Bag filters and cartridges filters are used for a variety of applications, from industrial processes to water
treatment and home use. Some common examples are:
Cartridge filters: filtering water that enters a home or an automobile oil filter
Bag filters: vacuum cleaner bag
Bag Filters
Bag filters are defined as a fabric filter designed primarily to remove particulate material from
fluids. Bag filters are usually non-rigid, disposable, and easily replaceable.
Bag filters are typically contained in a pressure vessel.
Bag filters can be used either individually or as an array of bags in the vessel.
Fluids usually flow from the inside of the bag to the outside.
The primary application for bag filters in water treatment is to remove Cryptosporidium oocysts and/or Giardia cysts from source water. Bag filters typically do not remove bacteria, viruses, or fine colloids.
Giardia cysts and Cryptosporidium oocysts are protozoan found in water. They can cause diarrhea and other health-related problems if ingested.
The use of coagulants or a pre-coat with bag filters is not usually recommended since removal of particulate material is based on the absolute pore size of the filter instead of the development of a layer on the surface of the filter to enhance its removal capabilities. Therefore, coagulants or apre-coat only increase the pressure loss through the filter,necessitating more frequent filterexchanges.
Applications
Industrial
Currently, bag filtration and cartridge filtration are more widely used for industrial purposes than in water treatment. Industrial uses include process fluid filtering and solids recovery.
Process Fluid Filtering: Process fluid filtering is the purification of a fluid by the removal of undesirable solid material. Process fluids include fluids used to cool or lubricate equipment. In mechanical equipment, or during the processing of a fluid, particulate material can accumulate. In order to maintain the purity of the fluid, the particles must be removed. The oil filter in your vehicle is a good example of a cartridge filter being used to maintain the quality of a process fluid.
Solids Removal/Recovery: Another industrial application is in solids recovery. Solids recovery is done to either recover desirable solids from a fluid or to “purify” the fluid prior to subsequent treatment, use, or discharge. For instance, some mining operations will use water to convey the minerals being mined from site to site. After the slurry arrives at its desired location, it is filtered to remove the desired product from the carrier water.
Water Treatment
There are three general applications for bag filtration or cartridge filtration in a water treatment plant. They are:
1. Filtration of surface water or ground water under the influence of surface water.
2. Prefiltration prior to subsequent treatment.
3. Solids removal.
Surface Water Treatment Rule (SWTR) Compliance: Bag filters and cartridge filters may be used to provide filtration of surface water or ground water under the influence of surface water. Given the nature of bag filters and cartridge filters, their application is likely limited to small systems with high quality source water. Bag filters and cartridge filters are used for: Giardia cyst and Cryptosporidium oocyst removal
Turbidity
Prefiltration: Bag filters and cartridge filters can also be used as a prefilter prior to other treatment processes. An example would be membrane filter systems which utilize a bag or cartridge prefilter to protect the membranes from any large debris that may be present in the feed water.
Most bag or cartridge filter systems consist of a prefilter, a final filter, and the necessary valves, gauges, meters, chemical feed equipment, and on-line analyzers. Again, since bag and cartridge filter systems are manufacturer specific, these descriptions will be generic in nature—individual systems may differ somewhat from the descriptions offered below.
Prefilter
In order for a filter to remove parasitic protozoan like Giardia and Cryptosporidium, the pore size of the filters must be very small. Since there are usually other larger particles in the water being fed to the filter system, removal of these larger particles by the bag filter or cartridge filter would tend to dramatically shorten their useful life.
To alleviate this problem, many manufacturers construct their systems with a prefilter. The prefilter can be either a bag or cartridge filter of somewhat larger pore size than the final filter. The prefilter traps the larger particles and prevents them from being added to the final filter. This increases the amount of water that can be filtered through the final filter.
As mentioned, the prefilter has a larger pore size than the final filter and also tends to be significantly less expensive than the final filter. This helps to keep the operational costs of a bag or cartridge filtration system as low as possible. The frequency of prefilter change-out is determined by the quality of the feed water.
It is possible that a bag prefilter could be used on a cartridge filter system or a cartridge prefilter be used on a bag filter system, but typically a bag filter system will use a bag prefilter and a cartridge filter system will use a cartridge prefilter.
Filter
After the prefiltration step the water will then flow to the final filter, although some filtration systems may utilize multiple filtration steps. The final filter is the filter that is intended to remove the target contaminant.
As mentioned, this filter tends to be more expensive due to its smaller pore size and it may undergo more stringent manufacturing procedures to assure its ability to remove the target contaminant.
Bag and cartridge filtration systems can be configured in many different ways. The configuration selected depends on a number of factors including source water quality and desired production capacity.
Bag Filter Systems
Bag filter systems can come in a variety of configurations. For each configuration, the PA DEP will require full redundancy of all filter stages.
Single Filter Systems: A single filter system would likely be somewhat rare in a water treatment application. A single filter system would only be applicable for extremely small systems with an extremely high quality source water.
Prefilter – Post Filter Systems: Perhaps the most common configuration of a bag filter system is a prefilter – post filter combination. By using a prefilter to remove the large particles, the loading on the final filter can be dramatically reduced and substantial cost savings can be realized.
Multiple Filter Systems: Intermediate filters are placed between the prefilter and the final filter.
Each filtration step would be finer than the previous step.
Filter Arrays: Some bag filter systems utilize more than one bag per filter housing. These are referred to as filter arrays. These filter arrays allow for higher flow rates and longer run times than systems with one bag per housing.
Post time: Jan-22-2024