Water treatment solutions –about filter bag and filter cartridge

In many filtering applications, a choice between the use cartridge filter or a bag filter has to be made. Both are sediment filters, that is to say they reduce the amount of sediments transported by the fluid trough filtration.
There are some differences between these two filter systems:

- The choice of cartridge filter depends on the application. Cartridge filters are preferable for systems with contaminations lower than 100 ppm, that is to say with contamination levels lower than 0.01% in weight.

Cartridge filter can be surface or depth-type filter: depth-type filters capture particles and contaminant through the total thickness of the medium, while in surface filters (that are usually made of thin materials like papers, woven wire, cloths) particles are blocked on the surface of the filter.
Surface filters are best if you are filtering sediment of similar-sized particles. If all particles are i.e. five micron, a pleated 5-micron filter works best because it has more surface area than other filters. Compared with pleated surface filters, depth filters have a limited surface area, but they have the advantage of depth.

It can be generally stated that if the size of filter surface is increased, higher flows are possible, the filter last longer, and the dirt holding capacity increases. Cartridge filters are normally designed disposable: this means that they have to be replaced when the filter is clogged.

- Bag filters are in general frequently used for dust removal in industrial applications. The flow can be from the outside to the inside of the filter (that means, the separation of particles happens on the external surface of the filter) or the other way around, depending on the application. The particles are normally captured on the internal surface of the bag filter.

Bag filters are in general not designed for replacement when they are clogged, but some bag filters for gaseous applications like dust removal can be cleaned, for example by mechanical shaking or by backwashing with compressed air (so called reverse-flow bag filters).

Bag filters are mostly surface-type filters.

- A rule of thumb is that for concentrations higher than 5 mg/m³ a surface filter is favored, while for concentrations lower than 0.5 mg/m³ a depth-type filter is preferred. In general surface filters can by backwashed and cleaned more easily, while depth-type filters normally have to be disposed when clocked.