Making Wastewater Safe for Urban Use: Why Multi-Stage Treatment Matters

The new article “Analysis of the impact of the advanced oxidation processes on reclaimed water microbiological safety” by Klara Ramm (Warsaw University of Technology, Poland) and Monika Wojciechowska (Savonia University of Applied Sciences, Finland) presents findings from the ReNutriWater project pilot studies, which explored water reuse for urban applications.

The study focused on a small-scale pilot water reclamation system in Kuopio, Finland, designed to treat effluents from municipal wastewater treatment plants. The research assessed microbiological safety, with particular attention to Escherichia coli (E. coli) as an indicator of fecal contamination, and compared results with multiple regulatory standards, particularly EU Regulation 2020/741.

The results showed that relying on a single disinfection stage was not sufficient to ensure stable water safety, as E. coli levels varied greatly depending on the conditions, making consistent safety difficult to achieve. The solution lay in a multi-barrier treatment approach. Pre-ozonation and activated carbon sorption proved essential for boosting water quality and reducing turbidity, which paved the way for more effective disinfection later in the process. Ozonation and activated carbon filtration successfully lowered E. coli counts, but it was only with the addition of ultraviolet (UV) irradiation that complete elimination was reached. A final step of chlorination then ensured this effect could be maintained, offering stable microbial safety.

Environmental conditions also played a major role in the system’s performance. During summer, higher E. coli levels highlighted the need for adaptive treatment strategies that can respond to seasonal variability. Cold weather, in turn, reduced the efficiency of biological treatment and contributed to unstable effluent quality, showing that low temperatures also posed a challenge. The study also emphasized the importance of turbidity management as a prerequisite for UV and chlorination to work properly. Moreover, the researchers observed that physicochemical indicators such as COD, BOD₅, turbidity, and total phosphorus closely followed microbial changes, suggesting these parameters may help predict microbiological outcomes.

Despite these challenges, the pilot system successfully achieved Class A reclaimed water quality under EU rules, demonstrating that safe water reuse is possible through a multi-stage treatment approach. The findings underline that reclaimed water can be a reliable resource for urban use, if treatment systems are carefully designed with attention to local conditions and seasonal changes.

Looking forward, the authors stress the need for economic evaluations, further technological optimization, and larger-scale trials. Their study not only advances the science of water reclamation but also points the way toward safer and more sustainable solutions for future cities.

Read the full research article here.

The paper was prepared as a part of the “ReNutriWater – Closing local water circuits by recirculation of nutrients and water and using them in nature” project financed by the European Regional Development Fund (ERDF), project no. #C016.

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