How to Build the Vacvina Biogas Digestor
This model was first introduced from Columbia to Vietnam in 1994 by doctor Reg Preston, rector of the University of Tropical Agriculture. The system consists of a long digestion bag or fermentation bag (Fig.2) and a gas reservoir bag. Both are made of two or three layers of polyethylene tubing, 21c thick and 1m in diameter. The length of the main bag determines the total digestion chamber volume (a 10 m bag results in a 7 cubic meter digestion chamber). The digestion bag is half buried in a trench and is connected at one end to a small mixing and collection tank and at the other end to a slurry holding pit. The gas reservoir has a capacity of 1,8 m3 and is usually suspended either in the kitchen or the stable (Fig. 3). As organic material is fed into the digestion bag, it slowly travels down the length of the tube while the fermentation process occurs and is then expelled at the other end as digested slurry. The biogas produced accumulates in the main bag and, through gravity as well as pressure, finds its way to the gas reservoirs. Thus biogas is stored partly in the digestion bag and partly in the gas reservoir. Users can easily judge the quantity of gas available by the level of inflation of the bags. Plastic bag digesters are inexpensive, easy to install and operate and scum formation can be controlled easily by gently slapping the scum to make it sink to the bottom. On the other hand, the polyethylene bags are somewhat fragile they can be perforated or torn and their life expectancy is but 3-5 years. They must be protected from wandering animals and also from sunlight as UV rays will degrade the plastic. In addition, the model requires a large area of 10-15m2 near the animal stable. This is a problem for many households in the Red River Delta where household compound is not large enough to allow its installation. File:Http://ease-vn.org.vn/images/VACVINA model.jpg
