Microplastics (MPs) from households, stormwaters and various industries are transported to wastewater treatment plants (WWTPs) where a high proportion of them are captured before discharging their residuals to watersheds. Although recent studies have indicated that the removed MPs are mainly retained in wastewater sludge, sludge treatment processes have gained less attention in MP research compared to water streams at primary, secondary and tertiary treatments. In this study, we sampled twelve different phases in a tertiary-level municipal WWTP in Central Finland. Our results showed that compared to plant influent load, three times higher number of MPs was circulating via reject water from the sludge centrifugation back to the beginning of the treatment process. Especially fibrous MPs were trapped in the dewatered sludge whereas fragment-like MPs were transported in aqueous stream. We concluded that sludge treatment is the main exit route for MPs into environment but also a return loop to the beginning of the process. Our sampling campaign also demonstrated that WWTPs with varying hydraulic conditions, such as the one studied here, benefit from disc filter based tertiary treatments in MP removal.

2024