Per- and polyfluoroalkyl substances — PFAS — encompass several thousand synthetic compounds used since the 1950s for their non-stick, waterproofing, and heat-resistant properties. They are found in firefighting foams, coatings, certain textiles, and packaging.

Microplastics refer to plastic fragments smaller than five millimeters. They originate either from particles manufactured as such or — much more frequently — from the gradual breakdown of larger plastics under the action of ultraviolet light, abrasion, and freeze-thaw cycles.

Connecting scattered housing to a centralized sewer network is expensive — the bulk of the cost lies in the piping, not the treatment plant. Below a certain population density, the logic reverses: treating water near the point of generation becomes more economical and more robust.