You need to know the reasons ...

You need to know the reasons for the scum formation; operational and/or organisms, especially filamentous bacteria. Options: You can introduce aerobic selector before the aeration tanks to reduce the amount of oxygen entering into them so as to reduce the growth of filamentous bacteria or you can introduce Chlorine to kill off the bacteria (depending on your final product goal). You can also reduce the Sludge Retention Time (SRT).