Tap water in Belgium (and all the Benelux countries) is rigorously tested and totally safe. Beware, however -- restaurants in Belgium -- at least in Brussels and Bruges -- will tell you that they either don't serve tap water or don't HAVE tap water -- which leads to the question, are you then never washing your hands, dishes, etc.?
I have finally decided, by the way, to walk out of restaurants that won't serve me tap water with a smile when I request it -- which probably means I won't be dining out much here in Belgium. Bottled water is ecologically murderous and financially dumb in regions where the tap water is safe. I'm not unreasonable, though; if restaurants here were to serve filtered tap water (which does taste better than water straight from the tap) I'd be happy to pay 50 cents or so for a 500 ml glass; but please, between five and six euros for a litre of bottled water, that's completely sick.
AND if the water is really so unsafe (which it is not), don't gouge patrons for the bottled water -- charge a little more than it costs the restaurant, perhaps double, but not five or six times more than the cost of the water.