Grab and composite samples: Variations in the analysis of microplastics in a real wastewater treatment plant in the South of Spain

In recent years, plastic pollution has become one of the most concerning problems. It has been demonstrated that microplastics (MPs) can be found in most environmental compartments and that wastewater discharges have been identified as one of the main pathways for these pollutants to enter the natural environment. Knowing the amount of MPs coming from wastewater is important to establish appropriate pollution management and mitigation. Most studies analyse grab samples collected at a specific time, whose MPs content can be affected by outlier events that deviate from the average values and may give a distorted picture of the number of MPs in wastewater, either by over- or underestimation. This study deals with the collection and analysis of grab and composite samples collected throughout the day and in different months of the year to determine the variability of MPs in a wastewater treatment plant in southern Spain. No relevant differences were observed in the predominant size range or microparticle morphology, but the results confirm the differences in the amount of microparticles and MPs between the grab samples collected at different times of the day, being advisable to collect composite samples to obtain values more representative of reality. The variability in the number of MPs associated with the time of the year when the sample is taken is also observed, a factor that may be considered when sampling and which is affected by human behaviour and climatic conditions. [Display omitted] •Collection of grab and composite samples in an urban WWTP in three different months.•Variations in the amount of microplastics at different times of the day and months.•Fibers were the most abundant microparticle shape (29.2 – 94.2%).•PE, PA, PET, PP, PS, PU and PVS were the most common polymers found in the samples.•Composite sampling avoids over- or underestimation of the amount of microplastics.