Any food grade material can be used as a filtration medium, encased in a housing. To filter drinking water, polypropylene is usually used.

The selection of porosity depends on the size of the suspended particles and their quantity in the water. In general, 5 micron filter candles are used to filter drinking water. In water where the particle size is less than 5 micron, a 1 micron candle has to be used as the 5 micron candle will not deliver the clear water.

Basically there are two types of filters.

Surface filter :Only the surface area of the filtration medium is used during the filtration process. These filters choke very fast and require frequent cleaning. The pressure drop is greater and the life of the filter candle is short.
Examples: cellulose candles, polypropylene moulded candles, ceramic candles, stainless steel mesh candles, etc.

Depth filters : The full depth of the filtration medium is used during the filtration process. Therefore the area available for filtration in these filters is more than that of surface filters. Consequently, the life of these filters is higher and the frequency of cleaning is lower. The pressure drop is also lower.
Example: polypropylene yarn wound candles.

The efficiency of the filter can be gauged through a visual comparison of the water pre and post filtration. To make a scientific comparison, the turbidity in the water pre and post filtration has to be measured. The acceptable turbidity level in drinking water laid down by the Bureau of Indian Standards is 5 NTU.

The life of the candle depends on the turbidity level in the water being filtered. If the turbidity in the input water is high, the life of the candle reduces drastically.