Duong Nguyen Khang, Chhay Ty, Soukarn Koenouchanh and  Preston T R 2008 Effect of effluent recycling on gas production in polyethylene biodigesters.  Proceedings MEKARN Regional Conference 2007: Matching Livestock Systems with Available Resources (Editors: Reg Preston and Brian Ogle), Halong Bay, Vietnam, 25-28 November 2007 http://www.mekarn.org/prohan/khang2.htm