Water Treatment Plant Processes

Water treatment plants follow a set of standard processes to prepare water for distribution. These processes vary from town to town and state to state.

The first step is aeration. This causes fine particles to clump together and become heavier so they can be removed.

The heavy clumps then settle in sedimentation basins. The clear water on top is then filtered to separate out additional solids such as chemicals, dust, parasites and bacteria.

Coagulation

All water contains small particles suspended in the liquid. These particles give the water a cloudy appearance called turbidity. Coagulation removes these particles and clarifies the water.

Coagulation is the process of making these tiny particles stick together into larger chunks (floc) that can be easily removed in a downstream sedimentation or filtration step. This step is accomplished by gentle agitation that is created by paddle-type mixers in a coagulation basin. The speed of agitation, the type and quantity of coagulant and coagulant aid used, as well as pH are important factors to optimize the coagulation step for best performance.

Inorganic coagulants such as alum and ferric chloride are often used to achieve coagulation. Hydrex plant-based coagulants offer several advantages: free of metal salts, they do not corrode the metallic parts in the treatment plant; they perform well over a wide range of pH and raw water temperatures; and they produce fewer chemical residuals in the final treated water.

Flocculation

Floculation is a process that involves the clumping together of fine particles to create larger aggregated flakes that can be easily separated from the solution. It can be achieved naturally or forced using flocculants.

In the first stage of flocculation, a coagulant such as aluminium sulphate is added to the water. The positively charged coagulant molecules neutralize the negatively charged particles in the water treatment plant wastewater. This allows the particles to collect together and bind into large masses, called flocs, which then settle at the bottom of the water stream for removal.

Once the coagulated particles are clumped together, they need to be increased in size so that they can be easily removed from the water by means of filtration or sedimentation. This is accomplished by a gentle mixing process called flocculation. Slow stirring causes the small coagulated floc particles to collide with each other, increasing their size to produce larger clumps known as pin-flocs. These are then ready for solids-liquid separation by filtration or sedimentation.

Sedimentation

Sedimentation is the natural process of allowing solids in water to fall out of suspension under gravity. This allows the heavier particles to sink to the bottom and the lighter ones to float to the top. The settled solids are known as sludge.

Sediments are moved based on the strength of the flow that is carrying them, their size, volume and density. As such, sedimentation is a key part of the water treatment process.

There are a number of advanced methods that can be used to enhance the sedimentation process. These include lamella settlers and ballasted flocculation.

The main objective is to make sure that the sludge is settled as quickly as possible. This helps to ensure that the purified water is available for use as soon as possible. Once the water treatment plant supplier sludge has settled, it is then pumped from the sedimentation tank and onto the next step in the treatment process.

Disinfection

Once the flocculation process is complete and the clumps are removed from the water they are sent to a tank called a sedimentation basin. There, they are kept for 20 to 30 days where bacteria digest them. This process helps to reduce odors and organic material that can cause disease in humans and animals. The resulting solid waste is then sent to landfills or used as fertilizer.

Once settled, the water is disinfected to kill any remaining pathogenic organisms. This step is usually done by adding chlorine to the water in Chlorine Contact Basins (the same type of water treatment system found at swimming pools). Other disinfection processes include chemical sterilants such as peracetic acid, ortho-phthalaldehyde and glutaraldehyde, or UV-radiation.

Next the water is filtered to improve taste and remove any remaining organic material. Once finished, the clean water is ready to be distributed throughout the city.

Filtration

Water goes through a filtration process which uses beds of granulated materials, such as anthracite coal and sand, to remove additional impurities that may not have been removed in the flocculation and sedimentation processes. This is accomplished by passing the water through a filter in a quick, upward motion (backwashing) which removes trapped dirt from the bed of material.

The water is then chlorinated and treated with a variety of chemicals, such as sodium hypochlorite for disinfection, zinc orthophosphate to control corrosion, and fluoride to prevent tooth decay. The water then flows into two, 2.75 million-gallon storage reservoirs.

The filtered water is then pumped to distribution systems for use by residential and commercial customers. The plant’s clear well contains 433,000 gallons of water. Water is tested on a regular basis to ensure that the water meets all regulatory requirements. If it does not, the system can be shut down to restore proper levels of disinfection and filtration.

Ozone Treatment

Ozone is a powerful oxidizing agent that kills bacteria and organic matter in water through a safe chemical reaction. It’s also cheaper and faster than chlorine or other disinfectants.

Ozonation produces fewer disinfection byproducts, such as trihalomethanes (THMs), than chlorination. However, some THMs are still suspected or proven carcinogenic.

Ozonation destroys organic carbon, and it reduces iron, manganese, odors, and sulfur in water through oxidation. It also controls biofouling of reverse osmosis membranes and ion exchange resin.

A specialized ozone generator creates ozone gas, which is dispersed in the plant’s water through point-of-entry devices. The ozone bubbles into the water, creating a high level of turbulence to ensure that it reaches every part of the water. The ozone then oxidizes any bacteria, viruses, and fungi in the water. It’s then filtered through a bed of anthracite or filter sand to remove any partially oxidized organic material.