In order to carry out the purposes for which the district is organized and created, the board of commissioners may borrow money at a rate of interest not exceeding six percent (6%) per annum and issue negotiable bonds therefor signed by the chairman or vice chairman and secretary when so authorized by the board of commissioners and may pledge all assessments of benefits for the repayment thereof. No bonds issued under the terms of this chapter shall run for more than thirty (30) years, and all issues of bonds may be divided so that a portion thereof may mature each year as the assessments are collected.