Ozonation before and after nitrogen removal: Consequences for ozone demand in light of the new urban wastewater treatment directive

Micropollutant removal is a key component of the revised Urban Wastewater Treatment Directive (UWWTD) and can be achieved with ozonation. During ozonation, the composition of the wastewater matrix and the targeted treatment level for individual micropollutants or groups of such compounds affect ozone consumption. In this work, it was hypothesized that nitrite scavenging, the point of dosing (before and after nitrogen removal), and the strategic selection of compounds from the UWWTD list are critical to ozone demand. Here, the relative contributions of these factors to ozone demand were compared. To this end, dose-response profiles were established for 27 organic micropollutants in wastewater from two large, coastal Swedish wastewater treatment plants (WWTPs) with separate steps for BOD- and N-removal. The results showed that nitrite consumption was dose-dependent and accounted for 15-40% of ozone demand. Ozone dosing was possible before nitrogen removal but required up to 15% more ozone. However, an unfavorable compound selection, still in compliance with the UWWTD, could result in up to 50% higher ozone demand. To also address elevated bromide concentrations (1 mg L^-1), which are common in coastal areas and were observed at one of the WWTPs, a risk assessment of bromate was performed for the marine environment, which found a low risk. These findings emphasize that nitrite formation should be carefully monitored and indicator substances wisely selected to minimize ozone demand. In WWTPs with separate BOD- and N-removal, ozonation can be moved upstream and integrated with nitrogen removal, potentially saving on infrastructure for post-treatment.