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One of the primary purposes of treatment methods employed for municipal waters is to filter and clarify the water, providing an important step in the protection of public health.

Surface waters tend to be more turbid, and contain more microbes, particles of vegetation, and silt, than ground waters.   For that reason, the principles of clarification and filtration play a key role in the protection of public health when surface water sources are used.  Not only do they play a role in removal of microbes, but also in the removal of other suspended solids that could affect subsequent treatment processes or disinfection steps.  Of primary significance in relation to public health, almost total removal is needed because remaining particles may shield the pathogens from disinfection.

Clarification is the process of using coagulant chemicals to produce larger particles that are settleable from nonsettleable solids such as microorganisms, color, fine silts and clays, and some vegetative matters.  The forming particles clump together with other growing particles and form floc, and are settled out in the clarification basin.

The primary coagulant chemicals, normally metallic salts, neutralize the electrical charges of the particles, causing them to clump together.  The coagulant aid chemicals, primarily polymers, and calcium and sodium compounds, add toughness to the clumps and add density to slow settling flocs. The chemicals are mechanically mixed with the water, and time and gentle agitation optimize floc formation by increasing the occurrence of collision between particles.  The success or optimization of coagulation depends on pH and adequate alkalinity.  For example, when alum is used as a coagulant, the optimum pH (because of amphotericity of aluminum) is between 5 and 7.  Optimum pH is 5 to 7 for many coagulants, along with enough alkalinity in the water to serve as a buffer to prevent major pH changes and aid in the coagulation process.

Sedimentation or clarifier basins provide the location for settling of the floc materials, as well as any other settleable materials.  The materials are allowed to settle, and are removed from the basin.  Basically, feed water with floc and other materials enters a basin.  Gravity pulls the floc to the floor, and clear water leaves at the other end.

There are single unit systems that may handle all phases of this clarification process, but the general principles are the same.  The method of removal of suspended solids is creation of floc material composed of these solids and coagulant chemical, and removal of the resulting particles.

Filtration, as discussed in the following paragraphs, is more mechanical in nature.

By definition, filtration is physically removing particulate matter from the water by forcing it through a bed of porous granular, material.  Depending on the coarseness and engineering of the media, and the type and effectiveness of the pretreatment, filtration can be relied upon to remove over 99% of some pathogens and other organisms from the water.  Also removed are silt, clay, and organic particulates of decaying plants. 

Granular-bed filters commonly use an ample depth of anthracite, sand, or a combination, engineered to handle a specific range of flows and qualities of waters.  The process is described as depth filtration because the trapped solids are removed within the granular material.  This method utilizes transport mechanisms to carry the particles, smaller than the size of the interstices of the granules, to contact the individual filter grains.  Attachment mechanisms, enhanced by chemical pretreatment, help retain the particles in the filter.

Precoat filters use a thin layer of very fine medium, such as diatomaceous earth, that is disposed of after each cycle, as the filter media.  This type of filtration is referred to as cake filtration because the solids are removed on the entering face of the granular material.  It strains the water at the surface to remove particulates. 

In summary, filtration utilizes mechanical principles, in various configurations, to trap microorganisms, decaying vegetable matter, and silt and turbidity.  Not only are pathogens removed physically, but also removal of the other substances allows for better utilization of sanitizing agents with less demand created.  Also, removal of organic materials helps reduce the level of THMs that could be produced with chlorination processes.  There are actually five different processes going on during filtration including, by order of importance: straining, sedimentation, absorption, flocculation, and biological metabolism.

 In a conventional system, the clarification process will do the greatest burden of particulate removal, with filtration providing additional and more thorough particulate removal.

In other articles on this site, methods of disinfection, and fluoridation, are discussed, which would normally be the next steps in conventional treatments of municipal waters. 

If the water is to be used for purposes other than basic drinking and household uses, reverse osmosis, activated carbon filtration, ion exchange, softening, ozonation, distillation, or other advanced treatment methods may be used.