Heavy industry uses of any kind not in operation on June 28, 1971, are prohibited in the coastal zone and no permits may be issued therefor. In addition, offshore gas, liquid or solid bulk product transfer facilities which are not in operation on June 28, 1971, are prohibited in the coastal zone, and no permit may be issued therefor. Provided, that this section shall not apply to public sewage treatment or recycling plants. A basic steel manufacturing plant in operation on June 28, 1971, may continue as a heavy industry use in the coastal zone notwithstanding any temporary discontinuance of operations after said date, provided that said discontinuance does not exceed 2 years. An incinerator is neither “public sewage treatment” nor a “recycling plant” for the purpose of this chapter.