The filter should be looked upon as the dividing part of the whole system. As much as it is there to filter the water it is also there to divide the system between the low pressure side (the water after the filter) and the high pressure side (the water before the filter).
This separation of the low and high pressure is needed due to the fact that this is what determines the size of the filter the pump and the piping system. A good system should have no more than 120 kpa on the high side and no lower than 60 kpa on the low side. This is determined by the designer/pool builder. There are many different filter types, models, names and of course different claims for the efficiency and quality of each. This complicates the issue of what filter type to purchase in the first place. In reality the efficiency of the filter depends on how much water can be forced through as well as released from the filter in a given time and for a given task.
The quality of the filter in general is determined by the capacity of the filter to spread the water equally over the top of the filterbed and to wash out the accumulated dirt from the top efficiently (I.e. when water is reversed in its flow) This is commonly know as backwash of the filter. (This mostly applies to sand filters and sophisticated D.E filters) A lot of domestic D.E filters don’t have the backwash facility as they need cleaning and the medium changed on a regular basis. I am in favour of sand filters because of there simplicity and ease of use, plus no consumables. Basically D.E filters are archaic in Australia as sand filters are now capable of doing just as good a job.
I am surprised at the amount of misinformation about backwashing that I have encountered in Thailand. The main error is in the amount of backwashing most people do. It is just as bad for the system to do too many backwash’s as not to do enough. The gauge on top of the filter (If working) is your best indicator for when a backwash is needed. When the needle is near the red line a backwash is needed.
The process involves reversing the flow to clean out the dirt from the filter medium. The system is turned off, then the valve is turned to backwash (wait 30 sec.) then turn pump back on. Most systems have a sight glass near the control valve. This is where you monitor the waste water. It will become dirty for up to 3 minutes. When it becomes clear for about 10 seconds you have done a proper backwash. It is best to run it clear for about 30 seconds. Turn off the system and wait for 30 seconds, then turn to rinse and turn on for 1 minute. Turn off, turn to filter, wait 30 seconds then turn back on. The number of backwash’s in a given month has a lot of determining factors I.e. Bather load, weather especially wind etc. If you are only using the pool on a casual basis I would be surprised if you need more than one or two backwash’s per month, however this is not a guide.
Doing short backwash’s coupled with air in the filter is one of the main reasons for mud balling or channelling causing poor filtration and resulting in cloudy water. Basically you create channels in the filter-medium and the water takes the least path of resistance, so the water doesn’t get filtered properly. If you have bubbles coming into the pool you have air in the system. This will also be detrimental to proper filtration. Call your service man to sort it out.
With sand filters the medium (sand) needs to be checked on a yearly basis and typically needs changing around this time as the sand here in Thailand is of poor quality and is soft. Most people are changing to “Eco Clear” recycle glass media now as it does a better job and lasts up to 15 yrs.
Typical media filter