Neb. Rev. Stat. § 16-123

Notwithstanding the powers granted by section 16-122 , no village or city of the second class may be annexed by a city of the first class when such village or city of the second class has its own sewage disposal plant, sewage disposal system, water well, water tower, water distribution system, and electrical distribution system or contracts for such services and facilities with an entity or entities other than such city of the first class.