**2. Studies of retaining microplastics in existing wastewater treatment plants**

#### **2.1 Standard functional units of WWTPs**

WWTPs are of different design and of different sizes (**Figure 1**). In general, most WWTPs start off with passing the wastewater through bar screens (screening) to remove large solids and an oil and grit removal tank as pre-treatment, before the water is left to settle in a clarifier, where solids are removed in form of sludge sunk to the bottom and in form of scum on the water surface as primary wastewater treatment. This primary treatment is followed by activated sludge treatment as secondary wastewater treatment. The treatment uses flocs of microorganisms that decrease the BOD (biological oxygen demand) of the water due to a decrease of

*The Effect of Wastewater Treatment Methods on the Retainment of Plastic Microparticles DOI: http://dx.doi.org/10.5772/intechopen.97083*

**Figure 1.**

*Schematic flow of microplastic (MP) particles through a wastewater treatment plant (WWTP).*

organic components through conversion to CO2 along with a decrease in nitrogen content by conversion of ammonia bound nitrogen to elemental nitrogen through a nitrification process and a subsequent denitrification process, involving both heterotrophic as well as autotrophic microorganisms and both aerobic and anoxic reaction zones. The activated sludge is separated from the treated water in a subsequent clarifier where some of the activated sludge is recycled. Activated sludge treatment can be conducted in a number of different ways. It is also possible to operate this process in a sequential batch reactor (SBR), where bioreactor and clarifier are run in a timed sequence within one vessel. Tertiary treatment methods can be added. They can include rapid sand filtration, membrane filtration, reverse osmosis, advanced oxidation processes and further biological methods [81]. Finally the treated water is disinfected, either by UV irradiation, addition of ozone (O3), of chlorine (Cl2) to produce hypochlorous acid (HOCl) or of chlorine dioxide (ClO2) [82], before it is discharged, mostly into a natural receiving water body such as a lake, river or the ocean. Especially in countries with extreme water scarcity or in large metropolises the water can also be recycled directly for consumption/use [83].

### **2.2 Early MP retainment studies in WWTPs**

More than 70 WWTPs have been studied to date as to their MP retaining capability [1]. Some of these studies were devoted to the assessment of microplastic accumulation in the sludge of the WWTPs [8]. While early studies were carried out in North America [80, 84–90], Europe [78, 90–106], and Australia [79, 107], a larger number of recent studies emanate from East Asia [108–123]. It must be noted that some WWTPs were handling sewerage and storm run-off separately, others were not. In some of the cases where sewerage and storm run-off were handled together, specific note was made of black fragments in the influent that derived from tire abrasion in form of microtires [124]. **Table 1** gives an overview of many of the published studies on MP retention efficiency of WWTPs around the world [77–80, 84–88, 90, 92–94, 97–100, 103–144].


#### *Promising Techniques for Wastewater Treatment and Water Quality Assessment*


#### *The Effect of Wastewater Treatment Methods on the Retainment of Plastic Microparticles DOI: http://dx.doi.org/10.5772/intechopen.97083*


#### *Promising Techniques for Wastewater Treatment and Water Quality Assessment*


### *The Effect of Wastewater Treatment Methods on the Retainment of Plastic Microparticles DOI: http://dx.doi.org/10.5772/intechopen.97083*

*Published studies on the microplastic (MP) retention efficiency of different WWTPs.*

In the following, we describe the outcome of some of the earlier studies (2012–2016) in differently-sized WWTPs in various parts of the world. In 2014, Magnusson and Norén [99] studied the MP retention in a smaller WWTP in Lysekil, Sweden (Långeviksverket, serving 14.000 inhabitants, flow rate of 5160 m3 /day). During the study time, the WWTP received per m3 15.1 ± 0.89·103 MP (10.7 ± 0.39·X 103 plastic fibers; 2.67 ± 0.77 X 103 plastic fragments; 1.78 ± 0.80·X 103 plastic flakes). Of these, only 8.25 ± 0.85 MP m−3 (4.00 ± 0.58 plastic fibers, 3.75 ± 1.25 plastic fragments, 0.50 ± 0.50 plastic flakes) could be found in the effluent, which was released into the sea. This amounted to 99.96% retention for plastic fibers. While MPs in the effluent were still appreciably higher than in the receiving water, Magnusson and Norén [99] found a steady decrease in fiber concentrations with increasing distance from the discharge point, from 1.82 ± 0.45 fibers/m3 at 20 m from the discharge point to 1.14 ± 0.38 fibers/m3 at 200 m from the discharge point. A larger sized WWTP was studied in 2012 by J. Talvite et al. [78] at Viikinmäki, Finland (serving 800.000 inhabitants in the Helsinki metropolitan area, flow rate 270.000 m3 /day). The influent carried 180 textile fibres L−1 and 430 synthetic particles L−1. After the primary sedimentation, the wastewater contained an average of 14.2 (±0.7) fibres and 290.7 (±28.2) synthetic particles L−1. After the secondary sedimentation, 13.8 (±1.6) fibres and 68.6 (±6.3) synthetic particles were still present. The remainder of the fibers and particles had settled in the sludge. Thus, most of the fibers were eliminated in the primary sedimentation process; most of the other synthetic particles were removed in the second sedimentation. As a tertiary stage, the WWTP also included a biological filtration, which removed further particles from the treated water. Removal of fibers in the second and third treatment stages was insignificant. The final effluent carried 4.9 (±1.4) fibres and 8.6 (±2.5) synthetic particles L−1. This means that 3.73 x 109 fibers were released daily, with the effluent [78] into the Gulf of Finland, Baltic Sea. In 2014, Talvitie and Heinonen [97] published a study on the Central WWTP of Vodokanal in St. Petersburg, Russia (serving about 4 million people, flow rate of 959.000 m3 /day), carried out in collaboration with HSY, Vodokanal of St. Petersburg and Water Research and Control Center. The influent was found to carry 467 fibers L−1, 160 synthetic particles L−1, in addition to 3160 less identified black particles L−1, most likely of synthetic nature. After pre- and primary treatment, these values decreased to 33 fibers L−1; 21 synthetic particles L−1, and 302 black particles L−1. After, secondary treatment, these values decreased further to 16 fibers L−1; 7 synthetic particles L−1, and 125 black particles L−1. Nevertheless, still 153.4 X 109 fibers were released daily with the effluent, some of which lastly will reach the Baltic Sea. Murphy et al. studied the MP retaining capability of a secondary WWTP in UK. Here, the influent contained on average 15.70 (±5.23) MP·L−1. This was reduced to 0.25 (±0.04) MP·L−1 in the final effluent, which is a decrease of 98.4% [100]. The team reported that about 45% of microplastics were removed in the grease and grit tank, while primary sedimentation in the first clarifier accounted for 34% removal [97]. 20% of the microplastics were removed in the secondary stage [100]. In 2016, M.R. Michielssen et al., investigated the retainment efficiency towards small anthropogenic litter (SAL) of a Detroit WWTP (Great Lakes Water Authority, serving 2.36 million people, flow rate 2.5 million m3 /day). SAL includes both plastic based and cellulose derived materials. Pretreatment at the Detroit WWTP removed 58.6% SAL. Primary treatment retained an additional 25.5%, secondary treatment (with activated sludge) an additional 9.7% SAL for a total of 93.8% SAL removal overall [85]. Nevertheless, the effluent was found to release about 8.94 billion fibers a day [85]. M.R. Michielssen et al. also looked at the much smaller Northfield WWTP (Michigan, serving 100.000 people, flow rate 1700 m3 day−1), which features sand filtration as a tertiary treatment [85]. Here, pretreatment was found to retain

*The Effect of Wastewater Treatment Methods on the Retainment of Plastic Microparticles DOI: http://dx.doi.org/10.5772/intechopen.97083*

35.1% SAL. Primary and secondary treatment held back a further 53.3% and 1.4% SAL, respectively. Sand filtration as a tertiary treatment method further reduced SAL by 7.4% of the total. Overall, the Northfield WWTP reduced the SAL load by 97.2%, with 8.9 million fibers released daily with the effluent [85]. In addition, M.R. Michielssen et al. studied the effect of an anaerobic membrane (AnMBR) test reactor, situated at the Northfield WWTP, as a stage directly after the pretreatment of the influent. Here, 99.4% of the inflowing MP were retained – this made for 64.5% of the total MP in the influent. Thus, pretreatment and AnMBR retained 99.6% of the MP in total [85]. Finally, in 2014–2015, Carr et al. [80] studied 7 tertiary WWTPs and one secondary WWTP in Los Angeles County, Southern California. The studied WWTPs had a gravity filtration as the tertiary stage. Bench studies of the group with water spiked with microplastics showed that all MPs could be retained by such filtration processes. This, however, did not hold up in the" reallife" scenario of the WWTPs. Nevertheless, in this study, Carr et al. showed that gravity filtration as a tertiary stage in a WWTP can give up to 99.9% MP retention, calculated over all stages [80].

### **2.3 Retention of MPs in preliminary and primary treatment stages of WWTPs**

P.U. Iyare et al. [5] showed that the bulk of the removal of MPs, at an average of 72%, comes during pre- and primary treatment. Dris et al. reported 69% MP retention in the pre- and primary stages of a WWTP in Paris [98]. Gies et al. saw that the MP concentration decreased from 31.1 ± 6.7 MP L−1 in the influent to 2.6 ± 1.4 MP L−1 (91.7% MP retention efficiency) in the primary effluent of a major secondary WWTP near Vancouver, Canada [90]. Michielssen et al. reported that screening and primary sedimentation removed 84–88% SAL ([85], see above) in studied US WWTPs. From WWTPs in Russia [97], Finland [78] and Canada [87] it was found that pre- and primary wastewater treatment removed 92–93% of fibres. In 2015, Ziajahromi et al. performed one of few studies on a WWTP with solely a pre- and primary treatment stage, receiving wastewater from over 1 million inhabitants in the Sydney area [107]. The pre- and primary stages were standard screening (mesh size of 5 mm), grit removal and sedimentation, with the effluent discharged in the deep ocean. Here, 1.5 MP L−1 were detected in the effluent. With a through-put of 300.000 m3 day−1, 460 million MP day−1 were being discharged from the WWTP into the ocean [107].

#### **2.4 Retention of MPs in the secondary treatment stage of WWTPs**

Different studies have looked at the MP retention in the secondary treatment stages of WWTPs, where the MP is then collected in the accumulating sludge. In most cases, the secondary treatment in a WWTP involves an activated sludge process, where different bacteria lower both the organic content as well as the nitrogen content of the water. Due to the different requirements of the bacteria, some of which are autotrophs and some of which are heterotrophs, some operating under aerobic conditions, some under anoxic conditions, the process involves different stages, where the water passes through aerated zones to anoxic zones and back. Different set-ups for such processes have been developed, involving different reaction chambers or a single batch reactor, where the different stages of the process run sequentially in time. After the process, the water needs to be separated from the sludge. This may be through passing the mixture to a settling tank or through a membrane in form of microfiltration or ultrafiltration, where the activated sludge is passed back to the bioreactor. This combination then is called a membrane bioreactor (MBR). MBRs have also be seen as a separate entity as a tertiary treatment method [125].

H. Lee and Y. Kim [124] looked at the efficiency of three different types of activated sludge processes, the A2O (anaerobic-anoxic-aerobic), the sequence batch reactor (SBR) and the Media process. As of 2013, these were the main processes used at public WWTPs in South Koreas with a capacity of over 500m3 /day [A2O 23.7%, SBR 34.8%, Media 22.8%] with membrane bioreactor (MBR), long term aeration, and special microbial processes accounting for the remainder of the processes [124]. The WWTP running the A2O process, with a throughput of 35.000 m3 day−1 (serving 67.700 inhabitants) reduced 29.85 MP L−1 found in the influent (taken before the pre-treatment) to 0.435 MP L−1 in the effluent after disinfection (98.5 overall retention efficiency), with 14.9 MP g−1 found in the ensuing sludge. The WWTP running the SBR process, with a throughput of 110.000 m3 day−1 (serving 235.700 inhabitants) reduced 16.45 MP L−1 in the original influent to 0.14 MP L−1 in the final effluent (99.1% overall retention efficiency), with 9.65 MP g−1 noted in the sludge. Finally, the WWTP running the Media process, with a throughput of 130.000 m3 day−1 (serving 245.000 inhabitants), reduced 13.86 MP L−1 in the original influent to 0.29 MP L−1 in the final effluent (98% overall retention efficiency), with 13.2 MP g−1 found in the sludge [124]. Based on the MP found in the sludge, the retention efficiencies of the secondary treatment alone were 49.3%, 44.7%, and 49.0% for the A2O process, the SBR process, and the Media process, respectively [121]. It is not clear, if the primary settling tank is included in these numbers as the fate of the sludge from the settling tank has not been discussed. If included, the numbers would compare well with the numbers given by Murphy et al., who reported a 53.8% MP removal in the primary settler and the secondary stage. Still, in the South Korean WWTPs 3 billion, 4 billion, and 11 billion MPs were discharged annually with the final effluent in the A2O process, in the SBR process, and in the Media process, respectively.
