I do not know what state you are in . Here in California it is a billion dollar industry to treat grey water.
To me, the idea that we can foresee the future and predict what is not here yet, as far as diseases that have the potential to travel in the grey water, is ludicrous. That we then spread this contamination over our parks and playgrounds is a liability that will one day haunt us.
Now your question is, How can I treat this water to make it a healthy grey? You can not, and my prediction is that the billions being poured into this by the municipalities of the world will prove to be futile. The one disease that sneaks by the processing plant, will be one disease to many.
Good Luck,
Bill
The Local Plumber
Tustin, California http://www.TheLocalPlumber.com