How a Water Treatment Plant Works

Most people don’t think about where their water comes from or how it gets cleaned before coming out of the tap. City water treatment plants are impressive systems.

The water begins in a pre-sedimentation building where it is exposed to air which helps dissolve hydrogen sulfide and other gases. It is also aerated and chemically treated with primary coagulants and polyelectrolytes.

Sedimentation

Sedimentation is a method for reducing suspended solids and pathogens in water by gravity. It is the first step in the water treatment process and it helps reduce turbidity, improves the visual quality of the water and increases its acceptance by consumers.

The primary sedimentation tanks, or settling basins, are water treatment plant rectangular and provide a calm environment for the solids to settle. Often, a sediment removal system is used to remove the settled solids from the bottom of the tank and transport them away for further processing. It is essential to maintain proper flow rate and detention time in the sedimentation tank to avoid overfilling and microbial contamination of the water.

Adding chemical or natural coagulants enhances the sedimentation process. These substances bind to the negatively charged particles that cause turbidity, neutralising their surface charge and allowing them to clump together. Chemical coagulants include aluminium sulphate, polyaluminium chloride (also known as PAC or liquid alum) and ferric sulphate. Natural coagulants include prickly pear cactus seeds, Moringa seeds and broad beans.

Plain or unaided sedimentation is a simple process that relies solely on gravity. It is used in reservoirs, grit basins or debris dams at the beginning of the treatment plant. The water flows into a horizontal tank where it is allowed to settle. The tank is usually built with a sloped floor to increase sludge build up, and an evacuation port pumps it to a hopper.

Flocculation

Flocculation is the process of clumping small, neutralized particles into larger flakes that can be easily separated from water. This happens naturally, or can be forced using chemical flocculants that have different charges, charge densities and molecular weights. Flocs are then ready for further water treatment processes like filtration and sedimentation.

Water is typically collected from lakes, rivers, and groundwater sources and then sent to a water treatment plant where it undergoes several stages of water treatment and purification. It starts with screening to remove any material that won’t float or settle on its own. This is followed by coagulation to neutralize the charges on particles and then by flocculation to bind them together into larger flakes that are easier to separate from water.

After coagulation, the clumps are then filtered to remove any remaining impurities in the water. This is done by passing the sludge through layers of sand, gravel and carbon in filters of various sizes to separate out the dissolved materials. The filtered water is then sent to the storage tank or drinking water distribution network.

The next stage of water treatment is osmosis, which uses membranes to separate out dissolved minerals and other substances. This helps make the water safer and more palatable for consumption. The final stage of water treatment is disinfection, which involves adding chlorine or chloramines to the treated water.

Stabilization

Once the coagulation, flocculation, and sedimentation processes have been completed, the water is ready to be disinfected. The process of disinfection involves pumping chemicals or ultraviolet radiation into the water to kill bacteria and viruses in it. This is done to prevent them from regrowing in the distribution system and causing diseases like food poisoning or diarrhea. Chlorine is often used to disinfect the water. Depending on the type of bacteria, ozone or ultra-violet radiation may also be used to kill them.

During this stage, the water is pumped through the distribution system in pipes and lifts to reach each building in the community. The water is also tested throughout the distribution path to ensure that water treatment plant supplier it still meets safety standards and does not contain any harmful bacteria.

The treatment plant also plays an important role in environmental preservation by reducing the amount of organic matter, nitrogen and phosphorus that is released into the environment. This is accomplished by a series of biological carbon and nitrogen conversion processes such as methanogenesis, denitrification and nitrification.

The centralized water treatment plant is generally located near the source of the drinking water that it treats. This allows it to benefit communities closer to the treatment plant. However, the transportation cost of supplying this water to regions that are far away limits its availability and benefits.

Disinfection

The disinfection process kills the pathogens in the raw water, thus eliminating many diseases that cause people to get sick. Examples of these include typhoid and paratyphoid fevers, cholera, salmonellosis and shigellosis. The treatment plant uses basic physics and high technology to purify the water so that it is safe to drink.

The oxidizing disinfectants in the plant, such as chlorine and permanganate, demolish organic material in the water and leave behind only inorganic substances. This prevents the microorganisms from being able to absorb essential nutrients, so they die out. Disinfection also provides a bactericidal effect against low level pollution in the mains distribution network and stops germs from passing on in resistant (endospores) or reproductive forms (cysts).

Water quality in the disinfection stage varies widely between ground and surface water sources, with different regional geochemistry affecting the water composition. Various treatments are used to remedy undesirable characteristics of the raw water, such as color, taste, and turbidity. These treatment processes can also affect the disinfection process itself, as they may influence the rate at which disinfectant reacts with and consumes the impurities in the water. This is known as the disinfectant demand of the water, and it must be satisfied to achieve adequate disinfection.

The concentration of the residual disinfectant in the water and the length of time it is in contact with the organisms in the water are important factors in determining how effective the disinfection is. The disinfection kinetics is governed by the CT concept (disinfectant concentration C, in mg/L, and time T, expressed in minutes, that the residual disinfectant remains in contact with the water).