I think I understand your ...

I think I understand your problem.  If you want to increase the sedimenation rate you need to reduce the zeta potential of the particles so that they will agglomerate and settle.  A lamella can help, but they are expensive to build and will plug if not maintained.  I believe that with a minor addition of a cationic polymer you may achieve the same result without the pre-clarifier.  There are a number of clarification aides which can help you, including alum and lime in small doses.  These chemicals will also help precipitate the phosphorous in the effluent.