Chapter 14: Treatment processes, filtration and adsorption

14.5 Cartridge filtration

The advantages of bag and cartridge filtration processes include low maintenance requirements, minimal operator skill and attention required, and low space requirements. The only routine maintenance required is filter replacement when a predefined terminal pressure drop or other operating parameter, such as filter age or volume treated, is reached. The operation of these systems is generally straightforward and requires little technical skill. In addition, the filter materials are relatively inexpensive and the housing system is not complex, resulting in relatively low capital costs.
An economic disadvantage of bag and cartridge filtration processes is that the filters must be replaced instead of being regenerated or washed. For larger flows, or water with higher particle loads, frequent filter replacement increases operation and maintenance costs. Additional pumps may be required to provide needed pressure.
A cartridge filtration plant consists of cylinders (or housings) packed with filter cartridges through which the water flows from the outside of the filter into a central collection duct. Cartridge filtration is used in other industries too, so care must be taken to ensure water supply filters are used, ie, the filtration units must comply with NSF/ANSI 53 Standard or similar standards.
A single filter unit or plant comprises the filter medium, its housing, and associated piping and valves. A housing may contain between 1–20 filters.
Unless the raw water is very clean, these systems usually incorporate some form of pretreatment to remove the bulk of the particulate matter to extend the cartridge filter life. If another cartridge filter is to be used upstream, it has been found that a 20–50 micron screening filter is quite effective, see sections 12.3.4 and 12.3.6 in Chapter 12: Treatment Processes, Pretreatment. A filter-to-waste component is recommended for any pretreatment pressure sand filters. At the beginning of each filter cycle and/or after every backwash of the prefilters a set amount of water should be discharged to waste before water flows into the bag/cartridge filter.
Some cartridge filters on the market are claimed to be able to be backwashed. This is not to be done because the washing process can progressively dislodge fibres from the medium, ultimately allowing more and more particles to pass through.
Cartridge types vary widely in size, cost and effectiveness. Testing has shown wide variations between different types. Unlike membrane filtration, there is currently no technique for direct integrity testing. For these reasons, the Drinking-water Standards for New Zealand 2005, revised 2008 (DWSNZ) require a safety factor over their validated protozoa removal performance. For cartridges that are validated to achieve 3 log removal of Cryptosporidium, 2 log credits may be awarded. See Chapter 8: Protozoa Compliance, section 8.4.3.4 for information about validation or certification. Refer also to Chapter 8 of the review draft LT2ESWTR Toolbox Guidance Manual (USEPA 2009) which discusses issues related to cartridge filtration.

Cartridge size

The standard size is 248 mm (9.75 inches) or 254 mm (10 inches) long and 64 mm (2.5 inches) in diameter. This size is used for most small applications. Some suppliers will provide dual housings that will fit two (or more) of these in parallel. Due to seating problems, it is not normally recommended to use cartridge filters in series. For example, instead of joining two cartridges together, it is better to purchase a double length unit, say 19.5 or 20 inches long.
There are larger sizes for applications producing higher volumes. The sizes vary according to the supplier; 150 mm (six inch) diameter is often the next step up. The length also increases in the larger cartridges: 20, 40 and 60 inch options are some common sizes on offer.

Filtration size

Filters are rated on their ability to remove particles of a specific size from a fluid, but the problem is that a variety of very different test methods are applied to specify performance. Because cartridge filters are fixed barriers, their effectiveness is expressed by the size of the smallest holes. This is usually measured in microns or micrometres (1000 microns being 1 mm). Pore size ratings refer to the size of a specific particle or organism retained by the filter to a specific degree of efficiency. A filter that is marked ‘10 micron’ has some capability to capture particles as small as 10 microns. However, this is meaningless unless there is a description of the test methods and standards used to determine the filter rating. Cartridge filters are often classified by either a nominal or absolute rating.

Nominal rating: is just that, it exists in name only. It is meant to represent the size (or more commonly mean size) of the particles which a filter will exclude, maybe with an efficiency as low as 60 percent! It does not guarantee to remove particles of the same size as the nominal pore rating. It is used only to give a comparison within the same manufacturer’s range.

Absolute rating: purports to provide some certainty (usually 100 percent or close to 100 percent) of removing particles of the size quoted. It is meant to represent the actual size of the pores of a filter. However, the material used in the testing may be different (for example, it may be more flexible) than the material being filtered. The test protocol needs to be checked before you can be sure of performance. Filters with an absolute rating are not usually ratified by an outside agency; the rating generally represents the manufacturer’s own assessment.
The ‘nominal’ and ‘absolute’ ratings are irrelevant for cartridges used for protozoal removal. They vary so much that each filter must pass a challenge test, see Chapter 8: Protozoal Compliance, section 8.4.3.4 Cartridge filtration.
To remove protozoal (oo)cysts such as Giardia and Cryptosporidium, 3 microns absolute is required, testing to NZS4348 (1995) is required. This size will not remove clays or silts, so most dirty water will remain dirty. To reduce the turbidity due to silt, 0.5 microns absolute is normally needed. For clay turbidity, smaller sizes may be effective. In all cases, it will be necessary to try out different sizes to find the rating required for the material.

Filter cartridge types

How the layers are arranged within the filter will affect how many solids can be held before the cartridge becomes too blocked to pass the flow. Wound or low cost pleated cartridges usually achieve at least 3 log removals in testing; it is usually the design of the physical construction of the housing or its assembly that causes the problems.

Pleated cartridges have a single sheet of filter membrane folded to and fro in a zigzag fashion. These are common in air filters for car motors. A further refinement of these is folded pleats, where the folds overlap each other, as if they were too big to fit up and down only. Pharmaceutical grade cartridge filters are often pleated, usually with a cage around them to make them rigid, ie, less susceptible to damage. Low-cost pleated cartridges can be damaged easily without it being obvious to the user.

Depth cartridges have material of a coarser weave on the outside and finer inside. This allows smaller particles to penetrate further into the filter, allowing more solids to be held before blockage occurs.

Wound cartridges have a continuous thread wound around a core to provide flow paths between the windings. They look a little like oversized bobbins of cotton, and are usually made from polypropylene. The earliest designs used a string material which was decomposed by micro-organisms in the water! Wound cartridges are now less common because they are susceptible to discharging when operating in the stop/start mode.

Filter material

Most cartridges now use polypropylene as the filter medium. Also used are glass fibre (bonded with a resin) and nylon. Various other compounds are also used, polyethersulphone is an example.

Filter housings

The key quality control issue is fitting the cartridges properly into their housings. Most seat on to knife-edge moulded end plates. It is very important to ensure that these are fitted properly into the ends of the cartridge elements, because a leak across this interface will break the whole filtration barrier. Always inspect the filter housing seal and the cartridge seal when changing a cartridge. It is very important to ensure that the cartridge is installed the right way up.
A pressure relief valve should be incorporated into the filter housing, and an automatic air release valve shall be installed on top of the filter housing.
Fitting the cartridge into the housing is covered by the requirements in Chapter 8 of the review draft LT2ESWTR Toolbox Guidance Manual (USEPA 2009) which discusses issues related to cartridge filtration, or the cyst/oocyst reduction conditions of NSF/ANSI 53, or Chapter 4 of EPA/NSF ETV (2002, updated 2005), or a standard formally recognised by the Ministry of Health as being equivalent.
Figure 14.3: Cutaway showing cartridge seal

Picture courtesy of Filtec Ltd.

This picture shows typical cartridge seals: Note there are knife edge seals at both the top and bottom of the cartridge.

Operating issues

Commissioning: verifying the assembly of a certified cartridge filter when it has been installed should be performed by demonstrating that the operating parameters necessary to achieve the specified performance rating, which have been previously established by challenge testing, are being achieved on site. The procedure should be as specified by the supplier.

Start-up: To allow the filtration process to settle down when starting up or restarting, it is strongly recommended to filter to waste for the first five minutes of the filter cycle. Record the initial headloss or pressure differential, and check that it lies within the manufacturer’s specification. Note in a log book all relevant information that may help in training and trouble-shooting, especially the date that the new cartridge(s) is installed so that its length of service (preferably volume treated too) is recorded.

Pressure testing for leaks: Correct installation of the cartridge needs to be demonstrated for each cartridge renewal. The most likely failure is a lack of proper seating, allowing flow to leak through the top or bottom seals. Another common cause for failure occurs when the cartridge bursts.

The way to demonstrate that the integrity of the unit has not been breached is to measure the pressure drop across it when operating at the maximum rate.

A major difficulty with this is the accuracy of the pressure gauge. Precise gauges are expensive, probably more expensive than the filter unit itself. A simple way to reduce costs is to arrange the pressure sample lines to feed back to the same gauge; this approach also allows the inaccuracy in the gauge to be cancelled out. See Figure 14.4.

The greater the number of cartridge filters in a housing, the more attention must be given to the pressure readings. One faulty cartridge could be passing (oo)cysts, but if it is one of several cartridges in the housing, its effect on the pressure drop may be slight.

The same gauge is used to measure both upstream and downstream pressures. After installation, the upstream pressure is measured by opening that valve and shutting the downstream valve. Then, with the flow unchanged, the downstream pressure is read. The pressure drop should not be greater than that advised by the cartridge supplier. If it is, there is probably a leak through one of the seals.

Record as much information on checking and general operation as possible in the log book.

Figure 14.4: Suggested arrangement for reading pressure differential across a cartridge filter



Pressure gauge recommendation

Minimum size (4”) 100 mm

Accuracy 1 percent of readable scale, as per EN837.1 European standard

Liquid-filled, this helps readability when vibrations are present

Average cost $150

Maximum readability of 1 kPa cannot be met due to limitations on number of graduations that can be fitted into one 100 mm gauge.

Pressure testing for clogging: If the clean cartridge pressure drop is recorded and the pressure drop is recorded during a filter run, the amount of clogging can be noted. The cartridge should be replaced when the pressure drop reaches or approaches the value advised by the cartridge supplier. Cartridge filters exhibit a knee shape curve of pressure drop over time; see Figure 14.5 (fictitious units). Cartridge filters do not load linearly; additional observation of the filter performance is required near the end of the filter run.

Figure 14.5: Typical pressure drop across a cartridge during a filter run

If a cartridge is not replaced at this point, the flow through it will be reduced. This may not be noticed at first if the flow is discharging into a tank or out through taps, because the change is not sudden or dramatic. Therefore, the pressure drop test should be repeated regularly, along with a simple flow test.

If the differential pressure is ever less than a previous reading, the cartridge must be assumed to have unloaded some of the trapped contaminants, and must be replaced, ie, the integrity of the filter has obviously failed.

Guarding against pressure changes: Cartridges will release particles when bumped, by sudden pressure changes, or as a result of sudden changes in flow. For this reason, the valves connected to the filters should be of the slow opening/closing type such as screw-operated stopcocks, not bar-operated ball valves. Pumps cutting in and out will cause pressure surges so should not be connected directly to cartridges.

Flow control: Cartridge filters must not be operated at flow rates above their stated design rate. If the installation is to meet the DWSNZ, flow control will be needed to limit flows above this. This may be an orifice or similar pressure-loss fitting, or the system may not be capable of excessive flows anyway. A simple flow test will demonstrate whether this is the case. Ideally, cartridge filters should be in continuous operation; restarting after a shutdown produces poorer quality filtrate for at least 30 minutes.

The flow through a cartridge filter should be as low as possible to lengthen filter run times and reduce surges. Alternatively, install a recirculating pump that pumps treated water back to a point ahead of the cartridge filter. Care must be taken to make sure there is no cross-connection between the finished water and raw water.

Disinfection: if the filter runs are very long, it will be advisable to check whether upstream chlorination is needed to prevent bacteria building up on, and subsequently sloughing off, the filter material and housing.

Monitoring

The compliance monitoring parameters for cartridge filtration are differential pressure, flow and turbidity. The tests and their frequency depend on the population served; see section 5.12 of the DWSNZ and Chapter 8 of the Guidelines.
Very fine particles in the raw water will pass through the cartridge filtration system. If these predominate, the turbidity of the filtered water could be almost the same as the raw water. If the turbidity of the filtered water is greater than the raw or feed water for more than a few minutes, it must be assumed that the cartridge is discharging (unloading) some of its accumulated contaminants, and therefore the operating conditions must be corrected immediately, or the cartridge replaced.
Should any operational requirement be exceeded, the operator should check whether:

the operating pressure across any one housing exceeded the manufacturer’s limit, generally not more than 1.0 Bar (15 psi) difference between inlet to outlet, in which case the filtering medium may have ruptured

a pressure differential reduction may be the result of damage to or bypass around the seal

the raw water quality (or prefilter performance) has deteriorated

there have been any flow surges or sudden pressure changes that may have dislodged particles

the seals no are longer seated correctly: this could be indicated by the pressure differential no longer increasing.

If the plant experiences turbidity problems following cartridge replacement, consideration should be given to incorporating a flush-to-waste step, five minutes is usually chosen.