Unaccounted Microplastics in Wastewater Sludge: Where Do They Go?

Abstract

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.

Durable Plastic Goods: A Source of Microplastics and Chemical Additives in the Built and Natural Environments

“…Microplastics <100 μm in size are often unretained by sampling and preparation techniques . The small size of microplastics and the complexity of wastewater-related matrices complicate accurate identification and quantification of microplastics. , Polymer types beyond the “usual suspects” (i.e., polyethylene, polypropylene, PET, PS, and polyvinyl chloride) often go unquantified. Understudied polymer classes include paint/coating-associated urethanes, acrylates, and butadiene-based rubbers common in tire wear. ,,, These may be toxicologically relevant (e.g., the tire antioxidant additive degradant described previously) .…”

“…Thus, consideration of the larger numbers and particle sizes (masses) of microplastics suggests that their contributions in sludges are underestimated. Koutnik et al postulated that 96% of the microplastics in wastewater sludge may go undetected due to methodological shortcomings . Once therein, stabilization practices, land application, and later environmental processes will fragment these into ever greater numbers of smaller particles, including nanoplastics.…”

Mikro‐ und Nanoplastik haben nur einen unwesentlichen Einfluss auf den vertikalen Stofftransport organischer Schadstoffe in landwirtschaftlichen Böden

“…Insbesondere landwirtschaftliche Böden werden durch zusätzliche Eintragsquellen belastet. Kompost oder Klärschlamm, die als wichtige Düngemittel aufgetragen werden, enthalten bereits erhebliche Mengen an Mikro-und Nanoplastik mit bis zu 300 000 Partikeln in einem kg Klärschlamm [1]. Zudem ist es gängige Praxis, Mulchfolien auf landwirtschaftlichen Feldern zur Ertragssteigerung zu nutzen.…”

Using Adhesives to Capture Microplastics from Water

“…Given the growing awareness and concern over microplastics pollution, studies are now appropriately focused on developing methods that prevent their broader release into the environment. , Wastewater treatment plants are a key location for such prevention. Recent studies have shown that these plants efficiently capture microplastics within nutrient-rich biosolid waste during coagulation and settling. − Unfortunately, this sludge is often sold as fertilizer, thereby releasing the captured microplastics into the environment. , As a result, methods that remove microplastics upstream of sludge formation are needed . One emerging technology aimed at removing microplastics earlier in the treatment process involves using magnets to separate magnetic materials that adsorb microplastics, including nanotubes, coagulants, nanoparticles, and core–shell particles .…”

“…26−28 Unfortunately, this sludge is often sold as fertilizer, thereby releasing the captured microplastics into the environment. 29,30 As a result, methods that remove microplastics upstream of sludge formation are needed. 24 One emerging technology aimed at removing microplastics earlier in the treatment process involves using magnets to separate magnetic materials that adsorb microplastics, including nanotubes, 31 coagulants, 32 nanoparticles, 33 and core−shell particles.…”