Abstract
Microplastic (MP) water pollution is a major problem that the world is currently facing, and wastewater treatment plants (WWTPs) represent one of the main alternatives to reduce the MP release to the environment. Several studies have analysed punctual samples taken throughout the wastewater treatment line. However, there are few long-term studies on the evolution of MPs over time in WWTPs. This work analyses the performance of a WWTP sited in Southwest Europe in relation with annual occurrence and fate of MPs. Samples were monthly taken at different points of the facility (influent, secondary effluent, final effluent, and sludge) and MPs were quantified and characterised by means of stereomicroscopy and FTIR spectrophotometry. The majority of MPs found in wastewater and sludge samples were fragments and fibres. Regarding to the chemical composition, in the water samples, polyethylene (PE), polyethylene terephthalate (PET) and polypropylene (PP) stood out, whereas, in the sludge samples, the main polymers were PET, polyamide (PA) and polystyrene (PS). The MPs more easily removed during the wastewater treatment processes were those with sizes greater than 500 µm. Results showed that the MPs removal was very high during all the period analysed with removal efficiencies between 89% and 95%, so no great variations were found between months. MP concentrations in dry sludge samples ranged between 12 and 39 MPs/g, which represented around 79% of the total MPs removed during the wastewater treatment processes. It is noticeable that a trend between temperature and MPs entrapped in sewage sludge was observed, i.e., higher temperatures entailed higher percentage of retention.
Highlights
wastewater treatment plants (WWTPs) are a major indirect source of MP emissions into the environment, due to the daily discharge of large quantities of MPs, from agricultural, industrial or urban activities, to the sewage system [1,2,3]
These microplastics can enter to the sewage system by surface runoff or stormwater, either because they are on the ground surface or deposited from the atmosphere [10,11,12]; wastewater could contain a high number of MPs, the MP concentration reported in WWTPs ranged between 0.28 and 3.14·104 particles/L [13]
Most of the studies dealing with the occurrence of MPs in WWTPs have been focused on wastewater and sludge samples collected over short periods, i.e., days or weeks [16,32,37]
Summary
WWTPs are a major indirect source of MP emissions into the environment, due to the daily discharge of large quantities of MPs, from agricultural, industrial or urban activities, to the sewage system [1,2,3]. MPs can be originated from the weathering and fragmentation of plastics due to disposal mismanagement or by the wear and tear of plastic items [7,8,9]. These microplastics can enter to the sewage system by surface runoff or stormwater, either because they are on the ground surface or deposited from the atmosphere [10,11,12]; wastewater could contain a high number of MPs, the MP concentration reported in WWTPs ranged between 0.28 and 3.14·104 particles/L [13]. The reported ranges of MP concentration in wet and dry mixed sludge were 400–7000 and 1500–170,000 particles/kg, respectively [17,18,19,20]
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