BACKGROUND: One of the main components of dairy wastewaters is cheese whey. Although different technologies have been used extensively in the past for cheese-whey treatment, constructed wetlands (CWs) applications are limited. Furthermore, the effect of crucial operational parameters (e.g. temperature, pollutant loading rate) have not been thoroughly studied. Having this in mind, two horizontal subsurface flow pilot-scale CW units (one planted and one unplanted) were used to treat secondary cheese whey, in order to examine the effect of different chemical oxygen demand (COD) influent concentrations (1200 to 7200 mg L-1), hydraulic residence times (8, 4, 2 and 1 day) and temperature (2.4 to 32.9 degrees C). RESULTS: During a 2-year operating period both pilot-scale units successfully removed organic matter, with COD removal efficiencies recorded at 91% and 77.2% for the planted and the unplanted unit, respectively. Hydraulic residence time affected COD removal efficiency only when limited to 1 day. Temperature significantly affected COD removal only in the unplanted unit. CONCLUSIONS: Constructed wetlands could successfully treat secondary cheese whey and provide COD effluent concentrations below EU legislation, when hydraulic residence time is above 2 days and COD influent concentration ranges from 1200 to 3500 mg L-1. (C) 2015 Society of Chemical Industry