There has been a lot of discussion on plastic bag digesters on the Anaerobic Digestion discussion group.
There are several basic reports on the making of such digesters, such as those by Reg Preston:
Low-cost biodigesters as the epicenter of ecological farming systems
A combined digester and gasholder - PVC plastic tube biogas unit.

There is a compendium of studies of such plastic digesters in various places (1997/1998/2002):
Tubular Plastic Bio-digesters in Tanzania, Viet Nam, Zimbabwe & China, selected & edited by John Furze

Here is a Vietnamese view from Bui Xuan An :
Biogas technology in developing countries: Vietnam case study

The references that David House (17/01/2010) suggested are:
"The Characterisation of Production and Function of a 15m3 Red-Mud PVC Biogas Digester
The Introduction of Low-Cost Polyethylene Tube Biodigesters on Small-Scale Farms in Vietnam (another from Reg Preston)
Gas Bio-digester Information and Construction Manual For Rural Families
and one in Spanish by Jaime Martí Herrero:
Guía de diseño y manual de instalación de biodigestores familiares.

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