What Are the Systems Associated with Water Treatment and How Are They Treated? – Cooling Towers

A cooling tower is responsible for cooling the water used in processes such as heat exchangers in factories and air conditioning in commercial buildings. You may recognise them from power stations like this:

Cooling Towers

These are huge cooling towers and are unlikely to be found anywhere other than at a power station. Cooling towers will vary in size depending on the work required but may look a bit more like this:

Smaller Cooling Tower

There are a number of different cooling tower designs but the following diagram should give you a better idea of how a typical cooling tower works:

Diagram of a Cooling Tower

Correct water treatment is a high priority when maintaining a cooling tower on a site as they can release an aerosol (fine, airborne particles).  It’s in this aerosol that the Legionella can escape the water system and be inhaled by people in the area. These particles can travel significant distances from the tower. As you can imagine this means that cooling towers need to be carefully, designed, maintained and treated in line with strict regulations to ensure that this does not happen.

There are four major problem areas associated with cooling tower treatment – scale, corrosion, solids and microbiological fouling (including Legionnaires disease).  Cooling towers are treated by chemicals and other water conditioning processes to prevent these problems.

The water from a cooling tower should be regularly sampled and tested for Legionella bacteria to ensure the treatment regime being used is working effectively. Cooling towers should also be cleaned at least 6-monthly to remove fouling.

When talking about the operation of a cooling tower, common phrases such as “drift” – the water driven or drawn out of the tower with the main flow of air, and “windage” – the water driven out of the cooling tower due to wind. With windage the droplets are often quite large, making it wet on the outside of the tower. Losses due to both drift and windage can reduce the chemicals in the water, increasing the risk of a Legionella outbreak.

These losses must therefore be taken into consideration along with evaporative losses and controlled bleed (see below), which together account for the required total make-up of a system.

Most modern towers have a more enclosed design and specially designed air inlet louvres to reduce this.  In the diagram above you can clearly see drift eliminators.

As previously mentioned Legionella is not the only reason for chemical treatment in a cooling tower. Scale and corrosion are also factors that need to be taken into consideration in order to protect the cooling tower and ensure it is working efficiently. Cooling towers have a “purge” or “bleed” to remove some of the recirculating water to keep the risk of scale and corrosion at bay.

All aspects of water treatment should be considered and actioned together with a regular monitoring programme to ensure good, efficient, economic and safe operation.

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What Else Affects a Water System?

As you will see below ultimately most water treatment comes down to Legionella control in the end. The following are problems associated with water systems that need to be treated:

Biofilm
Biofilm is where bacteria sticks to a surface, causing a slimy layer of bacteria to form.

Corrosion
Corrosion is the effect of metal coming into contact with water or oxygen. This causes corrosion deposits, commonly known as rust. In severe cases this will cause holes in pipework.

Scaling
Scaling is caused by hardness in water. The hardness drops out and onto the metal – think of your kettle at home.

Fouling
Fouling is general debris. Microbiological fouling is the debris of bacteria or biofilm.

All of the above must be prevented as they reduce the effectiveness of a system by enabling biofilm to grow – not only does biofilm prevent heat transfer, it also hides Legionella bacteria. There are various methods of preventing these from occurring which we will go into in future entries.

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Water Temperature Control

A high priority in any system is water temperature. As we have already covered, Legionella grows best in temperatures between 20 – 45°C. This means that maintaining outlet temperatures outside of this bracket is vital. For this reason ACoP L8 recommends that hot water should be stored at 60°C and should reach a temperature of 50°C after running for 1 minute from an outlet. This recommendation does not apply if biocides are in use in a water system (more information on biocides will follow), however the HTM 04- 01 suggests that temperature management should be a priority even when biocides are being used and we would have to agree that having as many preventative measures as possible is always the best option. For hot water, the risk of scalding must also be taken into consideration and 50°C is considered the safest temperature. For cold water outlets the temperature should be below 20°C after running the water for 2 minutes.

Certain sites may have outlets that are used infrequently such as showers or taps and we would recommend that these are removed. However there are procedures in place for sites where, for whatever reason, these outlets are still in place. As well as monitoring the temperature of these showers and taps they must also be flushed to reduce the risk of Legionella growing in peripheral parts of the system. ACoP L8 recommends weekly flushing of these outlets and HTM 04- 01 suggests that flushing twice a week is the best way to manage the risk of such outlets.

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What is Legionella?

Legionella is a bacteria that grows in water. It was first identified after an outbreak at an American Legion Convention in Philadelphia in 1976. Legionnaires’ disease is usually contracted by inhaling the Legionella bacteria in tiny droplets of water.

Legionnaires’ Disease is a form of pneumonia and fatal in approximately 12% of the 200-250 cases reported in the UK every year. Certain groups are more susceptible to contracting Legionnaires’ disease, for example men, those over 45 years old, smokers, alcoholics, diabetics and those with cancer or chronic respiratory or kidney disease.

Legionella appear to grow best in water between 20 – 45°C. For this reason it is vital that systems which have the potential to grow Legionella are closely monitored and treated. Unfortunately these systems are extremely common and present in most companies – including hotels and hospitals.

Cooling towers pose a significant risk and, as a result, every company with a cooling tower must register with the local council. This is due to the fact that cooling towers process water at a temperature within the 20 – 45°C bracket and produce an aerosol. There have been many Legionella outbreaks in the past which have been traced back to a cooling tower (one of the most well-known cases was the Barrow incident, which you can read more about here). More information on exactly what a cooling tower is and how to treat them will follow.

Hot and cold domestic water systems must also be closely monitored and treated. Taps, showers, water tanks and other hot or cold water outlets in any company are at risk if not correctly maintained. For example, showers consistently run within the 20 – 45°C bracket and water is left in the pipework to the shower and the showerhead itself may be left to stagnate. This presents a Legionella risk and if the shower is not used for a period of time they can be a significant threat. More information on treating hot and cold water systems will follow.

Legionella risk assessments must be conducted under ACoP L8, the guidelines for employers on Legionella and Legionnaires’ disease. These must be carried out on any open evaporative cooling system and any domestic system.

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What is a Legionella Risk Assessment and Why is it Important?


A Legionella risk assessment the first step in Legionella control on a site and is exactly that – an assessment of the Legionella risks associated with a water system. It is an important part of water treatment practise as many legionella outbreaks have been the result of poor water system management. A risk assessment also outlines protocol which should reduce said risks. Having a risk assessment also helps firms ensure that they are compliant with ACoP L8 standards.


A risk assessment should include the following:

  • The Inspection of all tanks to ensure they are compliant with all relevant legislation
  • The Inspection of water heaters and calorifiers to ensure they comply with all relevant legislation
  • The Inspection of water distribution systems to ensure they comply with all relevant legislation
  • Temperatures to be taken from around the site to ensure they are not at a level to promote the growth of bacteria
  • The production of an Asset register
  • Digital photographs of the main pieces of the plants water system
  • Schematics of all main pieces of a plant’s system
  • A Responsibility Structure so that all involved know the level of responsibility they have for the up keep and maintenance of the water system including chemical treatment
  • Recommendations that include the remedial work that is required to bring the system within the guidelines as set out in the ACOP L8 document
  • Process waters can be included if requested
  • A Log Book should be produced for regular monitoring and as a record.

The above would produce recommendations for a site that would ensure they comply with all relevant legislation including the Approved Code of Practice L8 & the Water Supply Regulations.

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Introduction

Every company in the world has water running through the building – whether this is for the radiators (known to us as a closed system), showers and taps (known to us as a domestic system), boilers or cooling towers (don’t worry – more on all of these to come!). Where there is water there is the potential for scale (think about your kettle at home), corrosion of metal pipework and, put simply, bugs – with Legionella being potentially the most problematic.

The potential for growth of Legionella bacteria is the main reason that every company in the UK has the responsibility to carry out water treatment in some form. Legionella is a bacteria that grows best in water with temperatures between 20 – 45°C. Legionella is a rare but dangerous bacteria and can lead to Legionnaires Disease – a sometimes fatal form of pneumonia (to read more about Legionella and Legionnaires Disease visit the ‘Legionella’ tab at the top of the page).

Aside from protecting employees and members of the public from Legionnaires disease, applying water treatment processes to a site can be extremely beneficial and can save a lot of time and money associated with equipment down time and replacement. For example, if water in a heating system runs through pipes in a ceiling with no chemicals added to prevent corrosion, the water will eventually corrode through the pipework. Not only is that pipework going to be expensive to replace, it is also another hazard for employees or members of the public – if the water in question is hot (going from a boiler, for example) there could be some sore heads on site!

What we’ve just described to you is a very basic explanation of why water treatment is vital to a company. There are many more reasons and plenty more terms to be explained – so keep visiting to learn more! Feel free to comment with any questions you may have, we’ll be happy to answer them.

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