CASE STUDY. In-Tank THM Removal System Achieves Over 50 Percent Reduction. Texas Utility Reaches Compliance for First Time in 5 Years.

Transcription

1 The Problem The City of Ballinger, Texas, provides water to a population of approximately 5, people. The City owns and operates a 2.5 million gallons per day (mgd) water treatment plant that was commissioned in 25, and two storage facilities. This new treatment plant has been unable to reduce disinfection by-product (DBP) precursors to levels such that DBP levels comply with regulatory limits. More specifically, THM (Trihalomethane) levels leaving the plant have been at or above the Maximum Contaminant Level (MCL) of µg/l, and at the compliance test location in the distribution system, the City has been above the MCL every quarter for 5 consecutive years. THM data collected in September 21 confirmed that levels leaving the clearwell between and ug/l. In the distribution system near their main storage facility, levels were measured between 13 and µg/l. Due to this extended period of non-compliance, the City has agreed to a consent decree to add reverse-osmosis treatment at the new plant, however this costly facility is just going out to bid and will not be online for 18 months or longer. The Solution PAX Water Technologies has pioneered the development of energy-efficient technologies to improve water quality in the distribution system. In partnership with Utility Service Company (the largest and most experienced tank maintenance company in the U.S.) PAX designed a new THM removal system that can be quickly installed and configured inside a storage tank to remove THMs. This THM removal system utilizes proprietary hydraulic and mass transfer models and a unique optimization process to arrive at a configuration of equipment that is ideally suited to each tank and its associated distribution system. Using extensive experimental validation, the percentage of THM removal can be predicted in advance of any equipment installation. For the City of Ballinger, the majority of the distribution system operates on one pressure zone which contains a single.25 million gallon (MG) multi-legged elevated steel tank. The average daily volume pumped to the tank is 3, gallons. A system targeting a peak THM removal of percent to 6 percent between the tank inlet and outlet conditions was designed and installed in early September 21. 2/ PAX Water Technologies, Inc. 1

2 Results: At the Tank Installation and start-up was completed on September 7 th, 21. Figure 1 presents THM concentrations of water entering the tank, and water at the end of the drain cycle leaving the tank. Data collected during the first three days demonstrated progressively lower levels of THMs in the effluent as the system worked to remove THMs from the water and lower average THM levels in the tank. Because water treated with the PAX THM removal system leaves the tank through the same pipes as it enters, the fill water feeding the tank from nearby in the distribution system shows progressively lower THM levels as water from the plant is mixed with treated water from the tank. Figure 1 illustrates that after 1 days of continuous operation, a peak reduction in THM levels of 59 percent throughout the tank was realized. Levels dropped from 89 µg/l in the fill water to 37 µg/l throughout the effluent at the end of the drain cycle. Entering the tank Leaving the tank 9/7/21 9/8/21 9/9/21 9/17/21 Figure 1. THM concentration entering and leaving the tank after system start-up Results: Current Stage 1 Compliance Location Treatment at the tank is one issue, but how are THM levels affected in the distribution system where compliance measurements are actually taken? Figure 2 shows both historical THM levels at the current Stage 1 compliance in comparison to levels with the THM removal system in operation. After 1 days of operation, the THM levels were reduced from 13 µg/l to 85.4 µg/l, just above the MCL the lowest level measured in 5 years. 2/ PAX Water Technologies, Inc. 2

3 Texas Utility Reaches Compliance for the First Time in 5 Years. 2 2 With THM removal system MCL = ug/l Q1 26 Q2 26 Q3 26 Q4 26 Q1 27 Q2 27 Q3 27 Q4 27 Q1 28 Q2 28 Q3 28 Q4 28 Q1 29 Q2 29 Q3 29 Q4 29 Q1 21 Q2 21 9/7/21 9/17/21 Figure 2. THM Concentrations at current location for distribution system compliance samples Results: Future Stage 2 Compliance Location The City of Ballinger must meet more stringent requirements defined in the Stage 2 DBP rule beginning in 213. At that time, the sample location for compliance will change to a new location identified as a worse case location. In addition, compliance determination will no longer be based on the annual average, but rather each discrete data point. Data were collected at this new location to measure the impact on THM levels at this future testing location as identified in the city s Initial Distribution System Evaluation (IDSE) report. Figure 3 on the following page presents THM levels at the Stage 2 compliance location. After 1 days of operation, the THM levels were reduced from 13 µg/l to 77 µg/l, just under the MCL. This result complies with the Stage 2 requirements the City will soon face. 2 2 MCL MCL = ug/l mg/l 9/7/21 9/17/21 Figure 3. THM concentration at Stage 2 location for distribution system compliance samples 2/ PAX Water Technologies, Inc. 3

4 Results: When System Is Turned Off On or around October 1 st, 21, the system shutdown due to an electrical outage. Samples taken on October 4 th (Figure 4) showed THM levels beginning to rebound. Once the TRS was turned back on, by October 18 th, 21, THM levels had returned to their pre-treatment levels. This outage confirmed the reduction of THMs observed was attributable to the PAX/USCI installation. Entering the tank Leaving the tank 1/4/21 1/18/21 Figure 4. THM concentration entering and leaving the tank after system shutdown Chlorine Residual A general concern with systems designed to remove THMs in tanks is the potential to lose chlorine residual. City staff have reported no observed changes in expected chlorine residual levels during routine testing at sample locations within the vicinity of tank in the distribution system. Chlorine residual data taken at two locations within approximately half a mile from the tank, presented in Figures 5a and 5b, confirm that the chlorine residual can be maintained while removing THMs. 612 Hutchings Street 39 3rd Street Chlorine Residual (mg/l) /25/21 9/21/21 Chlorine Residual (mg/l) /21/21 9/17/21. After. After Figures 5a. and 5b. Distribution system chlorine residual near tank location 2/ PAX Water Technologies, Inc. 4

5 Key Takeaways 1. Predictable and lasting THM reductions cannot be achieved by aeration equipment alone. 2. PAX s comprehensive solution for THM reduction relies on multiple technologies, each optimized for the specific needs and demands of each distribution system. 3. PAX combines energy-efficient technology, cutting-edge process modeling, and rigorous analysis to achieve substantial and lasting THM reduction in the distribution system. 4. Models used to develop case-by-case solutions have been shown to accurately predict performance. 5. PAX and USCI offer a turnkey solution to Stage 2 THM compliance that is predictable, reliable and backed by experimental evidence. Conclusion The tank which housed the TRS was taken offline in the Spring for a major rehabilitation and repainting. The TRS was reinstalled in late July 211 and has been operating continuously since then. Average aggregate THM removal rates have been greater than 5 percent (Figure 6) TRS Off Entering tank Leaving tank 7/12/11 7/25/11 8/8/11 8/15/11 8/29/211 9/12/211 9/26/211 1/1/211 1/24/211 11/14/211 Figure 6. THM Concentration entering and leaving the tank after rehabilitation and repainting. Because of the very high THM levels entering the system from the treatment plant, THM levels in the distribution system remain above the MCL. This is most likely due to high bromine levels in the source water and high TOC levels due to constraints in current plant operations. This illustrates the fact that no single technology can be the silver bullet for Stage 2 DBP Rule compliance: optimized plant operations, combined with effective mitigation strategies in the distribution system, are likely to be the best and most economical path to Stage 2 compliance. 2/ PAX Water Technologies, Inc. 5