The present invention provides a filter unit 10 for a capsule for preparing a beverage and/or nutritional product. The filter unit is designed to avoid clogging. The filter unit therefore comprises a second filter 50, which is provided upstream a first filter 40in respect to the liquid flow through the filter unit 10. The second filter 50has a pore size to hold back large particles such as scale, bio-film or iron oxide. The present invention is able to ensure a consistent liquid flow through the filter unit. The filter unit can be used with any kind of liquid source.
