Conflicting Challenges
A clearwell, also called a contact or disinfectant tank, is where the disinfection step takes place. It’s the last stage before potable water is distributed to the consumer. The clearwell is a large tank, with a volume as high as several millions of gallons. The tank is usually subdivided by a series of baffles which serve to control the path water travels through the tank. The goal is to encourage thorough mixing and to slow down incoming water so that it spends enough time in contact with the disinfectant before exiting. The minimum contact time is usually 30 minutes. Unfortunately, conventional clearwell configurations suffer from tendencies toward poor mixing and short-circuiting, and typically require a lot of energy to force water to travel in a different path.
Mixing and contact time are critical characteristics of a disinfection tank. It’s not enough to reach an average concentration of disinfectant to water, with variations within the sample. Microorganisms must be individually exposed to the disinfectant at high enough levels and for enough time so that each one is killed. No amount of water should be allowed to travel through the clearwell without meeting these minimums. If this happens, inadequately treated water will fail to meet mandated contaminant levels, triggering SDWA violations and posing a risk to the health of the community.
Still, it’s a matter of physics that flowing water seeks to travel the shortest path between two points. When incoming water travels directly from inlet to exit, it bypasses water trapped along the edges, creating areas of a tank with little or no flow known as dead zones. The still water in a dead zone doesn’t mix, so it may have disinfectant concentrations that are too high or too low and is likely to spend too much time in the clearwell. In contrast, the water that travels straight through spends too little time in the tank and fails to mix. This latter behavior is called short-circuiting.
One consideration further complicating the management of contact time in a disinfection tank is the need to limit the amount of time water spends in that same tank. When drinking water is exposed to certain concentrations of chlorine for too long, it reacts with naturally occurring molecules such as total organic carbon (TOC) and may form disinfection byproducts (DBP) known to cause cancer.
This means that water treatment facilities must manage operations so that water is both thoroughly and uniformly disinfected, without permitting the formation of harmful byproducts.
Choose Your Weapon
The U.S. Environmental Protection Agency (EPA) allows several methods for disinfecting potable water, including the use of several types of chlorine-based chemicals. Naturally, there is no single ideal chemical, and each type has its own set of advantages and disadvantages.
Free Chlorine
The most common disinfectant is chlorine, in the form of chlorine gas or sodium hypochlorite. Chlorine has a long history of use in treatment plants. It’s inexpensive, efficient, and a powerful oxidant, but it can interact with certain types of molecules that can persist after treatment or may be present in pipes along the distribution network. When chlorine interacts with these molecules, it can form carcinogens such as THMs (trihalomethanes), which are regulated by the EPA. Chlorine is also quickly consumed, and water that reflects acceptable concentrations as it exits the clearwell may be too dilute to kill germs it encounters further downline.
Chloramine
Chloramine is a compound that is formed when water containing ammonia is combined with chlorine. The most common type used in water treatment plants is monochloramine, which must be produced on-site at the water treatment plant. This requires additional equipment, and actively balancing the relative dosages of ammonia and chlorine adds complexity to the process.
Chloramine doesn’t evaporate as easily as chlorine and lasts longer in the distribution system. In fact, it’s commonly used as a secondary disinfectant to maintain residual levels during distribution. Initially, as EPA standards were established covering disinfection by-products (DBP) in drinking water, many utilities switched to chloramine as their chemical of choice, but other harmful DBPs such as HAAs (halo acetic acids) have been added to the regulatory standards more recently. HAAs are produced more prominently in chloramine systems and have been found to be even more carcinogenic than THMs.
Chloramine is cheaper than non-chlorine disinfection processes, but it is a weaker disinfectant overall than free chlorine and so it requires longer contact time to be effective. In situations with particularly difficult source water, that time could potentially extend to as much as several hours.
Disinfection Byproducts
Natural Organic Matter (NOM) is found in varying amounts in all surfaces, ground, and soil water. The amount has been rising over previous decades. Even fully treated potable water contains NOM. At the most basic level, it can create water quality issues like objectionable taste, color and odor, and the necessity of removing it can significantly affect water treatment processes.
In the disinfection stage, chlorine, chloramine and ozone all react with NOM and form hundreds of different disinfection byproducts (DBP). Many kinds of DBP are known to increase the risk of cancer and are linked to birth defects and miscarriage. Currently, the EPA regulates two classes of DBP: THMs (trihalomethanes) and HAAs (halo acetic acids). The actual types and amounts of DBP generated during the disinfection process are determined by a variety of factors, including the amount of NOM in the original source water, pH, temperature and the specific chemicals used.
During disinfection, water must remain in contact with disinfectant for a minimum length of time in order to be effective, but the longer TOC material is in contact with those disinfectants, the more DBP-producing chemical reactions occur.
The reduction and management of DBP is a significant concern for water treatment facilities. Fortunately, there are techniques to reduce the amount of DBP formed, even while maintaining safe levels of disinfection:
- Remove or reduce NOM during the treatment process. Coagulation and flocculation, followed by sedimentation and filtration are the cheapest and easiest processes to employ, although enhanced coagulation may be necessary when NOM levels are high.
- Improve mixing strategies in the clearwell stage to reduce overall exposure to disinfectants, both in time and concentration.
- Eliminate dead zones and ensure adequate turnover and mixing in clearwells
- Reduce “water age,” or the length of time water spends in the distribution system.
- Manage water temperatures; chemical reactions that form DBP occur faster in warmer water.
- Manage pH in the water; pH not only influences the effectiveness of the disinfectant, but it can also help determine the type of byproduct that forms.
