TRA CRWS Alternative Disinfection Evaluation Comparison to the Chlorine Gas Decision Tool from DHS/NACWA

Transcription

1 TRA CRWS Alternative Disinfection Evaluation Comparison to the Chlorine Gas Decision Tool from DHS/NACWA ABSTRACT Gennady Boksiner, P.E. Freese and Nichols, Inc North Market Street Suite 500 LB 51 Dallas, TX Patty Cleveland Trinity River Authority of Texas 5300 S. Collins St. Arlington, Texas Jeffrey E. Caffey, P.E. Alan Plummer Associates, Inc S. University Drive, Suite 300 Fort Worth, Texas Dawn R. Anderson, PE Chiang, Patel & Yerby, Inc Regal Row, Suite 200 Dallas, Texas The Central Regional Wastewater System Treatment Plant (CRWS) is the largest and most prominent of the Trinity River Authority of Texas (TRA or Authority) wastewater facilities. The CRWS plant has been serving the Dallas/Fort Worth metropolitan area since 1959 and is currently permitted to treat up to 162 million gallons of wastewater per day (MGD) on an average daily basis. The TRA CRWS treatment plant consists of preliminary, primary, and secondary treatment; effluent filtration and disinfection; solids handling; and odor control facilities. Historically, the CRWS has relied on rail delivery of liquid Chlorine and Sulfur Dioxide to achieve proper treatment of the plant effluent. Existing disinfection and dechlorination systems are nearing the end of their useful service life and are in need of rehabilitation. In August, 2007 TRA retained the services of Freese and Nichols, Inc., Alan Plummer Associates, Inc., and Chiang, Patel and Yerby, Inc. (Design Team) to perform an evaluation and comparison of alternative disinfection methods that would allow CRWS to continue to meet and exceed effluent permit as well as improve site security and safety of the facility. As part of the evaluation the Design Team developed the following major alternatives: Rehabilitate and expand existing Chlorine and Sulfur Dioxide systems, inclusive of the implementation of new safety and security improvements Utilize on-site generated ozone for disinfection, inclusive of the construction of new generation facilities and contact basins

2 Utilize Ultraviolet (UV) radiation for disinfection, inclusive of the construction of new UV contact basins Utilize on-site generated sodium hypochlorite solution, inclusive of the construction of new generation facilities and equipment and rehabilitation of the existing chlorine contact and dechlorination facilities Utilize bulk delivery of sodium hypochlorite solution, inclusive of the construction of new storage and feed facilities and equipment and rehabilitation of the existing chlorine contact and dechlorination facilities The developed alternatives were compared based on capital, annual operating and life cycle cost basis as well as on non-monetary criteria as selected by the Authority and the Design Team. The results of the evaluation were compared directly to the Chlorine Gas Decision Tool for Water and Wastewater Utilities as developed by the Department of Homeland Security (DHS) and the National Association of Clean Water Agencies (NACWA) in March This paper and presentation are intended to illustrate the evaluation approach taken by the Authority and Design Team to assess available alternative disinfection technologies and their applicability at CRWS as well as to compare evaluation results to DHS/NACWA s Chlorine Gas Decision Tool for Water and Wastewater Utilities. KEY WORDS Wastewater Disinfection, Water Disinfection, Alternative Disinfection, Chlorination, Dechlorination, UV, Ozone, Sodium Hypochlorite, Sulfur Dioxide, Sodium Bisulfite, Chlorine Gas, Decision Tool, DHS, NACWA BACKGROUND High quality, clean water is essential to human health and environment. Attempts to purify water for human consumption date back to 2,000 B.C. Many epidemics throughout human history have been proven to be caused by waterborne microorganisms in the drinking water supplies. The invention of the microscope and the identification of harmful microorganisms in water led to implementation of first drinking water standards in the United States in Supplementation of drinking water standards by numerous legislations, including the Safe Drinking Water Act of 1974, resulted in the virtual elimination of waterborne diseases such as cholera, typhoid, dysentery and hepatitis in the United Stated. Deactivation of the harmful waterborne microorganisms in the drinking water in the United Stated has been responsible for a large part of the 50 percent increase in life expectancy during the 20 th century. In the wastewater treatment industry, the disinfection did not begin to gain popularity in the United Stated until after the passage of the Federal Water Pollution Control Act of Today, virtually all wastewater treatment facilities in the United Stated employ some means of disinfection to meet stringent discharge permits. For nearly 100 years, chlorine has been employed as a disinfectant of choice for water and wastewater treatment facilities. The advantages of using chlorine gas that is typically supplied as a compressed liquid in volumes ranging from 150-lb cylinders to 90-ton rail cars are: Effective in killing pathogens

3 Simple and relatively cheap to produce and distribute Reasonably simple operation and maintenance of feed equipment The only disinfectant that can maintain residual for the distribution system in water treatment Prior to establishment of Stage 1 and Stage 2 Disinfectants and Disinfection Byproducts Rule of 1998 and 2005, chlorine gas had reached its peak popularity in systems that treat drinking water. And until recent developments in technology, chlorine gas was the predominant disinfectant of choice for wastewater treatment facilities. Despite chlorine s multiple advantages and popularity in water and wastewater disinfection, recent trends have dictated that many facilities consider alternative means of disinfection. The major drivers behind the recent shift in disinfection practices can be attributed to the following: Chlorine gas is an Extremely Hazardous Substance requiring extensive planning, managing and reporting. Release of chlorine gas into the atmosphere can result in severe environmental impairment, property damage and human injury and/or death. Following the events of September 11, 2001 and use of chlorine as a chemical weapon in Iraq, the Department of Homeland Security has been concentrating its efforts in minimizing the domestic use of chlorine gas. Multiple recent legislatures on a State and National level have attempted to minimize or eliminate chlorine for use in water and wastewater industries. To minimize their risk, railroad companies are attempting to discontinue transporting chlorine gas in 90-ton rail cars. Recent price increases in natural gas have resulted in sharp increase in the production and delivery of chlorine gas. In wastewater disinfection, chlorine residual must be eliminated prior to discharge. This is often achieved with other hazardous chemicals such as sulfur dioxide. Chlorine has been identified as a major reactant resulting in dangerous disinfection byproducts (DBP) in the drinking water. Studies have indicated that other disinfectants (such as UV, ozone or sodium hypochlorite) do not cause or significantly reduce the formation of DBPs. In addition to recent events to discourage the use of chlorine, several new technologies have emerged that have shown to be effective and cost competitive to chlorine in disinfecting water and wastewater. DHS CHLORINE GAS DECISION TOOL In March 2006, the Department of Homeland Security (DHS) by contract with National Association of Clean Water Agencies developed and released a publication in the form of a compact disc (CD) titled Chlorine Gas Decision Tool for Water and Wastewater Utilities (Decision Tool). The tool is made available to water and wastewater utilities to provide the means to conduct a simple assessment of alternatives to chlorine gas disinfection in hopes of encouraging utilities utilizing chlorine gas to consider alternative disinfection techniques on voluntary basis. The Decision Tool is currently a available free of charge through NACWA to

4 publicly owned water and wastewater utilities and cannot be obtained by consultants or manufacturers directly from NACWA. Figure 1 DHS Chlorine Gas Decision Tool Compact Disk The Decision Tool CD consists of two separate and independent components. The first component is geared for the water utilities and the second is for wastewater utilities. The major differences between the two components are: Dechlorination parameter is included in the wastewater section for chlorine-based alternatives and is absent in the water section. UV input parameters are slightly different for water and wastewater and for water treatment the use of sodium hypochlorite for a secondary disinfectant is provided. Ozone as the primary disinfectant and sodium hypochlorite as the secondary is only available in the water section. Taste and Odor is an additional non-monetary criterion that s present in the water section and is absent in the wastewater section. Each section contains a Users Guide, Microsoft Excel file that is the Decision Tool, a Microsoft Word template for report generation, and an example report with all the blank template parameters filled in. The Microsoft Excel Decision Tool is divided into two primary components. The first component of the decision tool is the Non-Monetary Criteria. The second component is the Cost Estimator Tool. The Non-Monetary Criteria consists of 13 items in the wastewater section and 14 items in the water section. The decision Tool user is required to assign proper weights to each Non-Monetary Criteria and rank the alternatives based on user s needs and experiences. The total of Non-Monetary Criteria points is then added for each alternative and displayed graphically in a column graph format. The Decision Tool does not have built-in flexibility that would allow the user to modify the pre-developed Non-Monetary

5 Criteria to the user-desired criteria. The Non-Monetary Criteria for water and wastewater components as it appears in the Decision Tool is presented in Figure 2. Figure 2 Decision Tool Non-Monetary Criteria for Wastewater (Left) and Water (Right) The Cost Estimator component of the Decision Tool consists of a worksheet with multiple parameters that the user is required to fill in. The list of required cost parameters for water and wastewater sections is as follows: Plant Flow Information o Average and peak flow Chlorination Information o Type of chlorine delivery currently employed (cylinders, bulk or rail) o Current chlorine average and peak usage o Chlorine residual (Water only) o Current system type and spacing information Dechlorination Information (Wastewater only) o Current dechlorination method o Current average dechlorination usage or cost Emergency Scrubber / Existing System Information o Current emergency scrubber information o Input on any capital improvements to the existing system currently needed o Current RMP/PSM costs Chemical Cost Information o Current chemical pricing in $/lb for: Chlorine gas Sulfur dioxide (Wastewater only) Sodium bisulfite (Wastewater only) Sodium hypochlorite

6 Food grade salt Oxygen gas (Water only) UV Specific Cost Information o UV transmittance o Cryptosporidium log inactivation (Water only) o Fecal coliform discharge limits (Wastewater only) General Cost Information o Current electricity cost o 20-year finance rate o Current year Engineering News Record (ENR) Cost Index o Estimated maintenance on facilities as a percentage of the equipment cost o Average maintenance mechanic hourly rate including fringe benefits After the Decision Tool user inputs all the required cost parameters, the Decision Tool calculates and outputs Capital Cost, Total Annual Cost, Present Worth of Annual Costs, and Total Present Worth for each of the alternatives in a table format. The Decision Tool developers claim an estimated accuracy of the costs at +50%/-30%. The example of the Cost Summary Table for the wastewater section is illustrated in Figure 3. Figure 3 Decision Tool Cost Summary Table for the Wastewater Section

7 When all the Non-Monetary and Cost information is complete, the Decision Tool calculates the Benefit of each alternative. The Benefit is a ratio of Non-Monetary Points to Present Worth. The results of the Benefit analysis are displayed graphically as illustrated in Figure 4. Figure 4 Decision Tool Benefits PW Chart for the Wastewater Section The alternative presenting the greatest Benefit is the one with the most Benefit points. TRA CRWS ALTERNATIVE DISINFECTION EVALUATION The Trinity River Authority of Texas owns and operates one of the largest wastewater treatment plants in the Dallas/Fort Worth metropolitan area, the Central Regional Wastewater System Treatment plant, or CRWS. The CRWS facility as shown in Figure 5 is currently permitted for 162 MGD average day flow discharger with an allowable 2-hour peak flow of 405 MGD. The CRWS facility handles the flow from nearly 1.1 million people over approximately a 475 square mile area. Various projects in planning, preliminary design, final design and construction stages are currently under way to expand CRWS to be able to handle year 2020 anticipated 189 MGD average day flow and 2-hour peak flow of 623 MGD. In May 2007, as part of the current expansion preliminary design work, a design team consisting of Freese and Nichols, Inc., Alan Plummer Associates, Inc., and Chiang Patel and Yerby Inc. was retained by TRA to perform alternative disinfection evaluation for the CRWS. Per scope of the evaluation, the design team was tasked to develop and compare the following alternatives for an average plant flow of 189 MGD and peak wet weather flow of 405 MGD: Rehabilitate existing chlorine/sulfur dioxide system to include a railcar enclosure and an emergency scrubbing system

8 Construct a new sodium hypochlorite, on-site generation system and rehabilitate existing sulfur dioxide system or convert to sodium bisulfite Construct a new sodium hypochlorite, bulk delivery system and rehabilitate existing sulfur dioxide system or convert to sodium bisulfite Construct a new UV system Construct a new ozone system Various combinations of UV/ozone and chlorine/sodium hypochlorite alternatives for dual disinfection system Figure 5 Central Regional Wastewater System Central Wastewater Treatment Plant Overall, 16 different alternatives were identified to be screened for further evaluation. As part of the screening procedure, Capital Cost, Annual Operation and Maintenance Cost, and Present Worth Value along with system schematics and site plans were developed for each alternative. Based on the design team s recommendation and TRA s experience and preferences, 4 out of the 16 initial alternatives were selected for further evaluation and pair-wise comparison. The alternatives selected for further evaluation consisted of: Rehabilitate existing chlorine/sulfur dioxide system to include a railcar enclosure and an emergency scrubbing system Construct a new sodium hypochlorite, bulk delivery system and rehabilitate existing sulfur dioxide system or convert to sodium bisulfite Construct a new UV system Construct a new UV system to handle the peak dry weather flows and a sodium hypochlorite bulk delivery system for peak wet weather flows

9 The selected alternatives were compared to each other and ranked on degree of importance based on the criteria selected by the Design Team and TRA as presented in Table 1. Table 1 TRA CRWS Alternative Disinfection Evaluation Pair-Wise Comparison Criteria Once all the criteria components have been compared for each alternative on a simple grater than = 1 point, same as = 0 points, or less than = -1 point, basis the alternative with the highest score is the one with the most benefit. Final results and the recommendation of the Alternative Disinfection Evaluation is pending mid to late Spring COMPARISON OF RESULTS Because the Non-Monetary Criteria in the Decision Tool does not match the pair-wise comparison criteria as selected by the project team for the TRA CRWS Alternative Disinfection Evaluation, and due to the fact that final results of the evaluation are still pending, only the Cost Items of the evaluation and Decision Tool can be compared directly. For purposes of this paper, Alternative Disinfection Evaluation cost items were adjusted to be more generic and to more closely resemble Chlorine Gas Decision Tool alternatives. In addition, the ozone alternative was generated with the water section of the Decision Tool as it is not available in the wastewater section. The summary of costs of the Alternative Disinfection Evaluation and costs as developed by the DHS Chlorine Gas Decision Tool for Water and Wastewater Utilities is presented in Table 2.

10 Chlorine Gas Sodium Hypochlorite Bulk Delivery Sodium Hypochlorite On-Site Generation Capital Cost Ultraviolet Disinfection Ozone Disinfection TRA CRWS Alt. Disinfection Evaluation DHS Chlorine Gas Decision Tool $14,930,000 1 $3,330,000 $14,740,000 $28,860,000 $58,970,000 $14,370,000 1 $3,590,000 $16,010,000 $22,760,000 $41,580,000 4 Deviation -3.9% +7.8% -8.6% -26.8% -41.8% Annual O&M Cost Net Present Worth TRA CRWS Alt. Disinfection Evaluation DHS Chlorine Gas Decision Tool TRA CRWS Alt. Disinfection Evaluation TRA CRWS Alt. Disinfection Evaluation DHS Chlorine Gas Decision Tool TRA CRWS Alt. Disinfection Evaluation $1,150,000 $2,690,000 2 $1,980,000 2 $900,000 $5,520,000 $1,230,000 $3,270,000 3 $2,530,000 3 $1,320,000 $1,990, % +21.6% +27.8% +46.7% % $28,070,000 $34,150,000 $41,220,000 $40,000,000 $117,350,000 $28,450,000 $41,750,000 $45,030,000 $37,890,000 $64,310, % +22.3% +9.2% -5.6% -82.5% 1 Contain items for rail spur relocation and rehabilitation required for chlorine and sulfur dioxide facilities 2 Based on sulfur dioxide dechlorination 3 Based on sodium bisulfite dechlorination 4 Based on water section of the Decision Tool with equivalent plant flow parameters Table 2 TRA CRWS Alternative Disinfection Evaluation vs. DHS Chlorine Gas Decision Tool Comparison of Cost Items It is important to note that the costs presented above are slightly different than the actual evaluation costs due to the following factors: Some capital improvements not typically associated with alternative disinfection have been omitted from the calculations. Life cycle cost analysis has been adjusted to reflect 0% inflation rate per Decision Tool.

11 Life cycle cost analysis has been adjusted to begin at year 0 and end at year 20. Actual evaluation considered initial investment in year 2011 with a 20 year life cycle brought back to 2007 worth. All other cost items in the evaluation are based on 2007 local equipment, chemical, construction materials, labor and electrical budgetary costs. CONCLUSIONS After performing an Evaluation of Disinfectant Alternatives for the TRA CRWS the results of the study were adjusted for comparison to those that were generated with DHS Chlorine Gas Decision Tool. The following are the conclusions regarding the comparison: Non-Monetary section of the Decision Tool does not allow the user to input parameters other than the ones that are inherently built into the tool. However, for simple comparison of the available technologies the provided parameters are applicable for general evaluation of most users. Cost estimates for the ozone alternative generated with the water section of the Decision Tool do not seem appropriate for comparison to the wastewater application. Therefore, the ozone alternative will not be considered in regards to the following discussion. Decision Tool seems to overestimate the annual O&M cost, however, the estimates fall within the stated error margin. Because a prior evaluation of alternative disinfectants took place, many parameters such as an electrical and chemical cost and hourly worker rate were pre-determined, that resulted in more accurate Decision Tool output. A user without a reasonable knowledge of these details can further reduce the quality of the results by using inappropriate values. Since the Decision Tool was developed in 2005, the end user should be cautious about underestimating capital cost with the tool that is three years old. Even though the Decision Tool has an economic inflation adjustment parameter in the form of ENR Cost Index, the ENR Cost Index is generally not the best indicator of inflation in the heavy civil construction industry. The life cycle cost portion of the Decision Tool s estimator is developed for comparison purposes only, and should not be relied for accurate life cycle cost estimates. Important parameters such as inflation rate and operating cost are not present in the Decision Tool and could result in underestimated life cycle cost. Generally, the accuracy of the Decision Tool is within the stated +50%/-30% estimated range. The Decision Tool presents a good comparative cost analysis of the available technologies if the data is normalized with respect to average cost of all alternatives. As shown in Figure 5, general cost patterns of the Decision Tool follow closely to those of the actual study. Normalization of data in Figure 5 was achieved by dividing each alternative s cost to the average of all alternatives (ozone excluded) cost.

12 Figure 6 Normalized Cost of Alternatives (Ozone Alternative Excluded)

13 Though the Decision Tool is a quick, easy and cheap way to gain an insight into the available disinfection technologies and their estimated comparative cost, it does not constitute a site specific evaluation. The cost analysis performed within the Decision Tool is cursory, containing +50%/-30% error. Such a large spread can result in significant monetary difference for a large utility and is not likely to provide a good insight into the actual cost of alternative disinfectant methods. The Decision Tool, however, does provide an accurate comparative, magnitude of scale cost between the available disinfection technologies. Coupled with the Non-Monetary component items and the cost to benefit analysis, the Decision Tool provides a powerful comparison means for evaluating available disinfection technologies to aid in short listing available options. A more definitive, site specific evaluation is recommended to better define anticipated costs and benefits in making a final selection.