One of the most basic distinctions in aquatic chemistry is between substances that are dissolved in water and particles that are suspended in water. Dissolved material, which includes ions and molecules, will stay in the water even if the water is totally still or is run through a filter. Suspended material, such as soil and sediment, will gradually settle out of still water and can be removed by filtration (typically using a filter with a size cutoff of 0.45 or 1 mm).
In reality, though, natural waters have substances with a spectrum of sizes, including some small particles / large molecules that don’t quite fit into either the dissolved or suspended category. These colloids—which are similar to the microscopic fat droplets found in milk—have sizes roughly between 1 nm and 1 mm (larger than typical dissolved material) and can make the water look cloudy, but they won’t generally settle out of water and they may or may not go through a filter, depending on the filter one chooses. The colloidal fraction often includes important pollutants such as bacteria, metals, and organic matter. Colloids can be removed from water by adding a flocculant: a chemical, such as Al2(SO4)3 (alum), that attracts multiple colloids to itself and allows them to come together into larger flocs that can then be removed by settling or filtration.