Testing Conditions

8.4.1.1 Full Scale Filter Testing

Challenge testing must be conducted on full-scale bag or cartridge filters and the associated filter housing or pressure vessel that are identical in material and construction to the filters and housing the system will use for removal of Cryptosporidium.

8.4.1.2 Challenge Particulate

Challenge testing must be conducted using Cryptosporidium or a surrogate which is removed no more efficiently than Cryptosporidium. The microorganism or surrogate used during challenge testing is referred to as the “challenge particulate.” The concentration of the challenge particulate must be determined using a method capable of discreetly quantifying the specific organism or surrogate used in the test; gross measurements such as turbidity may not be used (40 CFR 141.719(a)(3)). Key physical characteristics to be considered for identifying an acceptable surrogate include size, shape, and surface charge. Other factors include ease of measurement and cost. Chapter 3 of EPA’s Membrane Filtration Guidance Manual (U.S. EPA 2005) describes the characteristics of acceptable surrogates and lists potential and inert surrogates for

Cryptosporidium. Examples of possible microbial surrogates are P. dimunita and S. marcessans.

1 _{Specific sections of the EPA/NSF ETV Protocol that provide guidance for developing and conducting a challenge}

test for LT2ESWTR include: section 7.0, Characterization of Feed Water; section 11.0, Operating Conditions; section 12.3, Work Plan; section 13.0, Data Management; and section 14.0, QA/QC.

8.4.1.3 Test Solution Concentration

In order to demonstrate a removal efficiency of at least 3-log for bag or cartridge filters, it may be necessary to seed the challenge particulate into the test solution. A criticism of this approach is that the seeded levels are orders of magnitude higher than those encountered in natural waters, which could lead to artificially high estimates of removal efficiency. To address this issue, EPA set a limit on the maximum feed concentration applied to a filter during the challenge study. The limit is based on the detection limit of the challenge particulate:

Equation 8-1

Maximum Feed Concentration = 1.0 × 104 _{× Filtrate Detection Limit}

These concentrations allow the demonstration of up to 4.0-log removal for bag filters and cartridge filters during challenge testing if the challenge particulate is removed to the detection limit.

Example 8-1 - Determining maximum allowable feed concentration

If the detection limit of the surrogate test is 2 units/L, then the maximum feed concentration is 1 × 104 _{× (2) = 2 × 10}4

8.4.1.4 Challenge Test Duration

Each filter must be tested for a duration sufficient to reach “terminal pressure drop” (40 CFR 141.719(a)(6)). Terminal pressure drop is a parameter specified by the manufacturer that establishes the end of the useful life of the filter. Continuous challenge particulate feed is not required (i.e., intermittent seeding is permitted). At a minimum, removal efficiency must be determined during three periods over the filtration cycle:

• Within 2 hours of start-up of a new filter.

• When the pressure drop is between 45 and 55 percent of the terminal pressure drop. • At the end of the run after the pressure drop has reached 100 percent of the terminal

pressure drop.

The rule does not specify the number of samples that must be collected during each of the three periods. Because the effluent concentration is often very low and near the detection limit, it may be beneficial to collect more effluent than influent samples to obtain a more accurate

8.4.1.5 Water Quality of Test Solution

Water quality can have a significant impact on the removal of particulate contaminants, such as Cryptosporidium. In general, bag and cartridge filters in water treatment do not

experience influent turbidity concentrations much greater than 10 nephelometric turbidity units (NTU). For the application of the LT2ESWTR, they typically will receive filtered water and thus, very low turbidity.

A clean-water challenge test will generally provide the most conservative estimate of removal efficiency. However, since the challenge test must run until terminal head loss is reached, the challenge test solution should contain some solids to cause the head loss build-up across the filter, but not an excessive amount that will cause a rapid build-up. Particulate foulants that may be appropriate to add to the test solution include clay particles (such as bentonite or kaolin) or carbon powder, as long as they are not excessively fine-sized.

The following are recommended for the challenge test solution:

• High quality water with a low to moderate concentration of suspended solids should be used as the challenge solution. Suspended solids concentration should be high enough to achieve a reasonable rate of headloss buildup, but not so high that the headloss builds up too rapidly to conduct the challenges at the various headloss levels.

• No oxidants, disinfectants, or other pretreatment chemicals should be added to the test solution.

• Test water should be characterized with respect to basic water quality parameters, such as pH, turbidity, temperature, and total dissolved solids.

8.4.1.6 Maximum Design Flow Rate

The challenge test must be conducted at the maximum design flow rate for the filter as specified by the manufacturer (40 CFR 141.719(a)(5)).

8.4.1.7 Challenge Particulate Seeding Method

There are two basic approaches to seeding: batch seeding and in-line injection. In batch seeding, all of the challenge particulates are introduced into the entire volume of test solution and mixed to a uniform concentration. Batch seeding requires the entire test solution to be contained in a reservoir and for the reservoir to be well mixed to ensure a uniform concentration of the seeded particles. Generally, batch seeding is used for small scale systems that only require relatively small amounts of feed solution for testing.

In-line injection is the most common seeding approach used in challenge testing, allowing challenge particulates to be introduced into the feed on either a continuous or intermittent basis. While either could be used, intermittent seeding may be preferable to

continuous seeding for conducting the challenge test at the required intervals (i.e., a minimum of beginning, middle, and end-of-run). If intermittent injection is used, equilibrium should be achieved during each seeding event prior to the collection of feed and filtrate samples.

In-line injection delivers the challenge particles from a concentrated stock solution with a known feed concentration. Guidelines and examples for determining challenge test feed

concentration and stock solution delivery rates are provided in Chapter 3 of the Membrane

Filtration Guidance Manual (U.S. EPA 2005).

In-line injection requires additional equipment, such as chemical feed pumps, injection ports, and in-line mixers. These components should be designed to ensure a consistent challenge particulate concentration in the feed. A chemical metering pump that delivers a steady flow is recommended (pumps that create a pulsing action should be avoided). The injection port should introduce the challenge material directly into the bulk feed stream to aid in dispersion. An in-line static mixer should be placed downstream of the injection port, and a feed sample tap should be located approximately ten pipe diameters downstream of the mixer (U.S. EPA 2005).

8.4.1.8 Challenge Test Solution Volume

The volume of the test solution depends on filtrate flow rate, test duration, and hold-up volume of the test system. For intermittent, in-line injection, the seeded test solution volume can be considerably less than that required for batch seeding. Formulas for calculating test solution volume and examples are provided in Chapter 3 of the Membrane Filtration Guidance Manual (U.S. EPA 2005).

8.4.1.9 Sampling

An effective sampling program depends on a detailed sampling plan and the use of appropriate sampling methods, locations, and quality assurance/quality control (QA/QC) measures.

Samples can be collected using either grab or composite sampling methods. Grab

samples consist of pre-determined amounts of water taken from the feed or filtrate streams, while composite samples are of the entire process stream. Grab sampling is commonly used to

determine the concentration of challenge particulates in the feed solution, while grab or composite sampling is used to analyze the filtrate stream. Good sampling practices include flushing samples taps, using clean sample containers, and preventing cross contamination of samples. QA/QC measures include clearly identifying samples, collecting duplicates, and using blanks.

In many cases, it may be advantageous to collect more filtrate samples than feed samples, since the concentration of the challenge microorganism in the filtrate samples is expected to be very low and error of just a few particles could have significant impact on the demonstrated removal efficiency.

Sample port design is an important consideration and should ensure that a representative sample is obtained. Poorly designed ports contain large volumes where stagnation may occur (e.g., large valves and long sample tubes) and pull the sample from the edge of the pipe. A well designed port has a sample quill that extends into the center of the pipe to draw a more

representative sample.

Chapter 3 of the Membrane Filtration Guidance Manual (U.S. EPA 2005) contains additional information on developing sampling plans and provides schematics of typical sampling apparatuses.