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Biological Contamination in Cooling Systems

Yonatan Pe'er, B.A. // 09 Sep. 2020
Marcom Manager, Marketing Department

Anyone who’s ever passed by the general vicinity of a cooling tower will tell you – they’re hard to miss. You can also find smaller towers used in micro cooling towers, often installed on rooftops, that are used to cool smaller industrial buildings, such as data centers. But whether it’s gigantic towers that cool petrochemical plants and nuclear power station or their smaller siblings, keeping them running smoothly is no simple feat.

Cooling towers are used to remove heat from a part of a process (such as electricity production, refineries, etc.) This is accomplished through various methods that rely mainly on water for that purpose, as it’s a very effective medium for heat transfer. This process, however, usually adds a considerable quantity of contaminants to the water.

If left untreated, the contaminants will provide microbes with a comfortable substrate, and if allowed to develop, removal of microbial colonies can prove very difficult. These colonies reduce the heat transfer capability of the system, and so their formation needs to be prevented.

An effective way to prevent such occurrences is by eliminating the microbes’ food source – the contaminants in the water. This will effectively boost the performance of the cooling system.

If you think that the issue is not serious, know that even a biofilm that is thinner than 150 microns can reduce a system’s efficiency significantly. If left unchecked, it can very quickly grow into a serious problem, reducing pipes’ diameter or even completely blocking them. This, of course, means you need to either acid-flush the system (expensive) or replace it entirely (even more expensive). Some bacteria can also pose serious health risks for humans, particularly Legionella pneumophilia, that causes Legionellosis, colloquially known as Legionnaires’ Disease.

Prevention Through Treatment and Filtration

To prevent harmful bacteria from forming, industry regulations dictate that such cooling systems are to be chemically treated and monitored regularly, focusing on four key elements: scale, corrosion, biological fouling and suspended solids.

The first 3 elements can be kept under control with regular water treatment, but the last element – suspended solids, is harder to control. The solids can be carried into the system by the wind, corrosion of the pipes, contaminants created by production, changes in the raw water make-up, and more.

As most operators prefer to find a quick and easy fix, they tend to focus on a single aspect of water treatment but neglect others. It’s not uncommon to find system that are regularly chemically treated yet are still contaminated with an abundance of solids in the water.

The results can be blockages, poor energy transfer, and even full system shutdowns, incurring high costs for repairs or replacements.

Biofilm Formation

In most systems, cooling is achieved by running water through a system of pipework running through a cooling tower, a chiller, etc. The pipework is comprised of either metal, plastic, or stainless steel (or a combination of all 3). Around 80-90% of the particles in a cooling system are <10 micron in size. These particles lead to most of the problems in these systems.

The materials of the pipework have microscopically rough surfaces, allowing small particles (<5 micron) to attach themselves to the surfaces of the pipes. These particles (if biological) might be killed off by the chemical wash, but would stay physically attached to the surfaces, creating a biofilm over time. This biofilm works as a protective barrier, allowing bacteria to survive underneath it, and can later become a serious hazard for both the cooling process, as well as humans.

An effective and reliable method of preventing this from happening is through water filtration, which removes these elements even before they’re introduced into the system.

Water Filtration

There are 3 types of water filtration methods in cooling towers: side stream, full stream, and make-up.

In side stream filtration a volume of recirculating cooling water is passed through a water loop to reduce the overall loading of solids. The loop passes through a filtration system, be it self-cleaning screen filters, disc filters, or media filters. Generally, these systems filter up to around 20-100 micron.

In full stream filtration, a filter is installed after the cooling tower on the discharge side of the pump. This filter continuously filters all of the recirculating system water in the system. System filtration is generally 80-300 micron.

The make-up filtration process is meant to refill the water reservoir used for cooling, as water is lost during the process due to what’s called “evaporation blowdown”. Natural evaporation leads to higher concentration of dissolved solid content in the water (this can cause corrosive conditions and scaling). Draining the dissolved content is known as “blowdown”. The loss off water requires the user to make-up the missing water by introducing new water from an external source. The filter would be placed between the water source and the cooling tower, reducing the amount of solids in the water and increasing the number of cycles between blowdowns.

Sadly, most of the small-to-medium cooling systems won’t include a filtration system in advance, and will only install one or more after water fouling has already been established. However, before you install a new filter, you need to understand what level of filtration the system will need.

Choosing a Water Filtration System

Effective water filtration should be based on selecting the most appropriate technology for the application and making sure that it can effectively remove the type of solids in the water. There is a wide choice of technologies from which to choose, with each type having its own distinct pros and cons.

 Consideration should be given to how clean and efficient the cooling system needs to be – for example: an intermittent cooling requirement for a seasonal operator will have a completely different set of requirements from a site operating 24 hours a day all year-round and cannot be shut down.

Here are six main questions to consider when deciding on a filtration system for your cooling tower:

  • What is the nature of the particles? Organic or inorganic?
  • What level of filtration is really needed?
  • How much space is available? Would a filter which requires a smaller footprint be effective with the type of solids particles?
  • Is the site sensitive to water losses?
  • Do you need uninterrupted filtration?
  • What would be the cost of maintenance and the impact on operation?

Conclusion

Generally-speaking, deploying a “quick fix” can have serious implications on your system. While operators think they have solved the biological contamination, a lack of proper filtration means the eventual creation of biofilm and the potential for biohazards. The most effective prevention method for such occurrences is deploying a water filtration system that keep your cooler running efficiently, and health-code compliant.

For the full article by John Wilson, sales manager UK & Ireland of Amiad Water Systems UK, please click HERE.

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