Determination the effect of ozonation on micropollutant removal

In recent years, studies on micropollutants increase and micropollutants have become important subject to investigate. Micropollutants can be described as natural or synthetic substances in water in the range of ng/L to µg/L and they consist of many different groups. Pharmaceuticals, personal care products, industrial compounds and disinfection byproducts are the main groups of micropollutants. Micropollutants can enter into water sources via different ways, but most significant source of micropollutants is wastewater treatment plant discharges. As the wastewater treatment plant is not designed for micropollutant removal, micropollutants are discharged to the receiving environment without treatment. This condition creates a problem for aquatic ecosystems and human health. These substances may cause endocrine disruptive effects in aquatic ecosystems and may also undergo bioaccumulation. In general, the precise data on the effects of micropollutants have not been determined, but it is thought that they are not present in water as single compounds. They are in the form of mixture, so they can produce synergistic effects and cause chronic effects as a result of continuously discharge to water. For these reasons, it is necessary to provide control without entering the aquatic environment. At present, there is no legal regulation for the control of micropollutants. However, many countries have been working towards a control mechanism for micropollutants. Switzerland is one of those countries since they want to protect their natural water sources. In their research which is conducted by Federal Office of the Environment, they evaluated their WWTPs and created a timeline for the implementation of advanced treatment technologies for further removal of micropollutants. They investigated 700 WWTPs and made a decision to upgrade 100 WWTPs in order to achieve a removal efficiency of 80% for the majority of the micropollutants monitored. In this study, 13 micropollutants and 10 metabolites were measured. The micropollutants examined were as follows: Antibiotics (ciprofloxacin and sulfamethoxazole), anti-epileptic (carbamazepine), analgesics (ibuprofen, diclofenac and naproxen) and beta blockers (atenolol and propranolol). In addition, the stimulant caffeine, endocrine disrupting plasticizer bisphenol-A, hormones and NDMA which is disinfection byproduct were also monitored. There are a number of reasons why micropollutant cannot be removed at the wastewater treatment plant, which can be divided into factors related to micropollutants and factors originating from the wastewater treatment plant. Polar and non-volatile substances can escape from wastewater treatment plants. The sorption of the micropollutants depends on the hydrophobicity of the micropollutants and is usually expressed in terms of Kow. If log Kow value is lower the 2,5, it means low sorption potential, so micropollutants can be found in water, not in sludge. Acidity and chemical structure are other factors that affect the removal rate. Factors related to WWTP can be listed as hydraulic retention time, sludge retention time, pH, and temperature. Membrane processes, activated carbon adsorption, and ozonation are investigated as treatment options for micropollutants. Operation of the membrane processes and control of the remaining part after the treatment are problematic. In activated carbon adsorption, PAC is a good option for removal, yet, it is difficult to separate carbon from water and the process is slower than ozonation. When ozone oxidation is evaluated, ozone is a preferred option because of its high oxidation potential, and ozone also forms hydroxyl radicals at high pH values. Moreover, hydroxyl radical is not a selective oxidant, so it can oxidize more compounds compared to ozone. Ozone is used for drinking water, domestic wastewater and industrial wastewater treatment in terms of color, taste, odor removal and disinfection. The use of ozone as a disinfectant is more advantageous than chlorine. First of all, all the viruses can be removed more effectively than chlorine; besides, with ozonation the oxygen content in the water is increased and the natural water balance is protected. If ozone is used, the amount of chemical needed and the amount of sludge formed are reduced. The effectiveness of the ozonation process in the removal of micropollutants depends on the physical and chemical composition of the wastewater sample. Parameters such as conductivity, specific UV absorbance, total suspended solids (TSS) pH, alkalinity, organic matter (COD, DOC) are effective on the elimination of targeted micropollutants and on the applied ozone dose. Additionally, the affinity of micropollutants for ozone is another factor that affects the elimination efficiency. If a micropollutant's second-order rate constant is higher than 104 M-1s-1, it reacts with ozone quickly. Ozonation have some limitations because it can generate toxic transformation products. Formation of by-products depends on the characterization of wastewater. Although the ozonation process has some limitations such as by-product formation, transformation products can be controlled using sand filtration. Within the scope of this study, grab samples were taken from two different wastewater treatment plants, one accepting solely domestic wastewater (WWTP A) and the other one receiving a mixture of domestic and pretreated tannery wastewater (WWTP B). In this context, the experiments were conducted at two different pH (7 and 10) values and at six different ozone doses ranging from 0,2 to 1,5 mg O3/mg DOC to assess the removal efficiency of ozonation for both micropollutants and transformation products. This study was repeated seasonally throughout the year to observe the seasonal changes in both micropollutant concentration and elimination. As a result of the experimental studies, average removal efficiencies were found to be approximately 55% for the investigated WWTPs (for WWTP A at an optimum ozone dosage of 0,4-0,6 mg O3/ mg DOC and at the pH of 7; for WWTP B at an optimum ozone dosage of 0,8-1 mg O3/mg DOC and at a pH of 7). When micropollutants were evaluated singularly, maximum removal efficiencies were achieved at different doses. Therefore, they were evaluated cumulatively to obtain an optimum ozone dosage. Ozonation of some micropollutants; BPA, diclofenac, carbamazepine, epoxycarbamazepine, naproxen and sulfamethoxazole resulted in high removal efficiencies (more than 75%) throughout the year. Some micropollutants like caffeine and ibuprofen demonstrated moderate removal efficiencies (about 40-50%). Limited elimination rates were observed for some micropollutants (approximately 15-20%); namely hydroxy diclofenac and o-desmethyl naproxen. O-desmethyl naproxen constituted the majority of untreated total micropollutant concentration and this situation affected the total removal efficiency. The optimum pH value securing the most effective micropollutant removal was determined as 7 since the ozonation application at a pH of 10 yielded only a negligible incremental reduction efficiency. This observation could be attributed to wastewater constituents acting as hydroxyl radical scavengers. On the other hand, micropollutant removal efficiency by means of ozonation may vary depending on a wide variety of factors; such as micropollutant concentration, matrix effect. The systematic approach used in this study in terms of the ozonation of micropollutants can be applied to other domestic and / or municipal WWTPs at national level in order to impose micropollutant control at the point source.