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.
