Municipal Treatment Plants

Every time we open our taps to fill our glasses with drinking water, we expect safe, clean water to come out. But, do we know what happens to this water before reaching our homes? To most of us, our drinking water systems are out of sight and out of mind. What these systems do for us, however, is vital, and require important maintenance and a close coordination between numerous actors (i.e. plant operators, chemists, laboratory technicians, mechanics, instrument suppliers…).

Water undergoes extensive, multi-stage purification processes in large municipal treatment plants before it flows from our taps. These treatment plants come in all sizes and shapes, and no two are exactly the same. Nonetheless, while their design may differ, they all share the same purpose: provide safe, reliable drinking water to us. That is why we wanted to show how water is treated in these plants and what challenges they face.



There is no precise method for treating our water, because it depends on the type and quality of the water sources. For example, water from springs and boreholes may only need simple filtering and disinfection, while water from lakes and rivers may require extensive treatment as it is more likely to contain more pollutants. Besides, the level of treatment also depends on the water quality standards specified by the existing legislation.

Nevertheless, most municipal treatments plants have a fairly standard sequence of processes, which includes five main steps: screening, coagulation and flocculation, sedimentation, filtration and disinfection.


A.1. Screening

Before entering the treatment plant, water is passed through large mesh screens that remove debris (e.g. leaves, sticks, weeds and fish) and avoid the clogging of the water treatment systems. It is important that these screens are cleaned periodically to remove the objects stuck, so that they do not block them and impede water flow into the plant.

A.2. Coagulation and Flocculation

Colloids and other impurities (e.g. sand and dirt) are small enough to pass easily through the screens, as well as too minute to settle out quickly. For this reason, after the screening, water is passed through coagulation and flocculation basins where it is mixed with chemicals. Coagulants cause the tiny suspended particles to be attracted to each other and form “flocs”, while flocculants stick these small flocs to form larger ones. Thanks to the combination of these processes, the small particles present in the water agglomerate and former larger and heavier particles.

A.3. Sedimentation

Once flocs are large enough to be settled, water is moved into the sedimentation basins, also called clarifiers or settling basins. These tanks provide a quiescent, low velocity environment where the floc mixture can settle to the bottom. The water, now free of suspended solids, can be collected from the top without disturbing the bottom sediment layer (which is eventually removed).

A.4. Filtration

After the sedimentation basins, water is run through a series of filters, which trap and remove the remaining impurities. Conventional rapid sand filters are the common solution adopted, where water moves vertically through sand and has its particles trapped in pore spaces or adhered to sand particles. Another solution often implemented is slow sand filters, where water passes slowly through sand, and undergoes a biological treatment besides the physical filtration.

A.5. Disinfection

Now that water is largely free of particles and contaminants, it is disinfected to destroy any remaining disease-causing pathogens. This is commonly done through chlorination, ozonation or UV radiation. In this last step, it is important to provide a residual dose of disinfectant to kill potentially harmful microorganisms in the storage and distribution systems. Afterwards, water is ready to be sent to the pumping station for distribution to our homes and flows from our taps.



Today’s municipal water treatment plants face several challenges as result of the population growth and development. These issues are complex and require big investment, but must be tackled head-on in order to continue providing safe, reliable drinking water supply to our communities in the future. The main issues we need to solve in the short-term are three: aging infrastructure, new contaminants and need for technological innovation.

B.1. Aging infrastructures

Most of our treatment plants were constructed by previous generations and have served us consistently for numerous decades. However, many of these infrastructures are old and coming close to the end of their useful lives. Aging assets not only increase operational risks and costs, but also reduce the efficiency of the treatment systems.

For this reason, new investments are necessary to upgrade and fix today’s treatment plants in order to meet current needs and comply with stricter regulations. The longer we wait to address this issue, the more expensive and damaging it becomes.

B.2. Treatment of new contaminants

The conventional processes in our drinking water treatment plants eliminate a wide spectrum of contaminants at a fairly high standard. However, the truth is that today’s treatment plants do not remove all the chemicals and pharmaceutical that are building up in our water supply because they were not designed to eradicate them. What’s more, there is evidence that common processes in conventional drinking water treatment plants (e.g. chlorination) make some of this pharmaceuticals more toxic.

Therefore, it is necessary to accelerate research to encourage advances in risk assessment methodologies for this unregulated chemical and pharmaceutical contaminants in drinking water. This research will determine what adverse effects may be caused by exposure to these contaminants, and will lead to future treatment processes to reduce these effects.


B.3. Need for technological innovation

Our conventional water treatment technologies are effective at providing high-quality water supply, but not always efficient. It is becoming clear that new technologies are needed in the water treatment industry, in particular to address the health risks posed by the new contaminants and to optimize the treatment processes.

That is why new technologies need to be developed and implemented in the treatment facilities. Several international organisations, from water utilities to universities and governmental agencies, are already testing treatment technologies including nanomembrane systems and, with more frequency, water reuse strategies. This new technologies will not only improve the efficiency of water treatment plants, but also their sustainability.

Municipal water treatment plants are designed, built, operated and maintained with one purpose: provide clean, safe water to all city residents. To complete their role, they include several treatment processes, and to continue delivering clean water in the future, they need to be constantly upgraded.

By Fatine Ezbakhe
Civil Engineer specialized in Water

Published on 2015-06-12 12:44:09

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