Any member who obtains a minimum of 15 years of creditable service and who becomes totally and permanently disabled to the extent that he is unable to perform the duties of his employment shall be entitled to receive a disability retirement benefit equal to the full retirement benefit under subsection (b) of Code Section 47-4-101 which benefit shall be based on the number of years of service at the time he became disabled. The disability of any member applying for disability retirement benefits shall be determined by the board in the same manner and under the same procedure as disability of state employees is determined under the Employees' Retirement System of Georgia. Disability retirement benefits shall become payable within 30 days after such disability is determined by the board.