During this week’s American Water Works Association’s Water Quality Technology Conference, Amy Gao is presenting a case study on how one water utility in New Jersey is using tracer studies and other analytical testing to protect the community’s drinking water.

By: Amy Gao, CH2M Water Engineer

Amy is attending and presenting her paper, “Lessons Learned From Ozone Contactor Tracer Testing at Little Falls Water Treatment Plant”, co-authored by Russell Ford, CH2M Global Drinking Water Leader, at the American Water Works Association’s Water Quality Technology Conference (WQTC) in Salt Lake City, Utah. See the complete list of CH2Mers presenting at WQTC, and join the conversation on Twitter, using the #WQTC.

The Passaic Valley Water Commission (PVWC), located in Clifton, New Jersey, is committed to providing high-quality drinking water to its customers, which is why the utility has made it a priority to invest in modern technologies, ensuring water treatment and distribution system operations at its Little Falls Water Treatment Plant (WTP) remain a model for the industry.

Performing analytical testing of its water supply is one important way that PVWC monitors its water quality to provide the best possible drinking water to its community. PVWC enlisted the help of water quality specialists at CH2M to conduct a tracer study of the ozone contactors at the Little Falls WTP. The study was carried out in accordance with USEPA standards to comply with the New Jersey Department of Environmental Protection’s (NJDEP) requirement to conduct tracer studies for chemical disinfection treatment systems at potable water treatment plants.

Little Falls WTP treats water from the Passaic River using a sand-ballasted sedimentation process, ozonation and filtration through granular activated carbon and sand filters, and chlorination. The tracer tests provided information about the hydraulic characteristics of the chemical disinfection treatment systems. More specifically, the objective of the tracer studies was to determine the T10/hydraulic detention time (HDT) ratios (baffling factors) for the plant’s four ozone contactors at different flow rates.  T­10 represents the time when the first 10 percent of the water has passed through the contactor and the other 90 percent is still remaining in the contactor. Another method of looking at T10 is to consider it as the detention time that is equaled, or exceeded by, 90 percent of the fluid passing through the contactor. The T10/HDT ratios generated in the study were used by Little Falls plant staff as part of the calculations for determining disinfection credit as required by the NJDEP.

Tracer testing was performed using the step-dose method that involves introducing the tracer chemical at a constant dosage until the concentration at the downstream sampling point reaches steady state (i.e., no change in concentration over time) and should be approximately equal to the concentration at the inlet or the first sampling point. Eight tracer studies were completed (2 tests for each contactor, with one test conducted at a low flow rate of ~18 million gallons per day [mgd] and another test conducted at a high flow rate of ~28 mgd). The maximum design flow rate through each contactor is 29.25 mgd. T10/HDT ratios were obtained from plots of dimensionless time versus fractional concentration. Regression curve fits, utilizing data from the times corresponding to 0.0 to 0.5 fractional increase in the tracer chemical concentrations to obtain the best fit around the T10/HDT ratio, verified the results.

There were several lessons learned during the tracer testing. During the planning phase of the tests, it was important to allow sufficient time for several test runs prior to the actual runs. These initial test runs revealed that the planned location of fluoride dosing using the chlorine injection points in the contactor was not in the expected location just upstream of the contactor influent orifices. As a result, the location of fluoride dosing was changed to the clarifier effluent and adjustments had to be made for additional lag time. The initial tracer tests also revealed true as-built conditions, such as sample lines and injection points.

Another lesson learned for the planning phase is to account for the time needed for simple tasks, like bottle preparation and chain of custody sheets, which due to the number of samples taken during these tests, took more time than anticipated. Coordination was essential to allow the tests to run smoothly and keep the plant operational. Since the plant laboratory facilities and staff were utilized, it was important to coordinate with the lab and make sure that these additional samples did not negatively impact day-to-day operations. Safety is always important, especially when handling chemicals. Proper personal protective equipment was worn at all times and the Material Safety Data Sheet (MSDS) for chemicals used in the study was included with the safety plan.

Lastly, too much data can be both a good and bad thing. A lot of data provides a full story of the hydraulics through each contractor. However, as it is standard industry practice to use only one value for the T10/HDT ratio, the large amount of data collected from this study proved a challenge when trying to determine which final number should be used for disinfection credit. In the end, the T10/HDT ratio was selected from a representative dataset that demonstrates the greater spread or dispersion in the tracer. The results of the tracer study recommend that a T10/HDT ratio of 0.56 be used for the Little Falls WTP ozone contactors.

Amy Gao is a water engineer at CH2M. She has experience in planning and design projects for both water and wastewater treatment systems. She has been an active member of American Water Works Association (AWWA) and Water Environment Federation (WEF) for more than 6 years. She holds an M.S. in Environmental Engineering from the University of Texas at Austin and B.S. in Earth and Environmental Engineering from Columbia University.