I hate to be the one to tell ...

I hate to be the one to tell you. But you actually have 2 problems.  First you are using sand and mechanical filters. You may be partially cleaning the water but the residue will be in concentrate and be dumped somewhere.  Searching for a process  that augments a failed way of doing things makes no sense. Most of which you speak was 1950,s technology. Since  the late 70's microbial cleaning of waste and waste water is the preferred method and at 1% of the old technology's cost.  The major benefits are:  all waste is reduced into its elemental/nutritional form, there is zero residue to flush into rivers to the ocean, 40 years of monthly testing's had no pathogens, chlorine is no longer needed, all heavy metals are chelated into their elemental form. Lastly and most important the cleaned water is potable.  Not the answer you were looking for. But best solutions need to be at the top of your options list.  G'day mate.