Which is the best Wastewater Treatment technology?

Which new ​​technology is ​​the most ​​economic and ​​efficient for water treatment ​​plants according to you?  ​I am looking for some cost efficient ​​and ​​portable size,​​ easy to ​​operate​, long ​​shelf life technology or system.​​  ​ ​ Please give ​​your suggestions ​​and share your ​​experiences ​​with the existing ​​technologies in ​​your ​​treatment ​​plants. ​ ​