Cartridge filters are defined as fabric or polymer-based filters designed primarily to remove particulate material from fluids. They are usually rigid or semi-rigid and manufactured by affixing the fabric or polymer to a central core. Cartridge filters are disposable and easily replaceable.
Ø Cartridge filters are typically housed in a pressure vessel.
Ø Cartridge filters can be used either individually or as an array of cartridges in a vessel.
Ø Fluids usually flow from the outside of the filter to the inside.
Ø The primary application for cartridge filters in water treatment is to remove Cryptosporidium oocysts and/or Giardia cysts from source water. Cartridge filters typically do not remove bacteria, viruses, or fine colloids.
Ø The use of coagulants or a pre-coat with a cartridge filter 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 a pre-coat only increase the pressure loss through the filter, necessitating more frequent filter exchanges.
There are three general applications for 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:
Cartridge filters may be used to provide filtration of surface water or ground water under the influence of surface water. Given the nature of cartridge filters, their application is likely limited to small systems with high quality source water.
Cartridge filters are used for:
Ø Giardia cyst and Cryptosporidium oocyst removal
Cartridge filters can also be used as a prefilter prior to other treatment processes. An example would be membrane filter systems which utilize a cartridge prefilter to protect the membranes from any large debris that may be present in the feed water.
The use of cartridge filtration for water treatment is limited by two major factors.
Ø Source water quality
Ø System size
Source Water Quality
If the source water to be treated has high levels of turbidity, colloids, or algae, it will not be suitable for cartridge filtration. There are no hard and fast rules, but some general guidelines can be proposed.
Ø Turbidity: Less than 1 NTU (Nephlometric Turbidity Units).
Ø Fine Colloids: Even low levels of fine colloids or clays may make the source water unsuitable for cartridge filtration.
Even if you are using low turbidity sources waters there is still the concern the water intake could contain large objects such as fish or other debris. Intake screening devices may be used to prevent or minimize the entry of large objects or fish into the treatment facility.
Ø Algae: Although different manufacturers may have different algae loading criteria for their filter systems, all systems require source water with low levels of algae.
Although cartridge filter systems are modular and can theoretically accommodate any flow rate by increasing or decreasing the number of filter or filter arrays, economics tend to favor their use in smaller water systems. The following items must be considered when determining if a cartridge filter system would be feasible.
Ø Capital Costs: Pressure vessel(s), pumps, chemical feed, and analytical equipment.
Ø Operation & Maintenance (O&M) Costs: Filter change-out frequency.
Ø Flow Rate: System sizes in excess of 100,000 gpd may be too large for cartridge filtration systems to be economical.
Cartridge Filter Housing:
There are two basic styles of cartridge filter systems. Smaller systems usually use a single wound cartridge; larger systems usually consist of multiple cartridge filters which can be either pleated or wound. The type of system determines the configuration of the housing.
• Smaller cartridge filter systems such as those used in a home filtering system typically are constructed from some type of plastic or stainless steel. The body of the vessel usually is made of clear plastic (or stainless steel) the top of which is threaded to facilitate its connection to the lid or cap. The bottom of the body sometimes includes a barb or indentation which allows for the proper positioning of the cartridge in the housing. The lid usually has the female thread which accepts the male threads of the body and an O-ring to provide the seal between the two portions of the housing. The lid usually contains the inlet and outlet ports as well as the pressure relief valve used to depressurize the housing for filter change-out. Smaller cartridge filter systems may or may not contain taps or ports for pressure gauges.
• Larger cartridge filter systems can use either pleated or wound filters and usually use multiple filters in a single housing. From the outside, a multiple filter housing looks very much like the housing for a bag filter. Internally however, the housing has specific slots to accept the cartridges themselves. The housing itself consists of:
Ø The body. As with the bag filter housing, the body may include:
− Outlet ports,
− Taps or ports for pressure gauges,
− Legs or a stand to support the housing, and
− The necessary hardware to hold the lid or cap.
Ø The lid or cap. The lid may contain:
− An inlet port and
− Taps or ports for pressure gauges.
• There is usually some type of pressure relief valve on either the body or lid to allow for the housing to be depressurized for filter change-out.
• Cartridge filters are usually rigid. Therefore, they do not require the use of a support screen or basket.
Cartridge Filter Systems
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. Home water filter systems are usually single filter systems.
Prefilter - Post Filter Systems: This is one of the more common arrangements in a cartridge filter system. It is configured so that the feed water initially passes through a filter with a relatively large pore size and then is filtered through the finer post or final filter.
Multiple Filter Systems: A multiple filtration system is an extension of the prefilter – post filter
configuration described above. Rather than having a prefilter and a post filter, a multiple filtration system would consist of progressively finer filters plumbed in series.
Filter Arrays: Multiple cartridge filters are grouped together in a single filter housing to provide higher flow rates and longer runtimes between filter change-outs.
Ultimately the question of whether a cartridge filter system is suitable to a particular application is determined by the quality of the source water. There are many constituents in the source water that may affect the operation of the filtration system. These are:
Although each manufacturer’s source water quality requirements may be different, some general guidelines for source water quality can be offered.
Although each filter manufacturer’s standards for source water turbidity may be slightly different from one another, all cartridge filter systems require a high quality of source water. Generally, the lower the turbidity of the source water the better the application for cartridge filters. As the source water turbidity increases the amount of water that can be filtered decreases.
Some surface water sources may have a low average turbidity but get muddy after rainstorms, winter runoff, or other event. In these cases, it would be best to shut off the filter system and allow the source water quality to improve if possible. If this is not possible, the water can still be filtered but the life of the filter itself may be dramatically reduced. Some types of turbidity, especially those caused by clays or fine colloidal compounds may not be well removed by cartridge filters. If the filter cannot produce water with a turbidity of less than 0.3 NTU in at least 95% of the samples taken on a monthly basis and never exceed 1 NTU, a different filtration system should be considered. For this reason alone, a pilot test of the proposed cartridge filtration system should be conducted to confirm that the system can achieve these turbidity requirements. Even if you are using low turbidity sources waters there is still the concern the water intake could contain large objects such as fish or other debris. Intake screening devices may be used to prevent or minimize the entry of large objects or fish into the treatment facility.
There is no hard and fast rule as to the amount of color or colloidal content that a cartridge filter system can handle. The source water’s suitability hinges on two items:
Ø The filtration system’s ability to produce an acceptable filtered water quality, and
Ø The system’s ability to produce that quality economically.
Again, there is no substitute for a pilot evaluation of the proposed system to ascertain the system’s ability to operate effectively and economically.
Algae can be quite troublesome. It can create filtration, taste, and odor problems. Algae is present in nearly all surface waters. Piloting, especially during summer months, can be used to gauge the impact of algae.
A cartridge filtration system’s primary purpose is to remove Giardia and Cryptosporidium from source water. Fortunately, a cartridge filter will remove most Giardia and Cryptosporidium.
However, if the source water has extremely high levels of Giardia and Cryptosporidium, significant numbers of the parasites may remain after filtration. Although this may alarm you, it is important to remember that conventional, direct, and Diatomaceous Earth (DE) filters do not remove 100% of Giardia and Cryptosporidium. Compliance with the SWTR relies, in part, not in the number of Giardia in the filtered water but in the amount removed during treatment. SWTR regulations require a 3-log removal/inactivation of Giardia through the treatment process—not on a numerical level of the parasite allowed in the filtered water. Even 3-log means the removal of 99.9% of a target organism.
The second factor to consider when determining if a bag or cartridge filtration system may be appropriate for a particular application is to look at the required capacity of the system. Allowable flow rates through a cartridge filter are manufacturer specific and based on the available surface area of the filter. Surface area of filters vary from manufacturer to manufacturer.