Abstract: The horizontal trench is one of the main methods which are available to recirculate leachates. To study the unsaturated-saturated seepage laws during leachate recirculation process, different flow control equations are used for the saturated/unsaturated zones in bioreactor landfills. The saturated Richards' equation is adopted as the flow governing equation in the saturated area of the waste mass. For the unsaturated area of the waste mass, based on the law of conservation of mass, the modified Darcy's law and the Elagroudy’s settlement model, a new flow governing equation considering solid waste settlement is developed. Based on the two-dimensional saturated/unsaturated model which has been developed considering solid waste settlement, the migration laws of recirculated leachates are studied. The simulated results indicate that the various parameters (i.e., settlement of MSW, pressure head, initial void ratio, etc.) have effects on the zone of impact, pressure head, water content of MSW and recirculation leachate volume per m of trench length. The design method of horizontal trenches is proposed for the bioreactor landfills.