I think that the value given by Pierlorenzo is a bit high. What you will want is to assure a minimum water velocity of 0.3 m/s to keep the MLSS in suspension, and not perfect mixing - this will draw to much Oxygen into the tank.

More generally for you will need approx 7.5 w /m3 tank volume if you tank is not optimised this give  in you case 600 w equal 0.8 HP so with Motor loss the motor should be1.14 HP and the nearest standard size would be 1.5 HP, so to me you 1.75 sufficient.

Dear Kim;

Thank you for your useful response. Could you send me literature cited these values for design parameters.

Hello Amin,

The hp requirement will depend a lot on the style of mixer you use. Some have small diameter blades operating at high rpm, other have larger blades running at low rpm.  I have found that the various manufactuers of mixers have a lot of expertise on applying their products. You may be able to get some technical help from one of them. For example, Landia folks have been helpful to me in the past.

 http://www.landiaworld.com/

Best of luck to you!

Brian