The lien of any unpaid assessment upon which bonds have been issued continues until all the bonds have been paid in full.

If for any reason any part of the principal or interest of the bonds remains unpaid after enforcement of the assessment as provided in this division, the board shall order an additional or supplemental assessment to be made as provided in this division sufficient to pay the unpaid principal and interest.

The additional or supplemental assessment shall be enforced and collected in the same manner as the original assessment.

Where bonds of the district have been authorized to be issued on the assessments, all unpaid assessments bear interest at the rate of 7 percent per year from the date of the bonds issued thereon until the bonds have been fully paid and discharged.

The interest due at any time on unpaid assessments may be called without calling any installment of the assessment.

The word installment as used in this chapter shall be construed as applying to interest as well as to principal, as the case may be.