this is from the Orval website...
"At the end of the (primary) fermentation (the main fermentation lasts 4 to 5 days.), the young beer, still cloudy, is pumped into the holding tanks, where it matures, fines up, and takes on carbonic gas. The holding period lasts two to three weeks at a temperature of 15°C. It is in these same tanks that the yeasts, which have been very carefully kept in the Brewery laboratory, are added. As they develop in the later stages, they confer on the Orval beer its distinctive, slightly acidulous and so special character. A secondary fermentation in the holding tank gives the beer its incomparable taste, all the more so as fresh hops are added to heighten the bouquet."
"The most reputed varieties are the Bavarian Hallertau, the Slovenian Styrian Golding and the Alsacian Strisselspalt. These hops are used to flavour Orval beer. Before bottling, the beer is centrifuged to remove dead yeasts and hop particles in suspension."
"During this operation, liquid sugar and fresh yeasts are added so as to initiate the refermentation process in the bottle."
"For about 3 to 5 weeks, Orval beer re-ferments in the bottle in storage halls climatised at 15°C (56°F)."