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Wastewater treatment plants (WWTPs) could reintroduce microplastics into environments via biosolid application on land. Yet, the annual emission of microplastics via wastewater biosolids is unclear. Analyzing results from 76 studies, we estimate median concentrations of microplastics in influent, effluent, and sludge in various regions in the world and found that only 4% of microplastics removed in WWTPs are detected in the biosolids, and the remaining 96% could be unaccounted for. Unaccounted microplastics are attributed to limitations of current methodologies to isolate and identify small (<10 μm) microplastics in organic-rich sludge, although there is high variability in estimating the concentration in influent and effluent. A meta-analysis of microplastic data reveals that variability is high if a wastewater sample has low volume (<1 L, particularly for effluent), organic debris is not digested or digested without Fenton’s reagent, microplastics are isolated without density separation or using NaCl solution, and microplastics are counted using a microscope without spectroscopic identification. Based on the median concentration of microplastics in influent, effluent, and biosolids, land application of biosolids in the U.S. alone could annually release 785–1080 trillion microplastics, of which only 29–46 trillion are accounted for or detected. Thus, the true concentration of microplastics in biosolids could be significantly underestimated.

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