In the Project “Ozonation WWTP Neugut, Dübendorf – Optimizing Ozone dosage in a full-scale Treatment plant” the UV-absorbance difference between ozonation inlet and outlet is investigated to monitor the micropollutant removal efficiency as well as to control ozone dosage. To evaluate the cost and energy reduction of this method laboratory and pilot tests were carried out at Eawag.
Analytical analysis of micropollutants is time-consuming and cost-intensive. Thus, a control of ozone dosage based on such results is not realistic. Analytical analysis of several samples over the year can however be used by cantonal authorities checking the compliance of a treatment plant with the future water protection ordinance. However, continuous monitoring of the removal efficiency and control ozone dosage requires an online sensor. Tests at Eawag showed that the decrease of UV absorbance during ozonation is a potential method to quantify the removal of micropollutants. The conducted experiments showed a clear correlation between the decrease of UV absorbance, the ozone dosage and the micropollutant removal efficiency. For different types of wastewater the correlation between the UV-absorbance difference and the removal of micropollutants is very similar for some substances such as Carbamazepine and Propranolol. However, for other substances such as Primidon and Venlafaxin high variations exist for different wastewaters. Based on these results, a feedback control strategy can be derived. Nevertheless, parameters have to be adapted for a specific wastewater and the required elimination for the selected indicator substances. Among others benzotriazole, diclofenac, carbamazepin, mecoprop and sulfamethoxazole are discussed as possible indicator substances that have to be removed by 80% over the whole wastewater treatment plant (mechanical, biological and post treatment).
Benzotriazole and mecoprop show the lowest elimination rates during ozonation. Depending on the elimination during mechanical and biological treatment which is in the range of 40% for benzotriazole the eliminated in ozonation should be at least 50% to achieve the quality goal of 80%. In the wastewater treatment plant Neugut in Dübendorf this would lead to a required absorbance decrease of 45% at a wavelength of 254nm and of 70% at 366nm.
The successfully tested feed forward control of ozone dosage based on influent DOC-load could therefore be optimized by a feedback control strategy based on the UV-absorption difference.
To implement the decrease of absorbance as a feedback control parameter further investigations regarding measuring stability especially for the full-scale treatment plant are missing. The effects of rain events or seasonal variations have to be evaluated in future tests, carried out at the full-scale ozonation plant of WWTP Neugut starting in January 2014. Regarding measurement stability and maintanance, the online absorbance measurement device is further tested to improve the practical handling of the system.