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4.9 Exercises
4.1 a) You are to visit a biogas digester located at a farm or at a wastewater
treatment facility, and find out the organic loading rate, HRT, and biogas
(CH 4 ) production rate. Compare these data with those given in Table 4.11 or
4.14. Discuss the results.
b) Also check with the biogas digester operator about the common problems
of the digester and the remedial measures employed to solve these problems.
4.2 A farm has 100 cattle, each producing 40 L manure/day with 12.5% total
solids (125 kg TS/m^3 ), volatile solids (VS) is 75%TS and a C/N ratio of 15.
The manure is to be used for biogas production together with rice straw,
which has a C/N ratio of 80, moisture content of 50%, VS is 80%TS,
bulking density of 0.5 kg/L and N content of 0.9% dry weight. How much
rice straw is needed to be mixed with the cattle manure to reach a C/N ratio
of 25? At the VS loading of 2 kg/m^3 -day and an HRT of 40 days, determine
the volume of the biogas digester and the volume of dilution water required
to mix with the manure.
4.3 A biogas digester produces 5 m^3 of biogas per day, which contains 35% of
CO 2 by volume. The density of CO 2 is 1.80 kg/m^3 at 25°C and 1 atmosphere
pressure. Determine the daily requirement for quicklime (containing 95%
CaO) to remove the CO 2.
4.4 A 1000-pig farm plans to purchase 8-m^3 PVC bag digesters to produce
biogas using pig wastes in its own stock. Each pig weighs 80 kg. The