All special assessments provided for by sections 39-1601 to 39-1636 shall become due fifty days after date of levy and may be paid within that time without interest, but if not so paid shall bear interest as provided in section 39-1615. Such assessments shall become delinquent as stated in section 39-1615, or otherwise in equal annual installments over such period of years as the board of trustees may determine at the time of making the levy.