drinking water sources can become contaminated, causing sickness and disease from waterborne germs, such as Cryptosporidium, E. coli, Hepatitis A, Giardia intestinalis, and other pathogens.

The Stages ofWater Treatment

Preliminary treatment or pretreatment is any physical, chemical or mechanical process used on water before it undergoes the main treatment process. During preliminary treatment:

screens may be used to remove rocks, sticks, leaves and other debris; chemicals may be added to control the growth of algae; and a presedimentation stage can settle out sand, grit and gravel from raw water.

After preliminary treatment, the next step is coagulation.   Coagulation removes small particles that are made up of microbes, silt and other suspended material in the water.  Treatment chemicals such as alum are added to the water and mixed rapidly in a large basin. The chemicals cause small particles to clump together (coagulate). Gentle mixing brings smaller clumps of particles together to form larger groups called "floc". Some of the floc begins to settle during this stage.

During the flocculation stage, the heavy, dense floc settles to the bottom of the water in large tanks. As you can imagine, this can be a slow process! Once the floc settles, the water is ready for the next stage of treatment.

Coagulation and FlocculationCoagulation and flocculation are often the first steps in water treatment. Chemicals with a positive charge are added to the water. The positive charge of these chemicals neutralizes the negative charge of dirt and other dissolved particles in the water. When this occurs, the particles bind with the chemicals and form larger particles, called floc.

Clarification occurs in a large basin where water is again allowed to flow very slowly. Sludge, a residue of solids and water, accumulates at the basin's bottom and is pumped or scraped out for eventual disposal. Clarification is also sometimes called sedimentation Sedimentation

During sedimentation, floc settles to the bottom of the water supply, due to its weight. This settling process is called sedimentation.

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FiltrationOnce the floc has settled to the bottom of the water supply, the clear water on top will pass through filters of varying compositions (sand, gravel, and charcoal) and pore sizes, in order to remove dissolved particles, such as dust, parasites, bacteria, viruses, and chemicals.

Turbidity is a physical characteristic that makes water appear cloudy when suspended matter is present. The filtration process removes suspended matter, which can consist of floc, microorganisms (including protozoan cysts such as Giardia and Cyrptosporidium), algae, silt, iron, and manganese precipitates from ground-water sources, as well as precipitants which remain after the softening process.

These suspended materials are filtered out when water passes through beds of granular material, usually composed of layers of sand, gravel, coal, garnet, or related substances. (Measure turbidity with a Hach turbidimeter.)

DisinfectionAfter the water has been filtered, a disinfectant (for example, chlorine, chloramine) may be added in order to kill any remaining parasites, bacteria, and viruses, and to protect the water from germs when it is piped to homes and businesses.

During disinfection, disease-causing organisms are destroyed or disabled. Chlorine (Cl2) is the most common disinfectant used in the United States because it is practical, effective and economical.

Because chlorine dissipates rapidly, it is important to add the right amount of chlorine at the water treatment plant to make sure disinfection continues while the water is flowing through the distribution system. (Use a USEPA-approved Hach method to measure chlorine in water.)

Sedimentation

TYPE 1

(discrete

particle settling)

• settling of discrete particles in dilute suspensions

• particles have no tendency to flocculate

• they settle as individual entities and there is no

significant interaction with neighboring particles

Example:

removal of grit and sand in

wastewater treatment

TYPE 2

(flocculant

settling)

• settling of flocculant particles in dilute suspensions

• as particle settle and coalesce with other particles,

the sizes of particles and their settling velocity

increases

Examples:

• removal of SS in

primary sedimentation

tanks of WWTP

• settling of chemically

coagulated waters

TYPE 3

(hindered

settling)

or

(zone settling)

• settling of intermediate concentration of flocculant

particles

• particles are so close together that interparticle

forces are able to hold them in fixed positions

relative to each other and the mass of particles

settles as a zone at a constant velocity

Example:

biological floc removal in

secondary settling basins of

WWTP

TYPE 4

(compression

settling)

• settling of particles that are of such a high

concentration that the particles touch each other

and settling can occur only by compression which

takes place from the weight of particles

Examples:

• occurs in the bottom of

deep secondary clarifiers

• in sludge thickening

Facilities