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How to Manage Bacteria, Corrosion and Scaling in Closed Loop Water Systems

What can be done in closed loop systems to increase the system reliability and reduce the cost of water and treatment chemicals


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How to Manage Bacteria, Corrosion and Scaling in Closed Loop Water Systems

When we think about heat removal in semiconductors sites, we usually think first of big cooling towers that remove the heat from the fab process and facilities. While cooling towers are a significant water consumer and a critical system, there are more systems that are responsible for “carrying” the heat away from the Fab and facilities to the cooling towers. These are the closed loop systems, characterized by relatively stable chemistry composition resulting from minimal water loss.

Examples of the closed loops are: 

  • Process Cooling Water (PCW) – Removes heat from process equipment 

  • Chilled Water – Receives heat load from PCW and delivers it to the chillers 

  • Boiler Hot Water – Used primarily for heating Ultrapure Water 

  • Clean Dry Air (CDA) Cooling Loops – Responsible for maintaining a constant temperature within CDA compressors 

To understand the importance of these systems, imagine that you are a shipping logistics officer who needs to ship thousands of containers from one place to another. You ordered the biggest container ship that you could find and verified that it will be able to carry all the containers. You verify that the ship will arrive to your port on time, and you even added to the schedule the right amount of time needed to load this number of containers. But when you thought that all is in order and the ship has arrived in the port, you realize that the crane you have is not big enough or you don’t have enough cranes to load the ship. This crane’s job is exactly the same as the close loop water system’s role in any fab. You can design and operate your cooling towers in the best way, you can have the most advanced manufacturing technology, but if your system is unable to transfer the heat from the process equipment to cooling towers, none of this will matter. 

How are Closed Loops Different from the Open Loops?

Even though both open and closed loops are water recirculating systems, there are fundamental differences between the two, making each type of system unique and different from each other. Some notable differences with closed loops include:

  • No/Small Blowdown – As the name implies, a “closed loop” should have no incoming or discharge flow. This mode of operation provides an opportunity for maintaining a stable chemistry profile within the system. Although a stable chemistry profile is not specific to closed loops and can be maintained in the open loops, lack of water exchange in closed loops means that contaminating chemistries and system debris will be retained. 

  • Materials of Construction – In many cases a closed loop will have a larger number of materials of construction than open loops. This is driven by the fact that often hundreds of manufacturing tools require cooling, and each will have their own choices of materials exposed to PCW, for better cooling and other properties. This will impact the choice of the chemical treatment program to address the risk of galvanic corrosion. 

  • Sensitivity to External Contamination – A good practice for closed loop management is to avoid or minimize loss of water. As a result, any contamination that enters the system will remain in the system. Similarly, the byproducts of corrosion will accumulate in the system as well. System contamination will eventually be either filtered by side stream filters or discharged out of the system during “feed-and-bleed”, often used to troubleshoot contamination problems. 

What Do Good Operational Practices Look Like?

operational practices lifecycle

Make-Up Water Quality

  • Make-up water (typically provided during start-up or when water loss occur) should have minimal-to-zero impurities that could lead to scaling, biofouling, or enhanced corrosion.

Corrosion Management

What factors can influence corrosion rate within a closed loop?

  • Biological fouling and scaling that may lead to under-deposit or bio-induced corrosion

  • Dissimilar materials causing galvanic corrosion

  • Water quality incompatible with the material of construction resulting in pitting corrosion or other types of material deterioration

All of these factors can be managed by choosing the right quality of the make-up water, keeping your system clean from external contamination, having the appropriate corrosion control program, and having the correct biological control program.

Scaling Management

Scaling will happen when the concentration of specific salts exceeds their solubility limits under conditions of the system operation (i.e., pH and temperature). Since there is no evaporation, the concentration of incoming salts will remain constant. Therefore, so long as conditions within the loop don’t change, the risk of scaling is small. In cases where the make-up water contains hardness higher than trace levels (> 5 ppm as CaCO3), proactive risk analysis could help to prevent or at least mitigate the issue.

Biological Control

Biofouling can result in significant problems within a closed loop. While biological treatment can be a challenge in any water system, it is particularly problematic for the closed loops and especially for the critical PCW system. The following image is used to illustrate the challenge, understand the likely cause, and support decisions on how to address the issue.

biological control cycle in closed loop

Typically, the troubleshooting begins when the level of bacteria exceeds a certain target point. In some cases, the target itself can be an issue. For example, if the target is zero or very low, the Owner will extensively use non-oxidizing biocides to keep the level close to the target. As a result, the biocide and its byproducts will accumulate in the system, while bacteria get acclimated to the initially toxic compound. The new, more resilient type of organism will acclimate and grow in the system using the accumulated organics, posing a risk of clogging and corrosion over longer periods of time. Altering biocides levels may initially help, but will eventually lead to the same result, or even worse, as high concentration of the biocides (depending on the type), may have their own direct negative effect to the system and its materials.

Another typical cause for the vicious cycle described in the diagram is due to contamination coming from the external sources. The most common among them is the preservative used for new tool delivery or in site connection. Unless the preservative is thoroughly rinsed out of the tool, it will be washed into the PCW system after its hook-up. One tool and a relatively insignificant amount of contamination can trigger major long-term issues in the system, potentially leading to significant expenditure and eventually to a factory down time.

What can be done to break this cycle? What is the secret of Bacteria Control?  


  • Use the right make-up water and make sure any organic compound is kept to a minimum

  • Chemical treatment – use the minimum amount of appropriate chemicals, carefully chosen for the system conditions and materials of construction

  • Tool hookup – have the right procedure for thorough rinsing of new tools before hooking them up to any closed loops

  • Avoid the use of biocides. Biocides should be used as your last resort when troubleshooting requires some time to coordinate and conduct feed/bleed to recover the water quality in the system

It All Sounds Simple, So Why There Are So Many Problems?

Closed loop management may sound easy, but the number of variables make it difficult to control. Additionally, the complexity of the problem-solving limits the ability of the system owner to manage the stability of the system parameters. It is imperative that a strong integrated methodology is implemented throughout the facility to maintain proactive quality management.

If you are looking for additional information about best practices for close loop operation, please contact us.

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