This 365-day laboratory study examined the performance of a novel horizontal flow biofilm reactor (HFBR) system in the treatment of a synthetic domestic-strength wastewater. The HFBR system comprised two biofilm reactors positioned one above the other: Reactor I contained a stack of 8 horizontal plastic sheets with 25 mm high spacing frustums and Reactor 2, under Reactor 1, contained a stack of 18 horizontal sheets with 11 mm high frustums. The sheets were arranged so that when the wastewater was pumped onto the top sheet of Reactor 1, for 10 min every hour, it flowed along that sheet, and onto and back along the sheet underneath, and so on down through the system, resulting in biofilm development on the sheets. The hydraulic and filtered chemical oxygen demand (CODf) loading rates were 155 l/(m(2) d) and 47 g/(m d), respectively, based on the system plan area. The effluent CODf concentration was 33 mg/l. Nearly full nitrification occurred with 56% total nitrogen (TN) reduction at the loading rate of about 4.7 g TN/(m(2) d). During the study period, the solids yield was only 0.1 g SS/g CODf removed. The system was easy to construct and operate, did not clog and could be used for complete biodegradable COD removal, nitrification and partial nitrogen removal. (c) 2006 Elsevier Ltd. All rights reserved.