How Does Municipal Water Treatment Work?
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History and Purpose of Water Treatment
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Though the concept of delivering pure drinking water to every household originated in 18th century Enlightenment France, modern municipal water treatment in the U.S. really begins with the Clean Water Act of 1972. For the first time, it became law for every city in the nation to install a water treatment plant. The focus, however, was not as much on safe drinking water for households as it was the decontamination of lakes, rivers and streams that had been sullied by industrial wastes. The standards enacted were designed to regulate wastewater discharge into public waterways by cities and industries, not to provide the highest quality drinking water. As a result, even though more than a third of municipal water treatment facilities produce cleaner discharge than strictly required, most of America's tap water coming from centralized collection sources remains contaminated with pathogens, pesticides, hormones, pharmaceutical compounds and industrial sludge--not to mention the chlorine and fluoride that's added to "purify" the water.
Preliminary and Primary Treatment
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As the wastewater approaches a treatment plant, it is passed through screens of various sizes that serve to filter out the largest solid debris in the water. After this preliminary treatment, dissolved substances and suspended particles still remain. The water enters a gravity filtration tank where it is held still. Gradually, a layer of sediment settles on the bottom. This primary sludge is removed by various methods. What remains in the water after these physical methods of treatment are dissolved pollutants and very small particles that remain floating in suspension.
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Secondary Treatment
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The method of removing dissolved organic and inorganic compounds in water mimics the natural process by which water is purified gradually over time. In secondary treatment, the water is exposed to microbial bacteria, algae, fungi and other microorganisms that consume the contaminants in the water. In attached growth processes, the water is allowed to trickle over a medium covered with such microbial growth. The falling action of the water allows oxygen to enter the system, feeding aerobic bacteria that reproduce and eat the biodegradable pollutants. As the treatment progresses, the bacteria themselves are consumed by other biomass organisms, leaving relatively cleaner water. Alternatively, some plants use a suspended growth process in which the biomass is introduced to still water and then vigorously shaken. After secondary treatment, the water must enter another gravity filtration tank where any solids that might have entered the water can be filtered out.
Further Treatment
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In order to be considered safe for piping into homes, wastewater must have at least undergone primary and secondary treatment. Unfortunately, these processes do not remove contaminants that are not easily biodegradable, but which, if consumed, can produce serious diseases gradually instead of sudden illness. Advanced wastewater treatment processes that try to address some of these issues include land treatment, ultraviolet radiation, disinfection by the addition of chlorine, phosphorus and nitrogen control and carbon filtration. Carbon binds readily with organic compounds, which makes it useful in removing organic pollutants that can cause water to smell and can kill fish. Land treatment again mimics natural processes because the movement of water through porous and semi-porous rocks and sands is one of the most efficient water treatment methods. The addition of chemicals to water is controversial and can create harmful side effects. Identifying effective methods for removing pharmaceuticals and hormones introduced into the water supply through human urine is proving problematic.
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- Photo Credit Rama (CeCILL & CC-By-SA 2.0 France)
