A threaded coupling for receptacles comprising a tubular duct adapted to connect the internal chamber of the receptacle to an intake/discharge pipe, the duct having, at one end, a threaded portion arranged outside the dome-shaped part of the receptacle. The coupling has an externally threaded tubular collar, fixed externally to the dome-shaped part, a tubular insert coaxial thereto with an end arranged inside the dome-shaped part, a flange for locking the internal lining of the chamber by interposing a sealing element.