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