The deadline is looming: According to a state law adopted in 2018, the California State Water Resources Control Board has until July 1 to adopt a standard methodology for testing drinking water for the presence of microplastics; adopt requirements for four years of testing and reporting of microplastics in drinking water, including public disclosure of the results; and accredit qualified laboratories in California to analyze microplastics.

The problem? Testing for microplastics — which are defined as any plastic material less than 5 mm in length — is not quite ready for prime time. It is not yet precisely clear what effects microplastics have on animals, including humans, and at what levels they may be harmful. And it’s not entirely clear how microplastics get into our bodies — through water, air, food, or other sources, if not all of the above.

A law before its time?

Theresa Slifko, Ph.D., a chemistry unit manager at the Metropolitan Water District of Southern California — a regional wholesaler that provides drinking water to nearly 19 million state residents — says managing microplastics requires “a method to evaluate toxicity in animals; a method to evaluate occurrence in water, including drinking water; good studies to evaluate toxicity in animals and humans; and good studies to evaluate if microplastics are present in treated drinking water and what treatment can remove them.”

So the MWDSC is one of 40 labs — and the only public drinking water system — participating in a study coordinated by the Southern California Coastal Water Research Project that will report its findings to the state water board this summer. The board plans to use the information to help it select methods and develop a microplastics monitoring program for drinking water. “The recommendations will also be used to help California develop, adopt, and implement a statewide strategy for lessening the ecological risks of microplastics to coastal marine ecosystems,” Slifko says. “This legislation is not about developing best available treatment technologies to remove microplastics. We need to figure out if there is a problem and the potential extent that would need to be addressed first.”

The effect of ingesting microplastics from drinking water is “one of the questions we’re trying to wrap our heads around,” Slifko says. “Unfortunately, health assessments of microplastics pose unusual and difficult situations because those studies also require good analytical tools. (Researchers) need to be able to design the right studies, ask the right questions, and get the answers they need to inform decision-making. Those goals are also impacted by lots of uncertainty and missing information.”

Once researchers determine what levels of microplastics are actually harmful and in what ways, the next challenge will be determining how to test a given water sample to determine if it contains that threshold level. “One of the most important and key critical elements to monitoring drinking water for microplastics is the collection of a representative drinking water sample,” Slifko says.

Making it mainstream

Marie Enfrin, Ph.D., a postdoctoral research fellow at the Royal Melbourne Institute of Technology’s School of Engineering who focuses on microplastics research within the school’s department of civil and environmental engineering, agrees that there is much information yet to be determined. “We have been surrounded by plastics for decades, but we don’t know the effects of microplastics yet,” Enfrin says. “When we don’t know, we can’t say for sure whether the effects are serious or not. And because there’s this lack of knowledge, we still need to research the topic.”

But Enfrin is convinced that once some of the basic questions have been answered, a traditional water filtration method could be of use. Various types of membranes are effective at removing other contaminants this way, and part of Enfrin’s research is to develop a membrane system that can filter microplastics.

“Water treatment plants treat loads and loads of water every second,” Enfrin says. “It’s just not (feasible) to take 1 ml of water (out of the system) and take an hour or more to get an accurate estimate of microplastics concentrations” in that sample.

For now, though, microplastics’ varying sizes and resistance to settling are challenges to be overcome before any useful in-process treatment systems can be designed. “Most (microplastics) have the same density as water, which means they do not float and they do not really sink,” Enfrin says. This precludes the use of settling tanks or skimming procedures, two common methods used in water treatment today.

One possibility, Enfrin suggests, is finding a way to force the microplastics to coagulate into large enough particles to be filtered out, possibly toward the end of the treatment train.

For now, all that is certain is uncertainty: Testing and treatment are still far from ready to tackle the incidence of microplastics in drinking water — law or no law.