Integration of continuous biological and chemical (ozone) treatment of domestic wastewater : 1. Biodegradation and post-ozonation

The continuous treatment of domestic wastewater by an activated sludge process and by an integrated biological-chemical (ozone) oxidation process were studied in this work. Chemical oxygen demand (COD), biochemica- l oxygen demand (BOD), absorbance at 254nm (UV254) and nitrogenous compound content were the parameters followed in order to evaluate the performance of the two processes. Experimental data showed that both UV254 and COD reductions are improved in the combined biological-chemi- cal oxidation procedure. Thus, reductions of 59.1% and 37.2% corresponding to COD and UV254, respectively were observed after the biological process (hydraulic retention time=5h; mixed liquor volatile suspended solids concentration=3142 g/m3) compared with 71.0% and 78.4% obtained when a post-ozonation step (DO3=41.7g/m3) was included. During conventional activated sludge treatment, appropriate nitrification levels are only achieved with high hydraulic retention time and/or biomass concentration. Ozonation after the secondary treatment, however, allows improved nitrogen content reduction with total nitrite elimination. Post-ozonation also leads to a higher biodegradability of the treated wastewater. Thus, the ultimate BOD/COD ratio goes from 0.16 after biological oxidation to 0.34 after post-ozonation with 41.7 g O3/m3.