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Understanding municipal water filtration

A multi-cell gravity filter for municipal water filtration
This well house in Freeport, Illinois, features a multi-cell gravity filter.
The key objective of drinking water treatment and water filtration is to prevent water-borne illnesses and help communities access safe drinking water. The U.S. Environmental Protection Agency (EPA) regulates approximately 155,000 public water systems that provide drinking water to 90% of the country's population. The agency requires utilities to conduct frequent tests to examine the water quality and ensure drinking water treatment meets federal and local standards.
Depending on the municipality, the drinking water treatment process involves a series of steps, including coagulation, flocculation, sedimentation, filtration and disinfection. The following table highlights the various steps of a typical, modern municipal water filtration system.

 

Municipal drinking water treatment process

Steps

Description

Coagulation

Certain positively charged chemicals, including aluminum or iron salts, are added to the water to neutralize the negative charge of debris and other dissolved particles. 

Flocculation

The water is gently mixed to let the dissolved particles bind with the chemicals to form larger, heavier particles called flocs. 

Sedimentation

Flocs settle to the bottom of the water tank as separate sediments. 

Filtration

The clear water on top is filtered to separate additional, smaller solid particles from the water. 

Disinfection

One or more chemical disinfectants (typically chlorine, chloramine and/or chlorine dioxide) are added to the filtered water to eliminate any remaining biological contaminants in the water supply.  

Municipal water filtration is a key step that helps accomplish the desired purity of drinking water. Community leaders should consult water engineers to determine the right water filtration technology for their municipality.

Types of municipal water filtration

It is worth noting that depending upon the quality of the source water, the water treatment process varies. As a public water drinking source, surface water typically requires more treatment and filtration than groundwater, as rivers, streams and lakes contain more sediments and contaminants than groundwater. The types of municipal water filtration technology are determined accordingly.

Municipal water filtration types

Technology

Description

Traditional water filtration

  • This involves passing the water downward through the porous and granular beds of charcoal, sand and gravel. 

  • The pore spaces of the filter media trap the dissolved, suspended particles, including dust, debris, certain chemicals and biological contaminants. 

  • As the filters get saturated with trapped solids, they are backwashed, pumping clean water and air into the filter to remove impurities. The water carrying impurities is pumped into the sewerage system or discharged back into the source river after the settlement phase in a sedimentation tank.     

Activated carbon filter

  • Granular Activated Carbon (GAC) is the preferred filtration media across several conventional municipal plants, as it offers excellent mechanical filtration of particulate matter and organic compounds. 

  • The high adsorption qualities of activated carbon filters also remove any poor taste or foul odor in the water. 

  • GAC treatment is extensively used by public drinking water suppliers to remove PFAS chemicals. 

Ultrafiltration

  • This is used in addition to or instead of traditional filtration. 

  • The water goes through a hollow fiber or sheet filter membrane with very small pores. The super fine membrane technology mechanically filters small particles contained in the water. 

  • Ultrafiltration only lets through water and other small molecules, such as salt. 

Reverse osmosis

  • This advanced filtration technology is implemented in water treatment plants to remove dissolved inorganic solids.  

  • The water is forced through semi-permeable membranes under high pressure to filter most contaminants from the water. 

Some public water distribution systems, particularly those in the Midwest, may contain radionucleotides (small radioactive particles such as radium) that require specialized filtration through pre-formed Hydrous Manganese Oxide (HMO) filters. It is an EPA-recognized treatment method to remove radium from drinking water. The suspended manganese oxide particles have a high affinity for radium adsorption. As radium affixes to the HMO filter, it gets removed in the subsequent media filter along with any iron or sulfur particles.  

Water filtration is a complicated process that requires the technical expertise of professionals to comply with the EPA's drinking water standards. Communities often require securing funds to upgrade municipal water filtration processes. Partnering with an experienced team of water engineers helps municipal leaders determine affordable and effective water filtration methods and secure the means to implement them.

Partner with Fehr Graham to determine the right water filtration technique for your community

At Fehr Graham, we are committed to helping communities design custom, cost-effective and state-of-the-art municipal water filtration systems to secure safe drinking water. Our team of licensed engineers helps you design filtration systems to remove contaminants, oversee construction and secure funding for system upgrades. With Fehr Graham, you can be assured your water will comply with local drinking water regulatory standards.

To learn about how Fehr Graham can help you with developing municipal water filtration systems, contact us or call 815.235.7643.

Portrait of Derek Thompson Derek Thompson, PE, is a Professional Engineer with a proven track record in water, sewer, transportation and site design. He is also the Branch Manager of our Freeport, Illinois, office. Derek is client-focused. He builds and maintains strong relationships with community and municipal leaders while tailoring projects to their needs. Reach him at This email address is being protected from spambots. You need JavaScript enabled to view it..