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Conventional Coagulation-Flocculation-Sedimentation

Conventional coagulation–flocculation-sedimentation practices are essential pretreatments for many water purification systems—especially filtration treatments. These processes agglomerate suspended solids together into larger bodies so that physical filtration processes can more easily remove them. Particulate removal by these methods makes later filtering processes far more effective. The process is often followed by gravity separation (sedimentation or flotation) and is always followed by filtration.

A chemical coagulant, such as iron salts, aluminum salts, or polymers, is added to source water to facilitate bonding among particulates. Coagulants work by creating a chemical reaction and eliminating the negative charges that cause particles to repel each other.

The coagulant-source water mixture is then slowly stirred in a process known as flocculation. This water churning induces particles to collide and clump together into larger and more easily removable clots, or “flocs.”

The process requires chemical knowledge of source water characteristics to ensure that an effective coagulant mix is employed. Improper coagulants make these treatment methods ineffective.

The ultimate effectiveness of coagulation/flocculation is also determined by the efficiency of the filtering process with which it is paired.

Dissolved Air Flotation

Dissolved air flotation is a form of coagulation-flocculation technology that is used as a pretreatment. Employing this technique before water filtration reduces the clogging that causes maintenance problems of downstream filtration.

Dissolved air flotation is particularly well-suited for the removal of algae, unwanted coloring, and lighter particles that resist settling out of treated source water.

The process does not work well with highly turbid waters because heavier particles, like silt and clay, are not as easily floated to the water surface.

To begin the process, a chemical coagulant, such as iron salts, aluminum salts, or polymers, is added to the source water to facilitate bonding among particulates. Coagulants work by creating a chemical reaction, eliminating negative charges that cause particles to repel each other.

The coagulant-source water mixture is then slowly stirred in a process known as flocculation. The churning of water induces particles to collide and clump together into larger and more easily removable clots, or “flocs.”

Flocculated water is collected in a tank and is subjected to a large infusion of tiny, pressurized air bubbles. The action of these bubbles forces clots or flocs of particles to the water surface where they can be skimmed off.

Dissolved air flotation is an alternative to sedimentation. It performs a similar task by a diametrically opposed method—forcing contaminant clumps to the surface rather than allowing them to settle out on the bottom.

Flocculation-Chlorination

A system that incorporates coagulation-flocculation followed by chlorination has been developed as a point of use technology, especially for developing countries.

It uses a small packet of powdered ferrous sulfate (a common flocculent) and calcium hypochlorite (a common disinfectant). A user opens the packet, adds the contents to an open bucket containing about ten liters of water, stirs for five minutes, lets the solids settle to the bottom, strains the water through cotton cloth into another container, and waits 20 minutes for the chlorine to disinfect the water.

The combination of particle removal and disinfection appears to produce high removal rates of bacteria, viruses, and protozoa, even in highly turbid waters. There is considerable evidence that the system has reduced diarrheal disease significantly in various locations. There is also evidence that the flocculation process helps remove arsenic; however, these systems are not an adequate substitute for high-quality centralized treatment when it can be made available.

Lime Softening

Lime softening is primarily used to “soften” water—that is to remove calcium and magnesium mineral salts. But it also removes harmful toxins like radon and arsenic. Though there is no consensus, some studies have even suggested that lime softening is effective at removal of Giardia.

Hard water is a common condition responsible for numerous problems. Users often recognize hard water because it prevents their soap from lathering properly. However, it can also cause buildup (“scale”) in hot water heaters, boilers, and hot water pipes.

Because of these inconveniences, many treatment facilities use lime softening to soften hard water for consumer use.

Before lime softening can be used, managers must determine the softening chemistry required. This is a relatively easy task for groundwater sources, which remain more constant in their composition. Surface waters, however, fluctuate widely in quality and may require frequent changes to the softening chemical mix.

In lime softening, lime and sometimes sodium carbonate are added to the water as it enters a combination solids contact clarifier. This raises the pH (i.e., increases alkalinity) and leads to the precipitation of calcium carbonate. Later, the pH of the effluent from the clarifier is reduced again, and the water is then filtered through a granular media filter.

The water chemistry requirements of these systems require knowledgeable operators, which may make lime softening an economic challenge for some very small systems.

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