2015 American Water Works Association

Transcription

1 SUNNY WANG, MEHUL PATEL, BILL DUNIVIN, JAMES CLARK, STEVEN FOELLMI, AND CATHERINE (SPENCER) DIPIETRO Post-treatment Optimization to Enhance Groundwater Recharge Operations A RECENT EXPANSION OF ORANGE COUNTY (CALIF.) WATER DISTRICT S ADVANCED WATER PURIFICATION FACILITY ALLOWED THE ENHANCEMENT OF ENERGY-RECOVERY DEVICES ON NEW REVERSE OSMOSIS UNITS, SECONDARY EFFLUENT FLOW EQUALIZATION, AND ALTERNATIVE TREATED WATER QUALITY TARGETS. In California, where water has long been recognized as a scarce and valuable resource, the Orange County (Calif.) Water District (OCWD) has demonstrated commitment to advanced water purification and reuse for more than three decades. Completion of the initial expansion of OCWD s groundwater replenishment system (GWRS) early in 2015 made the world s largest advanced water purification system for potable reuse even bigger. OCWD s original GWRS, completed in 2008, has an advanced water purification facility (AWPF) that purifies secondary effluent through microfiltration, reverse osmosis (RO), and advanced oxidation using ultraviolet (UV) light and hydrogen peroxide. Purified water from the AWPF is injected into groundwater aquifers to minimize seawater intrusion and replenish local groundwater supplies. The initial expansion of the GWRS provided an opportunity to enhance AWPF processes while expanding production capacity from 70 to 100 mgd. Enhancements focused on energy-recovery devices on the new RO units, secondary effluent flow equalization, and alternative treated water quality targets, as well as new technologies to enhance the reliability and flexibility of posttreatment operations. Evaluation and enhancement of post-treatment stabilization processes at the AWPF were especially important for system reliability and flexibility. WANG ET AL. 107:5 JOURNAL AWWA MAY

2 The post-treatment regime at the AWPF consists of decarbonation (carbon dioxide removal) and the addition of lime to stabilize the purified water or final product water (FPW). However, overdecarbonation of RO permeate (ROP) often resulted in inadequate carbonates and thus inadequate alkalinity even after lime was added. Inadequate carbonate alkalinity made it difficult to control finished water ph; therefore, it was challenging to maintain ideal corrosion protection parameters and suitability for groundwater recharge. Improvements implemented as part of the initial expansion supported OCWD water quality objectives by enhancing post-treatment operation and reliability and producing more consistent finished-water quality. POST-TREATMENT OPTIMIZATION WITH THE END GOAL IN MIND Passing the entire feed flow through the RO membranes at the AWPF effectively removes contaminants along with most of the dissolved constituents in the water. The resulting ROP is highly corrosive TABLE 1 (low in dissolved solids, including hardness and alkalinity as well as ph) and requires post-treatment stabilization. In such instances, appropriate post-treatment is critical and must accomplish three primary water quality objectives: The post-treatment regime at the advanced water purification facility consists of decarbonation (carbon dioxide removal) and the addition of lime to stabilize the purified water or final product water. produce water that meets regulatory requirements, that is not corrosive to the conveyance system (e.g., to cement mortar lining on the GWRS conveyance piping), and with low scaling potential in order to prevent plugging at groundwater injection wells. The project team adopted a comprehensive approach for the entire post-treatment regime (from decarbonation through injection) to increase the FPW s alkalinity and reliability to meet water quality objectives. With this end goal in mind, an alternative FPW goal was developed for the GWRS that enhanced reliability and consistency of the FPW water quality for groundwater injection. The original and alternative post-treatment goals are summarized in Table 1. The benefits of changing the FPW target were as follows: Summary of current and recommended post-treatment FPW goals Parameter Original Goal Alternative FPW Goal ph Alkalinity mg/l as CaCO Calcium mg/l Hardness mg/l as CaCO Free carbon dioxide mg/l None <3 Buffer intensity None CCPP mg/l as CaCO 3 4 to 2 3 CaCO 3 calcium carbonate, CCPP calcium carbonate precipitation potential, FPW final product water The potential for calcium carbonate (CaCO 3 ) scaling at a seawater intrusion barrier would be controlled by maintaining ph below the values at which CaCO 3 forms (e.g., <8.0). The increased alkalinity and lower ph would increase buffer intensity (often referred to as buffer capacity). Buffer intensity is a measure of parameters such as inorganic carbonates or phosphates (if present) in the water that resist ph change. A higher buffer intensity could aid in stabilizing the treated water ph. The increased alkalinity and calcium levels would be more protective of the cement mortar on the cement mortar lined conveyance pipelines. The free carbon dioxide levels would be higher compared with the current average, ensuring that sufficient carbon dioxide is available for conversion to bicarbonate when lime is added. The FPW would be more representative of groundwater quality from natural recharge (rainfall). The alternative FPW goal would result in more stable product water (e.g., lower susceptibility to ph variations) and provide better protection for downstream conveyance pipelines. Research on ROP post-treatment (AWWA Research Foundation 1996) has demonstrated that finished water with calcium levels in excess of 20 mg/l as CaCO 3, alkalinity values in excess of 50 mg/l as CaCO 3, and slightly positive CaCO 3 precipitation potential values was more protective of cement mortar lined pipe than water with lower calcium and carbonate concentrations. Higher alkalinity and calcium levels correspond to lower rates of cement mortar degradation, even if calcium carbonate precipitation potential is slightly negative (AWWA Research Foundation 1996). Thus, it was recommended that OCWD use a potential CaCO 3 precipitation target that was slightly negative (i.e., 5 to 0 mg/l) in combination with 88 MAY 2015 JOURNAL AWWA 107:5 WANG ET AL.

3 the proposed finished water targets for calcium, alkalinity, and ph to protect against cement mortar corrosion. The project team recommended and implemented additional improvements to support the alternative FPW goal and increase operational flexibility and reliability for the initial expansion of the GWRS, including the following: A new bypass guideline for the decarbonation towers, following UV advanced oxidation process, was developed to ensure that adequate bicarbonate alkalinity and buffering capacity were maintained. A new lime slurry feed system was pilot-tested and incorporated to improve the quality and reliability of the entire lime feed system from dry hydrated lime feed through slurry makeup and lime saturation. DECARBONATION BYPASS The AWPF s current average bypass percentage is approximately 20 30%, with 70 80% of the UV product water (UVP) flow directed through the decarbonators. The primary reason for sending the majority of the UVP flow through the decarbonators is to minimize the costs of chemicals that raise the ph of the ROP or UVP. However, given the low alkalinity values of the water after RO and UV, coupled with low but variable ph, the decarbonation system must be operated to maintain sufficient carbon dioxide that can be TABLE 2 ph of UVP UVP bypass/decarbonation flow percentages as a function of UVP ph <5.9 UVP Bypass % converted to bicarbonate alkalinity when lime is added. New operating guidelines for the amount of flow bypassing the decarbonators were developed to increase alkalinity and support the alternative FPW goal. These guidelines, shown in Table 2, are used when the UVP ph ranges from 4.9 to 5.8. If the UVP ph is 5.9 or greater, as little UVP as possible should be decarbonated; if the UVP ph is below 4.8, all of the water should be decarbonated. LIME ADDITION A side-by-side pilot test was conducted to compare the performance of the original flow-paced lime slurry system against a weight-controlled lime-slaking system. 1 This test was UVP Decarbonation % Estimated ph of DPW 5.8 a a a a DPW decarbonated product water, UVP ultraviolet product water a Range of ph planned by Orange County Water District for future reverse osmosis operations essentially conducted at full scale with the original lime slurry system 2 feeding one saturator lime saturator A and a temporary weightcontrolled lime-slaking system feeding another saturator lime saturator B. Each lime slurry makeup system fed a separate lime saturator at the AWPF. The photograph on page 91 shows the pilot-test facility. On the basis of data collected from the pilot test, the weight-controlled lime-slaking system produced a more consistent and repeatable lime slurry than the original lime slurry system (see percent solids data in Table 3) with respect to the target percent concentration selected (7%). The standard deviation of the weightcontrolled lime-slaking system slurry TABLE 3 Comparison of lime slurry percent solids data a Weight-Controlled Lime-Slaking System Original Lime Slurry System Measure Slurry Feed Saturator Flow Lime Slurry % solids Slurry Feed Saturator Flow Lime Slurry % solids Average Minimum Maximum Standard deviation a Operation set point for both systems = 7% solids WANG ET AL. 107:5 JOURNAL AWWA MAY

4 FIGURE 1 Turbidity ntu /24/2009 FIGURE 2 Turbidity ntu Comparison of lime saturator effluent turbidity fed by existing flow-paced lime slurry system versus weight-controlled lime slurry system New weight-controlled lime slurry system New weight-controlled lime slurry system average 3.5 ± 1.7 Existing flow-paced lime slurry system Existing flow-paced lime slurry system average 5.8 ± /03/ /13/ /23/ /03/2009 Date 12/13/ /23/ /02/2010 Comparison of lime saturator effluent turbidity versus AWPF effluent flow New weight-controlled lime slurry system Linear (new weight-controlled lime slurry system) Existing flow-paced lime slurry system Linear (existing flow-paced lime slurry system) 01/12/ AWPF advanced water purification facility AWPF Production Flow mgd was only 0.08 compared with 0.70 for the original lime slurry system. The lower variability of the lime slurry concentration produced by the resulted in a more accurate lime solution produced to stabilize the finished product water. Performance of the two saturators with respect to percent solids, ph, conductivity, total dissolved solids, and alkalinity was comparable. However, turbidity of the saturated lime solution from saturator B (lime slurry fed by the weight-controlled limeslaking system) was two units lower compared with saturator A (lime slurry fed from the original lime slurry system) 5.8 versus 3.5 ntu. A plot of the two saturator effluent turbidities over the course of the pilot test is shown in Figure 1. The redshaded region indicates 1 standard deviation of the lime saturator effluent (saturator A) turbidity, fed by the existing flow-paced lime slurry system. The blue-shaded region indicates 1 standard deviation of the lime saturator effluent (saturator B) turbidity, fed by the new weight-controlled lime slurry system. The lower saturated lime effluent turbidity observed in saturator B suggests that the new produces a lime slurry that reacts more readily and dissolves better than the original flow-paced lime slurry system. The lower saturated lime effluent turbidity readings in lime saturator B were also supported by Secchi disk readings that were generally 1 ft clearer when compared with lime saturator A. The lower turbidity and better clarity in saturator B suggest that fewer particulate/colloidal impurities are produced with the because the system s slaker and aging tank ensure adequate time for the dry, hydrated lime to fully react with the dilution water. The effluent turbidity of the two saturators is plotted against AWPF production flow in Figure 2. The lime saturator fed by the original lime slurry system exhibited an increase in effluent 90 MAY 2015 JOURNAL AWWA 107:5 WANG ET AL.

5 turbidity with increasing AWPF production when a greater amount of lime was needed. The saturator fed by the was not affected by varying lime demand and AWPF production. During high-flow periods, more lime is needed to stabilize the FPW, and batch reaction time in the original lime slurry system dramatically decreases because it is a flow-paced system. Variable AWPF production and lime demand decrease overall reaction time and the lime slurry consistency produced by the original system during peak demand periods. Because the weightcontrolled lime-slaking system operates in a true batch mode and accommodates varying lime demand through the aging tank, consistent lime slurry is produced within the weight-controlled lime slaker regardless of lime demand or AWPF production. Pilot-test results (e.g., particle size distribution, turbidity, percent solids) showed that the new weightcontrolled lime slurry makeup system provides a more stable lime dose that is not affected by diurnal flow variations at the AWPF. The project team determined that the consistency and accuracy of the lime slurry produced by the weight-controlled lime-slaking system would provide OCWD with additional control over the posttreatment process at the AWPF. CONCLUSION The initial expansion of OCWD s GWRS facilitated a holistic approach to optimize post-treatment operations at the AWPF and to enhance reliability in producing water that meets regulatory requirements and will not corrode the conveyance and groundwater injection facilities. The alternative FPW goal and supporting process improvements will give OCWD operation staff more flexibility and reliability in meeting finished water quality targets to minimize corrosion and enhance groundwater recharge operations. ENDNOTES 1 RDP Technologies, Norristown, Pa. 2 ZMI Portec Chemical Processing Group, Sibley, Iowa Bulk bag loader Slurry-aging tank The lime-slaking pilot unit consists of a slaker tank, slurry-aging tank, and transfer pumps feeding lime saturator B during side-by-side testing. Photo courtesy of Orange County Water District. ABOUT THE AUTHORS Sunny Wang (to whom correspondence may be addressed) is a senior process engineer in Black & Veatch s Water Technology Group, 800 Wilshire Blvd., Ste. Transfer screw Slaker tank Purified water from Orange County Water District s (OCWD s) Advanced Water Purification Facility, highlighted here to distinguish it from the adjacent OCWD Wastewater Treatment Plant, is injected into groundwater aquifers to minimize seawater intrusion and replenish local groundwater supplies. Photo courtesy of Orange County Water District. 600, Los Angeles, CA USA; wangsunny@bv.com. He served as task leader for posttreatment improvements for initial expansion of Orange County Water District s (OCWD s) groundwater replenishment system (GWRS) and specializes in process optimization and design of WANG ET AL. 107:5 JOURNAL AWWA MAY

6 advanced water and wastewater treatment technologies. Mehul Patel is the GWRS program manager at the OCWD, Fountain, Calif., and is the former president of the American Membrane Technology Association. William Dunivin is the director of water production at OCWD, Fountain, Calif. James Clark is a senior vice-president with Black & Veatch, Los Angeles, Calif., and served as the project manager for the initial expansion of OCWD s GWRS. Steven Foellmi is a vice-president with Black & Veatch, Irvine, Calif., and served as the engineering manager for the initial expansion of OCWD s GWRS. Catherine (Spencer) DiPietro, who participated in the project as a process engineer with Black & Veatch, is now the president of Aquarius Engineering, Pownal, Maine. REFERENCE AWWA Research Foundation, 1996 (2nd ed.). Internal Corrosion of Water Distribution Systems. AWWA Research Foundation, Denver, Colo. SUGGESTED READING Faust, S.D. & Osman, A.M., 1998 (2nd ed.). Removal of Corrosive Substances. Chemistry of Water Treatment, CRC Press, Boca Raton, Fla. USGS (US Geological Survey), Microbial and Dissolved Organic Carbon Characterization of Storm Flow in the Santa Ana Basin at Imperial Highway, Southern California, Scientific Investigations Report USGS, Reston, Va.. pubs.er.usgs.gov/publication/sir (accessed Feb. 25, 2015). USGS, Ground-Water Quality in the Santa Ana Watershed, California: Overview and Data Summary. Water Resources Investigation Report USGS, Reston, Va. wri/wri /text.html (accessed Feb. 25, 2015). AWWA RESOURCES Potable Reuse: Developing a New Source of Water for San Diego. Steirer, M.A. & Thorsen, D., Journal AWWA, 105:9:64. Product No. JAW_ Internal Corrosion Control in Water Distribution Systems. AWWA Manual M58 (1st ed.), Catalog No PDF. Reclaimed Water: Optimize Groundwater Recharge Reliability. Bradshaw, G., Opflow, 35:3:22. Product No. OPF_ These resources have been supplied by Journal AWWA staff. For information on these and other AWWA resources, visit 92 MAY 2015 JOURNAL AWWA 107:5 WANG ET AL.