Fate of microplastics in wastewater and water treatment plants
Recently plastic litter contamination is considered one of the most serious manmade threats for the natural environment and hence a topic of emerging concern . Microplastics are called tiny plastic pieces, less than 5 mm to 1 nm in every dimension, primarily made of polyethylene (PE), polypropylene (PP), and other commonly used, poorly biodegradable polymers. Depending on their formation source, they can be generally classified into two different types, primary microplastics (PMP) and secondary microplastics (SMP). PMP are intentionally produced in small sizes and used as industrial pellets or microbeads added to personal care products and cosmetics. SMP are fragments obtained by mechanical abrasion (e.g. tire), and large plastic pieces disintegration, due to the influence of different biotic and abiotic factors of environment, such as solar irradiation, presence of dissolved oxygen in water, temperature, waving of water, activity of microbial etc. . This group also includes microfibers obtained by washing synthetic clothes and fabrics (but not cotton or cellulose fibers).
Table 1. Classification of microplastics based on their source
Moreover, substantial fluxes of microplastics coming from industrial plants [3, 4] or littering , the effluents of waste water treatment plants (WWTPs) are often considered as a major source for PMP and SMP. It is a plausible assumption due to the high likelihood of occurrence of macro- and microplastics in municipal and industrial effluents as well as urban surface runoffs. While large plastics are likely being removed in waste water treatment processing, used technologies are not specifically designed to retain small microplastic particles (MPs). So far, the effluents of WWTPs are studied rarely and only in some countries. Quantitative data are available from Australia , the UK , France , Finland  the United States  and Netherlands .
Significantly more MPs in the surface water were detected by McCormick’s research group  in a highly urbanized river in Chicago after the introduction of treated waste water (TWW), while the high removal performance of MPs in WWTPs was showed by Carr et al. , Talvitie et al.  and Murphy et al.  by examining influent and effluent. However, the comparability of data from different countries is limited since the research groups apply different methods for MPs sampling and analysis. In the next section the current and available knowledge about microplastics removal in WWTPs will be discussed.
2. Microplastics in urban wastewater
MPs including: (i) microbeads used in facial scrubs, toothpaste, shampoos and other personal care products, (ii) microfibers generated during washing of synthetic clothing, made of polyester and nylon and (iii) microparticles obtained by abrasion of plastic fragments from cleaning agents, go to municipal wastewater, which are transported to WWTPs. . Many of these products are used daily around the world. An estimated 6% of plastic microbeads used in personal care products are easily identifiable spherical or speckled microbeads, while the overwhelming majority are difficulty identifiable irregular microbeads (also called microparticles). The personal care product industry uses the term microbeads to describe any plastic particle (not only for smooth, and spherical pieces), regardless of size or shape, added to personal care products for use as an abrasive. Microbeads vary in size, with a median ranging from 0.2 to 0.4 mm in scrubs, while those found in toothpaste are about 100 times smaller, around 2 to 5 micrometers in size.
Table 2. Density of commonly used plastics
Microplastics can also vary in density based on the chemical composition and method of synthesis . The polymer particles (which include microbeads) can range in polymer densities from 0.9-2.10 g/cm3 (density of water at 25°C is approximately 1 g/cm3). In addition to polymer densities, the density of theentire particle will also be a function of other chemicals added during its manufacturing (e.g., additives to improve its usability, fillers, antipyrens etc). This variation in densities means that some synthetic polymer particles will float on water surfaces and others may be present in the water column or settle down and collect in the sediments.
To illustrate how many microparticles including microplastic particles are dumped by us daily into the municipal wastewater let’s analyze the content of toothpaste [14, 15]. Approximately 100 mg of white and blue particles is in 5.4 g of toothpaste. This amount represents about 1.8% of the total weight of the toothpaste. The blue and white particles have different densities and could be separated easily in water. The blue polyethylene fragments floated (density 0,917 g/cm3 lower than water density ); the white higher density component settled to the bottom. The white particles consist of mica (they aren’t microplastics). This example also shows how difficult it is to distinguish microplastics from other microparticles without advanced analytical technics or chemical knowledge. Without any further chemical analysis, particles determined via visual analyses show error values up to 70% [16,17].
One of the few data reported on microplastics used in the households was published in Sweden in 2010-2016. The yearly load of synthetic microparticles from personal care products, synthetic fibres from laundry and household dust that is discharged to Swedish municipal wastewater was estimated to 250-2 000 tons. The major part of this load remains in the wastewater treatment plants mainly in sludge and around 4–30 tons per year are released to the water surface. Most of these particles were > 300 μm and the fate of smaller particles is less known, in particular for those < 20 μm 
3. Efficiency of microplastics removal in Municipal Wastewater Treatment Plants
Municipal Wastewater Treatment Plants (WWTPs) collect mainly urban wastewater from domestic entities like households, shops, hospitals, kindergartens, schools etc. (about 70%), and to a varying degree industrial wastewater (about 30%) and stormwater. The wastewater from city and surrounding area are provided by sewer system to WWTPs.
Some municipalities have combined sewer systems whereas other have separate systems for urban wastewater and stormwater. The combined system it is the system, in which the sewer carry both sanitary and stormwater to WWTP and they are treated. If stormwater is carried separately from domestic and industrial wastewater, the system is called separate system. In this system the stormwater id not treated. The type of sewer system will have a large effect on the abundance and character of the MPs in the raw wastewater and in receiver of effluent below their dischrge.
Municipal wastewater treatment facilities are typically designed based upon a common schematic (Fig. 1), though each facility will differ slightly in the exact configuration of the same basic design. Primary treatment (mechanical treatment) is utilized to remove large debris items with screen mesh sizes of 6 mm or larger. It is also used to remove pollutants existing in wastewater as grease and suspended solids. This treatment utilize a series of screens, grinders, tanks, along with pumps, blowers, and other mechanical components, to treat wastewaters. At this stage screenings (textile, paper, rubber or plastic particles and large pieces of organic matter), sludge, grid or substances such as oil and fat are mainly removed from the wastewater. Secondary treatment is used to remove suspended and dissolved organic material as well as nutrients, largely through the incorporation of microorganisms within large tanks called bioreactor. It is often applied to remove remaining after primary treatment. The organisms include bacteria, protozoa, or other small microbes organized in so called “activated sludge” are used to break down dissolved in wastewater organic substances. Microbes also participated in the nitrogen and phosphorous compounds elimination, in aim to limit the eutrophication of water bodies. Activated sludge process is a proven biological treatment widely used for both domestic and some industrial wastewater, because they are biodegradable. Aeration is one of the stages in the process since bacteria and other organisms require oxygen to break down organic substances in the wastewater during treating. The decay of organic matter to carbon dioxide and water under aerobic conditions is called mineralization. Mineralization lead to purification of water from organic pollutants. Flocculates and settling tanks encourage the separation of sewage sludge from the post-processing effluent prior to any disinfection, polishing or advanced (tertiary) treatment, before being discharged into a nearby waterbody.
Fig. 1. Municipal wastewater treatment plant technology
The investigation on MPs removal from the Central Wastewater Treatment Plant in St. Petersburg in Russia , showed that the concentration of microplastics decreased substantially during the purification treatment, and finally a high removal of them from wastewater effluent (>95 %) was reached. The plant consists of mechanical-biological treatment with nitrogen and phosphorous removal technology (secondary WWTP). The authors identified fibers and particles mainly as microplastics in the incoming wastewater. Despite the high removal efficiency of microplastics from wastewater after biological treatment, a number of particles do remain in the effluent and enter the aquatic compartment. Equally high removal efficiency of microplastics (>90%) was found also in recent study in Paris, France by Dris et al., (2015). This research group reported that high levels of synthetic fibres were observed in raw wastewater (260 000–320 000 particles per m3). Specific to microbeads (from personal care products), New York State recently investigated a number of their wastewater treatment plants (WWTPs) and found that microbeads were present in the effluent of 25 of the 34 WWTPs sampled .
Carr et al.  present the most significant study to-date, having sampled 0.189 million liters of effluent at each of 8 different southern California facilities. All of these studies confirm that wastewater treatment facilities are quite efficient at removing microplastics from treated wastewater, with calculated removal efficiencies of 95-99%. It was  found that primary treatment removed 78% of MPs with subsequent secondary treatment removing an additional 20%. Moreover it was specifically noted that microplastic removal into wastewater sludge and other solids occurs largely as part of the skimming (which occurs only at some wastewater treatment processing facilities) and settling treatment  processes .
The example of microplastics removal in relatively small WWTP is plant with a load of 14 000 population equivalents (PE) in Långeviksverket in Lysekil at the Swedish west-coast. In influent to this WWTP was found to have a concentration of 15 000 MPs higher than 300 μm per m3 (3 200 000 MPs per hour). More than 99 % of the particles were removed from wastewater and retained in the WWTP sludge. The concentration in effluent was 1 770 MPs per hour. The shape of the particles effected the removal performance, and microplastic fibres were retained to a higher degree than particles of other shapes. 1.1 - 1.8 MPs/m3 were found in the effluent discharge compared to 0.45 MPs/m3 in the reference area of the recipient to not be directly affected by the effluent. The increase in microparticle concentrations were found close to the mouth of the tube compared to 200 m downstream. Only plastic fibres were found in the recipient .
The example of the fate of microplastics in the large secondary WWTP is plant in Glasgow located on the River Clyde. Municipal effluent discharged from this plant was suspected to be a significant contributor of microplastics to the environment. This plant discharges on average 260 954 m3 of treated wastewater every day (the population equivalent (PE) of approximately 650 000) to river Clyde. A secondary WWTP was analyzed for microplastic particles at different stages of the treatment process to ascertain at what stage in the treatment process the MP are being removed. In skimming tank, settling tank and biological stage were removed respectively 42, 20 and 30%. The total removal efficiency of MPs was 98.41%, and in the final effluent 0.25 MPs/L was observed. Despite this large reduction with the effluent 65 million microplastics were discharged into the recipient every day. Microbeads in amount of 19.67 MPs accumulated in 2.5 g of grease (oil and fat from kitchen and others), which was removed during treatment. This study shows that despite the efficient removal of MPs achieved by this modern treatment plant when dealing with such a large volume of effluent even a modest amount of microplastics being released per liter of effluent could result in significant amounts of microplastics entering the environment .
The main conclusion from these studies is that WWTPs usually effectively remove mincroplastics from the wastewater, despite this their concentration in surface water increases.
4. Fate of microplastics in wastewater treatment plants
In the surface water or wastewater treatment, the behaviour of MPs will change depending on the aggregation/disaggregation and agglomeration/dis-agglomeration behaviour as the microplastics interact with water or wastewater media. In wastewater are pollutants such as oils, fats, peptides, natural organic matter and organic pollutants, or microorganisms and suspending solids. which presence can support the microplastics aggregation process.
Microbeads in bodies of wastewater can accumulate toxic chemical pollutants on their surface  , such as polychlorinated organic compounds (e.g. pesticides, organic solvents etc.), endocrine-disrupting compounds, pharmaceuticals and personal care products, along with other persistent organic pollutants in aqueous media. Concentrations of them, which are detected at parts per trillion levels in many effluent samples [23; 24], could be adsorbed and enriched on the surfaces of MPs.
Interactions of MPs including microbeads with organic matter will have a strong impact on where they will finally reside in the water/wastewater column (Fig. 2). MPs with low densities, unless perturbed (e.g., by interacting with oil, fat, natural organic matter, other particulates, or micro-organisms), will float, while the denser MPs are expected to settle over time. The denser MPs will then undergo transformations e.g. agglomerating/aggregating, increasing in size and mass after interacting with dissolved/solid chemical species. Therefore, MPs are expected to be present in both water and sediment compartments. Synthetic microparticles surface can also be support for the microbial grow. As a consequence the MPs initially floated after fouling by microbial or chemical substances may eventually sink.
Some reports indicated that MPs were removed mainly in the primary treatment zones via solids skimming and sludge settling processes. In simulated partitioning tests, the majority of the microbeads were trapped with the solid toilet paper floc, while about 40% remained floating on the surface of the solution. When the floating microbeads were removed and a second vigorous shaking applied to the sample, a fraction of the trapped MPs resurfaced. The investigations suggested that removal of microplastics in primary treatment is accidental, non-verifiable and difficult to estimate.
 Skimming – removal of floating matter like oil, fat, grease by collection on the surface of water
 Sludge collection
 Population equivalent or unit per capita loading, (PE), in waste-water treatment is the number expressing the ratio of the sum of the pollution load produced during 24 hours by industrial facilities and services to the individual pollution load in household sewage produced by one person in the same time.
Fig. 2. The fate of microplastics during the wastewater treatment
The impact of biofilm on MPs was also examined using isolated blue MPs. In this experiment, a majority of the MPs floated to the surface in each vial spiked with mixed liquor. After more than 48 h of mixing, the blue particles in the autoclaved vial were still distributed primarily on the surface. The particles in non-sterile vial appeared to be more randomly distributed throughout the aqueous phase, due to density or other physical changes caused by the biofilm coating.
While large plastics are likely being removed in water processing, used technologies are not specifically designed to retain very small MP. Although it seems like a major portion of plastic pellets in personal care products are around 450 μm, other products may contain smaller sized particles.
It was observed that MPs in size 10-300 µm could be effectively retained by the media used in typical tertiary gravity bed filters. No breakthrough was observed after filtering 2 L of spiked secondary effluent. Greater than 95% of the spiked microbead particles representative of the full spiked range were recovered in the filtered backwash mix. In waste water treatment plant in Nürnberg as last treatment step, a sand filter is used to retain all microparticles. However in a Dutch study microplastic beads in tooth paste were mainly found to be lower than 10 μm and with a median size between 2 and 5 μm . These plastic particles would be much more likely to pass through the WWTP without being captured in the sludge.
It is worth to noting that not all wastewater is passed through piping systems and WWTPs with sufficient hydraulic capacity and well-functioning tertiary and secondary treatment. During episodes of overflow, untreated or moderately treated wastewater is passed on to recipient waters, which may occur for example in connection with heavy rain falls, the discharge of microplastic will then temporarily be higher. In year 2006 the overflow of untreated wastewater in Sweden was estimated to be 0.6% of the total wastewater volume in the piping system and 1.53% of the total wastewater volume at the WWTPs.
A benchmark for technologies of advanced wastewater treatment with the focus on MPs has not been formulated yet. In the secondary WWTP in Russia, determined a quantity reduction of 96% for MPs, the treatment performance in the Swedish WWTP was determined at the level of 99,9%, while for the American WWTPs the minimal effluent discharges of microplastics are suggested. Current studies indicated on significant amounts of MPs remaining in the sludge. There is a further need of research for this fraction to get verifiable results. Concerning the entry from combined wastewater overflow and discharged rainwater runoff currently there is no data available. In addition, for raw wastewater any valid concentrations are identified so that the degree of degradation for microplastics can just be estimated.
The reports of the spread of the MPs caused people's awareness of the threat of plastic litter has being increased. In recent years the public awareness has evolved into public pressure to companies and governments to regulate and ban the use of microbeads in personal care products.
Summarized, treatment plants are not designed to remove MPs from the wastewater stream. However, the effluent discharges from both secondary and tertiary wastewater treatment facilities may be contributing only minimally to the microplastic loads in oceans and surface water. Even if effective treatment technologies are found to be available, the potential cost and time necessary to equip wastewater treatment plants with such technology is likely to be substantial. Prevention of use in personal care products is a more efficient approach to address the emerging problem of microbead pollution in waters. Therefore the input of microparticles has to be avoided much earlier, i.e. during the production process.