Process and installation for treating a polluted aqueous liquid showing a COD value

The invention relates to a process and installation for treating a polluted aqueous liquid having a COD value caused by organic compounds present therein and a BOD/COD ratio smaller than 0.2. To reduce the COD value, the polluted aqueous liquid is percolated through a packed filter bed (7) of a carrier material, in particular of activated carbon, which is colonised with aerobic bacteria and which forms an adsorbent for at least part of said organic compounds. To provide a thin, fully aerated biofilm of bacteria on the carrier material so that no oxygen has to be dissolved under pressure in the liquid, the filter bed is kept at the most partially submerged in the liquid percolating therethrough. The percolate which has passed through the filter bed is collected and a portion of the collected percolate is recirculated to the filter bed whilst a further portion of the collected percolate is removed as treated effluent, preferably by means of a membrane filter (14) withholding the slowly growing specialist bacteria, enabling to remove recalcitrant COD, in the system.