Microplastics in the Tessel River: Counting Fibers at Steadmere

Hook, the jar demonstration (6 min): Before class, mix about one cup of table salt into a quart of warm water until no more dissolves, then let it cool. In front of students, add two tablespoons of sand and a pinch of nylon fishing line and clear plastic bag film to the jar, cap it, shake, and set it down. Ask students to predict what they will see in two minutes. Do not confirm predictions yet.

Density separation, discussion (10 min): While the jar settles, explain that sand grains are denser than the brine and sink, while many common plastics, such as bag film and nylon line, are less dense than brine and rise; polyester clothing fibers are denser than table-salt brine, so laboratories use denser salt solutions to recover them. Return to the jar, pour the surface layer through the strainer, and show the collected fibers and film. Tell students that laboratories use the same principle with a calibrated salt solution, then filter, dry, and count under a microscope. Ask why an analyst would rinse the sand twice rather than once, and take two or three answers.

Direct instruction, sources and plant performance (12 min): Identify the main sources reaching a plant: synthetic fibers shed in laundry, fragments from packaging and road dust, and beads worn off products. Walk through the treatment train and mark where particles leave the water, which is mostly in the primary and secondary settling steps, where they attach to floc and settle with the sludge. State plainly that a plant with strong removal concentrates those particles in its biosolids, so the question of where they end up does not disappear. Introduce removal efficiency as (influent minus effluent) divided by influent, times 100.

Guided practice, a worked load (12 min): Model the load calculation on the board with a set of numbers students will not reuse. Suppose a different plant discharges 8.0 million liters per day and its effluent averages 30 particles per liter. Then 30 x 8,000,000 = 240,000,000 particles per day, or 2.4 x 10 to the 8th. Ask the class what happens to that number if the concentration is halved, and confirm that the load scales directly with concentration when flow is unchanged. Emphasize that the mean of a handful of grab samples carries real uncertainty, so results should be reported as an estimate.

Independent practice (13 min): Students complete the Student Handout using Steadmere's own data, working through the mean, the daily load, removal efficiency, and the two proposed reductions. Circulate and check that students write units at every step rather than only at the end.

Closure and exit ticket (2 min): On an index card, students answer: "A plant reports that it removes 90 percent of microplastics. Where did that 90 percent go?" Collect the cards and confirm that at least three-quarters of the class names the sludge or biosolids rather than saying the particles were destroyed.