A device for filtering water; a straightforward compound noun.
This water filter purifies tap water.
Standard kanji spelling using 濾 (filter).
Mixed kana-kanji spelling; 濾 is sometimes written in kana because it is not a jōyō kanji.
Compound of 濾 (ro, 'filter') + 水 (sui, 'water') + 器 (ki, 'device').