Challenges and persistence of contact lenses in wastewater treatment plants: Environmental implications

Contact Lens (CLs) are often disposed of via toilet or sinks, ending up in the wastewater treatment plants (WWTPs). Millions of CLs enter WWTPs worldwide each year in macro and micro sizes. Despite WWTPs' ability to remove solids, CLs can persist and potentially contaminate watercourses and soils. This study evaluates whether different CLs degrade in WWTP aeration tanks. Six daily CLs (Nelfilcon A, Delefilcon A, Nesofilcon A, Stenfilcon A, Narafilcon A, Somofilcon A) and four monthly CLs (Lotrafilcon B, Comfilcon A, Senofilcon A, and Samfilcon A) were immersed in aeration tanks for twelve weeks. Their physical and chemical properties, including water content (WC), refractive index (RI), chemical properties (Fourier Transform Infrared Spectroscopy), and mechanical properties were assessed. Results show that all CLs maintained their physical appearance after 12 weeks. Neither Nelfilcon A nor Narafilcon A exhibited significant changes in WC and RI, (p > 0.05, Tukey test), while other daily lenses showed variations in at least one parameter. Among monthly CLs, only Senofilcon A showed significant differences in both WC ( p < 0.001, Tukey test) and RI ( p < 0.0001, Tukey test). No differences in Young's modulus were observed for any lenses (p > 0.05 Tukey test). However, Somofilcon A displayed significant changes in stress at break ( p < 0.0001, Tukey test), and Elongation at Break ( p < 0.05, Tukey test). No changes were found in the chemical structure of any CLs suggesting that twelve weeks in WWTP aeration tanks is insufficient for CLs degradation. These findings highlight CLs as a potential emerging pollutant, emphasizing their persistence in sludge or migration into watercourses and soils.