MEC Engineer Walks through the Clean Water Treatment Process
As the population grew in major cities during the Industrial Revolution, providing access to clean water and safely disposing sanitary waste became a public health discussion. But it took more than 50 years before the federal government addressed clean water issues, passing the Federal Water Pollution Control Act of 1948. Over the next 25 years, Americans became increasingly more aware of waterborne illnesses, pesticides, and industrial waste, and there was a growing desire to control them.
Sweeping water quality laws were approved in 1972, which gave the Environmental Protection Agency (EPA) the authority to implement pollution control programs, set water quality and wastewater industry standards, and fund construction of sewage treatment plants. Although the Clean Water Act went through some revisions in the 1980s and 1990s, it is still the basis of water quality standards in the United States.
Today, there are more than 151,000 public water systems, which are regulated by the EPA, and more than 15 million private drinking water wells. In the Kansas City metropolitan area, residents have access to several water sources, including bored, driven, and drilled wells, groundwater, and surface water like rivers and lakes. “We have a great underground river and some of the best tasting water in the world,” said Mark Pearson, senior project manager for McClure Engineering Co.
Clean water isn’t tapped at the source. Instead, it goes through a several steps of treatment before consumption, which can vary depending on the region. In Kansas City, water is treated through coagulation, sedimentation, filtration, and disinfection.
Water flows into a treatment plant where a splash aerator oxidizes iron into an insoluble particulate, which is removed by settling and/or filtration.
“There is iron, particularly in well water, and tannins in surface water that comes from leaves and decaying organic matter, which will add color and odor to the water,” he said. “Generally, the water flows into a treatment plant where there is a splash aerator. This will oxidize iron into an insoluble particulate that can be removed by settling and/or filtration. Activated carbon can be fed to water treatment systems to absorb and adsorb odor and color causing agents during periods when leaves fall and/or when reservoirs ‘turn over’ in spring and fall.” Both aeration and carbon addition improve the taste and odor of drinking water.
Adding lime and soda ash or other coagulants is the next step. The water is mixed with these chemicals and put in a special clarifier or the settling basin to remove the sodium and magnesium, which causes hard water. This step also raises the pH level. To lower the pH, the water goes through a recarbonation process where carbon dioxide is added in a reaction chamber.
“The water then moves through a filter to remove the particles, viruses, and bacteria. Chlorine will be added for disinfection purposes,” Pearson said. “The water will have what’s called ‘contact time’ to kill all viruses and bacteria that remain.” Residual chlorine must be maintained in the distribution system to prevent contamination in the distribution system between the plant and the tap.
Chlorine can react with organic material in pipelines, and the resulting compounds have been shown to be cancer-causing. This is more likely to occur when the water is very old due to long retention times in storage tanks or unflushed pipelines. “To prevent disinfection byproducts from developing in the distribution system, some communities will add ammonia to the water after contact time. These chloramines will persist longer in the distribution system and are less likely to form disinfection byproducts,” he said.
The storage tanks where the filtered water is housed must also be monitored. “There isn’t always good mixing in the elevated or groundwater storage tanks,” he said. “When that happens, there are stale areas where bacteria can grow if there isn’t enough chlorine, forming disinfection byproducts.” Sampling and testing can help show where disinfection byproducts are forming.
Water pipes may also need to be treated, depending on what they are made of. Many old water pipes are made of lead, which can leach into the water supply if preventive measures aren’t taken. While some water utilities are replacing lead pipes, the majority will add phosphate to the water supply to form a protective film between the pipe and water flowing through it. The City of Flint, Mich., is one example where phosphate was not added to control chlorine corrosion in the lead pipes, which contaminated the water supply.
From the source to the faucet, clean water goes through many steps of treatment before it is can be consumed, and water pollution continues to be an area under constant review. Engineers like Pearson continue their work to improve the aging water infrastructure, protect drinking water, and remove water-related health hazards.