Can a Hindu marry second time on the basis of written consent of his first wife ?

A Hindu cannot marry again during the lifetime of his wife, whatever her consent. Section 5(i) of the Hindu Marriage Act, 1955 requires that "neither party has a spouse living at the time of the marriage"; a marriage in breach of it is void under section 11 and punishable as bigamy under section 17. The first wife's written consent, her age or her not bearing children makes no difference.