BACKGROUND: The emissions at the Stimson Lumber Company plant, Gaston, Oregon, USA, were treated with a pilot scale biological system to remove low concentrations of volatile organic compounds (VOCs) (principally pinene, formaldehyde and methanol) emitted from the wet process hard board manufacturing section of the plant. The objective was to demonstrate the effectiveness of the system for optimal removal of VOCs and reduction of opacity (blue haze) of emissions from the wet press. RESULTS: Analysis of the air samples collected from the bio-treatment system showed a removal efficiency of 90% for total VOCs. The optimization experiments demonstrated that the VOC removal rates improved as the gas flow rate and VOC loading rates increased, and at higher recycle water flow rates, which suggested additional treatment capacity for the biofiltration unit when operated at 40 s empty bed retention time. The opacity of emissions measured using both the US EPA Method-9 and Wager 6500 opacity meter showed similar trends. CONCLUSIONS: The biological system demonstrated efficient removal of mixtures of forest products VOC emissions with elimination capacities in the range 810 gVOC m-3 h-1. The opacity reduction and VOC removal data were correlated demonstrating that inline opacity measurements may be useful for biosystem performance monitoring and troubleshooting. Copyright (c) 2012 Society of Chemical Industry