Foreword: JEM Spotlight: Monitoring the treatment efficiency of a full scale ozonation on a sewage treatment plant with a mode-of-action based test battery

David Sedlak

David Sedlak is a professor in the Department of Civil and Environmental Engineering at the University of California, Berkeley. His research interests include the fate of wastewater-derived contaminants in engineered and natural systems, advanced oxidation processes and the development and testing of technologies for potable water reuse.

Advances in environmental analytical chemistry over the past two decades have made it possible to quantify hundreds of different compounds in surface water and municipal wastewater effluent. As a result, a steady stream of papers is being published every week with new analytical methods for previously unknown compounds in wastewater effluent or new data on the ubiquitous presence of another compound in surface waters. As the novelty of reporting on the occurrence of trace concentrations of wastewater-derived contaminants passes, the scientific and regulatory communities have begun asking questions about whether these compounds pose risks to humans and aquatic ecosystems. If they do pose risks, it will be critical to determine which treatment technologies are best suited for controlling the problems and whether the treatment processes themselves pose any new risks.

While it would be tempting to try answering these questions by expanding the scope of monitoring for the chemicals that we have just learned how to measure, a brief conversation with a toxicologist will teach you that compound-specific analysis is unlikely to fully answer these questions. Aquatic toxicologists have repeatedly shown that organisms respond differently to mixtures than they do to single exposures to pure compounds. Furthermore, the consumer products and pharmaceuticals present in sewage undergo metabolism prior to excretion and are transformed during biological wastewater treatment. Many of the chemical treatments being considered for advanced wastewater treatment systems, such as ozonation and advanced oxidation processes, also produce transformation products that can be as toxic or more toxic than the parent compound.

Bioassays offer a complimentary approach to chemical analyses for detecting unrecognized contaminants, screening for toxicity of metabolites or transformation products and assessing the behavior of compounds that cannot be readily measured. However, traditional in vivo bioassays (e.g., whole effluent toxicity tests) are expensive and often fail to respond to the low concentrations of wastewater-derived contaminants that are responsible for our current concerns. They also pose logistical constraints for routine monitoring because they have to be conducted on site in flow-through systems that require considerable effort to operate. The following JEM Spotlight article by Beate Escher, Nadine Bramaz and Christoph Ort (DOI: 10.1039/b907093a) describes a suite of relatively simple bioassays that quantify the fate of contaminants that cause effects such as non-specific toxicity, estrogenicity, photosynthesis inhibition, insecticidal activity and genotoxicty in municipal wastewater effluent.

In this paper, Escher et al. apply the bioassays to a full-scale sewage treatment plant equipped with ozonation after biological wastewater treatment. While their results confirm expectations that conventional biological treatment removes many contaminants in sewage that have the potential to cause biological responses, it shows that compounds that affect some biological functions (e.g., photosynthesis inhibitors) are only partially removed during biological treatment. Their results also demonstrate that much of the residual toxicity remaining after biological treatment is removed during the ozonation step. Coupling multiple treatment processes provides a multiple barrier to contaminants and assures an extra degree of certainty with respect to the removal of contaminants.

The bioassays offer several important advantages over existing monitoring methods. First, they use extracts from the same types of solid-phase extraction cartridges that are used for routine chemical analyses, offering an opportunity to integrate the tests into chemical monitoring programs. The use of extracts eliminates the need to run the assays in flow-through systems or to conduct them immediately after sample collection. The use of extracts also eliminates artifacts due to the presence of wastewater components that affect in vivo assays (e.g., ammonia, residual chlorine, bacteria). Another advantage of the assays is that they are quantitative and highly reproducible, which allows for detailed assessment of the performance of individual unit processes. This is particularly important because many bioassays do not exhibit the level of precision needed to evaluate systems in which partial removal occurs. Finally, the assays are readily available through commercial suppliers or academic laboratories, which should make it easy for them to be incorporated into monitoring programs.

It's not time to abandon the chemical-by-chemical analysis of the myriad of wastewater-derived contaminants. However, the bioassays described in this JEM Spotlight article provide a complimentary tool to chemical analyses that can be used to screen for the effects of new products that will enter waste streams in the future. The new bioassay suite also provides insights into the behavior of some of the chemicals that we have yet to measure as well as a deeper understanding of the effect of advanced treatment on water quality.

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