As a consequence of the evolution of the European and the French regulations, many wastewater treatment plants (WWTP) are being upgraded. An additional biological treatment stage is added to enhance the removal of organic matter and nutrients. Among these sites, some plants in mountainous areas are considering the biofiltration process. This technique indeed seems to be best adapted, given its advantages: it is compact, which enables to use a reduced surface and to cover the units, and it is flexible, which allows to use a number of cells proportional to the load, which is an asset for treatment plants with strongly variable load. The design of such facilities is difficult especially for the winter period, given the very low influent temperature, the high ammonia removal performance which is required, and the strong load variations within short times. The monitoring of a site during two consecutive winter seasons enabled to study some aspects of the treatment, especially the maximum nitrification capacity and the operation mode to cope with load variations. The monitored site is on the Biostyr process located in the Southern French Alps. Its design capacity is 45,000 PE, with a load factor of 4 between low and high season. The design resulted in 8 biofiltration cells with a volumetric loading of 0.75 kgTKN/m3.d in order to obtain less than 12 mg TNK/l in the effluent when the raw influent temperature is at least 8°C. The main operating problem is to maintain the nitrification performance given the strong load variations in the winter due to the touristic activity. To solve this problem, OTV has developed a process control (RegulFiltre) which is installed on this plant. The main results obtained on this site have shown that: the maximum nitrification capacity was about 0.6 kgNO3-N formed/m3.d, while the temperature of the raw influent was about 7°C. The growth rate of the nitrifiers expressed as the increase of the nitrifying capacity was 0.04 kgNO3-N formed/m3 per day of feed. The quick and short load variations require a specific operation on the biofilters, in order to obtain an active biomass which is able to treat the peak load immediately. Among the various possible methods, the rotating feed of the reactors consists on the adaptation of the number of the filters to apply during low season a load similar to the one in high season, and thus to maintain the active biomass at a sufficient level. The rotating feed implies stopping times which might cause biomass loss. The duration and the management of these stops have been studied and confirm the values proposed by the builder: stopping times below 3 days allow to maintain the nitrification capacity of the biomass at a sufficient level. When the load peak factor is high (over 4), the rotation alone of all the biofilter units is not enough and has to be complemented by a specific preparation of the biofilters. The duration required to set up the autotrophic biomass after a long feed stop (several months) and under cold temperature has also been studied. The objectives of this article are: to better define the design basis for nitrifying WWTPs facing low temperatures and variable loads, to draw the attention of the involved parties to the main critical points during designto establish recommendations to make the operation easier while keeping a good treatment level with variable load and at low temperature.