Identification, characterization and the removal of Microplastic, a persistent neo-contaminant from Dairy Waste Water Treatment Plant (WWTP)

Plastic debris of size < 5mm are considered as microplastics and are serious concern in the present world due to its persistent nature and ubiquitousness in every spheres of the environment. Waste Water Treatment Plants (WWTP) are one of the main point sources of microplastic to the environment. The primary objective of this study was to identify and characterize microplastics present in wastewater from the dairy industry and to suggest effective management practices for their efficient removal before the effluent is discharged into the environment. The samples were collected from the influents to the WWTP, Aeration-tank, Clarifier, Final-effluent and sludge. The microplastic extraction were done by digestion (30%-H₂O₂) and density separation (NaCl and NaI) method. Micro-Raman spectroscopy, SEM and SEM-EDS techniques were used for the identification and characterization of microplastics. The findings indicated that the sludge from the WWTP contained a significantly higher particle count (2560 particles/g) compared to the water samples (38 particles/L). Microplastics of different shapes were identified in the study, its abundance is in the following order: fragments>films/sheets>pellets> foam. The size of microplastics ranges from 20µm to 2500 µm and the highest abundance observed in the range between 100-500 µm. Most of the microplastics were transparent (46.87 %), white (31.26%) and blue (15.62%) in color. Seven different varieties of microplastic such as Polyamide, Polyethylene, Poly-vinyl-chloride, Polypropylene, Low-density-polyethylene, Polyurethanes, Nylon were identified. Polyethylene is the predominant microplastic found in all the samples (62.49%) followed by Polypropylene (11.72%) and Poly-vinyl-chloride (9.37%) respectively. Polyurethane (7.81%) and Nylon (3.9%) were found only in sludge samples. SEM images showed cracks, pores (480 nm to 998 nm), fractures on the surface and are prone to breakdown. Some of the microplastics exhibit signs of being colonized by microorganisms or particle-like structures within cracks, signifying the presence of high surface area. It would increase the chance to attach contaminants, resistant microbes and other pollutants to microplastic when discharged/exposed to more complex environment and elevate its toxicity. SEM-EDS analysis shows microplastics association with metals (Mg, Al, Na, Si, Ca, Fe, Pd). The economical and expeditious solution for microplastic removal is to improve, the current treatment process instead of finding a new method. Some recommendations to enhance the removal of microplastics include lengthening the retention time in the sedimentation/skimming processes, altering the materials in the filtration-units, and improving the flocculation/coagulation methods. For example, aluminum-based coagulant is more effective in eliminating microplastic than Fe and polyacrylamide-based coagulant to reduce, comparatively high microplastics content in the influent and aeration-tank. The extraction of microplastic in fat-trap stage using grease and subsequent pyrolysis prevents larger particles to enter the system and helping to curb the elevated concentration of microplastic in sludge. Co-pyrolysis with biomass and hydrothermal reactions can also be adopted. Recommendations for efficient microplastics management practices were also proposed.