Bag and cartridge filter systems may contain anywhere from one to over twenty filter units. There is no maximum number of filters a system can include; however, membrane or other filtration processes become more practical for larger flows since bag and cartridge filters are generally replaced instead of backwashed or regenerated. A single filter unit is comprised of the filter media (bag or cartridge), housing, and associated piping and valves. Exhibit 8.4 shows a typical single filter vessel (housing).
Source: U.F. Strainrite
Systems with multiple filters may be designed as a manifold with connective piping between the individual filters in separate housing or alternatively as multiple filters in a single housing. Exhibits 8.5 and 8.6 show the manifold design and multiple filter vessel design, respectively.
Exhibit 8.5 Manifold Bag Filter Design
Exhibit 8.6 Multiple Filter Vessel
The designs of bag and cartridge filters are not complex; however, there are a couple of key issues that should be taken into consideration. First, the filter units should be designed integrally with their respective housing systems. Poor fittings can cause leaks and premature failure. Manufacturers can provide individual filter units that can be retrofitted into the existing process or complete filter houses that are skid mounted. It is important to adhere to the
manufacturer’s instructions on filter installation.
Second, the overall water treatment process design should minimize sudden changes in pressures applied to the bag or cartridge filters. Each time the flow to the filter is interrupted and then restarted, a sudden increase in pressure can occur across the filter unit unless steps are taken to allow for gradual pressure ramp-up. The particle load in the filter effluent often increases when the filter cycle begins. A study by McMeen (2001) reported that the increase in particle load could be occurring due to the seal at the top of the filter failing when the pressure suddenly increases. Bag filters are especially susceptible to cycling because these pressure fluctuations also increase wear on the fabric and seams, causing premature failure. Section 8.5.4 provides recommendations for reducing filter cycling.
8.5.1 Water Quality
As previously described, systems seeking compliance with the LT2ESWTR will most likely integrate a bag or cartridge filter process after the primary filtration process. As a result, influent water quality, with respect to high particulate levels, should not be an issue. However, for systems with existing processes that use coagulants, the presence of residual coagulant in the primary filter effluent may clog the pores of a bag or cartridge filter. Although this will not impair removal efficiency for Cryptosporidium, it will shorten the time until the terminal pressure drop is reached, thus reducing filter life.
Another water quality issue is the potential for biofilm growth on the bag or cartridge filter media. Systems can add a disinfectant prior to the bag or cartridge filters to prevent biofilm growth. (The filters must be compatible with the disinfectant.)
8.5.2 Size of Filter System and Redundancy
Systems should be adequately designed to handle maximum day or maximum
instantaneous flow, depending on the existing treatment process design. Prolonged operation at maximum flow velocity clogs the filter media faster than operating at lower flow velocities. The total volume throughout is greater when operating at a flow velocity lower than maximum flow velocity rated for the filter.
A minimum of two bag or cartridge filter housings should be provided to ensure continuous water treatment in the event of failure in the filter operation and to allow for filter maintenance and replacement. For water systems that do not require continuous operation, a state may approve a single filter housing operation. Redundancy in pumps is also recommended to ensure continuous operation.
8.5.3 Design Layout
Design layout features that should be considered for most designs are as follows: • Piping should be designed to allow isolation of the individual filter units or vessels for
maintenance and filter replacement.
• Common inlet and outlet headers for the filter units.
• Sufficient available head to meet the terminal pressure drop and system demand.
8.5.4 Filter Cycling
Filter cycling refers to the starting and stopping of the pump or filter operation. This can be problematic with bag filter processes (cartridge filters are not known to have this problem) in which 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, similar to wells for small systems, and the sudden increase in pressure across the filter causes premature wear and filter failure. For LT2ESWTR compliance, systems with bag filters in a series or followed by UV disinfection should consider the following recommendations for controlling the flow into the filter process to minimize filter cycling.
• Lengthen the filter runs by reducing the flow as much as possible through the filter. • Install or divert the flow to a storage facility (e.g., pressure tank, clearwell) after the bag
filtration process. The stored water can supply the frequent surges in demand and thus reduce the bag or cartridge filter cycling.
During filter start-up and other hydraulic surges, bag and cartridge filters often experience an increase in filter effluent turbidity. Systems should consider the following options to improve filtered water quality.
• Design for filter to waste capability. EPA strongly recommends filtering to waste for
the first few minutes of the filter cycle.
• Install a slow opening and closing valve ahead of the filter to reduce flow surges.
8.5.5 Pressure Monitoring, Valves, and Appurtenances
As previously mentioned, once the terminal pressure drop has been reached, the filter should be replaced. At a minimum, pressure gauges should be located before and after the bag or cartridge filter system and should be monitored at least daily. A valve or flow restricter should be installed on the inlet header pipe of the filters to maintain flows below the maximum operating flow for the filters.
8.5.6 Air Entrapment
An automatic air release valve should be installed on the top of the filter housing to release any air trapped in the filter. These valves should be checked routinely and properly maintained.
8.5.7 NSF Certification
All components used in the drinking water treatment process should be evaluated for contaminant leaching and certified under ANSI/NSF Standard 61.