Sand Filtration System: What it is and How it Works
In most swimming pools, both private and public, the most commonly used filtration system is sand filtration.
Why do everyone recommend using a sand filtration system in a swimming pool?
Danny, your pool expert, answers.
There are numerous reasons why a sand filtration system is highly recommended for swimming pools:
- good filtration quality
- easy maintenance
- long lifespan
- long interval between cleanings
- robustness and simplicity
- good value for money.
How to choose the right filtration system for your pool?
First of all, the filter must be proportionate to the pool, both in terms of filtering surface and filtration speed. A good recirculation system ensures that the entire volume of water in the pool is completely "recirculated" in 4/6 hours, meaning the entire volume of pool water must pass through the filtration system within this time period.
Another element that determines the values mentioned above is the circulation pump, which must also be calculated based on the volume of water in the pool.
To give a practical example, if our pool has a total capacity of 60 cubic meters, we will need a filtration system with a flow rate between 15 and 10 cubic meters/hour (a system that filters 15 cubic meters of water in one hour will filter all 60 cubic meters of pool water in 4 hours - a system that filters 10 cubic meters of water in one hour will filter all 60 cubic meters of pool water in 6 hours).
This means that both the pump and the filter must have this characteristic. Filter flow rates are calculated at a certain filtration speed; in the case of private pools, the value is in the order of 40-50 cubic meters/hour/sqm. Without going into too much technical detail, we can say that a lower flow rate improves filtration quality, while a higher one worsens it. From an economic point of view, we must also state that the lower the speed of a filter, the larger its volume, its size, and therefore its cost. For this reason, technically, an ideal compromise between costs and filtration quality is always sought. In public swimming pools, it is mandatory to refer to UNI standards, while in private ones, there are no predetermined rules, only the professional quality of the builder you have chosen.
In conclusion, the continuous search for a competitive price, by unscrupulous builders, often leads to the selection of undersized filtration components.
Initially, everything may seem to work properly, but after a few days, you will notice the difficulty of maintenance and the poor water quality. In general, we can state with good certainty that the data provided above are those that every builder should adhere to. To perform these checks, each sand filter should report the characteristics of speed and filtering capacity; knowing the size of the pool, calculate whether the system is capable of filtering all the water within a maximum of 6 hours.
How a Sand Filter for Pools Works
It is quite simple; usually, a sand filter consists of a cylindrical container, inside which sand is placed, either in a single layer (i.e., all of the same grain size) in private pools, or in multiple layers (i.e., layers of different grain sizes) or with variable grain size in public pools.
In filters commonly used in private pools, only one type of sand is used, with a grain size ranging from 0.4 to 0.8 mm.
The pump draws water from the pool and sends it to the filter, where it enters from the top. The water passes through the entire filtering layer and is collected at the bottom by "candles" or "plates" that convey it back into the pool through the return inlets.
During this passage, the water removes suspended particles, thereby cleaning itself. A pressure gauge is usually also provided on the filters, which will indicate the operating pressure; as this increases, it means that the filtering layer is getting dirty, and therefore the time for cleaning, or backwashing, is approaching. A multiport valve, located on top or next to the filter, allows for various operations such as backwashing, draining, rinsing, filtration, etc.
Maintenance of the Filtration System
In addition to regular backwashing, which is normally carried out every 10/15 days during the swimming season, it is also necessary to perform some more in-depth checks at the end and beginning of the season.
End-of-Season Checks
When we decide to close the pool for the season, it will be good practice to perform some checks after a thorough backwashing cycle. By opening the filter, we first check the condition of the sand; it should be loose, without large clumps caused by calcareous incrustations, which in some cases can even form actual "stones." If this is the case, we must remove the larger agglomerates and proceed to clean the sand by adding a good descaling product to the filter, such as our Sequat. We prepare a 30-40% solution and let it act overnight; the next day, we proceed with a thorough backwash. We also check the correct sand level and, if necessary, add any missing sand.
Once this is done, we can leave the filter to rest after emptying it of water to prevent winter damage caused by frost.
Beginning-of-Season Check
At the beginning of the following season, when starting up the pool, if we have not performed the previously described operation, we must do so now; otherwise, we can proceed with a thorough disinfection to eliminate any outbreaks of microorganisms. One of the most neglected maintenance tasks, which often causes serious problems during the swimming season, is precisely the disinfection of the filter.
How often should the filter sand be replaced?
If you have performed regular maintenance, carried out descaling procedures, and if backwashes have been performed regularly, the sand lasts a very long time, to the point that replacement can be done every 5-6 years or more, also to check the integrity of the collection candles. However, if the filter is systematically neglected, it may become necessary to replace it even after one year.
One thing we want to emphasize is that mechanical filtration performs almost 80% of a pool's purification work; the remaining 20% is done by chemicals. From this, it is easy to understand the enormous importance of good filtration, without which chemical treatments themselves become useless or ineffective.
For the filter sand to last even longer, the use of glass sand is highly recommended!