No new capacity for disposal of mixed municipal solid waste may be permitted in counties outside the metropolitan area without a certificate of need issued by the commissioner indicating the commissioner's determination that the additional disposal capacity is needed in the county. A certificate of need may not be issued until the county has a plan approved under section 115A.46. If the original plan was approved more than five years before, the commissioner may require the plan to be revised before a certificate of need is issued under this section. The commissioner shall certify need only to the extent that there are no feasible and prudent alternatives to the additional disposal capacity, including waste reduction, source separation, and resource recovery, that would minimize adverse impact upon natural resources. Alternatives that are speculative or conjectural are not feasible and prudent. Economic considerations alone do not justify the certification of need or the rejection of alternatives.