Anaerobic Digestion of Aqueous Waste for Methane and Hydrogen
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and pretreatment (Section 8.6), co-digestion (Section 8.5), and digester technology
(Section 8.7) on the biogas production. Some details of biogas purification (Section 8.9)
and the usage of produced biogas and digestate (Section 8.10) are also briefly examined.
8.3 miCrOBes and the eFFeCts OF OPeratinG COnditiOns
While the success of anaerobic treatment depends on the effectiveness of various
microbes, very little is known about how they work and the interactions between
them, and this lack of knowledge sometimes results in malfunction and failure of
biogas digestive process. Furthermore, only few percent of bacteria and archaea have
been isolated. However, with new molecular techniques, more information about
the community structure in the anaerobic processes can be obtained [41–44]. The
quantification of methanogens can be carried out by fluorescence in situ hybridization technique. Klocke et al. [43] detected 68 taxonomic groups by 16SrDNA analysis of samples from agricultural biogas plants and showed that hydrogenotrophic
methanogens dominate most of the agricultural biogas plants [5]. The effectiveness
of microbes in anaerobic digestion process depends on the temperature, ammonia
inhibition, pH, and presence of nutrients. The effects of these operating variables on
the digestive process are briefly described in Sections 8.3.1 through 8.3.3.
8.3.1 eFFeCTS oF TemPerATure And AmmoniA inhiBiTion
The digestive process can be operated at lower temperature, that is, mesophilic
conditions (temperature range of 35°C–42°C), or at high temperature, that is, thermophilic conditions (temperature range of 45°C–60°C). Generally, temperature
fluctuation decreases the biogas productivity. In general, under thermophilic conditions, the growth rate of methanogenic bacteria is higher, and the process is more
efficient and faster [42,45]. However, these bacteria are more temperature sensitive
and have difficult time adjusting to temperature variations. The faster rate allows
the operations to run at lower hydraulic retention time (HRT) than in mesophilic
operations. Mesophilic bacteria, however, can tolerate temperature fluctuation of ±3°
variations without a significant variation in methane production.
Since ammonia toxicity increases with temperature, thermophilic operations are
more susceptible to ammonia inhibition (particularly for the ammonia concentration
above 80 mg/l). An increase in ammonia concentration is, however, accompanied
by an increase in volatile fatty acid concentration [5]. This can lower the pH and
thus counterbalance the effect of ammonia. Many strategies to reduce the ammonia
inhibition effects have been examined [46,47]; the most stable digestive process was
observed when biomass was diluted with reactor effluent.
8.3.2 ph eFFeCT
The pH of the reacting solution has also significant effects on the effectiveness of
bacteria and methane production. The anaerobic digestion process best operates
between pH of 6.5 and 8.5 with an optimum value between 7 and 8 [5]. The process is severely affected when pH drops below 6 or increases above 8.5 [5]. While
ammonia accumulation increases the pH, and VFA (volatile fatty acid) accumulation