Updated September 19, 2022
The water in cooling towers can accumulate various contaminants over time. This can depend on the cycles of concentration, which is the amount of times you reuse the same water. The more times you reuse the water, the higher this will be as water will evaporate and the amount of mineral content in that water will increase as a result. If the level of minerals becomes high enough and reaches the saturation point of the water, scale will form, and it will cause various heat transfer and maintenance issues if not properly dealt with.
Thousands to millions of gallons of water are used annually during normal cooling tower operation. This number can increase or decrease based on the number of cycles of concentration being utilized. Higher cycles of concentration can be achieved with a properly run chemical water treatment program. This chemical water treatment program has tangible benefits from the chemicals used in the cooling tower water. For example, scale inhibitor would raise the saturation point of the water, allowing for more cycles of concentration before scale is a risk. These types of programs should pay for themselves and then some with the water, energy, and maintenance savings achieved with proper service. There are a few other methods like scale inhibitors that may save you money when you have to bleed your cooling tower water. These all rely on delaying the amount of cycles of concentration. Another one of these methods is using a water softener. Depending on the PH of your water, this may make a big difference. The PH of water is different from state to state. Softer water allows for more cycles of concentration as many of the minerals that cause problems are removed. It also helps alleviate problems such as corrosion and scale deposits. Additionally, acids also work as a way to reduce the PH of cooling tower water. Lastly, biocides are useful chemicals for controlling organic matter, bacteria, and filming that occur in tower water.
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The Necessity of Bleeding Tower Water
Although it is an ideal situation to keep cycles of concentration high and to use the water as much as possible, it eventually will have to be bled/blowdown. The water being bled is often toxic and has special regulations attached to it. It gets its toxicity from the bacteria that can accumulate in the reused water over time. There is a lot of heat transfer that occurs in a cooling tower, and often there are several ways organic debris can make its way into it. This causes an ideal environment for bacteria growth. The accumulation of bacteria can be maintained with a well-run biocide program, nonetheless, the discharge water still needs to be treated before being dumped in most jurisdictions. One of the ways to discharge this water is to dump it into a sanitary sewer. Sanitary sewers collect water that will eventually be treated and do not require pretreatment before dumping.
Unfortunately, many older and rural systems were designed to drain cooling tower water directly to drainage tile fields, collection ponds, ditches, creeks or other storm sewers. In an effort to protect the environment, the Environmental Protection Agency (EPA) enacted The Clean Water Act in 1987. The intention is to restrict and eventually eliminate the discharge of “pollutants” to storm sewers and other surface water sources.
The most acceptable means of discharging (i.e. bleeding) water from a cooling tower is to a sanitary sewer and onto a sewage treatment facility. So, what if there is no sewage treatment facility available? Under certain circumstances, a permit under a process called National Pollution Discharge Elimination System, or NPDES is required.
How Does the NPDES Process Work?
There are several factors considered in the NPDES permit approval. Most permits are issued on a case-by-case basis. Factors in the approval process include: location of the cooling tower, availability of sewage treatment, discharge water flow or volume, discharge temperature, receiving waterbody temperature, discharge pH, residual chlorine, other pollutants, waterbody location and seasonal/natural variation, the monitoring plan, etc.
As a resource for the application process, we highly recommend further information be obtained directly from the EPA at https://www.epa.gov/npdes.
For more information about our cooling tower services at Chardon Labs, as well as best practices in managing your water systems, please contact our Technical Support staff at 888-486-9263 or fill out our online contact form. We are always available to answer your questions and provide our expert recommendations!
Matt Welsh is the Vice President and Water Consultant at Chardon Labs. He helps consult a wide range of customers utilizing various methods of water treatment, from chemical to chemical-free approaches, large and small applications, and across a wide range of geographical influences. With 20 years of water treatment experience, including a wide range of troubleshooting and service in potable water and non-potable HVAC and industrial applications, he is an expert in water treatment chemistry for cooling towers, boilers, and closed-loop systems.