[1] He won two gold medals at the 2006 FINA Youth World Swimming Championships, and later represented Belgium at the 2008 Summer Olympics.

He achieved a FINA A-standard entry time of 49.09 (100 m freestyle) after finishing sixth from the European Championships in Eindhoven, Netherlands.

[9][10] Two days later, in the 50 m freestyle, Grandjean put up an astonishing swim with a fourth-place time and a Belgian record of 22.45 in heat nine, edging out Finland's Matti Rajakylä by a small fraction of a second.

His teammates Louis Croenen, Dieter Dekoninck, Pieter Timmers, and Glenn Surgeloose failed to reach the top 8 final, finishing only in twelfth place with a time of 7:14.44.

[13] At the 2012 European Short Course Swimming Championships in Chartres, France, Grandjean solidified his Belgian team (Timmers, Emmanuel Vanluchene, and Jasper Aerents) to take the bronze in the 4×50 m freestyle relay, posting their time of 1:25.60.