Chapter 14: Treatment processes, filtration and adsorption

14.6 Bag filtration

Bag filters comprise a disposable bag fitted into a filtered water receiver vessel or housing. They can be pressure or open (gravity) versions. They are much like cartridge filters in operation, in that they present a single barrier filter. However, unlike most cartridges, the flow path is from the inside of the bag to the outside.
Bag filters are commonly used to remove dust from air. Care must be taken to use filter bags manufactured for liquid applications only.
A single filter unit comprises the filter medium, housing, and associated piping and valves.
Unless the raw water is very clean, these systems usually incorporate some form of pretreatment to remove the bulk of the particulate matter and extend the filter life. Even at the finest mesh size available, bag filters will not remove colloidal colour or dissolved chemicals.
Some bag filters on the market are claimed to be able to be backwashed. This is not to be done because the washing process progressively dislodges fibres from the medium, ultimately allowing more and more particles to pass through.
Bags manufactured from needle felt, made from polyester, polypropylene or nylon, are suitable for pretreatment applications only. The nominal mesh size is from 1000 microns to less than 1 micron nominal. See section 14.5 for comments on nominal and absolute ratings. Bag filters manufactured from melt blown polymers offer higher efficiencies and test results should be provided to support the supplier’s claim of efficiency.

Bag filters may receive up to 1 log credit in the DWSNZ for protozoa removal. See Chapter 8: Protozoa Compliance, section 8.4.3.3 for information about certification requirements. The amount of log credit is less than has been achieved under test conditions because, as with cartridges, the chance of poor fitting and sealing is quite high. Also, as with cartridges, there is usually no continuous direct integrity test (although some have been installed with a turbidity monitor) so this potential loss of quality control has to be allowed for. Refer also to Chapter 8 of the review draft LT2ESWTR Toolbox Guidance Manual (USEPA 2009) which discusses issues related to bag filtration.

Bag filters are relatively common in industrial applications and have been used in some small water treatment plants, but are not common for larger towns. As at 2005 they are in use at Carterton, Pukekohe, and have been trialled at Okato (in Taranaki). The former sites are apparently operating reasonably well; the latter was not successful. Some trials fail due to a heavy load in the 3–10 micron range and low working head, resulting in reduced filter run times.

Operating issues

Integrity and monitoring issues are the same as for cartridges (see section 14.5), and the following issues for cartridges apply also to bags.

Filter cycling (starting and stopping of the pump or filter operation) can be problematic with bag filtration where the water is pumped directly from the source to the filter, and then out to the distribution system. In these situations, the filters operate on demand and the sudden increase in pressure across the filter causes premature wear and filter failure. Supplies using bag filters should consider the following recommendations for controlling the flow into the filter process to minimise filter cycling:

lengthen the filter runs by reducing the flow rate as much as possible through the filter

install or divert the flow to a storage facility (eg, pressure or storage tank) after the bag filtration process. The stored water can supply the frequent surges in demand and thus reduce the bag filter cycling

until a bag is replaced, the flow through it gradually reduces. This change is not sudden or dramatic; sudden changes indicate rupture or discharge of particles

the supplier sets the allowable pressure drop across the bag. If higher pressure drops are set up, the bag may rupture, creating a gap in the filtration barrier. Bag filters do not load linearly; additional observation of the filter performance is required near the end of the filter run

bags will release particles when bumped by sudden pressure changes due to sudden flow changes. Therefore, valves connected to the bags should be the slow opening/closing type. Pumps will cause pressure surges and should not be connected directly to bags. A pressure relief valve should be incorporated into the bag filter housing, and an automatic air release valve shall be installed on top of the filter housing

if the installation is to comply with the DWSNZ protozoa criteria, flow control is needed in order to limit high flows. This may be an orifice or similar pressure-loss fitting, or the system may not be capable of excessive flows anyway. Bag filters must not be operated at flow rates above their stated design rate. A simple flow test will demonstrate whether this is the case

to allow the filtration process to settle down when starting up, it is strongly recommended to filter to waste for the first five minutes of the filter cycle

ideally, bag filters should be in continuous operation; restarting after a shutdown produces poorer quality filtrate for at least 30 minutes.

Very fine particles in the raw water will pass through the bag filter. 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 bag is discharging some of its accumulated contaminants, and therefore the operating conditions must be corrected immediately, or the bag replaced.

The USEPA (2003d) found that different bags, even with the same stock and lot numbers, could exhibit a wide range of water treatment capacity. Some bags may treat many thousands of gallons of water while others may treat only a few hundred. Pore size gives only a general indication of a bag’s capability. Common faults are related to rupture of the seams; gaps in heat welded bags; gasket integrity; and bypass, usually near the lid, typically shown by significant discolouration of the bag. Once a bag begins to foul at 5 to 10 psi differential, the time until the bag must be replaced quickly approaches; bag rupture is more likely near the end of the filter run.

Monitoring

The compliance monitoring parameters for bag filtration are differential pressure, flow and turbidity, see section 14.5 for details. The tests and their frequency depend on the population served; see section 5.13 of the DWSNZ.
Should any operational requirement be exceeded, the operator should check whether:

the operating pressure across any one housing exceeded the manufacturer’s limit, in which case the filtering medium may be approaching rupture

a pressure differential reduction may be the result of a rupture or bypass

the raw water quality 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.