If any sidewalk adjacent to real property within the city shall not be cleared of obstructions or accumulations of snow, ice, mud and slush by the owner or occupant of said real property as required by this article, the city may cause the sidewalk to be cleared. The mayor and city council may levy a special assessment on such real property in an amount not greater than that sufficient to pay the costs and expenses of the removal of the obstruction or accumulation of snow, ice, mud or slush to the city. The special assessment shall be a lien on the real property from the date of levy, shall be due and payable to the city treasurer 30 days after such levy, and shall be collected as other real estate taxes and assessments are collected.