A clear well (clearwell) is an enclosed tank that comprises the final storage tank in a municipal drinking water treatment system. A clearwell can be used to mix treated water with specific disinfectants in order to keep the water free of dangerous levels of microbes not only as it exits the plant, but as it travels through pipes and other conduits to its final destination. It can also be used to store water, post-disinfection, that’s ready to be pumped out to customers.
In the disinfection stage, a clear well serves to provide contact time with the disinfecting chemical so that there is sufficient time for it to work. Water that is withdrawn from a clearwell must meet SDWA (Safe Drinking Water Act) standards for pathogens.
The EPA’s Safe Drinking Water Act
The Safe Drinking Water Act (SDWA) falls under the auspices of the EPA (U.S. Environmental Protection Agency) and is intended to protect the quality of drinking water through the US. The SDWA specifically addresses tap water from either surface water, such as lakes, rivers and reservoirs, or from groundwater sources like aquifers and wells.
For most public utilities, some level of water treatment is necessary for water drawn from any surface or groundwater source. This treatment may include screening and sedimentation to remove whatever solids might be found within the source water, like sand, soil particles, and animal waste. The next steps may include biological treatment to remove organic pollutants. This is often accomplished by facilitating decomposition with the help of beneficial bacteria and other microbes.
Once water has progressed through the primary and secondary treatment phases, it’s clear, but may still contain many waterborne pathogens, including enteric organisms like viruses, bacteria (such as Legionella) and parasites, including Giardia and Cryptosporidium.
Understanding that potable water systems are not sterile, the EPA has enacted a series of rules that requires public water systems to actively disinfect treated water, including ensuring a minimum level of residual disinfectant to inactivate (kill) harmful microbes. Common pathogens include bacteria and viruses that can cause gastrointestinal illnesses, including diarrhea and vomiting, as well as intestinal parasites such as Cryptosporidium and Giardia.
Controlling Pathogens
The disinfection process that comprises the final step in wastewater treatment is largely effective in reducing active pathogens. The EPA’s success in developing and enforcing water quality standards has lowered public exposure to microorganisms that cause infectious diseases. However, water can, from time to time, travel through disinfectant tanks (clearwells) too quickly. When this happens, the disinfection process is less effective and may produce water with contaminant levels that exceed levels set by the SDWA. If this happens, water utilities may be charged with regulatory violations and the local population may be at increased risk for illness.
One example is a drinking water regulation called the Total Coliform Rule (TCR) which sets specific limits for coliforms present in a public drinking water supply. The limits mandate that public water companies won’t detect any level of coliforms in more than 5% of the samples they take on a monthly basis. If that percentage is exceeded, the water company is required to report the violation to the state and the public. In some cases, after more specific testing, a “boil water” notice may be issued.
It’s important to recognize that the total coliform count is only a general marker for the sanitary condition of the water, since most coliform bacteria are not harmful. When TC is detected in samples beyond 5%, further tests are performed which include specific tests to detect fecal coliforms (E. coli). If any level of E. coli is detected, it represents a direct health risk and therefore requires rapid state and public notification.
The EPA is tightly focused on ensuring that waterborne pathogens are not present in our drinking water and dictates what types of disinfection processes and chemicals can be used. However, disinfectants can react with naturally occurring materials in the water which form byproducts which may cause cancer. No one wants the cure to be worse than the problem, so the EPA has established rules to limit public exposure to disinfectant byproducts (DBPRs). This adds significant complexity to the disinfection process and has required extensive modifications or complete replacement to many older clearwell tanks and disinfection systems.
