Elsevier

Chemical Engineering Journal

Volume 362, 15 April 2019, Pages 176-182
Chemical Engineering Journal

Transfer and fate of microplastics during the conventional activated sludge process in one wastewater treatment plant of China

https://doi.org/10.1016/j.cej.2019.01.033Get rights and content

Highlights

  • The abundance of MPs in WWTP declined sharply with a removal rate of 64.4%

  • Larger size fraction of MPs in the effluent was reduced compared to that in the influent.

  • Ellipse was abundantly seen in the influent with a percentage of 4.4%, while not observed in the effluent.

  • Polyamide was the main plastic component in wastewater with 54.8%.

Abstract

Municipal wastewater treatment plants (WWTP) are considered as a significant point source of microplastics (MPs) in the aquatic environment. The objective of this study was to investigate the transport and fate of MPs particles in one WWTP of China based on the conventional activated sludge process. The results exhibited that the abundance of MPs in wastewater declined sharply, from 79.9 n L−1 in the influent to 28.4 n L−1 in the effluent, with a removal rate of 64.4%. MPs removed were mostly transferred and stored into the sludge, and the abundance of MPs in dewatered sludge was 240.3 ± 31.4 n g−1 (dry sludge) with an average size of 222.6 μm. Larger size fraction of MPs in the effluent was reduced compared to that in the influent due to mechanical erosion and sedimentation into sludge. Fiber and fragment were main MPs particles in four wastewater sampling sites, with the average percentage ranged from 33.5 to 56.7% and 30.4 to 45.6%, respectively. An interesting finding is that the ellipses with the size ranged from 100 to 800 µm (average size of 348.1 µm), seldom reported before, were abundantly seen in the influent with a percentage of 4.4%, but not observed in the effluent. A higher fraction of microbead and foam in sludge (17.1% and 12.9%) indicates MPs with the smaller size (average size of 90.3 and 240.1 µm, respectively) in wastewater are prone to be adsorbed and transferred into sludge. Polyamide (nylon) was found to be the main plastic component in wastewater with 54.8% based on Raman spectra, indicating that the MPs particles are primarily originated from the wastewater discharged by washing clothes and polymer manufacturing and processing industries, followed by personal care products.

Introduction

The environmental problem of small plastic particles known as microplastics (MPs) is receiving high attention due to its potential threat to aquatic organisms. MPs were described as plastic particles smaller than 5 mm in size, mainly including primary MPs and secondary MPs [1], [2]. Primary MPs are those plastics manufactured to be the microscopic, mostly observed in many cosmetic and clothing products [3]. Secondary MPs are formed from plastics degradation through mechanical erosion by embrittlement and fracturing or photo-oxidative pathways by UV irradiation [4], [5]. Common plastic types are polyethylene (PE), polypropylene (PP), poly vinyl chloride (PVC), polyamide (PA, nylon), polyethylene terephthalate (PET), polystyrene (PS), polycarbonate (PC) [2], [6], [7].

MPs have been evidenced to pose a harmful effect on aquatic organisms because of their small size and low rate of degradation [8], [9]. So far, many studies pointed out that various organisms could ingest and accumulate MPs [10], [11], [12]. von Moos et al. [11] stated that MPs could be eaten and indigested by the Mytilus edulis. They may remain within the bodies of organisms, resulting in physical harm, such as by internal abrasions and blockages [13]. Thus, the exposure to MPs in the aquatic environment could pose adverse effects on the tissue of the organisms. It is urgent to provide adequate information on the fate of MPs due to their partly unknown or high-risk potential to organisms [2], [14]. To date, the occurrence and fate of MPs in aquatic (marine and freshwater) environment have been investigated worldwide within the last decade [8], [14], [15], [16], [17]. However, the sources of MPs and their transport are not adequately studied and understood. Plastic pollution might happen in receiving waters through many pathways, including stormwater surface runoff during stormwater period, wind advection, and effluent discharged from wastewater treatment plant (WWTP) [18].

Municipal effluent from WWTP has been proposed a known source of various types of MPs and testified to be one contributor of MPs to enter in the aquatic environment [6], [15], [19], [20], [21]. Actually, WWTP are regarded as the receptors of MPs derived from industry, agricultural, and domestic wastewater. Browne et al. [6] compared MPs in sediment samples of shoreline with the MPs extracted from effluent disposal site in a marine WWTP, and observed that polyester and acrylic fibers existed in both sample types. Meanwhile, many personal care products contain plastic microbead and synthetic clothing can cause the MPs to be discharged into WWTP [6], [15]. A few of researches have been undertaken on MPs in the effluent of WWTP and their removal during the treatment process [15], [21], [22], [23]. However, much information is still lacking and not well known on the transport and fate of MPS among varying treatment stages in WWTP, particularly in China. In this study, the variation in MPs at different treatment stages of one WWTP in Wuhan, China, was investigated. A general overview of the transport and fate of MPs was identified and quantified. The potential source of MPs in the wastewater was also analyzed to provide useful information and support for the alleviation of MPs pollution.

Section snippets

Sampling sites

Wastewater and sludge samples were obtained from one WWTP (20,000 m3 of wastewater were treated each day) in Wuhan City, China. The source of wastewater mainly contains the municipal wastewater from inhabitants and industry nearby, and effluent from WWTP was largely discharged into the Yangtze River via an effluent pipe. The flow chart of the treatment process (activated sludge process) and sampling sites of WWTP in this study were described in Fig. 1. Wastewater was sampled at four different

Abundance of MPs

The abundance of MPs in wastewater of WWTP was 79.9 ± 9.3, 47.4 ± 7.0, 34.1 ± 9.4, and 28.4 ± 7.0 n L−1 for W1, W2, W3, and W4, respectively (Table 1), being a total removal rate of 64.4% from influent to effluent. The wastewater characteristics were presented in Table 1. The wastewater turbidity, TN, TP, and COD were all significantly removed after the treatment process. TN, TP, and COD were mainly reduced during the activated sludge process, while the turbidity were both decreased sharply

Conclusions

MPs removed in this WWTP were transferred and stored to the final sludge, being an abundance of 240.3 ± 31.4 n·g−1 (dry sludge) with an average size of 222.6 μm. Larger size fraction of MPs in the effluent was decreased compared to that in the influent due to mechanical erosion and sedimentation into sludge. Fiber and fragment was two main MPs particles both in wastewater and sludge samples. The invaginated (or uncompleted) ellipses with an average size of 348.1 µm, seldom reported before, was

Acknowledgement

This work was grateful to the Funding Project of Sino-Africa Joint Research Center, Chinese Academy of Sciences (Y623321K01).

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