Biogas production in climates with long cold winters - SuSanA

2007). In such a reactor the HRT is the same as the SRT. The volume of the sludge filled part
of the digester is therefore equal to the retention time of the sludge times the flow rate. The
gasholder is adapted to fit the gas requirement of the end-user (family). Important to consider
is the daily pattern of cooking. The longer period between the meals, the more gas is produced
in between meals and subsequently the larger the gasholder should become.
The fixed dome digester is relatively inexpensive. It is simple, has no moving parts and has
therefore a long lifespan, up to 20 years (GTZ, 1999). The plant is suitable for cold climates
because most part is beneath the ground level. Therefore the plant is protected against low
temperatures occurring during night and in cold seasons. The temperature within the digester is
lower during daytime and higher during nighttime (GTZ, 1999). This fluctuation is beneficial
for the methanogenic bacteria and subsequently for the biogas production. The main
advantages and disadvantages:
Advantages of the digester
1. Relatively cheap and durable
2. Construction is labor intensive (local employment opportunities)
3. No moving parts
4. Well insulated
Disadvantages
1. High technical skills are required for a gas tight construction
2. Special sealant is required for the gasholder
3. Gas leaks occur when not designed well
4. Difficult to construct in bedrock
5. Amount of gas available for cooking is hard to detect
6. Enormous structural strength required for construction (Sharma et al, 1991)
Experiences for the China biogas program teaches us that special attention is required when
constructing a fixed-dome digester (Daxiong, 1990). GTZ only advices to construct such a
plant under the supervision of experienced biogas technicians (GTZ 2007). This should not be
taken lightly; Mohamed Nazir (1991) concluded in his study “biogas construction technology
for rural area” , that experience from Pakistan showed that the Chinese dome model failed
most cases as a result of low gas pressure due to persistent leakage problems and seepage
problems. Cleary, the design has many favorable aspects, but its success is dependent on the
input of high technical manufacturing.
2. Floating drum digester
Floating drum digesters are mainly found in India and this is semi CSTR type reactor. A
floating drum digester is shown next, figure 2.
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