Any person, firm or corporation that purchases, for installation in the Virgin Islands, or which constructs in the Virgin Islands a reverse osmosis water production plant shall, upon certification by the Commissioner of Public Works, be exempt from the payment of 50 percent of excise taxes on the purchase of the plant or on materials for the construction or installation of such plant and from 50 percent of the gross receipts taxes imposed by the Government of the United States Virgin Islands on such person, firm or corporation for a period of eight years or until the total exemptions equal to 50 percent of the cost of purchasing, constructing or installing such plant; Provided, however, That a person, firm or corporation which receives an excise or gross receipts exemption under any other provision of law shall not be eligible for an exemption under this section.