Optimizing the Zinc Orthophosphate Dosage in a Full-Scale Distribution System

Transcription

1 Optimizing the Zinc Orthophosphate Dosage in a Full-Scale Distribution System Eric C. Wert, P.E., and Samantha S. Stoughtenger, P.E. Southern Nevada Water Authority, P.O. Box 99955, Las Vegas, NV ABSTRACT The Lead and Copper Rule (LCR) established action levels of 1.3 mg/l for copper and 15 µg/l for lead. In order to comply with these action levels, the Southern Nevada Water Authority (SNWA) adds zinc orthophosphate (ZnOP) to the finished drinking water for corrosion control. Due to the increasing cost of this chemical, SNWA conducted a study consisting of a literature review, coupon testing and targeted monitoring. A targeted monitoring approach was developed to assess copper concentrations in residential plumbing before and after the ZnOP dose reduction. The monitoring plan was developed based upon volunteer participation from SNWA employees who have copper plumbing in their homes. It also included volunteers from the LCR sampling pool who were willing to collect quarterly samples for the study. Once the sampling approach was developed, SNWA discussed the approach with the Nevada Department of Environmental Protection (NDEP). Once the approach was approved by NDEP, the ZnOP dose was reduced from 2 mg/l to 1 mg/l (as product) and quarterly sampling began by the volunteers. Targeted monitoring results showed that the 90 th percentile copper concentration increased from 0.65 mg/l to 0.87 mg/l during the first month after the ZnOP dose reduction. In some cases, individual samples exceeded the 90 th percentile action level for copper. These sites were found to be influenced by seasonal groundwater, which does not have any ZnOP added. In subsequent testing, samples remained below the 1.3 mg/l action level for copper. Sampling is expected to continue through August 2010 to gather more information after the dosage reduction. To date, the ZnOP set point remains at 1 mg/l as product, and results thus far have indicated that SNWA will continue to operate with the reduced dosage and continue to experience the $500,000/yr cost savings. INTRODUCTION The LCR was promulgated in 1991 by the United States Environmental Protection Agency (USEPA) to protect public health by limiting the release of lead (Pb) and copper (Cu) into drinking water from household plumbing materials (USEPA 1991). The LCR established action levels (AL) of mg/l for lead and 1.3 mg/l for copper based on the 90 th percentile level of tap water samples collected. Lead and copper samples must be first-draw samples from 1

2 the cold water tap in households with a high risk of contamination. Exceeding the AL is not considered a violation but rather a trigger to institute further measures, such as water quality parameter monitoring, corrosion control treatment, source water monitoring/treatment, public education, and/or lead service line replacement. In 1990, the Southern Nevada Water Systems (SNWS) began a three year, multiphase Corrosion Abatement Project (CAP) to investigate corrosion control alternatives to comply with LCR requirements. Phase I of the CAP examined corrosion development within a closed copper pipe system in the presence of one of the following chemicals: ZnOP, sodium polyphosphate, silicate polyphosphate, sodium silicate, blended orthophosphate, or sodium hydroxide. Of these chemicals, sodium polyphosphate, blended orthophosphate, and ZnOP, at dosages of 2, 4, and 6 mg/l, were selected for further evaluation in Phase II. The final CAP report recommended a 2 mg/l dose of ZnOP to minimize lead and copper concentrations in the distribution system. However, ZnOP dosages less than 2 mg/l were never investigated during this study. In 1995, SNWS began using the 2 mg/l dosage of ZnOP as 10% product, which equates to dosages of 0.2 mg/l Zn and 0.2 mg/l PO 4. Over the past few years, the cost of ZnOP has risen from $287/ton in 2004 to $790/ton today. These cost increases have prompted the SNWS to investigate its corrosion control strategy. Reducing the zinc orthophosphate dosage from 2 mg/l to 1 mg/l as product would allow the SNWS to experience a cost savings of nearly $500,000 between both water treatment facilities, Alfred Merritt Smith (AMS) and River Mountains (RM). The goal of this study was to investigate whether the 2 mg/l ZnOP dosage could be safely reduced to 1 mg/l without violating the LCR. MATERIALS AND METHODS Lead and Copper Rule Compliance Monitoring The LCR compliance data was collected to evaluate how the distribution system is performing with the existing ZnOP dosage. Based on LCR criteria, SNWA purveyors currently monitor for both lead and copper on a triennial monitoring basis. Water samples throughout the purveyors areas are collected from taps by citizens. Sampling locations have remained relatively constant throughout the data set, as required per the LCR. The water quality data is sorted and the 90 th percentile concentration values are calculated for reporting to the State of Nevada. To date, the lead AL has been met with ease since ZnOP began in Most lead concentrations are below mg/l, which is well below the AL of mg/l. The 90 th percentile copper concentrations have exceeded 1 mg/l thirteen times since monitoring began in 1992, but have consistently remained below the AL of 1.3 mg/l. Therefore, this would indicate that a dose reduction could be reasonable given that, LCR compliance has not been threatened using the 2 mg/l ZnOP dosage as 10% product. Targeted Monitoring Approach The USEPA Corrosion Technical Team of Darren Lytle and Michael Schock recommended working with NDEP to establish a study group of targeted monitoring locations to evaluate the effect of a reduced ZnOP dose across the distribution system. The targeted monitoring approach would focus on residences with copper plumbing only, since none of the 2

3 SNWA purveyors have lead service lines, and homes constructed within the past 2 years will not have had any pipes and/or fittings with lead solder. Volunteers were requested from employees and current LCR sampling participants from the Las Vegas Valley Water District (LVVWD), City of Henderson (COH), and City of North Las Vegas (CNLV). Volunteer participants from each entity were as follows: employees (20), LVVWD (13), COH (12), and CNLV (9). Volunteers were required to have a home built between 1989 and present, have copper plumbing, and have a kitchen or bathroom tap that is not subject to home treatment, such as softening. Water quality samples were collected for lead, copper, and zinc from all of the LCR participants. While employee volunteers collected samples for orthophosphate and ph in addition to lead, copper, and zinc. The samples were to be collected after a 6 to 24 hour stagnation period. Baseline samples were collected in July 2009, before the ZnOP dose reduction. Then, quarterly sampling was completed (September 2009, December 2009, March 2010, and June 2010) to assess corrosion after the ZnOP dose reduction. After each sampling event, SNWS staff compiled the data and met with NDEP to review the results. These reviews confirmed that the reduced ZnOP dose continued to be in compliance with the LCR, and protective of public health. If any quarterly results exceeded an action level, the customer/volunteer was notified and the NDEP was consulted. SNWS and NDEP needed to mutually agree upon the determination of whether a specific result or group of results constituted failure of the dosage-reduction trial. If LCR compliance was jeopardized by the targeted monitoring study results, then the ZnOP dose would have been returned to the original dose of 2 mg/l as product (0.2 mg/l as PO 4 ). The targeted monitoring study data was not be included in LCR compliance calculations. However, the data has been provided to the affected public water systems (LVVWD, COH, and CNLV) and submitted (and flagged as non-regulatory data) to the NDEP. RESULTS Finished Water Quality The raw and finished water ph, alkalinity, TDS, temperature, and dissolved inorganic carbon (DIC) are important water quality parameters when evaluating the potential for lead and copper corrosion, as shown in Figures 1 through 5 respectively. The raw water ph for both the AMS and RM is typically between 7.9 and 8.3. The finished water ph is different between the facilities due to the use of chlorine gas at the AMS and the use of sodium hypochlorite at the RM. As a result, chlorine gas reduces the finished water ph at AMS to a typical range between 7.5 and 7.8. While the sodium hypochlorite used at RM produces a typical finished water ph ranging between 7.7 and 8.0. DIC is a calculated parameter determined from the ph, alkalinity, and total dissolved solids; it is important when comparing SNWS waters to other distribution systems. 3

4 ph 7.8 ph AMS Raw 7.2 RM Raw AMS Finish 7.0 Jun-97 Jun-01 Jun-05 Jun-09 RM Finish 7.0 Mar-02 Mar-04 Mar-06 Mar-08 Figure 1. Raw and finished water ph values for (a) AMS and (b) RM Alkalinity (mg/l) AMS RM Figure 2. Historical alkalinity concentration in the finished water of AMS and RM 4

5 AMS RM 700 TDS (mg/l) Figure 3. Historical TDS concentration in the finished water of AMS and RM Temperature ( C) AMS RM 10 Oct-95 Jul-98 Apr Oct-06 Jul-09 Apr-12 Figure 4. Historical temperature in the finished water of AMS and RM 5

6 Dissolved Inorganic Carbon (DIC) (mg/l) AMS RM Targeted Monitoring Figure 5. Historical DIC in the finished water of AMS and RM The volunteer participants collected samples during July and August of 2009 representative of baseline conditions. The baseline data was ranked and shown in Figure 6 for copper and Figure 7 for lead. The 90 th percentile copper value was 0.65 mg/l. All copper concentrations were below the action level of 1.3 mg/l, and range from 0 to 1 mg/l. In September 2009, the first set of samples was collected from the volunteer participants after the ZnOP dose reduction. Results showed that copper concentration increased at 45 of the 51 sampling locations. The 90 th percentile copper concentration increased from 0.65 mg/l to 0.87 mg/l. Lead results remained at similar concentrations to those during baseline sampling. In fact, the laboratory lowered their MRL to mg/l, explaining the apparent decrease in lead concentration on Figure 7. At a meeting on October 27, 2009, the corrosion control committee discussed these results with NDEP. The group decided to continue operating at the reduced ZnOP dose since the 90 th percentile value was still below the 1.3 mg/l action level for copper. Further, it was speculated that the two homes exceeding the copper AL were receiving service water with an appreciable amount of groundwater; however, the copper corrosion due to groundwater influence would not be totally realized until data from the December 2009 results were examined. In December 2009, the second set of samples was collected from the volunteer participants after the ZnOP dose reduction. Fewer data points were collected during this time frame, due in part to the following reasons: a) unwillingness for continued participation by LCR volunteers, b) inconvenience due to the holiday season, c) inadequate/insufficient labeling of sample collection bottles. Results showed that the copper concentration increased at 24 of the 46 6

7 sample locations. The 90 th percentile value was determined to be 0.85 mg/l in December compared to 0.87 mg/l in September. Therefore, the ZnOP dose reduction likely caused the increased copper concentration witnessed in September. Lead results did not appear to be much different than the previous two sampling events Jul-09 Sep-09 Dec-09 Action Level Copper (mg/l) Site Location Figure 6. Copper results from sampling of volunteer participants from employees, LVVWD, CNLV, and COH (Note: Site location numbers are the same for each sampling event) 7

8 0.016 Lead (mg/l) July Sept-Oct Dec-Jan Action Level Site Location Figure 7. Lead results from sampling of volunteer participants from employees, LVVWD, CNLV, and COH (Note: Site location numbers are the same for each sampling event) CONCLUSIONS After performing a literature review, utility survey, coupon testing, and targeted monitoring in the distribution system, the SNWA was able to safely reduce the ZnOP dosage from 2 mg/l to 1 mg/l without compromising LCR compliance. The reduction has resulted in a cost savings of nearly $500,000/yr to the SNWA. ACKNOWLEDGEMENTS The authors would like to thank the corrosion control committee and the NDEP for providing insightful comments throughout this study. We also thank Darren Lytle and Michael Schock for recommending the targeted monitoring approach. We also thank the laboratory division for providing the analytical support for the project, soliciting the targeted monitoring participants, and coordinating the targeting monitoring sampling events. REFERENCES USEPA (1991). National Primary Drinking Water Regulations: Subpart I - Control of Lead and Copper. Fed. Reg. 56:26460 (June 7, 1991). 8