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Swimming Pool Water Quality Control

Why it Requires Both Filtration and Sanitation to maintain the quality of swimming pool water

There is a lot of confusion surrounding Swimming Pool Water Quality and how it is maintained and this article sets out to reduce the level of confusion that seems to apply equally to both experienced and inexperienced  of private poolsowners.

In some cases it has even proven to apply in publicly owned and operated pools where a new pool has been installed and handed over to inexperienced staff to operate. 

What is the purpose of the filtration system?

The Swimming Pool Water Filtration Systems are designed to remove small particles from the water, but not the larger elements such as organic soil, dust, leaves, dead frogs that are easily visible. These and everything else that seems to accumulate in a swimming pool should be removed by a net and the vacuum point cleaning system that should be provided in all properly designed pools.  The purpose of the Swimming Pool Water Filtration system is to remove small particles that for the most part are not visible to the naked eye but that can cause the pool water to look cloudy and opaque. These particles are mainly of organic origin and will include hair, skin, faeces and dissolved materials including urea, fats and proteins.  These elements make the water look dirty but more importantly they are the food source that allows bacteria to grow  

What Filtration does

In a conventional pool filtration should:- 

  • Extract about 70% of the water in the pool from the surface of the pool water via small weir devices called skimmers
  • Extract the remaining 30% of the water in the pool from the deepest part of the pool through a grill covered opening in the pool floor that is normally called the main drain.
  • Pump the water through a strainer that will remove the rest of the large elements missed by the pool cleaning process
  • Pump the water at a pressure through a filter unit that can use sand, Zeolite (see other website article on the use of Zeolite in pool water filtration) or cartridge type filter mediums.
  • Return the water to the pool by pumped return lines that discharge the water from several points 400 or 500 mm below the water surface and at the other side of the pool opposite the skimmers
  • Complete theoretical filtration of the water system of a privately owned pool in a period of about 6 hours (pool water capacity divided by the theoretical pump capacity in cubic metres per hour)

Periodically the sand filter must be backwashed, or the cartridge changed to remove the filtered out waste that reduces the efficiency of the filtration unit. 

There is always bacteria in the filter

However it is impossible to stop bacteria from growing in the sand filter because chlorination only works when the bacteria are free in the water. The bacteria in the sand filter excrete alginate that binds the sand together. This requires periodic "backwashing" by reversing the water flow direction through the filter. Eventually  the sand will need to be replaced following filter performance degradation and this must be carried out periodically to ensure that the filtration system continues to work.

Cartridge systems do not suffer from this drawback because of the periodic renewal of the cartridge - but these are always very costly for a large pool.

Water Sanitation by Chlorine injection etc  should be carried out concurrently with the filtration, using oxidation by chlorine or bromine to kill off the bacteria such as Legionella, Salmonella, Leptospira, Streptococcus, viruses like Hepatitis and Cysts like Cryptosporidium that are so dangerous for human health. 

The important thing to remember is that the chlorine will only do its disinfecting job properly if the filtration system is working properly.  

Public pools that are often full of children need much higher filtration capacity. How many times do children get ear infections? I just wonder how much of this is caused by ineffective Swimming Pool Water Treatment? 
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