Minimization of sludge production in biological processes: an alternative solution for the problem of sludge disposal

A combined system associating activated sludge and ozonation was evaluated for the treatment of urban wastewater. Experiments have shown that 70% reduction in sludge production can be reached (compared to a reference system running in low loaded conditions Y_obs = 0.28 g VSS.g COD^-1) by applying an ozone dosage of 0.05 g O₃/g VSS_treated. Recycling of the ozonated sludge to the aeration tank induces a slight increase in effluent COD, but the biological treatment performance is maintained. Nitrification capabilities are not altered by the sludge reduction process and active biomass measurements revealed that autotrophic biomass seems to be less affected than the heterotrophs. Significant improvements in sludge settling characteristics are observed.