Toxins and Reused Water Quality

We are starting to look into toxins that compromise water quality.
Looking at a decentralized potable water reuse system – we know the list of contaminants is long and growing, so we would like to start with the must-be-addressed

    Input: what toxins must be treated? Output: what are byproducts that must be addressed?
In addition, if you are aware of any relevant guidelines and research - please do share links with us. Thank you!