The pilot plant study comprised the construction and monitoring of a new vertically moving biofilm system (VMBS) for treating municipal wastewater. The system operated on site for I I months. The biofilm module in this system, consisting of high surface area plastic media, was vertically and repeatedly moved in cycles up into the air and down into the wastewater. The vertical movement of the biofilm module supplied sufficient oxygen for the removal of the organic carbon in the wastewater. The overall physical oxygen transfer coefficient (Kla) measured at the cycle speed of six cycles per minute was 2.53 per hour. During the pilot study, dissolved oxygen (DO) concentrations in the bulk fluid were in the range of 1.5-5 mg/l. It was found that the areal removal rate of filtered chemical oxygen demand (COD) was up to 35 g COD/(m(2) day) and the bulk fluid volumetric filtered COD removal rate was 2.62 kg COD/(m(3) day). The field experiment showed that clogging commonly found in other biofilm systems did not occur in this system. The power consumption was in the range of 0.09-0.25 kWh/m(3) wastewater flow, 0.40-2.19 kWh/kg COD removal and 1.24-1.74 kWh/kg BOD removal. The new biofilm system offers potential for reduced reactor volumes, energy saving, simple construction and easy operation. (C) 2003 Elsevier Science Ltd. All rights reserved.