The Institutes may offer a course of at least one week’s duration which may be attended by sewage treatment operators or employees or public water supply operators or their employees, as the case may be, who have been newly appointed, and a refresher course of at least 2 days’ duration which may be attended by all other sewage treatment operators or employees or public water supply operators or their employees, as the case may be. Attendance at courses offered by the Institutes shall be restricted to sewage treatment operators and their employees or public water supply operators or their employees, as the case may be.