In addition to the above amounts, the council may raise by special assessment in sewer districts and special assessment districts, for the purpose of grading, paving, curbing, graveling and otherwise improving the streets and for constructing sewers and drains and making other local improvements chargeable upon the lands and property in the district according to frontage or benefits and for all other purposes for which the main sewer funds and special assessment funds are constituted, such sums as they shall deem necessary.