Page 44 - Energy from Toxic Organic Waste for Heat and Power Generation

32 Energy from Toxic Organic Waste for Heat and Power Generation

for optimum biogas production. Therefore, it is often necessary to period-
ically monitor the pH of digester to keep track of the changes in alkalinity
throughout the anaerobic digestion process.
Gupta et al. [57] investigated the biogas production in batch reactors,
with detoxified Mahua seed cake as feedstock. They observed that the di-
gester containing cattle dung and water in the ratio 1:1 gives a pH range
between 6.4 and 7.2. On the other hand, the digesters containing cake
mixtures showed an initial decrease in the pH (5.9–6.1) for about 6–9 days,
indicating a fast hydrolytic and acetogenic phase, and then moved toward
the alkaline range.
Desai and Madamwar [58] reported that the amount of CO 2 and
volatile fatty acids produced during anaerobic digestion affect the pH of
the digester slurry. For a smoother anaerobic digestion, the concentration
of volatile fatty acids and acetic acid in the feedstock should be below
2000 mg/L.

Hydraulic Retention Time
It is the average time consumed by the input feedstock inside the digester
before it comes out after digestion. Generally, the HRT depends upon the
tropical climate condition. A shorter retention time is likely to face the risk
of less active bacterial action, while a longer retention time requires a larger
volume of the digester, and hence, a higher capital investment.
Baserja [59] observed a maximum gas production of 2.2 L/day
(CH 4 = 62%) at an HRT of 10 days having a loading rate of 6 g TS/L,
while treating a mixture of cattle dung, poultry waste, and cheese with
ratio of 2:1:3. The retention time ranges between 30 and 60 days, and
only about 1/3 of the tank volume is used for active digestion. If an-
aerobic digestion is to be completed with other MSW (municipal solid
waste) disposal options, then the retention time must be lower than the
current standard of 20 days. Garba [50] indicated that, for mesophilic di-
gestion, the temperature range varies from 25°C to 40°C and the HRT
may be longer than 20 days.

Solid Concentration
The amount of raw materials fed into the digester depends upon the solid
concentration. Desai and Madamwar [58] documented their investigation
results related to biogas production from cheese whey, poultry waste, and
cattle dung. They observed that the best acceptable solid concentration for
a faster anaerobic digestion was in the range of 7%–9%. The total solid level