Public Utilities Code § 33009

All accrued interest and premiums received on the sale of bonds issued by the district pursuant to this chapter shall be placed in the fund to be used for the payment of principal of, and interest on, those bonds. The remainder of the proceeds received on the sale of the bonds shall be placed in the treasury to secure those bonds or for the purposes for which the debt was incurred.

When the purposes for which the debt was incurred have been accomplished, any money remaining shall be either (a) transferred to the fund to be used for the payment of principal of, and interest on, the bonds or (b) placed in a fund to be used for the purchase of those outstanding bonds of the district, from time to time, in the open market at the prices and in the manner, either at public or private sale or otherwise, that the board determines. Bonds so purchased shall be canceled immediately.