The specific dilution rate of the feedstock (manure) is important for biogas production from small
digesters due to the effect of dilution on hydraulic retention time. The model presented here can be
used to construct digesters with suitable dimensions tailored to the number of animals and the local
climate. According to our calculations, the currently practiced manure:water dilution ratio of 1:9 is not
advisable if the digester must be heated in the winter. In the subtropics, where winter temperatures can
fall below 20 °C, using biogas to heat simple digesters buried in the soil means that less gas is
available to households. Using a combination of solar provided heat and biogas to heat the digester
(the latter during periods without solar heat) can provide enough surplus biogas to meet household
requirements on a standard pig-producing farm in Vietnam. For example, heating the digester to 35 °C
with the biogas contributing 10% of the heat means that a standard dome digester can almost produce
the energy needed by the household during a “normal” winter by increasing the volume of gas
delivered by approx. 50%. To meet family energy needs, 17 pigs will be needed instead of the standard
number of 16 pigs. If biogas contributed 20% of the energy used to heat the digester, then the manure
from 22 fatteners would be needed on a farm with a “standard” retention time of 35 days to meet the
energy requirements of the household. Very few data are needed to carry out the calculations of this
model, therefore, the calculations with the model will be very useful when advising farmers to invest
in biogas digesters, and also if the farmers have to decide between constructing a joint larger
communal digester that could be heated instead of one digester per farm.
The specific dilution rate of the feedstock (manure) is important for biogas production from small
digesters due to the effect of dilution on hydraulic retention time. The model presented here can be
used to construct digesters with suitable dimensions tailored to the number of animals and the local
climate. According to our calculations, the currently practiced manure:water dilution ratio of 1:9 is not
advisable if the digester must be heated in the winter. In the subtropics, where winter temperatures can
fall below 20 °C, using biogas to heat simple digesters buried in the soil means that less gas is
available to households. Using a combination of solar provided heat and biogas to heat the digester
(the latter during periods without solar heat) can provide enough surplus biogas to meet household
requirements on a standard pig-producing farm in Vietnam. For example, heating the digester to 35 °C
with the biogas contributing 10% of the heat means that a standard dome digester can almost produce
the energy needed by the household during a “normal” winter by increasing the volume of gas
delivered by approx. 50%. To meet family energy needs, 17 pigs will be needed instead of the standard
number of 16 pigs. If biogas contributed 20% of the energy used to heat the digester, then the manure
from 22 fatteners would be needed on a farm with a “standard” retention time of 35 days to meet the
energy requirements of the household. Very few data are needed to carry out the calculations of this
model, therefore, the calculations with the model will be very useful when advising farmers to invest
in biogas digesters, and also if the farmers have to decide between constructing a joint larger
communal digester that could be heated instead of one digester per farm.
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