If it shall be decided to do the said work, it shall be competent for the directors to order the rate percent agreed upon for the cost of said work, to be paid in annual installments, in such amounts as may be convenient for the accomplishment of said work; otherwise the whole amount of said assessment of the rate percent shall be payable immediately upon the approval by the landowner of the agreed amount of rate percent, and shall be a lien upon the lands assessed within the district until paid, and shall bear interest at the rate of not to exceed eight percent per annum until paid, and such interest may be collected and enforced as part of the assessment or annual installment; said assessment or annual installment shall be levied and collected in the same manner as provided in section 245.445.