**1. Introduction**

A number of reviews have appeared on the topic of wastewater treatment and microplastic retainment [1–17]. Over the last 10 years, the understanding of microplastics and their impact on the environment has developed as have the analytical techniques to identify and quantify microplastics. In this respect, the focus has shifted to even smaller plastic particles dubbed "nano plastics". The identified sources of micro(nano)plastics have increased to include secondary plastics created by such mundane processes as opening a package [18] or making tea using plastic tea bags [19]. On the other hand, the importance of the individual sources of primary microplastics has shifted, with the ban of plastic microbeads in cosmetics coming into effect in many regions [20–25], changing the attention more to microtyres [26, 27], synthetic fibers [28, 29] and to secondary micro- and nano plastics [30]. There will be a shift of the sources of secondary microplastics as the ban in certain regions of plastic bags [31–33] and single use plastics [34, 35] comes into effect, as both are potential materials for microplastics due to subsequent degradative fragmentation processes. Plastics already existing in the environment degrade very slowly [36]. Furthermore, the examination of food articles and drinking water [37–40] for micro- and nano plastics has increased, as micro- and nano plastics have been found in foods and drink as diverse as table salt [41], soft drinks [42], beer [43, 44], and meat [45].

Microplastics (MPs) are defined as plastic particles of ≤5 mm in size [46–48]. For smaller particles, of size ≤1 μm, the term nanoparticles (NPs) is often used [46, 49]. Some authors define NPs as particles of up to 100 nm in size [50]. Plastic particles include polymeric films and synthetic fibers. Plastic microparticles come from different sources. They can be degraded and fragmented materials from tires (tires and road-wear) [51], clothing [52, 53], plastic bags [30] and packaging [18], where larger pieces of plastic are exposed to wear or weathering [54]. These are secondary MP. Primary MP are materials that are produced industrially at this small size. These include solid micropellets in cosmetic formulations, such as in facial cleaners and body scrubs [20, 55], microspherules in toothpastes (2–5 μm in size) [56], microparticles in washing powder/detergents [57, 58] and scrubbers used for air-blasting surfaces to remove paints and rust [59, 60] in paints and coatings themselves [58], and in drilling fluids in oil and gas exploration [1]. Drug delivery systems have used plastic micro−/nanoparticles, also – these are often biodegradable materials [61]. The amounts of materials used as primary MP and secondary MP stemming from the degradation of meso- and macroplastics on-land have been estimated in different studies commissioned by different European countries [62–65] and by the European Community [66]. Often, sediments of water bodies [67], especially oceans [68, 69], and terrestrial soil are some of the places where MPs may end up when released into the environment. There are a number of ways that MPs can enter the world's oceans that include direct run-offs into the oceans or into rivers that lead to oceans. Additionally, atmospheric transfer of MPs [70], which has been largely neglected until relatively recently, has been found to contribute to the accumulation of MPs in rivers, lakes [71] and oceans [72]. Terrestrial acquisition of MPs in soils can also happen in a number of ways that again includes atmospheric transport, but can also occur through fertilizer and even irrigation water [73]. Plastic mulching also contributes [74]. In both the dispersal of MPs to the aquatic and the terrestrial environment wastewater treatment plants (WWTPs) play a major role. On the one hand, WWTPs play a major part in retaining MPs from the sewage water, on the other hand, MPs can enter the soil through the application of sewage sludge [75, 76]. In the treatment of wastewater, WWTPs themselves can become point sources of MPs [77–80], releasing MPs into the receiving water. Thus, many examples have been found where the concentration of MPs downriver of a WWTP was higher than upriver. As the volume of wastewater is bound to increase over the years with an increase of population, new methods of wastewater treatment are being developed that help retain MPs better. This comes against the background of studies that assess the retaining capabilities of different treatment methods in existing WWTPs. Both are topic of the current review.
