Water Quality Study Part 2 Water Treatment Plant Residuals

Transcription

1 2121 Main Street, Suite 300 Dallas, Texas Water Quality Study Part 2 Water Treatment Plant Residuals November 2010 Report Prepared By: Malcolm Pirnie, Inc Coit Road Suite 1200 Dallas, Texas (972)

2 Table of Contents Contents 1. Introduction Introduction Background Objectives Approach Background Enhanced Softening Residuals Calculations Theoretical Calculation of Enhanced Coagulation Residuals Production Density of the Residuals Water Quality Findings Existing Flows Volume Calculations and Residuals Production Comparison Conclusions Conclusions List of Tables Table 2-1 Predicted Range of Chemical Doses for Enhanced Organics Removal Table 2-2 Average Monthly Influent and Effluent Turbidity Table 3-1 Historical WTP Flow Rates Used to Estimate Residuals Production for Enhanced Softening and Enhanced Coagulation Table 3-2: Current Project Residuals Lagoon Storage Capacity Table 3-3: Current Projected Residuals Lagoon Remaining Useful Life List of Figures Figure 3-1: East Side WTP Residuals Production for Current (Enhanced Softening) and Future (Enhanced Coagulation) Operation Figure 3-2: Elm Fork WTP Residuals Production for Current (Enhanced Softening) and Future (Enhanced Coagulation) Operation Figure 3-3: Bachman WTP Residuals Production for Current (Enhanced Softening) and Future (Enhanced Coagulation) Operation i

3 1. Introduction 1.1. Introduction (DWU) is planning to move forward with changes to their water treatment plant processes to convert from an enhanced softening approach to an enhanced organics removal strategy. Malcolm Pirnie recommended this process change as part of the Treated Water Quality Study (Part 1) to increase the chemical and biological stability of the finished water. This change is expected to have an impact on solids production at the treatment plants. Therefore, Malcolm Pirnie conducted a review of the potential impacts based on projected operating parameters for enhanced organics removal. This memorandum presents findings of the residuals production analysis Background DWU generates a significant volume of residuals while producing potable water at the Bachman, Elm Fork, and East Side water treatment plants (WTPs). The WTPs currently use lagoons to temporarily store residuals prior to ultimate disposal. Part 1 of the Treated Water Quality Study included an assessment of the amount of residuals produced with the current enhanced softening treatment process and the effectiveness of the current storage practices. A change to practicing enhanced coagulation is expected to alter the amount and nature of residuals produced. Therefore, this evaluation assesses the impact of the process change on residuals production and lagoon capacity Objectives This memorandum initially explains the reasons for the differences in residuals production between the softening process and the enhanced coagulation process. The memorandum then presents the calculated changes in water residuals production at the three different WTPs and compares predicted residuals production to the existing residuals production. 1-1

4 2. Approach 2.1. Background A new equation for calculating residuals is required to account for changes in water chemistry and to compare the residuals production from the existing softening process to the proposed enhanced coagulation process. The specific reasons for this are discussed further below. The difference in residuals production between the new process and the existing process is compared on an average monthly basis and on cubic yards of residuals produced per million gallons of treated water. A change in the lime dose is the primary difference between the existing enhanced softening process and the new enhanced coagulation process. The Water Quality Study Part 2 Chemical Systems Evaluation (Malcolm Pirnie, November 2010) memorandum provided a range of doses that will be required for the new enhanced coagulation process pending confirmation from ongoing pilot testing. The pertinent chemicals for residuals production include lime, coagulant (ferric sulfate), and polymers. Table 2.1 shows the expected range of chemical doses: Table 2-1 Predicted Range of Chemical Doses for Enhanced Organics Removal Dose (mg/l) Chemical Low Med High Ferric Sulfate Lime Coagulant Aid Polymer Filter Aid Polymer* *Filter aid polymer may not be used, if the plants practice biological filtration with granular activated carbon (GAC) media 2.2. Enhanced Softening Residuals Calculations Chapter 7 of the Treated Water Quality Study, Part 1 Report, Treatment Plant Residuals (January 2008), includes recommendations that DWU adopt a new equation for estimating water treatment plant residuals production based on a more fundamental evaluation of water chemistry. This equation differed slightly from DWU s historical method of calculating residuals production. The fundamental difference was the use of change of hardness instead of change in alkalinity in the formula. The equation for the calculating the residuals production (as recommended in Part 1 study) is shown below. 2-1

5 Section 2 Approach Weight of Residuals (lbs) = Q * 8.34 * [β (Turbidity Removed) + (Hardness Removed) * Lime Dose *Ferric Sulfate Dose + Polymer Dose] Q = Plant flow rate (million gallons (MG)) β = beta factor relating turbidity to solids produced All doses are presented in mg/l 1.79 & 0.57 are required for the stoichiometry Chapter 7 of the Part 1 Report demonstrated that this method was a more accurate predictor of the quantity of solids produced by the three WTPs, by using historical data and comparing the calculated residuals volume to the surveyed quantity of residuals in the treatment plant residuals storage ponds Theoretical Calculation of Enhanced Coagulation Residuals Production Because DWU now has a more accurate measure for estimating residuals production from their current enhanced softening process, an equation for estimating the residuals production based on the new enhanced coagulation process is also required. The proposed modification to the water treatment process requires a significantly lower lime dose, as the purpose for the lime is to provide ph adjustment and maintain alkalinity across the process rather than for softening. This change will affect two fundamental parameters in the residuals production equation, hardness and lime dose. First, because the dose of lime will not create a sufficiently high calcium concentration to precipitate at the target operating ph of 7.5 in the treatment plant to 8.7 in the distribution system, there will be no significant reduction in hardness and quite possibly a slight increase in hardness across the plant as the lime stays in solution. Second, when softening, the lime dose is assumed to precipitate out with the softening process and since the treatment plants will no longer be softening much of the lime dose will remain in suspension. Typically 10% of applied lime is insoluble so 10 mg/l of lime produces about 1 mg/l of dry solids. Based on these water chemistry changes, the residuals production equation can be modified as follows: 1) eliminate the hardness removed component and 2) modify the coefficient of the lime dose. The modified residuals production equation for the enhanced coagulation process is shown below: 2-2

6 Section 2 Approach Weight of Residuals (lbs) = Q * 8.34 * [β (Turbidity Removed) + 0.1*Lime Dose *Ferric Sulfate Dose + Polymer Dose] Q = Plant flow rate (MG) β = beta factor relating turbidity to solids produced All doses are presented in mg/l 0.1 & 0.57 are required for the stoichiometry 2.4. Density of the Residuals The equations in Section 2.2 and 2.3 provide the means for calculating the weight of residuals produced for the enhanced softening and the enhanced coagulation process respectively. However, the volume of residuals produced will be of much greater concern to DWU since it determines the useful life of the treatment plant lagoons and will impact lagoon operations and maintenance (O&M). The volume of residuals is measured in cubic yards (CY) and density is measured in pounds per cubic foot (pcf). The equation for the volume of residuals produced is given below: Volume (CY) = Weight (lbs)/ Density (pcf) / 27 (ft 3 /yd 3 ) The density of the solids can be variable and is difficult to precisely determine and estimate. To obtain accurate density numbers, extensive sampling at all three treatment plants would be required. However, the density values in the literature are approximately that of water or perhaps slightly higher depending on the solids concentration of the residuals. The density of water is approximately 62.4 pcf. In part of the Part 1 Study, Nathan D. Maier (NDMCE) determined the density of the residuals based on calculating the weight (lbs) of residuals produced and correlating to lagoon survey data. Using the weight of residuals calculated from the equation in Section 2.3, and the volume occupied by the survey data, the resulting calculated densities of the residuals for the East Side, Elm Fork and Bachman WTPs were 42, 46, and 40 pcf, respectively, based on the larger volume occupied by the lagoons. However, theoretical value of the density of the residuals is expected to be closer to 62.4 pcf (as discussed above). The lower calculated densities results from larger volume occupied in the lagoons than predicted by residuals alone. Besides the volume contribution by residuals, there are other volume contributions due to basin blowdown and spent filter backwash wastewater. Based on the volume of residuals calculated from the equation in Section 2.3 and density of 62.4 pcf, and comparing with the survey data, it appears that approximately 70 percent of the volume contribution is due to the residuals and approximately percent of the volume contribution is due basin blowdown and spent filter backwash wastewater. 2-3

7 Section 2 Approach Given the change from softening, it is more probable that the density of the residuals from the new treatment process will be more approximately that of water or slightly higher. For estimating the residuals production of the new enhanced coagulation process it is assumed in this assessment that the density of the residuals will be 62.4 pcf Water Quality The water quality parameter included in the residuals production equation that will affect residuals production for the new enhanced coagulation treatment scenario is turbidity. In Chapter 7 of the Treated Water Quality Study Part 1 Report, Treatment Plant Residuals (January, 2008), the influent and effluent turbidities from January, 2001 to December, 2006 were presented. These monthly averages were used for the residuals calculations in this report. The average turbidities are presented in Table 2-2 below: Table 2-2 Average Monthly Influent and Effluent Turbidity Treatment Plant Turbidity (NTU) Influent Effluent East Side Elm Fork Bachman

8 3. Findings 3.1. Existing Flows To estimate the residuals production the average monthly flow rates from the three WTPs was determined to estimate the residuals production. Data from July 2005 to August 2006 was used to determine the average monthly flow rates from the plants. Table 3-1 provides the flow data for the three plants. Table 3-1 Historical WTP Flow Rates Used to Estimate Residuals Production for Enhanced Softening and Enhanced Coagulation Water Treatment Plant Bachman Elm Fork East Side Month Total flow MG Total flow MG Total flow MG Jul-05 1,441 7,825 8,435 Aug-05 * 8,378 9,141 Sep-05 2,542 6,937 8,303 Oct-05 2,456 6,438 7,457 Nov-05 2,499 3,920 6,989 Dec-05 1,315 3,794 6,823 Jan-06 * 5,652 6,847 Feb-06 * 4,481 5,060 Mar-06 2,087 4,420 5,078 Apr-06 3,243 3,431 6,800 May-06 3,103 4,335 8,002 Jun-06 2,767 5,373 9,580 Jul-06 3,063 6,092 10,126 Aug-06 3,042 6,355 10,719 Sep-06 1,970 5,757 8,251 Oct-06 2,521 5,376 6,891 Nov-06 2,278 4,062 6,491 Dec-06 * 2,710 5,779 Average Monthly Flow 2,452 5,296 7,598 (MG) * Data unavailable 3-1

9 Section 3 Findings 3.2. Volume Calculations and Residuals Production Comparison Using the recommended calculation method for the enhanced softening process (Section 2.2) and for the density of the residuals (Section 2.4), NDMCE estimated residuals production per MG of treated water and presented that number in the Part 1 Report. The residuals production estimates were independently verified by Malcolm Pirnie. The change in the water treatment process from softening to enhanced coagulation will result in a significant reduction of residuals production for all three WTPs. Figure 3-1, 3-2, and 3-3 present the current (enhanced softening) and anticipated future (enhanced coagulation) residuals production, in cubic yards (CY) of residuals per million gallons (MG) of water treated, based on the plants average monthly flow rates. For the future treatment process, only the average and maximum chemical usages are presented, as these will provide DWU the requisite planning scenario for their lagoon management practices East Side WTP Filter Backwash/Basin Blowdown Residuals CY/MG Current Operation Future High Dose Future Avg dose Figure 3-1: East Side WTP Residuals Production for Current (Enhanced Softening) and Future (Enhanced Coagulation) Operation 3-2

10 Section 3 Findings Elm Fork WTP Filter Backwash/Basin Blowdown Residuals CY/MG Current Operation Future High Dose Future Avg dose Figure 3-2: Elm Fork WTP Residuals Production for Current (Enhanced Softening) and Future (Enhanced Coagulation) Operation Bachman WTP Filter Backwash/Basin Blowdown Residuals CY/MG Current Operation Future High Dose Future Avg dose Figure 3-3: Bachman WTP Residuals Production for Current (Enhanced Softening) and Future (Enhanced Coagulation) Operation Using the existing monthly average residuals production, the existing lagoon survey data, and the time elapsed since each survey; an estimate of the current residual storage lagoon available volume was calculated. Table 3-2 projects the residuals lagoon storage capacity for each of the three plants. 3-3

11 Section 3 Findings Table 3-2: Current Project Residuals Lagoon Storage Capacity East Side Elm Fork Bachman Pond Volume (CY) Surveyed Volume (CY) Residuals produced since survey (CY) Estimated Occupied Volume (CY) Cell #1 456, , ,000 Cell #2 476, , , ,565 Cell #3 486,000 65,000 65,000 Cell #4 493,000 16,000 16,000 Cell #5 480,000 46,000 46,000 Total 2,391, , , ,565 Total Remaining volume = 1,508,435 Pond A 146, ,000 22, ,000 Pond B 123,800 71,000 62, ,000 Pond C 75,300 1,000 56,886 57,886 Total 345, , , ,886 Total Remaining volume = 10,714 Pond 1 90,500 7,400 29,422 36,822 Pond 2 308,000 12,500 29,422 41,922 Total 398,500 19,900 58,843 78,743 Total Remaining volume = 319,757 Based on the projected remaining lagoon volume, estimates were made for the remaining useful life of the residual storage lagoon at each of the three WTPs, as shown in Table 3-3. The useful life estimated above is based on the total volume reaching the lagoons, which includes residuals, spent filter backwash wastewater, and basin blowdowns. The useful life of the lagoons presented will increase if the spent filter backwash wastewater is isolated, treated, and diverted away from the lagoons. Table 3-3: Current Projected Residuals Lagoon Remaining Useful Life Total Remaining Volume (CY) Useful Life - Existing Softening Process (YR) Useful Life - Enhanced Coagulation Average Dose (YR) Useful Life - Enhanced Coagulation Maximum Dose (YR) Treatment Plant East Side 1,508, Elm Fork 10, Bachman 319,

12 4. Conclusions 4.1. Conclusions The conclusions that can be derived from the data analysis are that the DWU water treatment plants could realize a percent reduction in residuals generated when converting from enhanced softening to enhanced coagulation treatment process. The reduction in residuals production will increase the life of the residuals storage lagoons and should decrease the O&M costs of residuals management. 4-1