Designing a lamella ...

Designing a lamella clarifier requires many calculations that would start with chemical precipitation chemistry for the specific wastewater. For instance if you want to remove Cu, a simple pH adjustment to 8.5 will do the job, if the Cu is not chelated. However, if your wastewater contains Cr+6, then you need to reduce that to Cr+3 by adding Na2S2O5.  Otherword, you need to create insoluble precipitates, followed by either natural or synthetic organic polyelectrolytes, which would adsorb onto the particulate surface, to accelerate particle aggregation. Depending upon your wastewater characteristics, the chemistry part of the design may also require other coagulants like PAC or FeCl3. 

Then you need to do particles dynamics and separation design calculations. This would give you inlet, outlet and settling zone parameters. Then under the settling zone you need to design the discreet particle, flocculant, hindered and compression settling zones, which would include inclined plates as a minimum. 

Therefore, to design a lamella clarifier is not something that can be done on a short message. The better way is to just purchase one as we have gone through number of modifications to get our clarifiers to perform properly (http://www.solutionsrecovery.com/wastewater-treatment-products/chemical-precipitation-water-treatment/).