Whenever any local improvement provided in any ordinance passed by virtue of this Division 2 consists of a system of waterworks or a bridge or viaduct, or extension of water mains which are a part of any municipal waterworks system, any portion of the cost of which is to be defrayed by special assessment, it may be provided by the ordinance for the improvement or by ordinance passed at any time before the confirmation of the assessment roll, that the aggregate amount assessed and each individual assessment, and also the assessment against the municipality for public benefits and on account of property owned by it, may be divided into not exceeding 30 installments in the manner provided in Section 9-2-48. The provisions of this Section 9-2-19 shall not apply to any city having a population of 500,000 or more.