A bike is 100% necessary in Antwerp, save yourself the hassle of finding someone you can buy a bike from (often in bad state) and then having to resell it with a loss at the end of your stay. Having experienced this particular problem on their own exchanges, three students from the University of Antwerp got together and came up with an excellent sollution: Uni-Cycle.
Uni-Cycle rents vintage bicycles which are both good looking and in an excellent state. They are the all time cheapest if you need a bike for just one semester and on top of that, you get an additional 10% discount with your ESNcard.
Their idea took an amazing start, all their bicycles were rented out previous semester so they expanded their stock. If you want a sturdy bike without much trouble, you better act fast.
If you want to reserve a bicycle or you have you more questions, check out their website: http://www.uni-cycle.eu/