Abstract
Water, a vital resource for the environment, human well-being, and economic development, is increasingly threatened by anthropic activities. Global freshwater use has surged sixfold over the last century, with developed countries showing a remarkably high per capita consumption. Unequal global freshwater distribution results in water stress for over two billion people. Global warming worsens the situation by increasing water demands and decreasing river flows. Water quality also faces threats with the usage of numerous chemical compounds in water by economic activities like agriculture (pesticides and pharmaceutical products), industry or households (pharmaceutical and personal care products, disinfectants,and other chemicals). Wastewater (WW) reuse after adapted treatment represents one of the existing solutions to decrease tension over water resources. However, it is associated with possible health and environmental risks caused by the presence of micropollutants and pathogens. Ozonation is then considered as a potential tertiary WW treatment for the elimination of these pollutants due to the strong oxidation capacity of ozone and hydroxyl radical generation during the process.The aim of this study was to determine the effectiveness of ozonation as a cost-effective tertiary treatment for the removal of emerging contaminants and organic matter in the context of WW reuse. We investigated the elimination of eight pharmaceuticals and two pesticides (native concentrations) from treated wastewater (TWW) using low ozone doses and studied the formation of transformation products (TPs).Ozonation experiments were done with TWW sampled from Claira wastewater tratment plant (WWTP), located in south of France. Water quality analysis were conducted based on global parameters, including Dissolved Organic Carbon (DOC), UV254 nm absorption, Biological Oxyen Demand in 5 days (BOD5), Chemical Oxygen Demand (COD), and Total Suspended Solids (TSS). Additionnaly, both target and non-target analyses of micropollutants were carried out using High Performance Liquid Chromatography coupled to a High-Resolution Mass Spectrometer (HPLC-HRMS). Ozonation experiments were performed in a 3 L ozonation pilot glass stirred semibatch reactor (Vreactor = 3 L, 250 rpm) under thermostatic control (20 ± 0.5 ◦C). The total gas flow was 60 NL.h−1 and the inlet ozone gas concentration ([O3]G,inlet) was 5 ± 0.2 g.Nm−3, monitored by an ozone gas analyzer (BMT 964, BMT). The ozone enriched oxygen was immediately injected in the glass reactor by the action of solenoid electrovalves. Global parameters assessed in the TWW effluent revealed compliance with both French and European water REUSE regulations with the following parameters well within acceptable limits: TSS < 10 mg/L, BOD5 < 10 mg/L, Turbidity < 5 NTU, COD < 60 mg/L. It is worth highlighting that ozonation further improved the water quality by effectively degrading the effluent organic matter (OM). Hence, COD was slightly decreased under specific transferred ozone doses of 0.6 mgO3.mgC-1, above this dose the demand significantly decreased, up to 25 %. UV254nm absorbance decreased steadily between 0 and 0.4 mgO3.mgC-1, where it reached a minimum value of 0.04. As for DOC, ozonation did not show any significant impact on the parameter. These results show that ozone oxidated OM and thus reduced the wastewater organic content. The DOC content remained unmodified, showing that ozonation did not lead to OM mineralization. Finally, the decrease in OM aromaticity, as demonstrated by UV254nm absorbance reduction, showed that ozonation modified OM by generating simpler molecular structures.Micropollutants degradation was significant even at low ozone doses. Hence, the elimination was superior to 80 % for nine out of ten compounds at an ozone dose of 1.68 mgO3.mgC-1 (corresponding to 30 minutes of ozonation). The only compound that was refractory to ozonation was diuron. Other compounds, even with low reactivity to molecular ozone were quickly eliminated, e.g. Irbesartan (k03=23 M-1.s-1) decreased from 5587.4 ± 41.6 ng.L-1to 35.7 ± 7.7 ng.L-1 in 30 minutes of ozonation. These results demonstrated that ozonation can efficiently eliminate pharmaceuticals and pesticides even at very low ozone doses. However, their oxidation generates TPs that may be toxic. Some of these TPs have been identified using non-target analysis, leading to a better understanding of the ozone process and its implications for WW reuse.Ozonation decreases the TWW organic load and can significantly degrade micropollutants, even at acceptable costs. Thanks to new approaches such as non-target analysis, generated TPs are becoming better known, as well as associated risks in the context of TWW reuse. Finally, the possibility of coupling ozonation with a complementary nature-based solution should be evaluated, as it may be useful for TPs elimination in the WW reuse context.