Managing plastic microparticles in water treatment processes

Masses of large waste plastics floating in the oceans have been an issue of concern for many years. More recently, microplastic particles have been detected in seawater, freshwaters and drinking waters, including bottled waters and foods. Environmental plastics are a risk to aquatic organisms from ingestion; they are unsightly and indicative of gross mismanagement of disposal of plastic wastes. Plastics degrade very slowly over many years, and eventually  produce microparticles generally roughly in the size range of  about  1 to 10 µm (micrometer or micron, 1/1,000,000 of a meter) to perhaps as large as 5 mm (millimeter, 1/1,000 of a meter). Particles smaller than 1 µm could be called nanoparticles. Microplastic particles in surface freshwaters and especially treated drinking waters tend to be in the smaller size ranges  (considerably less than 100 µm).

Apart from larger sea animals mistakenly ingesting large pieces, fish flesh has been found to contain plastic particles and filter feeders like clams and oysters can accumulate plastic microparticles. Studies of shellfish reported 10 microparticles per gram in some shellfish. Filter feeders are known to accumulateCryptosporidiumoocysts (4 to 6 microns), bacteria and viruses. Human food exposures are likely dominated by fish and shellfish and some plant products.

Adverse human health effects from ingestion of plastic microparticles have not yet been demonstrated. Concerns have been raised about ingestion of the microparticles, leaching of unreacted monomers into the environment or after ingestion, ingestion of absorbed trace organic chemicals from the water, and even colonization of microbes on biofilms that might form on the microparticles. Several of these are probably not real or unusual particulate-related concerns, but they do stimulate research grant applications for studies, so there will be many more publications and visibility on the topic.

What are plastics?

Plastics are synthetic chemicals made by linking together many small chemicals (monomers) to produce very high molecular weight polymers, averaging perhaps 5,000 or more linked monomers. There are many natural polymers like proteins, natural rubber, fibers and cellulose, but they are not usually included in the particles of current interest. Some examples include polyethylene, polypropylene, polystyrene, epoxys, polycarbonates, polyesters, polyamides (nylon), polyacrylates, polyurethanes, polyvinyl chloride, and elastomers like polyisoprene (synthetic rubber). The corresponding reactive monomers, respectively, include: ethylene, propylene, styrene, bisphenol A and phosgene, diacids and glycols (dialcohols), some amides like caprolactam, acrylates, diisocyanates, vinyl chloride, and  isoprene. The plastics almost always contain additives like antioxidants for protection from oxygen degradation and plasticizers for maintaining flexibility. Solar UV and thermal fragmentation produce more brittleness from environmental aging, contributing to extractive loss of plasticizers that would make them more likely to degrade to particulates.

Analyses of microplastics

The analytics for microparticles in water is a formidable challenge; standard sampling and analytical methods have not yet been established. The published numbers that one sees (10 or 100, or 1,000 per liter) may seem large, but the masses involved are very small, sometimes nanograms or less. Size and dimensions of the microparticles vary and the data are driven by the type of filtration process used for sample collection and processing. Surface water samplers are commonly trawl screens with larger mesh (typically 300 µm), so the collected particles and fibers tend to be on the larger scale. Microfiltration processes are applied to other types of water.

The first challenge is to separate the microparticles from the water, then distinguish the microplastics from the usually much larger amounts (perhaps 1,000 times) of inorganic and organic natural and other particles that will also accumulate in the separation process. These can include algae, microorganisms, colloids, suspended particulates, natural fibers and others.  Then the challenge is to determine the chemical identities of the mix of small particles of different sizes and chemical compositions. One recent detailed objective evaluation of more than 50 published analysis reports of water contamination by Koelmans et al.¹, concluded that 46 of the studies could not be considered fully reliable because of one or more deficiencies in sample processing, lack of controls, chemical identification problems  and several other criteria.

Freshwater and other contamination sources and quantities

The principal sources of plastic microparticles entering surface freshwaters are wastewater treatment plant discharges, and possibly combined sewer overflows, surface runoff, agricultural runoff where sewage sludge has been applied to the land and air deposition. Wet and dry atmospheric particle and fiber suspension and deposition can occur from tire wear, fibers from textiles and other releases. Indoor air may contain more particles and fibers than outdoor air. Estimated plastic particle amounts reported in freshwater in several world locations range from about 1 to 12 per liter. Most U.S. averages were less than 1 particle per liter with some exceptions.

Efficacy of water treatment

Depending upon surface charges or biofilms and aggregations, plastic microparticles should be susceptible to standard particle removal water treatment technologies. An early study demonstrated the capabilities of water treatment and conventional filtration to significantly reduce numbers and sizes of all types of particles in treated water. That analysis compared  total (not specifically plastics) particle sizes and concentrations in a variety of waters from a river to secondary sewage effluent, to conventionally treated drinking water (Trussell and Tate, 1979). They showed that both particle numbers and sizes were distributed almost linearly from small to large, and were significantly reduced by standard water treatment. The secondary effluent ranged from about 1,000 particles/mL at about 2.5 µm, to 10 particles/mL at about 150 µm;  treated drinking water was at about 5 particles/mL at 2.5 µm to less than 0.05 particles/mL at 50 µm.