Emerging pollutants in wastewateraquatic toxicity and ozonation

Résumé

The increasing worldwide contamination of freshwater ecosystems with thousands of chemical compounds is one of the key environmental problems facing humanity. While concentrations of so-called Persistent Organic Pollutants (POPs) and Priority Pollutants are declining, numerous emerging substances, such as pharmaceuticals, personal care and house cleaning products amongst others, are commonly detected in the aquatic environment as complex mixtures. Aquatic toxicity of two mixtures of emerging substances with inherent antimicrobial properties (personal care product preservatives and antibiotics) were assessed on indigenous biological communities of the aquatic compartments (activated sludge microorganisms and natural limnic biofilms) in order to provide ecologically a more realistic data and to improve the knowledge about their environmental risk. The results showed that the preservative mixture (iodopropynyl butylcarbamate, bronopol, diazolidinyl urea, benzalkonium chloride, zinc pyrithione, propylparaben, triclosan and a mixture of methylchloroisothiazolinone and methylisothiazolinone) displayed a potential risk to the microorganisms present in an STP aeration tank and consequently, to the process performance of activated sludge. Among them, benzalkonium chloride is the most problematic of the studied preservatives as is the risk driver of the mixture. The result from a screening level risk assessment of antibiotics (doxycycline, erythromycin, ofloxacin, sulfamethoxazole and trimethoprim) towards bacterial periphytic communities showed potential risk to the aquatic ecosystem for the mixture under a Spanish STP scenario and for single ofloxacin, the risk-driver of the mixture toxicity. Emerging pollutants enter the aquatic environment mainly through conventional STP, where most of them are not efficiently removed. One way of minimizing the input of these micropollutants into surface waters is to integrate an additional treatment step at STPs such as ozonation. In the present study, continuous ozonation is shown as a suitable technology for upgrading conventional STPs both as a pre-treatment stage and a polishing step of activated sludge process. Ozonation effectively removed compounds that pose environmental risk (benzalkonium chloride, ofloxacin and six-antibiotic mixture) in synthetic water and real wastewater as a result of the combined attack of molecular ozone and hydroxyl radicals, being the optimum ozone dose strongly water matrix-dependent. Ozonation did not lead to a complete mineralization of the organic compounds with the consequent accumulation of transformation products (TPs), which were identified using mass spectrometry coupled to liquid chromatography (LC-ESIMS(TOF), LC-ESI-MS(QTOF)). The further oxidation of TPs gave rise to low molecular weight by-products such as carboxylic acids. These ozone refractory compounds are easily assimilable and consequently, constitute a special concern for the proliferation of microbes downstream of an ozonated-wastewater discharge point. The current study demonstrated that copper-catalysed continuous ozonation, in both synthetic and real wastewater, significantly improves organic acid mineralization, which is mainly due to its high performance in oxalic acid depletion. Nonetheless, the water matrix has a notable influence on the optimum catalyst dose necessary to achieve a given degree of mineralization. In addition to the chemical analysis, aquatic toxicity of ozone treated waters should also be taken into account to assess ozonation in a comprehensive manner. Ecotoxicity assessment was conducted combining different levels of biological complexity with the aim of providing an accurate indication of the toxic effects of ozonated wastewaters on exposure biological systems: bioassay batteries of single species belonging to different trophic levels (the bacteria Vibrio fischeri and Pseudomonas putida, the protozoan Tetrahymena thermophila, the alga Pseudokirchneriella subcapitata and the crustacean Daphnia magna) and indigenous biological communities (microorganisms from an STP aeration tank and natural limnic biofilms). The results indicated that during ozonation, the aquatic toxicity of wastewater decreases in proportion to the disappearance of the studied emerging pollutants. It can be assumed that toxicity is dominated by the parent compounds, and the TPs were not relevant for the aquatic hazard assessment. However, the degradation of emerging substances that interacts with nanoparticles, such as benzalkonium chloride, caused an increase in toxic-metal leaching from the nanomaterial and consequently, led to a toxicity enhancement of treated wastewater.