5 (O-Thong et al. 2008). On the other hand, methanogenic
archae can function within a relatively narrow pH interval between 5.5-8.5 with an optimum pH
interval between 7.0-8.0 (Boe, 2006). The pH level also affects acid-base equilibrium of different
compounds in the digester. At high pH, the accumulation of free ammonia produced during
degradation of proteins or by the present of ammonia in the feed can cause weak base, while low
pH, the accumulation of free VFA can cause weak acid inhibition.

-
Bicarbonate (HCO 3 , pKa 6.3) is often the main buffer in anaerobic digesters to resist the
pH in the anaerobic digester change. Furthermore, the present at high concentration of ammonia
2-
+
-
(NH 4 /NH 3, pKa 9.3), hydrogen sulfide (H 2S/HS /S , pKa 7.1 and 13.3), and hydrogen