When a city or a county or combination of cities or counties provides for a facility or facilities to recover materials or energy as a part of an approved solid waste management plan, any city, county or state agency may enter into a long-term contract to supply solid waste to the resource recovery facility or facilities; to construct, operate and maintain or construct or operate or maintain such facilities; to contract with a private entity for the construction, operation and maintenance of such facilities; to market materials or energy recovered from such facility or facilities; or to utilize such facility or facilities to conserve materials or energy by reducing the volume of solid waste. For the purpose of this section “long-term” shall mean a period of not less than 10 nor more than 30 years. All long-term contracts negotiated under this section shall be reviewed and approved by the attorney general before becoming effective.