Use of advanced oxidation processes for removal of micropollutants

One of the big challenges of modern water treatment is the handling of micropollutants. These are compounds found in very low concentrations, often at ppt or ppb level, but are still capable of having a potent effect on the environment, and possibly humans as well. One of the emerging technologies for removal of micropollutants is the use of advanced oxidation processes (AOPs). AOPs use highly reactive hydroxyl radicals to degrade the micropollutants, but the processes are very energy intensive, which may limit their applications.

To investigate the feasibility of introducing AOPs in the Danish water treatment industry, a study was set up at Vejle Waste Water Treatment Plant (WWTP). Here the WWTP had employed a UV system to disinfect treated waste water, and it was decided to investigate the effect of applying titanium dioxide (TiO2) surfaces to the system. TiO2 will produce hydroxyl radicals through photocatalysis when illuminated with UV light, and it may furthermore be arranged so the photocatalyst is immobilized on the UV quartz tubes by coating, which removes the need for a constant addition and subsequent removal of TiO2 to the system. The effect of the current system, and the TiO2 modified system was investigated by degradation of the synthetic estrogen 17α-ethinylestradiol (EE2). EE2 was used as the model compound since it is a very potent endocrine disruptor that has been found to have endocrine effects on fish at ppt levels. Also, the disinfection capability with photocatalysis was investigated to determine the effect of the modification of the UV system on its original purpose. The degradation experiments were carried out at pilot scale in the laboratory, and then compared to data collected at the WWTP.

TiO2 was found to improve the rate of degradation of EE2 by 66 % compared to the photolytic degradation with UVC light alone. It was as such a significant improvement. However, even with TiO2 coated surfaces, a relative large amount of energy is required to degrade EE2. By assuming the UV system in Vejle to be operating at maximum capacity, a photocatalytic system would be capable of removing between 1 and 5 % of the EE2 concentration within the measured flow regime. Coated TiO2 surfaces were found to reduce the disinfection efficiency compared to pure UV light. This was assumed to be because of differences in the disinfection mechanisms. Where UV light directly affects the DNA of the microorganisms, photocatalysis works by oxidizing the cell membrane for microorganism adsorbed to the coated surface, which is a more inefficient process per mole UV light.