Before bottled water and whole-house filtration systems were available, people drank tap water that had not been cleaned with reverse osmosis or charcoal filters. Taps that are connected to individual wells require purification systems, but people who live in municipalities most often connect to community water-treatment systems. The mechanisms vary depending on the geology and history of an area but follow the same general steps to guarantee safe water for consumers.
Aeration
Water drawn from a source like a lake, river or artesian well is oxygenated to allow aerobic bacteria to act on compounds like ammonia and nitrites. Nitrosomonas and nitrobacter bacterias use oxygen as a catalyst to react with nitrites, a common contaminant in water drawn from surface sources.
Coagulation
The goal of coagulation is to neutralize or bond chemical and biological impurities into larger, heavier compounds. It may require widely differing treatments depending on the area. Impurities ideally form flakes and feathery sediments after this step. Some systems use a two-stage process called flash mix and flocculation. A polyelectrolyte is added to begin coagulation and is supplemented by a variety of compounds. Where phosphates are present from fertilizes or detergents, ferric sulfate may be added. Aluminum sulfate is another common coagulant. Anaerobic bacteria that function without oxygen react with nitrates to form nitrogen gas, which can be released into the air where it is a primary element.
Sedimentation
Some coagulation processes precede and some may follow the sedimentation phase depending on what type of pretreatment is used. Sedimentation tanks hold the water until the heaviest precipitates settle to the bottom of the tank. The settled water is removed from the tank above the sediment level and pumped through a series of valves into the first or rough clarifier. Sodium hydroxide, lime or other chemicals may be added at this point to adjust pH. Sediment is periodically removed from tanks and dried before disposal.
Clarifying
Water travels from the sedimentation tanks through a series of filters of increasing fineness. Rough filters may begin with screening and move on to adsorption, activated carbon or ion-exchange filters. Finer filtration, using sand filters, reverse osmosis or diatomaceous earth filtration follows. Filters must be backwashed--run backwards--on a regular basis to keep them functioning properly.
Disinfection
The final step of the process introduces a chemical like chlorine that kills organisms that may have survived the coagulation and clarifying steps. Again, practice varies; some systems, like that in New Orleans, introduce chlorination before or during sedimentation. Many, however, finish with chlorine disinfection, often with an ammonia "chaser" to break the chlorine down into chloramines. Newer disinfection technologies using ultraviolet radiation and ozone eliminate the problems inherent in the use of environmental chlorine but are more expensive to operate.
Member Comments