Sedimentation and Filtration
When raw water is brought into a treatment system to be made safe for drinking, the first step in purification (after adding chemicals to adjust pH, if necessary) is to promote sedimentation, in which large-particle contaminants settle to the bottom of the water. Among the first substances introduced to raw water for this purpose are iron salts such as ferrous sulfate or ferrous chloride. These are byproducts from heavy industrial operations that have been oxidized by exposure to chlorine or hydrogen peroxide. The iron salts induce small particles suspended in the water to bind together and become heavier so that they will sink. Other substances, such as alum or certain synthetic polymers, can be substituted for iron salts to perform the same function. This process, called flocculation or clarification, generates sludge on the bottom of the water containment that must be periodically removed and detoxified. The water is then filtered through many different substances, which may include clay, silt, sand, activated carbon, manganese, iron, hard (anthracite) coal, organic or microbial layers, and pressure filters. Filtration promotes disinfection and increased water clarity. In filtration, as in sedimentation, sludge builds up, necessitating regular backflushing of the various filtration media.
Ion Exchange and Adsorption
Up to this point, nothing has been done about dissolved inorganic chemical pollutants that are impervious to sedimentation or filtration. Some of these substances include lead, nitrates, arsenic, chromium, mercury, radium and even uranium. The process of ion exchange introduces resins, polymers, clay or other substances to induce a chemical reaction that further clears and purifies the water, and can also treat hard water (ion exchange occurs in the home in the action of water softeners and laundry soaps). In many cases, substances such as silica gel, alumina or activated carbon are introduced to promote a process called adsorption, in which suspended particles are made to bind to a solid surface. Adsorption is primarily used to remove particles causing undesirable odors or taste in the water.
Disinfection and Monitoring
Finally, the clarified water must be treated to remove biological contaminants. Chlorine, ultraviolet radiation and ozone are used in the disinfection of drinking water. In many cases, chlorine has already been introduced as the water entered what is called the "treatment train." The treatment process itself may now be over, but the job of monitoring the treated water for contaminants has just begun. Federal, state and local statutes mandate periodic testing for more than 80 contaminants, including organic compounds, inorganic compounds and microbes. Such testing is particularly necessary for urban water treatment systems that draw their supply from surface water, such as lakes, rivers and reservoirs, that can be affected by runoff, acid rain and industrial discharges. The needs of smaller municipalities are often met by groundwater that is generally far less polluted than surface water. The last step in the purification and monitoring process concerns the water pipes that carry drinking water from the treatment plants to users. If a water main breaks, for instance, a boil alert must be issued so that consumers can disinfect their own water, which has passed beyond the reach of treatment systems.
Tags: iron salts, activated carbon, bottom water, drinking water, introduced water