Introduction | Waterborne Pathogens and the Diseases They Cause | Microbial Water Contamination and Human Exposure | Drinking Water Treatment | Improvements ... and What You Can Do
When we turn on the tap in our households, most of us take for granted that clear, clean water will flow out. Before reaching our homes, however, that water goes through a long journey that may involve many steps.
Drinking water comes from two types of sources: surface waters (lakes, rivers, and reservoirs) and groundwater (wells). How your water’s treated depends on where it’s from.
Odds are that if you live in or near a big city, your water supplier relies on surface waters to obtain large quantities of water quickly and easily. Because surface waters are exposed to environmental elements like wildlife droppings, urban and agricultural run-off, and trash, they require extensive treatment.
Water treatment facilities add chemicals that combine small dirt particles into larger, heavier particles that sink to the bottom. This allows the clear water to be filtered and then disinfected.
Courtesy of L.M. Pope, U.S. Geological Survey.
Groundwater has historically been assumed to be safe without treatment to kill microorganisms. Layers of soil act as a natural filter, removing microbes and other particles as water seeps through.
Groundwater is pumped from wells drilled into underground water reservoirs known as aquifers. The water undergoes a natural filtering process as it trickles through layers of soil and sand particles before collecting in the aquifers.
Groundwater is more protected from environmental elements than surface water, meaning it requires fewer treatment and purifying steps. Some groundwater systems need to add a disinfectant like chlorine; others (such as those from particularly deep reservoirs) require no further treatment.
Illustration of aquifer courtesy of the Gulf of Maine Aquarium.
Chlorine is the most common means of disinfecting water in the U.S. The addition of a small amount of chlorine is highly effective against most bacteria, viruses, and protozoa. But cysts (durable seed-like stages) formed by parasitic protozoa such as Cryptosporidium and Giardia can survive chlorine.
Mechanical failure can also lead to contamination. Water treatment facilities maintain constant water pressure in distribution pipes to prevent microbes from getting "backwashed" into the distribution system. Even a momentary lapse in pressure can result in a temporary backflow that can allow microbial contamination of already treated water.
All major water suppliers must comply with state and federal water safety regulations. (Different regulations apply to suppliers with fewer than 25 people or 15 connections a year.)
The Environmental Protection Agency (EPA) administers and enforces the two principal federal water safety laws: The Safe Drinking Water Act (SDWA) and the Clean Water Act (CWA).
The Safe Drinking Water Act protects water intended for consumption by setting standards and rules for levels of various pollutants that can contaminate water.
The Clean Water Act protects the nation's bodies of water, including rivers, lakes, and coastal zones. The Act regulates the discharge of pollutants into waters, and includes standards for wastewater (sewage) treatment.
This indirectly affects the safety of our drinking water, because the bulk of treated wastewater is released into rivers or other natural bodies of water, which in turn are often used as sources for drinking water.
Water Treatment Resources from the Environmental Protection Agency
Introduction | Waterborne Pathogens and the Diseases They Cause | Microbial Water Contamination and Human Exposure | Drinking Water Treatment | Improvements ... and What You Can Do
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