There is no real way to completely eliminate the air in the overflow using a standpipe....that would defeat the design and compromise the cintinuous siphon. By maxing out the return flow from your sump you can reduce it dramatically, if the water is moving fast enough it will draw less air in. However you run the risk of something obstructing the overflow pipe and leaving you with water on the floor. The same applies to restricting the overflow pipe with a valve, while it will help reduce air it has to stay in near perfect balance.