(a) Construction. Construction material for aeration basins and clarifiers must be resistant to the effects of a corrosive wastewater environment.(1) Aeration basins and clarifiers must not be buoyant when empty.(2) Structures using a common wall must be designed to accommodate the stresses generated when one basin is full and an adjacent basin is empty.(3) Aeration basin and clarifier walls must be watertight.(b) Freeboard.(1) An aeration basin must have a minimum freeboard of 18 inches at the peak flow.(2) A clarifier must have a minimum freeboard of 12 inches at the peak flow.(c) Redundancy and Flow Control.(1) A wastewater treatment facility with a design flow equal to or greater than 0.4 million gallons per day must have a minimum of two aeration basins and two clarifiers. Aeration basins are exempt from this requirement if the aeration equipment, including the diffusers, is removable without taking the aeration basin out of service.(2) Internal and interconnecting pipes must be capable of hydraulically handling the peak flow without overflow while either the largest clarifier or the largest aeration basin is out of service.(3) Each aeration basin and clarifier must have gates or valves to allow it to be hydraulically isolated.(4) Each aeration basin and clarifier must have a dedicated means for draining.