Organic Waste Recycling

Biofuels production 175

Figure 4.12 Fixed-dome digester with separate gas holder

Separate gas holder digester

This kind of system could be any of the digesters so far explained. The
difference is that a separate tank is added for the purpose of gas collection
(Figure 4.12). In addition, the gas holding tank can be used as a common
reservoir for a few digesters and therefore the system as a whole can serve larger
communities or industrial plants. The chief advantage of this kind of system is
that it provides a continuous supply of biogas (even in the case of batch system),
furthermore, the gas pressure can be regulated and therefore, appliances and
engines may be designed and used at their optimum working conditions.

Table 4.11 Design criteria and performance data of dispersed growth biogas digesters

Temperature inside digester (°C)

pH

Alkalinity (mg CaCO 3 /L)

C/N ratio

Loading rate

(kg VS/m^3 -day)

(kg COD/m^3 -day)

Feed solid content (%)

HRT (days)

Gas production (m^3 /day per m^3 of digester

capacity)

Fixed-dome digester

Floating gas holder digester

Volume for gas storage (% of 1-day gas

production)

Seeding material at start-up

25-40 (optimum 35, mesophilic)

6.6-7.6 (optimum 7.0-7.2)

2500-5000

25-30

1-4

1-6

5-10

10-60 (no sludge recycle)

0.15-0.40 (part of digester for gas

storage)

0.30-0.60

50-100

>50% of influent feed

Gas pipe

Water

Floater

Inlet

Gas

Slurry

Loose

cover

Bamboo cement

gas holder

Displacement

tank

Removable manhole cover

sealed with clay