This study investigated the performance of a horizontal-flow biofilm reactor in treating domestic-strength synthetic wastewater (SWW) under three hydraulic and chemical oxygen demand (COD) loading rates (phases 1 to 3) ranging from 152 L/m 2 Â· d and 58.9 g COD/m 2 Â· d to 497 L/m 2 Â· d and 192.7 g COD/m 2 Â· d, respectively; the rates were based on the top surface plan area (TSPA) of the reactor. The reactor comprised a stack of 30 horizontal polystyrene sheets. The SWW, intermittently dosed onto the unit, flowed over and back along sequential sheets in the stack before exiting at the bottom of the unit. Maximum average removals of >98% biochemical oxygen demand and 96.2% total ammonium-nitrogen (NH 4-N) were observed. The TSPA COD and NH 4-N removal rates were high for all phases. The unit provides a high-performance, economic, low-maintenance, and flexible alternative for removing carbon and ammonium from low-flow point sources.