By Rick Andrew
Conceptually, contaminant reduction testing under the NSF/ANSI Drinking Water Treatment Unit (DWTU) standards is simple. Contaminated water is introduced into the DWTU system and the product water is analyzed to establish whether the system is effectively reducing the contaminants for which the test is being done. Still, taking contaminant reduction testing from concept to reality can get rather complicated. Contaminants must be introduced at specific concentrations as dictated by the standards.
Other characteristics of contaminant challenge water are also dictated and vary from test to test. The test may require cycling of water through the systems at specific intervals. Samples must be collected at specific intervals that vary from test to test, standard, and sometimes on specifications of the product. Contaminants themselves have different properties that create issues. Some contaminants may be volatile and tend to evaporate from solution; others have low solubilities in water while others can be reactive. Finally, a test laboratory must be prepared to run multiple tests for different contaminants on different test systems under varying standards. This article examines how two different types of contaminant reduction test apparatus may be used to implement creative solutions to dealing with all of the variables described above.
One tank at a time
The most fundamental contaminant reduction test apparatus that most people identify with is the batch or tank apparatus. At NSF, this method of testing is referred to as “tank rigs” or “batch rigs.” The apparatus basically consists of a 500-gallon tank, mixing and recirculation pumps, a water delivery pump, a piping manifold with bladder tank, pressure regulators, solenoid valves and computerized control systems. The NSF laboratory has eight of these apparatus. The contaminant challenge solution is prepared in the 500-gallon tank by mixing together a “recipe” of contaminant and any other necessary reagents to create the solution required by the standard for the particular test being conducted. The tank is stirred with mixing and recirculation pumps to ensure that both the batch of challenge solution is homogeneous and all dry reagents are truly dissolved and don’t fall to the bottom of the tank as solids. Sample collection to verify the water characteristics and influent challenge isn’t done on water drawn from the tank directly, but rather from a point comparable to the challenge water flow to the filters undergoing tests. This is critical, as the piping arrangement between the tank and the filters can impact on the water characteristics and challenge level.
The water delivery pump, bladder tank, and piping manifold are used to deliver a constant pressure contaminant-reduction challenge water. The pressure regulators allow the higher-pressure challenge solution to be regulated to the level prescribed by the specific standard prior to delivering water to the test systems. Plus, the solenoid valves and computerized control systems allow any cycling of the flow to be controlled according to the specific standard, and ensure samples are collected at the proper points.
Batch contaminant reduction testing has certain advantages. It’s conceptually the simplest form of testing and the easiest to implement. Certain tests are required by the standard to be performed on batch rigs. These include Standard 42 – chloramine reduction, Standard 53 – pentavalent arsenic reduction, and Standard 53 – lead reduction, pH 8.5. These tests have such complicated test water requirements that they can only be properly controlled by preparing the challenge water batch by batch, and analyzing each batch to confirm that the water is correct. For some of these tests, specific recipes for batches of contaminant challenge water are included in the standards. There are other tests that, although not required to be conducted on batch test rigs, are much more easily conducted in this way due to the complicated water characteristics. Standard 55 – UV Class A and UV Class B testing would fall into this category.
The drawbacks
There are also certain disadvantages to batch contaminant-reduction testing. First, batch rigs take up quite a bit of space. A 500-gallon tank is not small! Smaller tanks could be used, which would take up less space, but then batches would have to be made more frequently. Each batch must be analyzed to ensure that all water characteristics are within the range prescribed by the standard. Increasing the number of batches through use of smaller size tanks would increase the time of the test as well as cost.
There also are other disadvantages. Volatile contaminants can evaporate from batch solutions that lead to fluctuations in the influent contaminant levels. The result may be an influent concentration that drifts out of conformance of the standard. Other contaminants may tend to plate onto the tank wall, which can also lead to fluctuating influent contaminant levels. Plating onto the tank can also cause problems with future testing on the same rig if the plated contaminants return to solution. If the subsequent tests are for other contaminants, the laboratory will not likely be analyzing for the previous contaminants now being added to the challenge water unknowingly. This can have an impact on the performance of the system.
Somewhere in the batch rig review, there should be a comment regarding the importance in taking the influent challenge sample from the piping just prior to injection into the water filtration device. This is critically important as it’s not at all uncommon that sample results taken directly from the tank will be different than sample results taken from the line just prior to the filtration unit.
Injection testing
Injection contaminant reduction testing is much more complicated than batch testing, but overcomes many of the disadvantages of batch testing. As implemented at NSF, injection contaminant-reduction testing consists of a manifold that delivers two types of test water to over 15 different test stands. This manifold is highly automated. As water is drawn through the manifold to meet the demands of various tests, new water is continuously generated. Online monitors and reagent pumps maintain the specific general test water chemistry for each contaminant reduction test. The water is constantly recirculated through the injection rig manifold to each individual test stand. Bladder tanks are used to maintain a constant pressure in the manifold, which is regulated down at each test stand as required by the specific standard. At each stand, the applicable type of the two available test waters is selected with valves.
At each stand, there’s also an injection pump that injects a concentrated contaminant solution into the test water to deliver the appropriate contaminant challenge solution. Inline mixers are used to ensure turbulent flow results in homogeneous contaminant challenge water. The pumping rate is tied to the system flow rate such that the injection rate remains constant as the system flow rate may change during the life of the test.
Reliance on the computer
This is a complex system. Computer control is essential. Each test stand is controlled by special monitors and programmable software developed by NSF for this application. These computer controls allow the flexibility to conduct many different types of tests, yet ensure that sample collections aren’t missed and test parameters are maintained.
Injection testing solves many of the problems that arise with batch testing. Because there are only two tanks of water for the entire manifold, it takes up much less space per test than batch testing. Once the injection configuration has been initially optimized, the test can continue for many gallons without the constant analysis required each time a batch is prepared. Volatile contaminants don’t have a chance to evaporate from solution, as they aren’t exposed to air. Contaminants don’t have an opportunity to plate to a tank because they’re injected inline very close to the test systems and are constantly flowing when injected. In addition, a different test can be run at each test stand to meet very diverse testing needs.
Batch contaminant reduction testing is the simplest. Injection testing is more complicated, but overcomes some of the drawbacks of batch testing. By utilizing both technologies, laboratories can have the greatest flexibility in conducting all of the various contaminant reduction tests specified by the DWTU standards in the most proper, best controlled and most economical way possible.
Conclusion
Manufacturers are encouraged to meet with their product-testing laboratories while understanding specifically how their products are being evaluated. There are many details to understand with some directed by the standard and others designed and engineered by the laboratory. No two test rigs, whether batch or injection, are likely to be the same. By visiting the laboratory, manufacturers will better understand the testing and can be assisted in designing their own in-house product evaluation and testing processes.
About the author
Rick Andrew is technical manager of the Drinking Water Treatment Units Program at NSF International. He has been with NSF for over four years while working with certification of residential drinking water products. His previous experience was in the area of analytical and environmental chemistry consulting. Andrew has a bachelor’s degree in chemistry and a master’s degree in business administration from the University of Michigan. He can be reached at (800) 673-6275 or email: [email protected]