Removal of micropollutants and ecotoxicity during combined biological activated carbon and ozone (BO3) treatment.

Ozonation is a viable option to improve the removal of micropollutants (MPs) in wastewater treatment plants (WWTPs). Nevertheless, the application of ozonation is hindered by its high energy requirements and by the uncertainties regarding the formation of toxic transformation products in the process. Energy requirements of ozonation can be reduced with a pre-ozone treatment, such as a biological activated carbon (BAC) filter, that removes part of the effluent organic matter before ozonation. This study investigated a combination of BAC filtration followed by ozonation (the BO3 process) to remove MPs at low ozone doses and low energy input, and focused on the formation of toxic organic and inorganic products during ozonation. Effluent from a WWTP was collected, spiked with MPs (approximately 1 µg/L) and treated with the BO3 process. Different flowrates (0.25-4 L/h) and specific ozone doses (0.2-0.6 g O3/g TOC) were tested and MPs, ecotoxicity and bromate were analyzed. For ecotoxicity assessment, three in vivo (daphnia, algae and bacteria) and six in vitro CALUX assays (Era, GR, PAH, P53, PR, andNrf2 CALUX) were used. Results show that the combination of BAC filtration and ozonation has higher MP removal and higher ecotoxicity removal than only BAC filtration and only ozonation. The in vivo assays show a low ecotoxicity in the initial WWTP effluent samples and no clear trend with increasing ozone doses, while most of the in vitro assays show a decrease in ecotoxicity with increasing ozone dose. This suggests that for the tested bioassays, feed water and ozone doses, the overall ecotoxicity of the formed transformation products during ozonation was lower than the overall ecotoxicity of the parent compounds. In the experiments with bromide spiking, relevant formation of bromate was observed above specific ozone doses of approximately 0.4 O3/g TOC and more bromate was formed for the samples with BAC pre-treatment. This indirectly indicates the effectivity of the pre-treatment in removing organic matter and making ozone more available to react with other compounds (such as MPs, but also bromide), but also underlines the importance of controlling the ozone dose to be below the threshold to avoid formation of bromate. It was concluded that treatment of the tested WWTP effluent in the BO3 process at a specific ozone dose of 0.2 g O3/g TOC, results in high MP removal at limited energy input while no increase in ecotoxicity, nor formation of bromate was observed under this condition. This indicates that the hybrid BO3 process can be implemented to remove MPs and improve the ecological quality of this WWTP effluent with a lower energy demand than conventional MP removal processes such as standalone ozonation.

The effect of organic matter fractions on micropollutant ozonation in wastewater effluents.

Organic matter (OM) is the most important factor influencing the effectivity and efficiency of micropollutant (MP) ozonation in wastewater effluents. The importance of the quantity of OM is known, because of this, total organic carbon (TOC) is generally used to determine the required ozone dose for any water sample. Still, the effect of OM type on MP ozonation is not well understood. In this study, effluents from five wastewater treatment plants were collected and the organic matter in these effluents was fractionated using membranes (F1-4) and resin (HI, HOA, HON and HOB). Fractions were diluted to the same TOC concentration, spiked with MPs and ozonated at three ozone doses. Our results show that all five effluents had comparable OM compositions and similar MP removal, confirming the suitability of OM quantity (TOC) to compare the ozone requirements for wastewater effluents. From the 19 analysed MPs, three groups were identified that showed similar removal behaviour. The strongest differences between the groups were observed around MP ozone reactivities of 102, 104 and 106 M-1 s-1. This indicates the presence of three OM groups in the samples that interfere with the removal of different MPs. MP removal in the resin fraction HON were higher for MPs with high and medium ozone reactivity, indicating a low interference of OM in this fraction with MP ozonation. OM in the resin fractions HOA and HI showed higher interference with MP ozonation. Therefore, removing the HOA and HI fractions prior to ozonation would result in a lower required ozone dose and a more efficient removal of the MPs. MP removal correlated with the OM characteristics A300, SR and fluorescence component comp 2. These characteristics can be used as inline tools to predict the required ozone dose in water treatment plants.