Bonjour Hélène As Gerard ...

Bonjour Hélène

As Gerard Van Gils says (he shows you a very useful graph), If you reach pH=9, you will remove Ni until you get about 3.5 mg/L in solution; if pH=10, then remaining Ni in solution is about 0.13 mg/L, and so on. 

You can add NaOH, Ca(OH)2, Mg(OH)2... tot get those pH, but if you don't want to use chemicals you can remove Ni (and other metals simultaneously) using OneWater system (www.onewatertech.com).