This section discusses two key issues associated with operating bag or cartridge filters, pressure changes and water quality monitoring.
8.6.1 Pressure Drop (Inlet/Outlet Pressures)
The pressure drop across the filter directly relates to the amount of particle build-up on the filter material and to the time when the filter should be replaced. Typical pressure drops across a clean filter are 1 to 2 psig (pounds per square inch-gauge) and can increase to a differential of 20 to 30 psig when the terminal pressure drop is achieved. The pressure
differential does not increase linearly with run time; the differential pressure increases at a faster rate with the duration of the run or as more material accumulates on the filter. The time between filter replacement is primarily dependent on flow rate, but also on influent water quality and filter material (i.e., size of pores).
The differential pressure between the inlet and the outlet header should be monitored to determine when the filter needs replacement. Also the differential pressure should be monitored immediately after replacing a filter and placing the unit back in service to verify that the filter was properly installed to prevent bypassing around the seals. An alarm could also be linked to the pressure gauges to ensure the operator is alerted when a filter needs to be replaced due to fouling or failure of the filer or associated seals.
8.6.2 Water Quality Monitoring
In addition to monitoring the pressure drop across the filter, the influent and effluent turbidity or particle count can be monitored to assess performance and indicate possible process upsets with the bag or cartridge filter or other upstream processes. The recommended monitoring frequency depends on the influent water quality and its variability. At a minimum, the pressure differential and effluent turbidity can be checked daily. If the filter is used to meet the treatment requirements of IESWTR/LT1ESTWR, turbidity monitoring is required and the state will set a turbidity performance standard. During the initial start-up phase of a newly integrated bag or
cartridge filtration system, monitoring can be more frequent and then can be reduced once the operator becomes familiar with the system. If continuous monitoring of turbidity and/or pressure differential is employed, the output from the sensors should be sent to an alarm to warn operators of sudden changes in operation, or if the filter element needs replacing.
EPA recognizes turbidity has limitations as an indicator of filter failure or pathogen breakthrough. However, in the absence of a better indicator, monitoring both influent and effluent turbidity over a full run (i.e., from start to end of the filter life) can provide a
performance baseline. The baseline can then be used to indicate process upsets. This method may not be applicable to systems with very low raw water turbidity or where the influent has been filtered; the difference between influent and effluent turbidity may be too low to provide meaningful data.
Particle counters can be another valuable monitoring tool. If available, periodic checks of influent and effluent particle counts are also recommended to ensure the filter is removing particles in the appropriate size range (i.e., 4-6 microns).
8.7 References
McMeen. 2001. Alternate Filtration: Placing New Technology in an Old Regulatory Box. American Water Works Association, Membrane Conference Proceedings.
NSF International. 2005. Protocol for Equipment Verification Testing for Physical Removal of
Microbiological and Particulate Contaminants. 40 CFR 35.6450.
http://www.epa.gov/etv/pubs/059205epadwctr.pdf.
U.S. EPA. 2005. Membrane Filtration Guidance Manual. Office of Water. EPA 815-R-06-009. November, 2005. http://www.epa.gov/ogwdw/disinfection/lt2/compliance.html.
U.S. EPA. 2006. Ultraviolet Disinfection Guidance Manual for the Final Long Term 2
Enhanced Surface Water Treatment Rule. Office of Water. EPA 815-R-06-007. November,
2006. http://www.epa.gov/safewater/disinfection/lt2/compliance.html.
U.S. EPA. 2007. Simultaneous Compliance Guidance Manual for the Long Term 2 and Stage 2
DBP Rules. EPA 815-R-07-017. March, 2007.
LT2ESWTR Toolbox Guidance Manual 9-1 April 2010
9.1 Introduction
The Long Term 2 Enhanced Surface Water Treatment Rule (LT2ESWTR) 40 CFR 141.719(c) describes second stage filtration as the use of a rapid sand, dual media, granular activated carbon (GAC), or other fine grain media unit process applied in a separate stage following rapid sand or dual media filtration. Applying an additional layer of media, such as a GAC cap, on an existing single stage filtration unit does not qualify for this credit.
This chapter is organized as follows:
9.2 LT2ESWTR Compliance Requirements - discusses criteria and reporting requirements that systems must meet to receive Cryptosporidium removal. 9.3 Toolbox Selection Considerations - discusses issues specific to second stage
filtration that water systems should consider when selecting toolbox options. 9.4 Design and Operational Considerations - discusses hydraulic issues, backwashing,
and turbidity monitoring for systems that integrate a second stage filtration in their treatment train.
9.2 LT2ESWTR Compliance Requirements 9.2.1 Credits
Under the LT2ESWTR, a system that employs a second, separate filtration stage may receive 0.5 log credit for Cryptosporidium removal (40 CFR 141.719(c)) under the following conditions.
• The first stage of filtration is preceded by a coagulation step.
• The second stage of filtration is comprised of rapid sand, dual media, GAC, or other fine grain media.
• Both filtration stages treat 100 percent of plant flow.
• The state must approve the treatment credit based on an assessment of the design characteristics of the filtration process.
Under the LT2ESWTR, a system integrating a slow sand filtration process for the second stage of filtration can receive 2.5 log credit for Cryptosporidium removal (40 CFR 141719(d)) under the following conditions.
• No disinfectant residual is present in the influent to the slow sand filtration process. • Both filtration stages treat 100 percent of plant flow.
• The state must approve the treatment credit based on an assessment of the design characteristics of the filtration process.
9.2.2 Reporting Requirements
To receive Cryptosporidium removal credit for compliance with the LT2ESWTR, systems must verify that 100 percent of the flow was filtered through both stages and that the first stage was preceded by a coagulation step (40 CFR 141.721(f)).
Reporting for LT2ESWTR does not take the place of the IESWTR and LT1ESWTR reporting requirements. Specifically, the turbidity of the combined and individual filter effluent from the first filtration stage must be reported as required by the IESWTR and LT1ESWTR (40 CFR 141.74, 40 CFR 141.174(a), 40 CFR 141.551, and 40 CFR 141.560).
9.3 Toolbox Selection Considerations
Plants already employing a second unit process that meets the requirements for this toolbox option (e.g., GAC columns to meet dissolved organic or taste and odor treatment goals) are in the ideal position to seek credit. Other plants that have enough excess filtration capacity or unused filter beds (e.g., built in anticipation of unrealized plant expansions), may be able to convert piping to enable these filters to operate in series for relatively low cost. However, many plants will find that integrating second stage filtration into an existing treatment train poses significant additional space, capital, and hydraulic requirements. These systems may want to consider this option if the additional treatment provides other benefits. For example, systems that use chloramination and/or ozone could run the second stage under biological filtration conditions to reduce assimilable organic carbon (AOC), which promotes biofilm growth and nitrification (for chloraminating systems) in the distribution system.
Additionally, plants experiencing taste and odor problems or dissolved organic contaminants in their raw water might consider installing GAC columns to alleviate these problems and also receive the Cryptosporidium removal credit.
Slow sand filtration plants who wish to consider this toolbox option should either have sufficient excess filtration capacity to allow filters to operate in series (with possible piping modifications) or have sufficient land area to build additional filters.
LT2ESWTR Toolbox Guidance Manual 9-3 April 2010
9.3.1 Advantages
The advantages of a second stage filtration process are the same for both rapid and slow sand plants and include operator familiarity with the process, ease of operation, and potential to reduce disinfection byproducts. For plants with existing processes and infrastructure meeting the two-stage requirements, implementation costs are likely to be relatively low.
9.3.2 Disadvantages
The disadvantages associated with second stage filtration apply primarily to those plants that do not have existing processes in place or cannot easily convert built-in infrastructure. In addition to the capital cost for new filters, these plants may need the following improvements to integrate a second stage of filtration:
• Space if there is currently no room for expansion in the existing plant grounds.
• Additional pumping to compensate for head loss associated with an additional filtration process.
• Increased backwash supply and treatment.
For those plants that have existing infrastructure available for a second stage of filtration, they still may have to account for an increased volume of backwash and loss of head due to the second stage.
Systems with rapid sand filtration plants that are considering integrating slow sand filtration into their treatment process should be aware of the following differences in operation and performance of slow sand plants compared to rapid sand plants:
• More space required for slow sand plants.
• Decreased filtering performance with cold temperatures.
• Maintenance of filters requires draining and scraping a thin layer off the top of the filter.