image: Contact lenses recovered from treated sewage sludge could harm the environment. view more
Credit: Charles Rolsky
BOSTON, Aug. 19, 2018 -- Many people rely on contact lenses to improve their vision. But these sight-correcting devices don't last forever -- some are intended for a single day's use -- and they are eventually disposed of in various ways. Now, scientists are reporting that throwing these lenses down the drain at the end of their use could be contributing to microplastic pollution in waterways.
The researchers are presenting their results today at the 256th National Meeting & Exposition of the American Chemical Society (ACS). ACS, the world's largest scientific society, is holding the meeting here through Thursday. It features more than 10,000 presentations on a wide range of science topics.
The inspiration for this work came from personal experience. "I had worn glasses and contact lenses for most of my adult life," Rolf Halden, Ph.D., says. "But I started to wonder, has anyone done research on what happens to these plastic lenses?" His team had already been working on plastic pollution research, and it was a startling wake-up call when they couldn't find studies on what happens to contact lenses after use.
"We began looking into the U.S. market and conducted a survey of contact lens wearers. We found that 15 to 20 percent of contact wearers are flushing the lenses down the sink or toilet," says Charlie Rolsky, a Ph.D. student who is presenting the work. Halden, Rolsky and a third member of the team, Varun Kelkar, are at the Biodesign Institute's Center for Environmental Health Engineering at Arizona State University (ASU). "This is a pretty large number, considering roughly 45 million people in the U.S. alone wear contact lenses."
Lenses that are washed down the drain ultimately end up in wastewater treatment plants. The team estimates that anywhere from six to 10 metric tons of plastic lenses end up in wastewater in the U.S. alone each year. Contacts tend to be denser than water, which means they sink, and this could ultimately pose a threat to aquatic life, especially bottom feeders that may ingest the contacts, Halden says.
Analyzing what happens to these lenses is a challenge for several reasons. First, contact lenses are transparent, which makes them difficult to observe in the complicated milieu of a wastewater treatment plant. Further, the plastics used in contact lenses are different from other plastic waste, such as polypropylene, which can be found in everything from car batteries to textiles. Contact lenses are instead frequently made with a combination of poly(methylmethacrylate), silicones and fluoropolymers to create a softer material that allows oxygen to pass through the lens to the eye. So, it's unclear how wastewater treatment affects contacts.
These differences make processing contact lenses in wastewater plants a challenge. To help address their fate during treatment, the researchers exposed five polymers found in many manufacturers' contact lenses to anaerobic and aerobic microorganisms present at wastewater treatment plants for varying times and performed Raman spectroscopy to analyze them. "We found that there were noticeable changes in the bonds of the contact lenses after long-term treatment with the plant's microbes," says Kelkar. The team concluded that microbes in the wastewater treatment facility actually altered the surface of the contact lenses, weakening the bonds in the plastic polymers.
"When the plastic loses some of its structural strength, it will break down physically. This leads to smaller plastic particles which would ultimately lead to the formation of microplastics," Kelkar says. Aquatic organisms can mistake microplastics for food and since plastics are indigestible, this dramatically affects the marine animals' digestive system. These animals are part of a long food chain. Some eventually find their way to the human food supply, which could lead to unwanted human exposures to plastic contaminants and pollutants that stick to the surfaces of the plastics.
By calling attention to this first-of-its-kind research, the team hopes that industry will take note and at minimum, provide a label on the packaging describing how to properly dispose of contact lenses, which is by placing them with other solid waste. Halden mentions, "Ultimately, we hope that manufacturers will conduct more research on how the lenses impact aquatic life and how fast the lenses degrade in a marine environment."
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A press conference on this topic will be held Monday, August 20, at 9:30 a.m. Eastern time in the Boston Convention & Exhibition Center. Reporters may check-in at the press center, Room 102 A, or watch live on YouTube http://bit.ly/ACSLive_Boston2018. To ask questions online, sign in with a Google account.
The researchers acknowledge support and funding from ASU's Human Health Observatory and the Virginia G. Piper Charitable Trust.
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Title
Chemical and physical changes in a variety of contact lenses during the wastewater treatment processes
Abstract
Contact lens use has increased steadily over the last decade. The usage of soft contact lenses specifically has grown in popularity driven by research on the physiological demand of the cornea and comfort of use. These attributes include higher oxygen permeability, typically attained by use of plastic hydrogel soft lenses which are replaced and disposed of either daily, bi-weekly, monthly, or yearly. Much is unknown about the practices of contact lens disposal by consumers and their fate as contaminants in domestic wastewater. If flushed down the sink or toilet, these lenses will travel to local wastewater treatment plants (WWTPs) where treatment processes may lead to material degradation and fragmentation of macroplastics into microplastics. Over 45 million people use contact lenses in the US, and the number of users of so called 'daily disposable lenses' is constantly increasing. To better understand disposal and fate of lenses in WWTPs, we created a three-part study consisting of (1) an anonymous survey of 139 individuals, wearers and non-wearers, to assess disposal techniques. We learned that 19% of the wearers flush their contacts down the sink or toilet. To understand their journey through WWTPs and degradative forces which they may encounter, we (2) conducted density experiments on 11 types of contact lenses and on WWTP solids/wastewater to identify where the contacts will be retained or separated within the treatment process. Lastly, we (3) subjected a variety of contact lenses of different plastic types to unit operations constituting conventional wastewater treatment and observed degradative effects on the contact lenses. Study findings suggest that disposable contact lenses are emerging contaminants of concern that cause environmental pollution, present a potential physical threat to susceptible aquatic biota, may contribute to microplastic pollution, and have the potential to adsorb, accumulate, and transport harmful persistent organic pollutants into aquatic and terrestrial environments via, respectively, sanitary sewer overflows and the application of treated municipal sludge (biosolids) on land.