Closed filters allow managing unwanted elements such as suspended matter, iron, manganese, nitrates, pesticides, taste or smell...

They can contain 

• Sand: for water that is slightly loaded with solid suspended matter
• Active carbon: to hold, by adsorption, organic molecules giving water displeasing tastes and smells.
• Exchange resin: for softening, demineralization, denitration or decarbonation
• Neutralizing mass: to solve problems of calco-carbonic balance