Studies On The Tertiary Treatment Of Domestic Wastewater By Low Concentration-ozonation And BAC Filtration

Tertiary treatment of domestic wastewater is conducted for water reclamation at Beishiqiao wastewater Purification Center, Xi'an Municipality, with a process of low-concentration-ozonation followed by biological activated caibon (O3-BAC) filter in pilot scale. The secondary effluent was introduced into a vertical flow ozone reactor with an average contact time of 16 min at an ozone dose of 1-1.5 mg/L, and then led into a 1.6-meter-high BAC filter at a rate of 3-6 m/h.The process achieved a remarkable effect of tertiary treatment of domestic wastewater. It removed 92.3% of color and 80.5% of turbidity. The average removals of organic substances in terms of UV254, TOC and CODcr were up to 78.5%, 47.0% and 64.0% respectively. The filtered water was completely free from offensive odor with turbidity lower than 0.6 NTU, color below 4 c.u., TOC and CODcr about 4.9mg/L and 8mg/L respectively. Quality of the treated water is good enough to meet the needs of water reuse for various purposes.In spite of the low concentration of ozone applied, a remarkable color removal was achieveed Ozonation couldn't result in a substantial removal of organic substances, but according to the change of the molecular weight distribution and UV/TOC ratio before and after ozonation, it was found that organic matters with larger molecules were broken into smaller molecules, which improved the biochemical degradability of organic substances in the water.Color, turbidity and organic substances were further removed after the ozonated water flowed through the BAC filter. Meanwhile, 53.9% of NH3-N was removed but the removal effect was not steady under the influenced of NH3-N concentration of the raw water. The organic substances and NH3-N were removed mainly in the upper 40cm of the BAC filter.The biomass was found to decrease along the depth of the BAC filter. After the biofilm wasmathurated, the biomass in the upper 10cm was 315.9 nmolLipid-P/g activated carbon, but only 35.9 nmolLipid-P/g activated carbon in the lowest part. In the upper 10cm the measured biomass growth curve showed an S shape with time, which is similar to the growth process of suspended microorganism, characterized by an acclimatization period, a logarithmic growth period and finally a steady period.Based on the analysis of bioreaction kinetics, a three-stage kinetic model was proposed to characterize TOC removal in the BAC filter. Because the biomass decreases with the depth of the BAC filter, the rate coefficient of the first-order reaction decreases with the filter depth as well.