At such time as the contract for said improvement is let, or in the event no contract is let, at such time as the improvement is completed, the county board shall determine the exact amount of each assessment and direct the county clerk to certify the same to the county treasurer. This certificate shall establish the due date for payment for said assessments. Said assessments shall bear interest after the due date at the rate of seven percent per annum and shall be a lien upon the real estate assessed against to the same extent as general taxes, prior to all other liens, and may be collected by sale of the same as for delinquent general taxes. Where the lot or land assessed belongs to any public or governmental corporation, no interest shall be charged until after a claim has been filed by the county treasurer with the appropriate payment agency of such public or governmental corporation. Where title to lands assessed is in the State of Nebraska, payment shall be made under provisions of Chapter 31, article 6.