A method and apparatus for monitoring a level of silicon dioxide in a liquid and removing the silicon dioxide using polishers is disclosed. In an embodiment, two polishers that absorb carbon dioxide and silicon dioxide, but which have a greater affinity for carbon dioxide, are placed in series along a conduit containing the liquid for use in an immersion lithographic apparatus. The upstream polisher absorbs carbon dioxide and silicon dioxide until it is saturated, at which point it desorbs the silicon dioxide in preference for the carbon dioxide. Silicon dioxide continues down the conduit and is absorbed by the downstream polisher. Once the upstream polisher is saturated with carbon dioxide, carbon dioxide present in the liquid flows downstream where it is absorbed by the downstream polisher. A sensor between the polishers senses the presence of carbon dioxide and initiates a request for the one or more of polishers to be cleaned or replaced.