1.) Screening

Surface water (water from Angat Dam and Ipo Dam) often contains large debris, such as sticks, logs, leaves, fish, and trash. These objects can clog the water-treatment system and therefore must removed before the water enters the treatment plant. As such, treatment facilities that use surface water have large screens that cover the site of water intake. The debris is too large to pass through the holes in these screens. So as the water enters the facility’s tunnels and aqueducts, the large debris is removed. These screens, however, must be cleaned periodically to remove any objects that have become stuck and prevent the screen from becoming clogged and impeding water flow into the facility.

2.) Pre-Chlorination (Disinfection)

This step effectively kills any organisms (pathogens) in the water that may maybe harmful to humans. This is done through the addition of chorine, and is repeated after all other treatment sequences to ensure the destruction such organisms.

3.) Rapid Mixing

As the first step in which the water is being treated, this involves intense vigorous mixing and promotes the dispersion of coagulation chemicals (Aluminum Sulfate and Polymer) in the raw water.

A series of chemical reactions occur to begin the formation of a “floc”, which is a cluster of coagulative chemicals and impurities, such as suspended solids and organic matter. The intensity of the mixing is then reduced, allowing well-defined pinhead-sized particles of floc to begin to form.

4.) Flocculation

These small non-rigid particles are made to come in contact and agglomerate with one another by mixing the water. When the agglomeration of the particles gets large enough, the aggregate will then settle in still water by sedimentation.

The larger particles continue to combine, or flocculate, into much larger and heavier particles. These particles become too heavy to float and begin to sink and settle. Other suspended particles that do not agglomerate well by flocculation are removed from the water through the use of polymers.

After flocculation, the water is ready for the next step in the treatment process, sedimentation.

5.) Coagulation         

Aluminum sulfate is added to the water entering the plant. The water is mixed, rapidly at first and then more slowly as the process continues. Mixing causes lightweight particles in the chemically treated to clump together, or coagulate, into much larger particles.

Coagulation is the process of gathering particles into a cluster or clot, often achieved by the addition of special chemicals known as coagulants. The most common coagulant used in water treatment facilities is aluminum sulfate (alum, Al2 (SO4) 3). Other Al and Fe salts, including polyaluminum chloride, ferric chloride, and ferric sulfate, may be used as well. As this precipitate forms, other particles are caught in the solid and form a mass.

6.) Sedimentation

Other suspended insoluble particles, such as sand and dirt, are often small enough to pass easily through the screens. Therefore, these particles must be removed from the water by another process known as sedimentation. When water is allowed to sit, heavy suspended particles, such as sand, will settle to the bottom over time, as they are denser than water. The water, now free of the suspended impurities, can be collected from the top without disturbing the layer of sediment at the bottom, which is eventually discarded.

7.) Filtration         

Oftentimes, however, the particles generated by the previous precipitation reactions are too small to settle efficiently by sedimentation. One strategy frequently employed to remove these solids is gravity filtration.

In this process, water containing solid impurities is passed through a porous medium, typically layers of sand and gravel pieces. The solids formed from precipitation get stuck in the pores, and are thus removed from the water as it passes through the porous medium. The water that remains at the bottom of the filter no longer contains those solid impurities.

These gravity filters are essentially a pipe feeding into the under-drain, or the bottom layer where the clean water is collected. By adding water to the filter through this pipe, clean water can be forced upward through the filter to remove the solids that have collected in the filter. This process is used to clean the filter.

8.) Disinfection

In many water supplies, the most serious health threats are posed not by chemicals, but by infectious organisms, such as bacteria, in the water. Chlorine (CI2) is a major disinfectant that is affordable and kills most of the serious disease-causing bacteria in the water. Other methods are also employed to remove the organisms, including through coagulation, sedimentation, and filtration.

Schematic Flow Process of a Water Treatment Plant

The processes of screening, flocculation, sedimentation, filtration, and disinfection work together to remove the unwanted substances from our water supply, making it safe to drink and appropriate for other uses.  Once the water is treated, it is sent to storage reservoir and then distributed to household consumers, businesses, and industries.