Constructed wetlands are recognized as facilities for wastewater treatment. Traditionally, constructed wetlands are developed mainly in northern Europe and in the USA where their need for large areas can be easily met. In order to reduce such areal need, the use of active substrates was tested in a subsurface flow constructed wetland for the treatment of wastewaters from a medium-size cheese-making plant, located near the town of Aosta (North West Italy), in alpine environment (MAAT = 10.4 °C). System seasonal performance was assessed by weekly analysis of chemical properties (BOD5, Ntot, Norg, N-NH4+) of inflow and outflow wastewater. The average removal efficiency was equal to 90% for BOD5 and comprised between 50 and 60% for nitrogen forms. The BOD5 removal efficiency was less affected by the seasonal changes, while nitrogen forms removal revealed a greater seasonal variability, especially for ammonium. The use of selected materials allowed a reduction of surface requirement, maintaining a high removal efficiency of wastewater organic matter, slightly influenced by seasonal climatic and operational conditions.