The evolution of European legislation has led to the rehabilitation of many wastewater treatment plants, sometimes through the installation of a biological complementary treatment stage. Among these sites, some plants in mountain areas are considering a biofiltration process. The design of such plants, especially for winter, appears to be tricky because of the very low influent temperature, the high performance requested for ammonia removal and the important and short term variations of the influent loads. The monitoring of a site during two consecutive winters has allowed us to study some aspects of the treatment. The major results are: (1) a maximal nitrification capacity of about 0.59 kg of formed N-NO3-.m-3 of material.d-1 with an influent temperature around 7°C at the plant inlet, (2) a nitrifying biomass growth rate, expressed as nitrifying capacity increase, of 0.03 kg of N-NO3-.m-3 of material.d-2, (3) quick and short terms load variations require a specific operation of the filters prior to the load increase, in order to grow enough active biomass to be able to treat the peak load immediately.