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Removal of microplastics from urban WWTP effluent: Ultrafiltration tertiary treatment combined with anaerobic digestion
Summary
Scientists tested a filtering technology that can completely strip microplastics out of treated wastewater before it's released into rivers and streams, good news for reducing plastic pollution in our water. But there's a catch: those captured microplastics don't fully break down during the sludge treatment process afterward, meaning the leftover material (often used as fertilizer) ends up with much higher plastic concentrations. This suggests we need better solutions for what happens to microplastics *after* they're removed from water, especially if that leftover sludge is being spread on farmland where it could re-enter our food system.
Although conventional wastewater treatment plants (WWTPs) retain most microplastics (MPs) in the sludge, a significant proportion escapes with the effluent, leading to their release into the environment and potential adverse impacts. The application of ultrafiltration (UF) as a tertiary treatment has demonstrated complete removal of MPs, producing high-quality effluents. However, MPs accumulate in the resulting waste stream, compromising its potential use as an organic soil improver after stabilisation. This study aims to evaluate the fate of MPs from a UF reject stream, assessing their potential biodegradation through anaerobic digestion (AD). For this purpose, a UF pilot plant treating the secondary clarifier effluent of an urban WWTP was operated for five months. Two laboratory-scale anaerobic digesters were used to determine the biodegradability of the produced waste: one (AD₁) was fed with a mixture of mixed sludge and UF waste at a 30:1 ratio based on COD load, while the other (AD₂) was fed exclusively with mixed sludge and served as a control. The UF plant achieved complete MP retention from the WWTP effluent, concentrating MPs in a waste stream that represented approximately 7% of the AD influent flow, which meant no significant differences in biomethanisation were observed between the lab-scale digesters (59% vs. 56% for AD₁ and AD₂, respectively). However, differences in MP removal were observed mainly due to the higher load received by AD₁ (2069 mg·L⁻¹), compared with 332 mg·L⁻¹ in AD₂. Consequently, the digestate from AD₁ showed a much higher MP concentration (850 mg·L⁻¹) than AD₂ (69 mg·L⁻¹).