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Microplastics Removal from Treated Wastewater by a Biofilter

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Microplastics Removal from Treated Wastewater by a Biofilter water Article Microplastics Removal from Treated Wastewater by a Biofilter Fan Liu 1,*, Nadia B. Nord 2 , Kai Bester 2 and Jes Vollertsen 1 1 Department of the Built Environment, Aalborg University, Thomas Manns Vej 23, 9220 Aalborg Øst, Denmark; [email protected] 2 Department of Environmental Science, Aarhus University, Frederiksborgvej 399, 4000 Roskilde, Denmark; [email protected] (N.B.N.); [email protected] (K.B.) * Correspondence: fl@civil.aau.dk Received: 18 March 2020; Accepted: 9 April 2020; Published: 11 April 2020 Abstract: Microplastic (MP) pollution is a global environmental issue, and traditionally treated wastewater has been identified as a source of land-based microplastics into the aquatic environment. This study evaluated the performance of a pilot-scale biofilter to polish wastewater treatment plant (WWTP) effluent before it enters the environment. The filter was divided into four zones, allowing the concentration of microplastics to be followed through the filter. It was fed with secondary effluent from a conventional WWTP in Denmark. The raw effluent from the WWTP contained 917 items 3 3 m− which corresponded to a mass concentration of 24.8 µg m− . After the top layer of the biofilter, 3 3 the concentration had decreased to a median value of 197 item m− and 2.8 µg m− , indicating an overall removal efficiency of 79% in terms of particle number and 89% in terms of particle mass. We also observed a tendency that MP of larger size and higher particle mass were more likely to be retained. After the last filtration zone, all MP larger than 100 µm had been removed. The results of this study demonstrate that biofilters are able to lower the MP abundance in treated wastewater significantly, but a complete removal is not ensured, hence some MP, particularly small-sized ones, can still be discharged into the receiving environment. Keywords: microplastics; wastewater; biofilter 1. Introduction Plastic litter is a global pollution issue both in the aquatic and terrestrial environment [1–3]. Microplastics (MP), often defined as plastic particles <5 mm in length [4], are of particular concern due to their persistence in the environment, and their potential to transport and release chemical compounds [5]. Nowadays, MP has been detected not only in various water bodies [6–8], soils and sediments [9–11], but also in indoor air [12], table salts [13], and even in the atmosphere in remote areas [14]. More recently, ingestion and entanglement of MP was also found by freshwater and deep-sea organisms [15–18], which raised a concern that the human food chain is under risk of MP contamination [19,20]. Land-based sources such as households, industries, and mismanaged plastic waste have been suggested as the main cause of plastic pollution of the aquatic environment, the atmosphere, and the terrestrial environment [16]. The pathways for the plastic pollution includes stormwater runoff, municipal and industrial wastewater, and wind [8,21,22]. Here wastewater plays an important role, as it receives and conveys plastic litter from residences, industries, agriculture, and commercial activities. Several studies have shown that wastewater treatment plants (WWTPs) designed for removal of organic matter and nutrients are efficient in removing MP, with reported efficiencies generally above 90% [23–26]. Despite this, large amounts of MP are still released with the effluents [27,28]. Water 2020, 12, 1085; doi:10.3390/w12041085 www.mdpi.com/journal/water Water 2020, 12, 1085 2 of 11 generally above 90% [23–26]. Despite this, large amounts of MP are still released with the effluents Water 2020, 12, 1085 2 of 11 [27,28]. Some tertiary treatment technologies can further polish the effluent from WWTPs, for instance, membraneSome tertiarybioreactors, treatment rapid technologies sand filters, can and further disc polish filters the [28–30]. effluent Another from WWTPs, technology for instance, which membranepotentially bioreactors,can polish rapid WWTP sand effluents filters, and are disc biof filtersilters, [28 –designed30]. Another to technologydegrade specific which potentiallydissolved canpollutants polish WWTPsuch as epharmaceuticalsffluents are biofilters, [31]. Their designed efficiency to degrade in retaining specific MP dissolved has though pollutants not yet such been as pharmaceuticalsstudied. The aim [31 of]. Theirthis study efficiency was into retainingcontribute MP knowledge has though on not the yet beenperformance studied. of The biofilters aim of this in studyremoving was MP to contribute from effluents knowledge of nutrient on theremoving performance WWTPs. of A biofilters pilot-scale in removing biofilter designed MP from to e fflremoveuents ofpharmaceuticals, nutrient removing personal WWTPs. care Aproducts, pilot-scale and biofilter other organic designed micropollutants to remove pharmaceuticals, from WWTP personaleffluents carewas chosen products, for andthe study. other organicThe evaluation micropollutants includes fromthe removal WWTP efficiency effluents wasin terms chosen of MP for themass study. and Thenumber, evaluation as well includes as details the on removal sizes, masses, efficiency and in polymer terms of composition MP mass and of number, the particles as well retained as details by the on sizes,filter. masses, and polymer composition of the particles retained by the filter. 2. Materials and Methods 2.1. Experiment Setup 2.1. Experiment Setup A pilot-scale biofilter was set-up in the fall of 2017 at Avedøre WWTP in Hvidovre, Denmark. A pilot-scale biofilter was set-up in the fall of 2017 at Avedøre WWTP in Hvidovre, Denmark. Avedøre WWTP applies biological nutrient removal and secondary clarifiers before discharging the Avedøre WWTP applies biological nutrient removal and secondary clarifiers before discharging the effluents to the Oresund. The biofilters were packed in circular 1 m3 stainless steel tanks, and from top effluents to the Oresund. The biofilters were packed in circular 1 m3 stainless steel tanks, and from to bottom consisted of a drainage layer of approx. 1.1 m of stone wool (ROCKWOOL®, Hedehusene, top to bottom consisted of a drainage layer of approx. 1.1 m of stone wool (ROCKWOOL®, Denmark), 40 cm of Filtralite® CLEAN HR 3-6 (Leca Rælingen, Norway), and 10 cm of granite gravel Hedehusene, Denmark), 40 cm of Filtralite® CLEAN HR 3-6 (Leca Rælingen, Norway), and 10 cm of (11–15 mm grain size). The stone wool filling consisted of 6 separate stone wool boards, where the top granite gravel (11–15 mm grain size). The stone wool filling consisted of 6 separate stone wool boards, board was 0.1 m thick and the following 5 boards were 0.2 m. Each stone wool board was cut into where the top board was 0.1 m thick and the following 5 boards were 0.2 m. Each stone wool board four even pie-shaped pieces, and rotated relatively to each other to avoid preferential flow through was cut into four even pie-shaped pieces, and rotated relatively to each other to avoid preferential the filter (Figure1). The di fferent materials (stone wool, Filtralite, gravel), and sampling valves on flow through the filter (Figure 1). The different materials (stone wool, Filtralite, gravel), and sampling the side of the column were separated/covered by a layer of glass fiber mat (CSM 300 gr. Emulision valves on the side of the column were separated/covered by a layer of glass fiber mat (CSM 300 gr. m 2, Lintex) to avoid migration of materials. Four sampling valves were mounted on the side of the Emulision− m−2, Lintex) to avoid migration of materials. Four sampling valves were mounted on the tank, each covering different levels of the inside material (Figure1). Valve 1, 2, and 3 allowed sampling side of the tank, each covering different levels of the inside material (Figure 1). Valve 1, 2, and 3 of the effluents which passed through the filtration of the top, middle and bottom part of the stone allowed sampling of the effluents which passed through the filtration of the top, middle and bottom wool layer, respectively. Valve 4 allowed sampling after the stone wool and the Filtrilate layer. As the part of the stone wool layer, respectively. Valve 4 allowed sampling after the stone wool and the layer below valve 4 consisted of gravel, which was unlikely to further retain microplastics, the water Filtrilate layer. As the layer below valve 4 consisted of gravel, which was unlikely to further retain sampled at level 4 represents the effluent of the biofilter. microplastics, the water sampled at level 4 represents the effluent of the biofilter. Figure 1. Overview of the biofilter. The effluent water from the treatment plant entered at the top (Figureinlet), and1. Overview was discharged of the biofilter. at the bottom The effluent (outlet). water from the treatment plant entered at the top (inlet), and was discharged at the bottom (outlet). Water 2020, 12, 1085 3 of 11 Prior to the experiments, the setup was fed with raw effluent for 2.5 months to ensure that biofilms had sufficiently matured. The filter was kept submerged at all times due to a higher inlet flow than the outlet (Figure S1). The flow through the setup was driven by gravitation. 2.2. Sampling Samples for quantifying retainment of microplastics were collected in June 2018, by means of a custom-made filtering device. The device was constructed of stainless steel, and equipped with a removable 10 µm stainless steel filter of 100 mm effective diameter. Samples were drawn one at the time from the 4 levels of the biofilter and from the outlet by a plastic-free positive displacement pump (Creusen Roermond, Dordrecht, The Netherlands). A pressure gauge was placed between the pump and the filter to indicate the hydraulic capacity of the filter. The pump was stopped when the pressure had risen to 2 bar, and the filter changed. Further details on the filtering devise are found in Simon et al.
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