The Effect of Low-Concentration Ozone on the Structure of Organic Matter and Their Removal by

1. The Effect of Low-Concentration Ozone on the Structure of Organic Matter and Their Removal by a BAC Filter for Tertiary Wastewater Treatment Introduction The most important effect of ozone, among its multi-functions, is to convert non-biodegradable organic substances into biodegradable organic substances, but not limited to direct decomposition of organics. Therefore, ozonation is usually followed by a biofilter using sand, anthracite, granular activated carbon (GAC) or other materials as filter media, to achieve a high organic removal and/or to improve the bio-stability of the treated water. It was found that complete decomposition of organic matters required higher ozone dose, but significant conversion of the structure of non-biodegradable organics into biodegradable ones could be achieved at a low concentration. It is thus suggestible that effective organic removal could be made possible by combining low-concentration ozonation process with biofiltration. In this paper, this idea was practised in a pilot study of tertiary treatment of domestic wastewater for water reclamation using a low-concentration ozonation (ozone dose as 1 – 2 mg/l) and biofiltration process. Results & Discussion Profiles of biomass, NH3-N and NOX-N along the BAC filter Effect of ozonation on the structure of organic matter According to the distribution curve of the quantity of biomass along the filter depth (Figure 3), it can be estimated that more than 80% of the biomass is accumulated in the upper half of the filter. The BAC filter can also achieve a removal of inorganic nitrogen to certain extent (about 40%). In Figure 3, the profiles of ammonia (NH3-N), nitrate (NO3-N), and nitrite (NO2-N) along the filter depth are compared. It is interesting that in the upper half of the filter down to the depth about 90 cm, there is a decrease of NH3-N concentration accompanied by an increase of NO3-N along the filter depth, while in the lower half of the filter, NH3-N almost keeps unchanged but NO3-N decreases dramatically along the filter depth. Regarding NO2-N, only a slight increase is found along the filter depth especially in the lower part. The ozonated water entering the BAC filter is full of dissolved oxygen (usually higher than 9.0 mg/l), which results in a good aerobic environment at the top part of the filter. Sufficient dissolved oxygen and substrates supply provides a favourable condition for the process of nitrification where NH3-N is oxidized to NO2-N and/or NO3-N under the action of nitrifying bacteria. Down to the depth of 90 cm, the concentration of dissolved oxygen drops to about 5.0 mg/l and this value almost keeps unchanged in the lower half of the filter. Under the condition of laminar flow through the filter, the efficiency of oxygen transfer is often very low from the flowing water to the biofilm on the surface of the filter media. Therefore, an anoxic environment may be thus resulted in the filter layer especially inside the biofilm. This is a condition suitable for the process of denitrification where NO3-N is consumed to provide oxygen for the growth of the denitrifying bacteria and finally forms gaseous nitrogen (N2). As shown in Figure 1, it is identified that in addition to certain kinds of aromatic acids, cyclic hydrocarbons are dominant in the raw water, secondary effluent from the wastewater treatment plant. After ozonation, there is an apparent decrease in the amount of cyclic hydrocarbons while many chain hydrocarbons, aliphatic acids and some newly formed organic matter with oxygenated functional groups become dominant. Figure 1 GC-MS chromatograms of organics before and after ozonation BDOC/DOC Ratio at Various Stages of the Treatment Process BDOC analysis results have provided evidence for explaining the role of ozonation in improving the biodegradability of organic matter. As shown in Figure 2, although noticeable decrease has not been found in the total concentration of DOC of the water after ozonation, the BDOC/DOC ratio has increased from 0.04 to 0.27, which indicates a 23% increase in the biodegradable portion of the dissolved organic matter. Figure 2 also shows the changes in the BDOC/DOC ratio after the ozonated water has entered the BAC filter. At a filter depth of 30 cm, the ratio drops to 0.17, and then to 0.07 and 0.03 at 60 cm and 90 cm, respectively. The BDOC/DOC ratio is as low as 0.01 in the finished water, indicating an almost complete removal of the biodegradable portion of the dissolved organic matter. The whole process can achieve a total removal of DOC as about 50%. Figure 3 Profiles of biomass, NH3-N and NOX-N along the BAC filter Layer The process of low-concentration-ozonation and biological activated carbon (BAC) filtration that applied for tertiary treatment of domestic wastewater for water reclamation can achieve a very high removal of SS, colour, offensive odour, and dissolved organic matter, resulting in high quality treated water. Figure 2 Comparison of BDOC/DOC ratio at different stage Conclusions A low concentration ozonation can achieve an effective conversion of organic functional groups from cyclic hydrocarbons and aromatics into chain hydrocarbons and aliphatic acids. This in due increases the BDOC/DOC ratio showing a significant improvement of the biodegradability. Therefore a remarkable organic removal is achievable by the subsequent BAC filter. The top section of the BAC filter plays the main role of organic removal and nitrification, while the lower section of the filter performs the function of denitrification. High quality treated water is obtained by the low-concentration ozonation and bio-filtration process. Acknowledgement: This study is supported by the National Natural Science Foundation of China (Grant No. 50138020)