BACKGROUND: Anaerobic digestion of liquid manure from flush dairy operations is an alternative to manure management practices with environmental benefits and producing biogas. A unique sequential reactor system was used to study the anaerobic digestion (AD) of flush dairy manure with the aim of determining (1) the effects of organic strength of the influent on AD performance; (2) whether phase separation is beneficial to treatment and AD design; and (3) whether there is any difference between completely mixed and plug-flow approaches to implementing the AD process. RESULTS: The influent with high organic strength resulted in higher levels of biogas production and chemical oxygen demand (COD)/solids removal. When the manure stream is used as influent for the anaerobic digestion, the phenomenon of two-stage anaerobic digestion process does not exist because the stream contained high levels of alkalinity and volatile fatty acids. The biogas production and COD/solids destruction achieved by plug-flow and completely mixed reactor were very similar when treating the liquid manure stream. CONCLUSION: The organic strength of the influent significantly influenced AD performance. The plug-flow reactor and the completely mixed reactor had a similar AD performance. For the type of manure and organic loading rate investigated, the results give indications for designing an AD process in flush dairy operations. (c) 2007 Society of Chemical Industry.