Investigation of Abundance, Characteristics, and Fate of Microplastics in Water and Sludge of Water Treatment Plants: A Narrative Review

Abstract

Microplastics (MPs) as an emerging pollution with a size of less than 5 mm, can act as carriers of chemical and biological pollutants and have destructive effects on the environment and human health. Drinking water is one of the methods of exposure and entry of these particles into the human body. The importance of water treatment plants is to eliminate and reduce the maximum of existing microplastics and prevent them from entering water sources. Optimizing the removal of microplastics in drinking water treatment plants can reduce the risk of human exposure to these substances. This study reviews the frequency and characteristics of microplastics in water and sludge of drinking water treatment plants and the efficiency of different processes in their removal. This study is a narrative review. To identify the studies that investigated microplastics in water and sludge of a water treatment plant, Persian and English studies in the period from 2013 to 2024 with two groups of Persian keywords including microplastics, water, sludge, water treatment plant, and English keywords. Including Drinking water treatment plants, DWTP Sludge and Microplastic. Google Scholar, PubMed, John Wiley, and ScienceDirect sources were searched. A total of 816 articles were obtained. After removing duplicate articles, the number of articles was reduced to 798 articles. Then, by checking the titles and abstracts, the number of articles was limited to 38. By reviewing the articles whose full text was available, 12 articles were finally left, 9 articles were about microplastics in water, 2 articles were about microplastics in water treatment plant sludge, and 1 article was jointly about microplastics in water. Water and sludge from the water treatment plant. The results showed that the number of microplastics in raw water with the origin of surface sources (reservoirs and rivers) varied between 4.23±1.26 to 3918±425 numbers per liter and the size distribution of microplastics in water can be different depending on the source of raw water. Investigations showed that the predominant size of these particles was less than 100 micrometers. In addition, the most abundant types of microplastics include polyethylene, polypropylene, and polyethylene terephthalate with two forms of fragment and fibers. The number of microplastics in sludge is very high and can be as high as 86,000 particles per gram of dry sludge. Also, common water purification processes including coagulation, flocculation, sedimentation, and sand filtration can remove microplastics by 39-70%, but if granular activated carbon is used in the continuation of the conventional purification process, the efficiency will reach 97%. These particles can eventually be swallowed by humans or aquatic organisms as a result of water consumption or left on the ground as a result of the use of sludge in agriculture. This study showed that microplastics with different characteristics in terms of size, shape, color, and polymer are present in raw water, and most of them accumulate in sludge. Therefore, to prevent the harmful environmental effects of these particles, it is necessary to purify the sludge from the water treatment plant before entering the environment.

Keywords

• sludge
• water
• microplastics
• treatment plant
• drinking water