On or before the first Monday of September, or on or before the first Monday of May in each year where the council has decided to have the taxes levied and collected in 2 installments as authorized in section 2 of chapter 30, the board of public works shall submit to the common council careful estimates in detail of the amount of money, which, according to the judgment of the board, will be needed for the waterworks fund, the light fund and the sewer fund during the ensuing year, which estimates may be increased, modified or adopted by the said common council as in its judgment may seem justifiable.