Read the article "Design and ...

Read the article "Design and management of conventional fluidized sand biofilters" by Steven T Summerfelt (available - just Google) where they note that for a sand with SG of 2.65, effective size of 0.1 to 1.0 mm (yours is 0.96 I assume), and a uniformity coefficient of 1.3 to 1.8, the pressure drop required across the sand bed to ensure fluidization of at least 20 to 30% is 0.87 to 0.98m for every meter of static sand depth.  So in your case the pressure drop across the bed will need to be approximately 1 to 1.1m, plus the total static head of 1.1m + 1.5m (assuming your overflow launders are at the maximum water depth of 1.5m).  Summerfelt's article gives good information to determine pressure drop in the underdrain system, etc.