The residual toxicity of the wastewater after ozonation is very low, i.e. treatment with ozone had already brought about a significant improvement. Only in isolated cases, for example, was a low mutagenic effect detected. These negative effects could be further reduced or eliminated by the various post-treatment processes. After the fresh GAC filter, the effects in the photosynthesis inhibition test were reduced by a further 66 percent. This is probably due to additional removal of micropollutants by the GAC filter. The loaded GAC filter also brought a slight improvement. Overall, however, the low residual toxicity after ozonation made it difficult to assess additional effects of the post-treatment in the bioassays. The results for the sand filter, fluidised bed and fixed-bed post-treatments were inconsistent and did not enable any conclusion to be drawn.
New sensitive methods
A tissue analysis of rainbow trout conducted at the University of Tübingen showed that after ozonation, the tissue condition of trout livers was still worse than in unstressed control animals. Treatment with fresh GAC resulted in better tissue condition, while the other post-treatments did not lead to any detectable improvement. Environmental toxicologist Stephan Fischer from Eawag also investigated biomarkers at the gene expression level in the trout. These indicate whether and to what extent organisms react to different groups of pollutants. The biomarkers studied are involved in reactions such as general stress response, oxidative stress, conversion of foreign substances, regulation of the immune system, hormonal activity and reactions with heavy metals. Most biomarkers were elevated after the biologically treated wastewater. Ozonation reduced this signal. The different post-treatments achieved further changes in the biomarker patterns, making them more similar to those of the control trout, but less clearly different from each other. The most significant positive effect was after the relatively fresh GAC filter or after the fixed bed.
Conclusion and implementation
The bioassays used confirmed that ozonation significantly reduces the pollutant load of the wastewater and thus ecotoxicological effects at the Neugut WWTP. Since the wastewater was only very slightly toxic after ozonation, hardly any differences in the efficiency of the various post-treatments could be shown. Summarising the results of the bioassays, only the fresh GAC filter provided a significant additional cleaning performance to ozonation, which is probably due to the additional removal of micropollutants by sorption to the filter. In further post-treatments, no consistent improvement could be demonstrated with most of the methods used. Isolated mutagenic effects were reduced by all post-treatment processes, indicating their efficiency in removing labile reaction products.
In principle, biological post-treatment is required in Switzerland after ozonation to reduce the biodegradable dissolved organic carbon and potentially toxic products generated during ozonation. This study has shown that the combination of ozonation and activated carbon (GAC filter) achieves the greatest reduction of ozonation reaction products and ecotoxicity in the treated wastewater. This treatment combination has become increasingly important in recent years and is already being used at the Altenrhein wastewater treatment plant in Switzerland.