By nature of their action, cooling systems may suffer considerable levels of system contamination. This can be caused by suspended solids in the make-up water, the ‘scrubbing action’ of cooling towers, or by process leaks encouraging microbial activity.
With operators wanting to employ what they believe to be a ‘quick fix’, what we regularly see are operators focussing on just one aspect of the water treatment regime.
Little do they realise that this could in fact make the situation worse as is carefully pointed out by the Health & Safety Executive’s (HSE) own advice:
“An appropriate cooling water treatment programme must be capable of controlling not only legionella and other microbial activity, but also corrosion, scale formation and fouling, and include appropriate measures, such as regular physical cleaning and disinfection, to maintain the system’s cleanliness. This is very important since these aspects are often interrelated and failure to control one aspect will often lead to other problems and will increase the legionella risk.”
In reality, what we commonly see are cooling system installations that have focused upon compliance, with operators throwing extensive chemical treatments at the problem in the belief that this will ensure they do not fail microbiological tests.
Rather than considering the efficiency or indeed the implications of running with a heavily contaminated system, conventional chemical techniques employed may involve adding inhibitors to control corrosion and scale formation, biocides to control microbial growth and dispersants to control fouling.
Here lies the hidden and often ignored issues of poor energy transfer, blockages and system shutdowns that result from a poor-quality cooling water being circulated.
To help combat these, many operators feel safe having installed traditional technologies such as side-stream filtration, where a volume of recirculating cooling water is passed through a ‘side-stream’ loop to remove these contaminants from the water and in so doing reduce overall loading of solids.
Typically these are kinetic or centrifugal separators, or other technology such as self-cleaning screen filters, disc filters or even conventional media filters. As a standard practice such systems are then combined with chemical treatment.
By fitting a side stream filtration solution will of course dramatically improve the overall performance and allow the chemicals to work more effectively. However, we need to fully understand the implications of what lies within the cooling water before a correct side stream filtration solution can be implemented.
Generally, when you look at a cooling system you will find that 80-90% of the particles are less than 10 micron in size and it is these particles which lead to most of the problems.
In most systems the process cooling water is passed through a system of pipework and then if a cooling tower is present, it is passed over the cooling area of the tower, or through a chiller or heater.
Now, even in the best of systems, and most don’t fall into this category, the pipework will be plastic, steel, stainless or, as in many systems, a combination of all three. All of these materials have a single thing in common, their surfaces are not completely smooth. Larger particles, having a larger surface area, collect into a mass which is swept along through the system by the water flow.
However, looking though a microscope you will see surface inclusions and roughness and it is into these very locations where the smaller particles, typically less than 5-6 micron, affix themselves. What you now have are smaller particles lodged in the pipework and system cooling surfaces.
Some may well be killed off by chemicals but they still hold their position, often building up to give a smooth internal pipe surface. This is what we call biofilm, something with which any operator will familiar.
And it gets worse. Underneath this now smooth biofilm remains a rough surface area which is now colonised with live bacteria and it is this live bacteria where the problem lies. Not only are they protected from chemicals but the bacteria happily colonises due to being kept warm and fed on a continuous basis.
Where biofilm has formed you can expect major problems with plant and equipment as well as the stability of the cooling and there is increased risk of breaching present HSE laws.
The question is should we filter to less than 5 micron? Well, not necessarily.
This is where you need to get some meaningful water analysis done. It is quite common for operators to occasionally do a Total Suspended Solids (TSS) test which only tells you how much solid particulate you have in the water.
What you really need to understand is the volume of solids and the size of particles that are present. A Particle Size Distribution (PSD) will help you understand what volume of solids you have within the cooling water.
When you look at a PSD you will often see that a considerable proportion of the solid particles are very small and as low as 1 micron in size. However, it is not just particle size that matters but the volume of the particles.
For example, you may find your particle count for <5 micron particles is very high but this only represents a small fraction of the volume. The following graphs illustrate that where almost 95% of the particles are <10 micron in size this actually only accounts for less than 10% of the particles volume.
Effective side stream filtration should be based upon selecting the most appropriate technology for the application and ensuring that it can effectively remove the type of solids in the water.
Here is a short list to consider when choosing a side stream filtration and the options available:
- What is the nature of the particles? Organic or inorganic?
- How good a level of filtration is really needed?
- How much space is available?
- Is the site sensitive to water losses?
- Do you need uninterrupted filtration?
- What would be the cost of maintenance and the impact to operation?
Like most types of equipment there is a wide choice of technology from which to choose with each type having its own distinct pros and cons. The table below provides a guide on various filtration options available and how they perform in different applications.
|Filtration Technology||Size of particles removed||Footprint||OPEX Backwash / replacements||Applications|
|Traditional Sand Filters||20 micron||High||High||Particles to 20 micron, organic & inorganic|
|Automatic Screen Filters||10-800 micron||Low||Low||Wide range of applications good for non-deformable solids|
|Centrifugal Separators / Hydrocyclones||>100 micron||Low||Low||Removal of larger, heavy particles, requires constant operation|
|Automatic Disc Filters||20-400 micron||Medium||Low||Very good for organic particles|
|Consumables Bag / Cartridge Filters||Down to 0.1 micron||High||V. High||Limited to smaller flow rates, good for small or temporary installations|
|Automatic Disc Filters||Down to 1 micron||Medium||Medium||High filtration performance, low backwash compared to sand filters|
In conclusion, there needs to be a better understanding of the implications of a ‘quick fix’ approach to cooling tower and cooling water filtration. Operators may feel they have achieved biological control, but biofilm in the pipework, unless thoroughly treated, can suddenly appear and cause major problems not just to a process but more alarmingly to public health.
The best and most efficient side stream filtration system is the one that provides the best possible return on investment by improving plant efficiency, reducing both operational and maintenance costs and most importantly meeting HSE requirements.