What special provision must be included for a sewage y-valve when the discharge seacock is not lockable?

In situations where a sewage y-valve's discharge seacock is not lockable, it is crucial for the safety and compliance of the system that the handle of the valve be lockable specifically in the direction of the holding tank. This provision serves to prevent accidental discharge of sewage overboard, which could happen if the valve were to be mistakenly opened while the vessel is underway or in a sensitive area. By ensuring that the valve can only be locked in the position that directs waste to the holding tank, it mitigates the risk of environmental contamination and enhances adherence to regulations regarding sewage discharge.

The other options, while they may contribute to overall safety or functionality in certain contexts, do not address the essential operational requirement of securing the discharge route when the seacock is not capable of being locked.