Page 45 - Energy from Toxic Organic Waste for Heat and Power Generation

Energy Extraction From Toxic Waste Originating From Food Processing Industries 33

Table 3.3 Rate of biogas production and methane yield for some digestible materials
3
Feedstock Biogas yield (m /kg TS) CH 4 %
Animal barnyard manure 0.260–0.280 50–60
Pig manure 0.561 45–68
Horse droppings 0.200–0.300 55–65
Green grass 0.630 70
Flax straw 0.359 55
Wheat straw 0.432 59
Leaves 0.210–0.294 58
Sludge 0.640 50
Brewery liquid waste 0.300–0.600 58
Carbohydrate 0.750 49
Lipid 1.440 72
Protein 0.980 50

below 7% was unstable, and it was overloaded when it was > 10% [59].
When a proper solid concentration between 7% and 10% was maintained,
3
3
the biogas yield increased to 0.46 m /day at 37°C and 0.68 m /day at 55°C
[59]. The rate of biogas production and methane yield for some digestible
materials is given in Table 3.3.

Agitation
Rubia et al. [60] investigated the effect of the particle size of sunflower
oil cake and agitation on biogas production in a batch type digester. They
have also observed that, for an ultimate improvement in biogas production,
stirring of digester slurry was essential. Stirring of the slurry improves the
contact between the microorganism and the substrate. Hence, the micro-
organisms can yield more quantity and quality biogas. The stirring can be
done by installing several mixing devices like stirrer, blower, etc. Also, the
agitation can be increased by flushing the slurry from the digester by insert-
ing the nozzle again into it.

Seeding of the Biogas Plant
It is often necessary to introduce enriched seeding bacteria into the digester
for starting up the anaerobic digestion process. Generally, the digested slurry
from a running biogas plant or material from well-rotted manure pit, or
cattle dung slurry, was used as the seed. If the volatile fatty acids were ac-
cumulated during the operation due to overloading, then these fatty acids
could be used by reseeding or temporarily stopping the feed to the digester
or by adding lime in a requisite quantity [41].