IV.
Figure 4. Removal of selected trace organic chemicals by biological activated carbon filters; empty bed contact time is indicated in the legend (average of 3 independent values ± standard deviation). No bar means a removal could not be calculated because concentrations were either too low or below the limit of quantification. Letters in brackets indicate removal generally observed in WWTP estimated from Onesios et al. (2009): P=poor (<20%); I=intermediate (20-80%); G=good (>80%). However, there was no clear increase between 18 and 45 minutes EBCT. Given that the filters have been in use for several years and have filtered many tens of thousands of bed volumes, it is thought that their adsorption capacity is exhausted and they rely only on biological degradation for the removal of organic matter. However, most of the compounds known to be poorly or moderately removed in WWTP were significantly removed in the filters, even with an EBCT as short as 9 minutes. We also previously observed high removal of TrOCs over a period of two years in BAC filters treating non-ozonated and ozonated wastewater. These observations suggest that the bacterial community adapts to the biodegradation of compounds refractory in WWTP as it has been shown in simulated aquifer recharge (Rauch-Williams et al., 2010). But even though it is hypothesised that the adsorption capacity of the activated carbon in the filters is largely exhausted, the removal of specific TrOCs is not correlated with the removal of bulk organic matter and TrOCs breakthrough can be observed much later than DOC breakthrough. Moreover, TrOCs with different properties can show varying breakthrough times separated by tens of thousands of bed volumes. The removal mechanisms of TrOCs in biological activated carbon filters remain unclear at this stage and could be a combination of adsorption and biodegradation, depending on the compounds. The estrogenicity levels were so low after ozonation that it was not possible to accurately assess the effectiveness of BAC
filtration. However, some reduction was observed in all three plants and the levels after BAC filtration were close to or below the quantification limit (0.03 ng L-1). The baseline-TEQbio was further reduced after the BAC filtration by 33±13%, 51±15% and 54±13% compared to after ozonation in Landsborough, Gerringong and Caboolture, respectively. In parallel, the DOC was reduced only by 17±3%, 48±10% and 24±6% respectively, indicating that compounds contributing to the non-specific toxicity are preferentially removed or transformed to metabolites with lower toxic potential.
CONCLUSIONS The investigation of three full-scale reclamation plants using ozonation followed by BAC filtration showed that: I.
The combination of chemical and biological treatment processes can improve treated effluents quality by removing DOC up to 50% and a wide range of TrOCs by more than 90%. It can also reduce non-specific toxicity by up to 70% and estrogenicity by more than 95%;
II.
The non-specific toxicity of the ozonation by-products mixture was 30% to 40% lower than the parent compounds mixture, suggesting that the by-products have a lower toxic potential;
III.
The BAC filtration is capable of further removing some of the TrOCs remaining after ozonation by up to 99% and also reducing the non-specific toxicity of the by-products mixture by up to 54%;
Increasing the ozone dose and filtration EBCT generally has a positive influence on the removal of DOC and TrOCs as well as on the reduction of non-specific toxicity, but there is no direct linear relationship. Therefore, increasing the ozone dose and EBCT further will not necessarily lead to substantive gains in water quality.
In regards to chemical analysis and E-SCREEN assay, ozonation as a single step would be sufficient for the removal of selected TrOCs and reduction of estrogenicity. However, the non-specific toxicity shows us the interest of subsequent BAC filtration because this bioassay integrates the effect of all TrOCs present in the sample. Most transformation products cannot yet be quantified with chemical analysis and, as discussed, will only marginally contribute to estrogenicity, but can still substantially contribute to non-specific toxicity. This is an important point and justifies the parallel application of bioassays when investigating the removal of TrOCs in various wastewater treatment processes. Based on these results, it can be concluded that the combination of ozonation and BAC filtration could be employed to upgrade WWTPs for environmental protection or in water recycling schemes. Placed before high-pressure membrane filtration, it would provide an additional barrier to organic contaminates and would improve the quality of the waste stream. The removal mechanisms of TrOCs in the BAC filters are still unclear. Further research is needed in this area to determine to what extent these compounds are adsorbed and/or biodegraded and if transformation products are formed.
ACKNOWLEDGEMENTS This work was funded by the Urban Water Security Research Alliance under the Enhanced Treatment Project. The National Research Centre for Environmental Toxicology (Entox) is a joint venture of the University of Queensland and Queensland Health Forensic and Scientific Services. The Authors acknowledge the following institutions and individuals who contributed to this study: Unitywater; Veolia Water Australia; Sydney Water Corporation and their staff for providing access to their reclamation plants and help with sampling; and Dr Beatrice Keller and Dr Jelena Radjenovic for their help in establishing the analytical method at the AWMC.
FEBRUARY 2013
WATER
WASTEWATER MANAGEMENT AND TREATMENT
65
Technical Features