Suspended particles cannot be removed completely by plain settling. Large, heavy particles settle out readily, but smaller and lighter particles settle very slowly or in some cases do not settle at all. Because of this, the sedimentation step is usually preceded by a chemical process known as coagulation. Chemicals (coagulants) are added to the water to bring the nonsettling particles together into larger, heavier masses of solids called floc. Aluminum sulfate (alum) is the most common coagulant used for water purification. Other chemicals, such as ferric sulfate or sodium aluminate, may also be used.

Coagulation is usually accomplished in two stages: rapid mixing and slow mixing. Rapid mixing serves to disperse the coagulants evenly throughout the water and to ensure a complete chemical reaction. Sometimes this is accomplished by adding the chemicals just before the pumps, allowing the pump impellers to do the mixing. Usually, though, a small flash-mix tank provides about one minute of detention time. After the flash mix, a longer period of gentle agitation is needed to promote particle collisions and enhance the growth of floc. This gentle agitation, or slow mixing, is called flocculation; it is accomplished in a tank that provides at least a half hour of detention time. The flocculation tank has wooden paddle-type mixers that slowly rotate on a horizontal motor-driven shaft. After flocculation the water flows into the sedimentation tanks. Some small water-treatment plants combine coagulation and sedimentation in a single prefabricated steel unit called a solids-contact tank.

Filtration

Even after coagulation and flocculation, sedimentation does not remove enough suspended impurities from water to make it crystal clear. The remaining nonsettling floc causes noticeable turbidity in the water and can shield microbes from disinfection. Filtration is a physical process that removes these impurities from water by percolating it downward through a layer or bed of porous, granular material such as sand. Suspended particles become trapped within the pore spaces of the filter media, which also remove harmful protozoa and natural colour. Most surface water supplies require filtration after the coagulation and sedimentation steps. For surface waters with low turbidity and colour, however, a process of direct filtration, which is not preceded by sedimentation, may be used.

Two types of sand filters are in use: slow and rapid. Slow filters require much more surface area than rapid filters and are difficult to clean. Most modern water-treatment plants now use rapid dual-media filters following coagulation and sedimentation. A dual-media filter consists of a layer of anthracite coal above a layer of fine sand. The upper layer of coal traps most of the large floc, and the finer sand grains in the lower layer trap smaller impurities. This process is called in-depth filtration, as the impurities are not simply screened out or removed at the surface of the filter bed, as is the case in slow sand filters. In order to enhance in-depth filtration, so-called mixed-media filters are used in some treatment plants. These have a third layer, consisting of a fine-grained dense mineral called garnet, at the bottom of the bed.

Rapid filters are housed in boxlike concrete structures, with multiple boxes arranged on both sides of a piping gallery. A large tank called a clear well is usually built under the filters to hold the clarified water temporarily. A layer of coarse gravel usually supports the filter media. When clogged by particles removed from the water, the filter bed must be cleaned by backwashing. In the backwash process, the direction of flow through the filter is reversed. Clean water is forced upward through the media, expanding the filter bed slightly and carrying away the impurities in wash troughs. The backwash water is distributed uniformly across the filter bottom by an underdrain system of perforated pipes or porous tile blocks.

Because of its reliability, the rapid filter is the most common type of filter used to treat public water supplies. However, other types of filters may be used, including pressure filters, diatomaceous earth filters, and microstrainers. A pressure filter has a granular media bed, but, instead of being open at the top like a gravity-flow rapid filter, it is enclosed in a cylindrical steel tank. Water is pumped through the filter under pressure. In diatomaceous earth filters, a natural powderlike material composed of the shells of microscopic organisms called diatoms is used as a filter media. The powder is supported in a thin layer on a metal screen or fabric, and water is pumped through the layer. Pressure filters and diatomaceous earth filters are used most often for industrial applications or for public swimming pools.

Microstrainers consist of a finely woven stainless-steel wire cloth mounted on a revolving drum that is partially submerged in the water. Water enters through an open end of the drum and flows out through the screen, leaving suspended solids behind. Captured solids are washed into a hopper when they are carried up out of the water by the rotating drum. Microstrainers are used mainly to remove algae from surface water supplies before conventional gravity-flow filtration. (They can also be employed in advanced wastewater treatment.)