3.9.4 Retention Time
Hydraulic retention time is the time which depicts the number of days the liquid
fraction stayed in the digester before they pushed through the outlet. The HRT is
determined by the ratio of active slurry amount to biomass input flow rate.
High HRT--------------------more biodegradation:
3.9.5 Solid Retention Time
This is the number of days solids remain in the digester. Solids are retained due to
sedimentation in the case of unstirred digesters.
High SRT-------------------decrease HRT:
So the manual removal of solids is important for desired HRT and gas production
(Verma 2002).
3.9.6 Carbon-to-Nitrogen Ratio
The C/N ratio is the measure of relative amount of carbon and nitrogen in organic
materials. It is important to estimate the nutrient (C) insufficiency and process
inhibition by ammonia by adding extra “C” source. The reported optimum C/N is
16:25.
High C=N-------lower gas production quick consumption of nitrogen by methanogens
ð
Þ :
Low C=N--------increased pH lethal to methanogens
ð
Þ :
3.9.6.1 Sample Raw Materials C/N Ratio
To maintain the desirable level of C/N ratio, different materials with high C/N ratio
are mixed with those with low ratio, such as municipal waste, biosolids, animal
manure, etc. (Karki et al. 1994). The C/N ratio of different types of frequently
available raw materials is shown in Table 3.9.
3.9.7 Stirring
Stirring is the process which increases the biogas production. Mixing combines the
fresh feed with digestate and the microbes. So the stirring increases the fermentation.
Mixing should be done in such a manner that air (oxygen) doesn’t enter the digester.
Violent or mechanical agitation stops the digestion. Stirring is advantageous in some
types of digesters. If not agitated properly, scum will be formed negatively affecting
92
S. Elangovan et al.
Hydraulic retention time is the time which depicts the number of days the liquid
fraction stayed in the digester before they pushed through the outlet. The HRT is
determined by the ratio of active slurry amount to biomass input flow rate.
High HRT--------------------more biodegradation:
3.9.5 Solid Retention Time
This is the number of days solids remain in the digester. Solids are retained due to
sedimentation in the case of unstirred digesters.
High SRT-------------------decrease HRT:
So the manual removal of solids is important for desired HRT and gas production
(Verma 2002).
3.9.6 Carbon-to-Nitrogen Ratio
The C/N ratio is the measure of relative amount of carbon and nitrogen in organic
materials. It is important to estimate the nutrient (C) insufficiency and process
inhibition by ammonia by adding extra “C” source. The reported optimum C/N is
16:25.
High C=N-------lower gas production quick consumption of nitrogen by methanogens
ð
Þ :
Low C=N--------increased pH lethal to methanogens
ð
Þ :
3.9.6.1 Sample Raw Materials C/N Ratio
To maintain the desirable level of C/N ratio, different materials with high C/N ratio
are mixed with those with low ratio, such as municipal waste, biosolids, animal
manure, etc. (Karki et al. 1994). The C/N ratio of different types of frequently
available raw materials is shown in Table 3.9.
3.9.7 Stirring
Stirring is the process which increases the biogas production. Mixing combines the
fresh feed with digestate and the microbes. So the stirring increases the fermentation.
Mixing should be done in such a manner that air (oxygen) doesn’t enter the digester.
Violent or mechanical agitation stops the digestion. Stirring is advantageous in some
types of digesters. If not agitated properly, scum will be formed negatively affecting
92
S. Elangovan et al.