Integration of continuous biological and chemical (ozone) treatment of domestic wastewater:: 2.: Ozonation followed by biological oxidation

A combined chemical-biological treatment, ozonation plus activated sludge, of domestic wastewater was studied and the results were compared with those shown in a previous study where the sequence of oxidation was reversed. Experimental results indicated wastewater preozonation to be an effective pretreatment step for domestic sewage. The goals of ozonation were the improvement in effluent biodegradability and the removal of certain biorefractory organics as aromatic and unsaturated compounds. The percentage of biodegradability was measured by the BOD/COD ratio (eg 0.69 was obtained for an ozonated effluent (D-o3 = 40 g m(-3)) while a ratio of 0.57 was measured for raw sewage). Based on continuous experiments of a single activated sludge system (hydraulic retention time = 5 h; mixed liquor volatile suspended solids = 1.6 g dm(-3)) the combined oxidation leads to an improvement of BOD and COD percentage reductions (88% and 66% observed after the combined process for BOD and GOD, respectively, compared with 80% and 47% obtained without preozonation). Preozonation also improves the total kjeldahl nitrogen (TKN) removal during subsequent single stage activated sludge treatment. The fraction of autotrophic biomass (f(nit)) was more important when preozonation took place, so nitrification rates were enhanced in the combined process. The efficiency for the individual and combined process was computed and compared. Among the three processes studied, the sequence of ozonation plus biological oxidation resulted in the highest percentage GOD, TKN and UV254 reduction. (C) 1999 Society of Chemical Industry.