Elimination of Organic Micropollutants in a Municipal Wastewater Treatment Plant Upgraded with a Full-Scale Post-Ozonation Followed by Sand Filtration

TLDR

The study proposes a conceptual approach that enables direct upscaling from laboratory to full‑scale wastewater treatment systems and is applicable to similar facilities. The authors quantified removal of 220 micropollutants in a municipal WWTP upgraded with post‑ozonation and sand filtration using kinetic and hydraulic modeling, noting an energy cost of 0.035 kWh m⁻³ (≈12 % of a typical plant). Post‑ozonation achieved complete removal of fast‑reacting micropollutants and >85 % removal of more resistant ones at a medium ozone dose, leaving only 11 of 55 detected compounds above 100 ng L⁻¹; carcinogenic by‑products NDMA and bromate remained below drinking‑water standards, and sand filtration further reduced biodegradable by‑products such as NDMA.

Abstract

The removal efficiency for 220 micropollutants was studied at the scale of a municipal wastewater treatment plant (WWTP) upgraded with post-ozonation followed by sand filtration. During post-ozonation, compounds with activated aromatic moieties, amine functions, or double bonds such as sulfamethoxazole, diclofenac, or carbamazepine with second-order rate constants for the reaction with ozone >104 M−1 s−1 at pH 7 (fast-reacting) were eliminated to concentrations below the detection limit for an ozone dose of 0.47 g O3 g−1 dissolved organic carbon (DOC). Compounds more resistant to oxidation by ozone such as atenolol and benzotriazole were increasingly eliminated with increasing ozone doses, resulting in >85% removal for a medium ozone dose (∼0.6 g O3 g−1 DOC). Only a few micropollutants such as some X-ray contrast media and triazine herbicides with second-order rate constants <102 M−1 s−1 (slowly reacting) persisted to a large extent. With a medium ozone dose, only 11 micropollutants of 55 detected in the secondary effluent were found at >100 ng L−1. The combination of reaction kinetics and reactor hydraulics, based on laboratory- and full-scale data, enabled a quantification of the results by model calculations. This conceptual approach allows a direct upscaling from laboratory- to full-scale systems and can be applied to other similar systems. The carcinogenic by-products N-nitrosodimethylamine (NDMA) (≤14 ng L−1) and bromate (<10 μg L−1) were produced during ozonation, however their concentrations were below or in the range of the drinking water standards. Furthermore, it could be demonstrated that biological sand filtration is an efficient additional barrier for the elimination of biodegradable compounds formed during ozonation such as NDMA. The energy requirement for the additional post-ozonation step is about 0.035 kWh m−3, which corresponds to 12% of a typical medium-sized nutrient removal plant (5 g DOC m−3).