TDS Impacts on Wastewater Treatment and NPDES Permit Compliance


  • Mary Sadler, PE, Robbie Graham, Clarke Wallcraft, Janice Carroll, PE

The WET test was designed in two parts. The first part consisted of a single salt spiked test to see if there was an individual salt that may be more problematic that the others. The spiked salt concentrations were chosen based on known toxicity thresholds for those salts. The second part was a matrix of spiked TDS concentrations in the wastewater effluent.

None of the single salt spiked concentrations exhibited any signs of acute or chronic toxicity.

Another surprise was the results of the TDS matrix test. The large red dash indicates the threshold for chronic toxicity in 2013, and the small red dash indicates the threshold for which no chronic toxicity was observed. The green dashes show the no chronic and chronic thresholds in the 2006 test. The threshold for which chronic toxicity was not observed increased significantly from 2006, from 900 mg/L to 1,500 g/L. The 2006 test did see acute toxicity at the 4.9 g/L matrix spike, but no acute toxicity was observed in the 2013 test.

Many wastewater treatment plants are faced with a significant challenge of an industrial contribution of total dissolved solids (TDS) in plant influent. TDS can dramatically affect plant operation or compromise a facility’s ability to meet NPDES permit limits. TDS ions are not regulated pollutants, but the concentration of TDS in plant influent may have a significant effect on plant processes and the ability to meet permit limits. New or expanding industry to an area facilitates economic development. Most utilities do not desire to turn away high TDS-discharging industries from relocating to an area; however, the impact to a wastewater treatment facility’s operation and the increased likelihood of NPDES permit violations is a significant concern.

There are options to evaluate the impact of a high TDS industry on plant operation and effluent quality to determine the critical TDS concentration. Initially, an assessment of the collection system industrial contribution is required, including updated industrial waste surveys to define industrial discharge characteristics and a statistical analysis of industrial metals data. With respect to treatment, an evaluation of structural/corrosion concerns, aeration, nitrification, sludge settling, solids precipitation, biosolids disposal, UV disinfection, metals, and whole effluent toxicity (WET) are critical factors that must be considered prior to accepting a high-strength TDS industry. The WET evaluation requires a series of increasing strength TDS dilutions and then assessing the resulting acute and chronic toxicity at each dilution. The final analysis requires an evaluation of the limitations from the process analysis and WET testing to determine the critical TDS pollutant load to a treatment facility.

This paper will present two case studies relative to TDS industrial contribution relative to NPDES permit limits, whole effluent toxicity, and pretreatment. Additionally, this work will offer insight on the technical challenges of managing and regulating TDS in a pretreatment program.

For more information, please contact the author at

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