Municipal water treatment is a highly controlled sequence of steps designed to remove contaminants from source water and make it safe for public use. These processes happen at large-scale facilities to ensure that water drawn from rivers, lakes, and underground sources meets strict quality standards before entering the distribution system. Removing impurities like suspended solids, organic matter, and harmful microorganisms is a multistage undertaking that combines chemical and physical methods to achieve high water clarity and safety.
Coagulation and Flocculation
This preparatory stage addresses the tiny, suspended particles that cause cloudiness, or turbidity, in the source water. Since these particles, such as fine clay and silt, often carry a negative electrical charge, they repel one another and remain in suspension. Coagulation is initiated by adding positively charged compounds known as coagulants, such as aluminum sulfate or ferric chloride, which neutralize the negative charges.
Once the charges are neutralized, the destabilized particles begin to stick together, forming micro-flocs. The water then moves into the flocculation phase, where it is gently mixed in large basins. This slow, controlled agitation increases collisions between the micro-flocs, causing them to bind together and grow into larger, heavier aggregates called “floc.”
A polymer can be used as a coagulant aid to further strengthen the bonds of this newly formed floc. This combined chemical and physical process transforms microscopic contaminants into masses large enough to be separated in the next step.
Sedimentation
Sedimentation is a purely physical process that relies on gravity to remove the large floc particles created previously. The water is channeled into large sedimentation basins where the flow rate is significantly reduced. This reduction in velocity allows the heavy, dense floc particles to settle without being kept in suspension.
Gravity pulls the macro-floc to the bottom of the tank, where it accumulates as a layer of sludge. This process effectively removes the majority of physical impurities and turbidity from the water. The clearer water, called the supernatant, flows off the top of the tank toward the next stage of treatment.
Effective sedimentation removes a high percentage of suspended solids, often ranging from 50% to 70%. This greatly reduces the particle load and the burden on the subsequent filtration process.
Filtration
Following sedimentation, the water undergoes filtration to remove any remaining fine particles and impurities. The water passes through deep filter beds typically composed of multiple layers of granular media. These layers are arranged with coarser material on top, like anthracite coal, followed by progressively finer materials such as sand and garnet.
The layers act like a complex sieve, trapping residual floc, clay, silt, and organic matter as the water passes through. This process removes particles too small to be efficiently removed by gravity alone. Filtration may also incorporate activated charcoal, which is effective at adsorbing dissolved organic compounds that cause taste and odor issues.
Rapid sand filters are widely used in municipal plants to ensure nearly all remaining suspended matter is captured. The result is water with significantly improved clarity and purity, ready for the final step.
Disinfection
The final step is disinfection, which is designed to eliminate harmful microorganisms that cause waterborne diseases. While previous steps remove physical contaminants, this process targets bacteria, viruses, and parasites like Giardia and Cryptosporidium. The most common method is chlorination, involving the addition of chlorine or chlorine compounds like sodium hypochlorite.
Chlorine works by penetrating the cell walls of microorganisms and interfering with their internal enzymes, effectively killing or inactivating them. Alternatives such as chloramine, a compound of chlorine and ammonia, or ultraviolet (UV) light can also be used.
A crucial aspect of this stage is maintaining a residual disinfectant, most commonly free chlorine, as the water leaves the treatment plant. This residual, typically maintained between 0.2 and 1.0 milligrams per liter, provides continuous protection against microbial regrowth or recontamination as the water travels through the distribution system. This safeguard ensures the water remains safe until it reaches the tap.