If you add this before your primary clarifier you will get some additional BOD and TSS removal, this is not without risk of precipitating too much phosphorus which is needed in biological treatment. Alum will also react with alkalinity in the system so you need to monitor the alkalinity and pH.

Polymer won't give you any problems provided it's dosed accordingly. These are commonly dosed into final clarifiers where the sludge settles out and the RAS is returned to the system. not a problem.  You will produce more sludge with alum than you would without it. It really just comes down to what's most efficient cost wise and compliance wise.

best regards,

Ryan