A method for treating metal-working fluid comprises reducing a concentration in the metal-working fluid of at least one metal by passing the metal-working fluid through an ion-exchange filter. The at least one metal may comprise cobalt or one or more other metal(s). The ion-exchange filter may include an ion-exchange resin, and the resin may comprise a sulfonated divinyl-benzene-cross-linked polystyrene ion-exchange resin. The metal-working fluid may be recirculated through the ion-exchange filter, from a holding vessel, from a sump or reservoir, or from any other container of the metal-working fluid.