noun
filter medium; filter material
Technical term for the material used in a filter, such as activated carbon, sand, or fabric.
この浄水器のろ材は半年ごとに交換が必要です。
The filter medium in this water purifier needs to be replaced every six months.
フィルター refers to the filter device as a whole, while ろ材 specifically means the filtering material inside it.
Compound of ろ (濾, 'filter') and 材 (ざい, 'material').