Except as provided in this section, no new septic tanks shall be permitted to be installed in the city after April 3, 1990. If a sewer main line of the city is not located within 150 feet of a parcel of property, the owner of the property may install a septic tank to service structures to be built upon the property until a sewer main line is installed within 150 feet of the property. Requests to install septic tanks shall be decided on a case-by-case basis. Installation of septic tanks shall be approved by the city only to relieve a hardship and shall not constitute a change of the city's comprehensive plan. Whenever a sewer main line is installed within 150 feet of a property serviced by septic tank, the owner of such property shall be required to discontinue use of the septic tanks and shall connect to the city sewer system within 60 days.