To remove small solid ...
Published by Drewes Zuur, Chemisch Technoloog
To remove small solid particle you can look into continuous sand filtration:
http://en.paques.nl/products/other/astrasand
For highly purification you can look into membrane technology like MBR. Grudfoss has aslos developed a system: http://www.grundfos.com/about-us/news-and-press/news/turning-domesticwastewaterintoirrigationwater.html
https://stateofgreen.com/files/download/121