Wastewater treatment plants are designed primarily to eliminate nutrients. However, attention is increasingly being focused on micropollutants, since even low concentrations of these substances can have adverse impacts on aquatic ecosystems. Ozonation of wastewater appears to be a promising option for removing micropollutants at treatment plants.
Ozone, which consists of three oxygen atoms, is a reactive gas. It is a powerful oxidizing agent and forms hydroxyl radicals in contact with water. While ozone itself only oxidizes certain substances, the hydroxyl radicals attack a wide range of substances. Ozonation thus leads to the breakdown of various complex compounds, making substances much more susceptible to subsequent biological degradation. During this process, not only pollutants but also microorganisms are destroyed; for this reason, ozone is frequently used as a disinfectant.
Ozone is produced in ozone generators by electrical discharges from purified and dried air, or directly from industrial oxygen. It thus arises from oxygen and, over time, also decomposes into oxygen. A process of the same kind occurs naturally in thunderstorms.
Ozonation followed by granular activated carbon filtration is already a standard method for the treatment of raw surface water to produce drinking water (see Box). Here, the main purpose is disinfection, but also the elimination of organic substances and odour compounds, and colour removal. The experience gained in the drinking water area can be used in applying the method to wastewater treatment. In addition, ozonation is a straightforward process involving only a small number of components, and it can be readily adapted to local conditions.
Laboratory tests with wastewater have shown that the ozonation method can be successfully used to eliminate a wide range of micropollutants. In the scaled-up process, ozone is added to the wastewater stream in a special reaction vessel, ensuring adequate mixing and contact time. Various options exist for technical implementation. A simple and reliable method – used at the Wüeri wastewater treatment plant (WWTP) in Regensdorf – is the injection of fine (gas) bubbles in a closed contact tank. Any remaining ozone not consumed in the process would be destroyed in the off-gas with the aid of a thermal catalytic unit.
At the Wüeri WWTP in Regensdorf, an ozone reactor connected to a sand filter is being installed after the biological treatment step. The sand filter, which includes a biofilm, is designed to permit further biological degradation of pollutants following the breakdown of complex compounds by ozonation. The planned studies will focus on measurement and ecotoxicological analysis of pollutants which it has not previously been possible to eliminate (at all or in full) at WWTPs.
This is the case, for example, for benzotriazoles (used as anticorrosive additives, e.g. in dishwasher detergents), diclofenac (analgesic and anti-inflammatory agent) and carbamazepine (antiepileptic). To date, it has also only been possible for natural and synthetic hormones (estrone, estradiol) to be partly removed, under certain conditions (sufficiently high sludge age).