21. The municipality is not liable for damage caused to an immovable or its contents if the owner of the immovable neglects or omits to install an apparatus intended to reduce the risk of malfunction of a water supply system or sewer system in accordance with the by-law adopted under section 19. Such a by-law may apply to an immovable already erected if it prescribes a minimum period of one year to allow the owner to comply with that obligation.