Occurrence and Characterization of Microplastics in Urban Wastewater, A Case Study: Sari Wastewater Treatment Plant

Document Type : Original Research

Authors
S. S. Alavian Petroody 1
S. H. Hashemi 2
1 PhD candidate, Department of Environmental Pollution, Environmental Sciences Research Institute, Shahid Beheshti University, Iran
2 Associate professor, Department of Environmental Pollution, Environmental Sciences Research Institute, Shahid Beheshti University, Iran.
Abstract
Plastics are a group of relatively high molecular weight organic materials that are obtained from the polymerization process. Plastics have diverse applications due to variety, lightness, strength and transparency. Given the immense benefits of this valuable commodity efficient cause increased worry environmental concerns. The major concern is for smaller pieces or microplastics (MPs) in the oceans that are not seen with the naked eye with a size less than 5mm. Plastics produced in microscopic size are called primary MPs. Primary MPs are composed of microscopic particles of plastic, also MPS that are transferred to the water ecosystem during industrial activities, physical, chemical or biological degradation of macroplastics and various human activities such as using of scrubs and cosmetics are referred to as secondary MPs. Microplastics have a small size that can be eaten and absorbed by the primary organisms in the food chain. The extensive presence of MPs in the environment has been shown by various studies. However, neither MPs concentrations nor their sources are completely known. Wastewater treatment plants (WWTPs) are considered as significant point sources discharging MPs to the environment. This paper is the first to report on the role of an urban WWTP in Iran, as a source of MPs pollution. Composite 30-liter/24-hour samples in 3 replicates took after the grit removal, during one day of winter and spring 2018. Samples were passed through a series of sieves in size of 500, 300 and 37μm (mesh 35, 50 and 400) and transferred to the laboratory for further processing. The sampled materials on each mesh screen were rinsed into a glass bottles with 1000ml ultrapure water depending on the fouling of mesh screens. In the laboratory, the glass sampling bottles were emptied into clean beakers, and dried at 70°C to concentrate the volume to 100ml. The beakers were placed on magnetic heater stirrers at 60°C and hydrogen peroxide (H2O2) solution (30%) was added to beakers to digest of organic matter that was present in the samles, including algae and other organic materials. After digestion of the organic matter and full hydrogen peroxide evaporation, 15 mL of sodium iodide (NaI) solution with a density of 1.7-1.75 g/cm3 were added to the dried sample for to density separation of the MPs from the sand particles. MPs floating in the NaI solution were collected by centrifugation and filtering the supernatant over a 37 μm screen. Then, the specimens were centrifuged and the floating particles were filtered using a screen size of 37μm (400 mesh) and washed with distilled water. To further minimize the overestimation of the suspected MPs, a staining method was applied using the Rose-Bengal solution. After extraction of MPs, their morphology and structure were examined by microscope and micro-Raman. The result showed that the wastewater contained 5188.9±560 and 12666.7±754 MPs/m3 in winter and spring, respectively, with the total numbers of MPs/m3 differing between the two seasons. The dominant type of MPs in the wastewater was microfibers with 4922.2±544 and 12022.2±655 per m3 in winter and spring, respectively. In both seasons, fibers and particles sizes of <300μm were the most abundant in comparison to larger sizes, and given the properties of MPs in the absorption of organic pollutants and heavy metals. Smaller microplastics have a higher surface-to-volume ratio, and thus they will have the greater ability to absorb the contaminants and more risk to the organisms. The predominant type of fibres and particles in this study was polyester and polyethylene, respectively, that these fibers and particles are likely to originate from the washing of synthetic clothing or carpet washing industry wastewater and microbeads in toothpaste and cosmetics, Also, The dominant color of the fibres and particles were identified as black, blue, respectively. The results showed that the number of microplastics entering the treatment plant in two spring and winter seasons were different, which could indicate the effect of climate change and also the activities of people in the two seasons on the number of microplastics released into the wastewater.

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Modares Civil Engineering journal
Volume 19, Issue 6
November and December 2019
Pages 145-154