Page 200 - Advances in Eco-Fuels for a Sustainable Environment

164 Advances in Eco-Fuels for a Sustainable Environment

which contain no detectable levels of pathogens (Water Environment Federations,
2004). The anaerobic digester that operates at the higher temperature range of
50–60°C is known as thermophilic digestion. The main advantage of thermophilic
digestion depends on the fact that a higher temperature reduces pathogens and these
temperatures provide more quick reaction rates when compared to mesophilic diges-
tion. Biosolids of quality classify A can be produced by preserving thermophilous
beaches of temperature when the criteria of temperature-times stated in the US
EPA Left 503 are filled [38]. The disadvantage of thermophilic anaerobic digestion
is the associated higher maintenance cost. The benefits of thermophilic digestion
(Water Environment Federations, 2004) are a reduction in volatile solids, more rapid
reaction rates for smaller retention times, a higher ratio of output gas volume to tank
capacity, improved dewaterability of the digested biosolids, and increased destruction
of pathogens. Fangs et al. [39] investigated that food waste feedstock with a temper-
ature range of 30–55°C produces gas production and COD removal rate at 50°C.
Pawlowski et al. state that maximum biogas and methane yield are produced at a tem-
perature of 300°C and 450°C, using seaweed as a feedstock at different temperatures
of 20°C, 35°C, and 45°C [40].

6.4.2 Pressure

The process of anaerobic digestion and the production of methane depend on the pres-
sure inside the digester. In general, pressure change brings more complications in
interactions among operational conditions and activities of microorganisms in a
digester. So to reduce the complications, the gas pressure is commonly maintained
slightly above the atmospheric pressure, preferably 0.02bar.
The activity of the microorganism present in the anaerobic digester depends on var-
ious parameters such as temperature, pH, and pressure inside the digester. Because the
activity of the microorganism is affected by pressure, the microorganisms are also
characterized into two groups namely, piezosensitive, and piezophilic microbes.
Piezosensitive microbes have ideal growth at atmospheric pressure, and they cannot
reproduce around 500bar. Because most microbes in anaerobic digesters are immu-
nized from wastewater treatment sludge, excrement, or sewage slurry under atmo-
spheric pressure, they are usually piezo-sensitive. It shows that these microbes’
growth rates are hardly repressed by pressure up to 10bars. Both piezo-tolerant and
piezophilic microbes are bacteria that can reproduce and multiply, sustaining pressure
up to 1000bar. The best growth rate of piezo-tolerant microorganisms, however,
occurs at atmospheric pressure [41].
The solubility in the liquid phase of the digester of some compounds depends on its
pressure. It is important to know that some of these compounds such as CO 2 or ammo-
nia have acid-basic reactions that control the pH of the digester. Therefore, the toxicity
effect of nonionized ammonia or nonionized H 2 S can be avoided directly by making
use of their high gas solubility under pressure. Therefore, there is a growth in a
pressure-reduced ammonia inhibition; on the other hand, a reduction in pressure
reduces free hydrogen sulfide inhibition [23].