Cities coming under the provisions of this act are hereby authorized to elect a waterworks board having the powers and duties hereinafter set forth upon the following terms and conditions: The question of electing such waterworks board shall be submitted to a vote of the electorate of such city upon the filing with the city clerk of a petition signed by at least fifteen percent of the qualified electors of such city as shown by the total number of votes cast for all candidates for mayor at the last preceding election. Upon the filing of such petition, it shall be the duty of the mayor within ten days thereafter to issue a proclamation calling a special election for the submission of such question. Such election shall be held within thirty days of the date of the proclamation: Provided, That if a general city election is to be held in not less than thirty nor more than ninety days thereafter, such question shall be submitted at such general election.