174 Organic waste recycling: technology and management
Similar to the plug-flow digester is the 'bag digester' which is commonly used
in Taiwan and Korea. The early digesters of this type were made of neoprene
coated nylon, which was expensive. The present ones are made from red mud
plastic (RMP) which is residue of aluminum refineries. There are bag digesters
made of PVC in Central America. This is a very light weight digester, easy to
install and durable (estimated at 20 years). The Chinese have also started
producing this kind of digester. A rapid expansion in the use of bag digesters is
expected in China, due to its simplicity, durability, and low cost - US$8/m^3 of
digester volume (Figure 4.11). This digester is fed semi-continuously. The inlet
and outlet to the digester are at opposite ends. The biogas is accumulated at the
top of the digester and collected through the gas pipes. The digester is easily
installed by excavating a shallow trench slightly deeper than the radius of the
digester. This digester is designed to receive mostly swine manure, but other
agricultural residues can be added. The reported values of gas production is very
much temperature dependent. For swine manure these values are 0.14 m^3 /day
per m^3 of digester capacity at 8°C to 0.7 m^3 /day per m^3 of digester capacity at
32 °C (Hao 1981). A higher gas production rate of 1.53m^3 /day per m^3 of digester
volume was reported by Yang and Nagano (1985) for the RMP digester
operating with slurry recycle at the ratio of 0.25 of the influent flow rate.
Anaerobic lagoons covered with plastic sheets can be considered as plug flow
digesters. Biogas is stored in the plastic sheets and pumped to the heating
facilities. Besides energy recovery through biogas production, covered
anaerobic lagoons help to minimize the release of green house gases, such as
CH 4 and CO 2 , to the atmosphere.
Figure 4.11 Bag digester
INLETGASOUTLETGAS PIPEd
LEVELED
SURFACELAYER OF ABOUT
70mm COMPACTED
BACKFILL150mm approx.
SLURRY