The biological filter treatment process – i.e. Biofilters - is fixed-bed biofilm reactors using expanded clay or polystyrene materials (Fig. 1a). It aims to filtration of particles, and biological conversion of pollutants, with removal of carbon, ammonia (nitrification) or nitrate (denitrification). It is used in more than 300 plants around the world, and the treatment capacity has doubled in the last five years. During treatment phase, biofilm growth and particles accumulation, decreases gradually the porosity and increase the head loss of the filter, leading to the requirement of daily backwashing phase (ie succession of alternative phases for air and water supply at high rates) to eliminate the accumulated material. However, in some cases, mechanical constraints of backwash phase are insufficient to recover the initial head loss (Fig. 1b). Clogging is reached, due to refractory aggregates remaining in the reactor, expecting chemical supply (ex. sodium hydroxide) to be removed by operators. Such phenomenon widely reaches French biofilters for instance (Urroz, 2007). This work aims at determining the reasons why biofilters used for wastewater treatment clog. It first proposes a state-of-the-art of the different chemical, biological and physical interactions that influence the formation and the cohesion of aggregates. It proposes an aggregate strengthening scenario, and aims at linking clogging with the operational conditions. It also presents the strategy developed to study aggregates' characteristics.