Changing your aeration ...

Changing your aeration technology to a fine and ultrafine bubble generator will provide a significantly better performance in coagulation and flocculation and thus improve filtration performance.

However, I think your sand filter is failing if TDS is still high after tertiary treatment. I suspect that the filter bed has wormhole channeling due to the formation of biofilm. In this case, switching to Activated Filter Media would fix this problem because biofilm cannot form on the media bed. I have found this to be the most common reason why sand filters fail. I have experience in this process for food processing in Asia and Australia.

If you would like to know more, please email me at andrew@waterandoilsolutions.com.au