<p>A two-step process for microbiological purification of contaminated water, comprising a first step using anaerobic conditions in a reactor (2) having a stationary bed (3) and a biofilm fixed thereon followed by conventional microbiological purification. In the first step the water is supplied at the upper end of the reactor (5) and is brought to trickle over the fixed biofilm without drenching the bed and while maintaning a continuous gasphase within the bed.</p>
