I have sand flowing back into my pool. Is this coming from the filter and how has this happened?

Maybe! It is a common mistake for people to think brown algae on the pool floor is actually sand. If it is sand from the filter it will only be in one place in the pool and that is just below or slightly in front of the pool inlets (returns). This is because sand is heavy and it will sink to the bottom of the pool. When you brush sand it moves but doesn’t float. There will also be sand in the multiport valve making it hard to turn - you will feel it and hear it grinding.

Brown algae can be spread out anywhere in the pool and may hug the seams of the liner. When brushed it will float into a brown cloud.

The only causes for sand to enter the pool via the filter are either the laterals in the pool base or the internal pipework are damaged. Sand may also be in the pool from an external source such as building or landscaping.