The filtration sand goes back into the pool

First, you will need to confirm that it is really filtration sand. It is multicoloured and you can hold it in your hands. In most cases, it is algae in the form of dust at the bottom of the pool.  If it is filtration sand, you will probably need to replace the filter lateral or place a service call.

https://www.trevi.com/en/tips-and-advice/technical-support-pool/filtration-sand-in-the-bottom-of-your-pool