Tube Gas Digester
The tube gas digester is a low-cost tubular plastic digester which was developed by CONDRIT (Consultorias el para el Desarrollo Integrado del Tropico) located in Cali, Columbia in the early 1980s. The digester was based on a design first found in Taiwan, called the Red Mud PVC biogas disgester. The cost of the tube gas digester is relatively low, varying according to size and location. For instance, in Colombia the cost per m3 of liquid volume is around $US30, taking into account that this includes the container and its connection, cement boxes for the inlets and outlets, plastic gas reservoir, stove, labour to prepare the trench and installation of the biodigester.
In Viet Nam the average cost per square metre is around $US7 (materials only), giving a total cost for one biodigester of 5.4 m3 of US$37.80, including two burners. In Western countries the cost polythene tubes are also quite cheap.
The design is very simple although a trench has to be dug to receive the tube gas digester. The walls of the trench must be firm and the floor must be flat or with only a minimum of slope. There must also be no sharp stones or protruding roots in the floor which might perforate the tube gas digester. Long tubular polyethylene that is used in agriculture is produced in most countries. The choice of fittings and related materials will be limited to what is available locally on farms or in rural markets. A simple safety valve is made from a transparent plastic bottle, a T-piece and 3 PVC tubes (one 6cm and the other two of 30cm). Water is poured into the bottle and maintained at 5cm depth (above the mouth of the tube). All these details are in the construction plans. Plans for construction of various systems are available to download for free in the Construction section of the site.