I can tell you that in an ...

I can tell you that in an experiment I did during my master thesis, we were using biological treatment with MBBR technology. An anaerobic reactor was followed by an aerobic one (simple PAOs bacteria technology) and as carbon source for the anaerobic we used the carbon in the wastewater. The removal was good, but from 19 mg/l we never went down to 1mg/l; if I remember well it was something around 40-50% removal. Therefore I think you can't achieve that goal ONLY biologically, or at least unless you have a very complex system (maybe lots of Anaerobic/Aerobic couples of reactors with external carbon source, but this solution is a simple and probably wrong idea I had now). And even if you do, I don't think you can grant it always, especially if unwanted nitrification happens in the aerobic reactor.