PROCESS CONTROL STRATEGIES FOR THE SAWS BRACKISH GROUNDWATER DESALINATION PROJECT'S ENHANCED RECOVERY RO TREATMENT PROCESS.

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1 PROCESS CONTROL STRATEGIES FOR THE SAWS BRACKISH GROUNDWATER DESALINATION PROJECT'S ENHANCED RECOVERY RO TREATMENT PROCESS Jarrett K. Kinslow, P.E., Tetra Tech, 201 E. Pine Street, Orlando, FL Phone: Duane E. Bryant, PE, San Antonio Water System, San Antonio, TX Saqib H. Shirazi, PE, PMP, San Antonio Water System, San Antonio, TX Abstract Technology continues to change our everyday lives, creating paradigm shifts in how we work, learn, and interact as a society. In dealing with modern day challenges of water and our environment, the concept of One Water has similarly made impacts on how utilities view the use and management of water resources. Membrane treatment technologies such as NF and RO have become essential tools both in water desalination and in recycled water treatment. Both processes utilize spiral wound membrane elements that generate a product water or permeate stream and both will generate a concentrate stream that can be either reused or disposed. While the typical recovery rates of these treatment processes have historically ranged from 75-85%, there is a persisting need for technological advances to allow higher recovery of existing supplies which is further amplified in times of water shortage and scarcity. San Antonio, along with many parts of Texas is experiencing double digit growth rates every 10 years. San Antonio Water System (SAWS) must be able to provide adequate water supplies in order to meet this growth. SAWS, currently, gets the majority of their drinking water from the Edwards Aquifer, but water levels are decreasing, resulting in limitations on water withdrawals imposed by regulatory agencies. This reality has resulted in SAWS looking for ways to diversify their water sources, including brackish groundwater located in the Lower Wilcox Aquifer. To meet their water supply needs, SAWS is developing a proposed Brackish Groundwater Desalination (BGD) Program consisting of a reverse osmosis (RO) treatment facility that is currently under construction and scheduled to be in service by October At its proposed buildout capacity of 30 MGD the SAWS BGD project will be the largest municipal RO treatment facility in the State of Texas and one of the largest in-land potable desalination facilities in the United States. A key component to making this alternative water supply option a reality for SAWS was the need for a treatment process that could maximize the use of raw water supply resources while minimizing the concentrate disposal flow. Due to the high costs and limited capacity of in-land concentrate disposal wells, the implementation of an enhanced recovery RO process was essential to the feasibility of the project. This paper will provide insight into challenges faced in the RO process design of large capacity in-land desalination facilities. Specifically the paper will discuss unique treatment process optimizations that were made in response to capacity constraints in the concentrate disposal 1

2 facilities. Control strategies for the 3-stage RO treatment process that utilizes a separate 3 rd stage array will be discussed and compared to conventional 3-stage RO designs. Introduction and Project Background For several years leading up to the decision by SAWS to begin implementation of a brackish groundwater desalination program, a series of studies and preliminary testing were conducted by the Texas Water Development Board and various engineering and hydrogeological consultants for the SAWS. These studies included pilot testing of membrane treatment utilizing RO membranes, pipe corrosion loop testing using various blends of RO permeate, brackish raw water, and water from existing treatment facilities that utilize the Edwards aquifer as a supply source, treatment facility feasibility studies, and evaluations of the proposed brackish groundwater supply source by hydrogeological consultants. Pilot testing was conducted using water available from the Wilcox aquifer from test wells that were constructed during initial exploratory hydrogeological work. The water quality of the test wells indicated slightly brackish water on the order of 1,300-1,600 mg/l TDS, with some variability between wells. In general, the test well water quality was high in sulfate, sodium, and calcium hardness but relatively low in chloride when compared to what is typically seen with relation to sodium levels. Color and organics in this supply source were also observed to be low. Reverse Osmosis (RO) treatment was quickly identified as a potential treatment technology for the proposed brackish water source. In Texas, RO is considered an innovative treatment technology by the Texas Commission on Environmental Quality (TCEQ), which has historically required that all proposed membrane facilities undergo pilot testing in order to obtain permits for treatment facilities that provide drinking water supply. Pilot Testing Pilot testing of one of the test wells was conducted by Carollo Engineers for SAWS in Four phases of initial pilot testing were carried out that included both 2-stage and 3-stage RO pilot operations utilizing Toray membrane elements. Primary objectives of the BGD Membrane Pilot Test included: 1. Determine practical limits to RO recovery of the brackish groundwater supply 2. Evaluate a closed to atmosphere versus open to atmosphere raw water transmission system and any needs for pretreatment in addition to standard cartridge filtration. 3. Confirm chemical pretreatment doses at varying recovery rates. 4. Obtain 90-days of RO membrane operating data, including 30 days of data at steady-state operation to utilize for full-scale facility design development. 5. Evaluate product water quality and any potential post treatment processes that may be necessary. 6. Provide sufficient data to satisfy TCEQ membrane pilot testing requirements and enable SAWS to proceed with implementation of a full-scale BGD RO facility. 7. Evaluate concentrate water quality to determine impacts on various concentrate pipe materials and injection well formations. 2

3 Following initial pilot testing SAWS opted to conduct additional pilot testing of RO membrane elements from Dow FilmTec and Hydranautics in order to obtain TCEQ approval for a total of three (3) different membrane manufacturers to facilitate competitive bidding of the full scale system. The pilot units consisted of a 3-stage RO membrane array that utilized 4-inch diameter elements in the 1st and 2nd stages, and 2.5-inch diameter elements in the 3rd stage. The results of the pilot testing indicated that a recovery rate of 90% was attainable utilizing a 3-stage array, with some scaling observed in the 3rd stage that was manageable with periodic cleaning. Supply and Disposal Well Capacity Considerations Preliminary evaluations of the feasibility of concentrate disposal wells for the BGD project indicated that the capacity and cost of such wells would be a substantial consideration for overall project feasibility. The initial assumptions were that the individual injection well design would be limited to gpm per well at an injection pressure of up to 750 psi. Additionally, as exploratory production well construction and aquifer testing of the Wilcox aquifer was undertaken, it also became evident that the available supply productivity of each brackish well was not as abundant as initially projected in previous studies conducted by the Texas Water Development Board. This meant that higher drawdowns would occur in each well and a higher quantity of supply wells would need to be running to provide the raw water capacity that was planned at the treatment facilities. The combination of these factors of limited supply and limited disposal both indicated a need for a higher recovery RO process to be implemented. Production Well Water Quality Considerations Following initial exploratory well construction and testing, SAWS proceeded with mobilizing a well drilling contractor for the construction of eight (8) production wells for the proposed facility, with the remainder of the required wells for the first phase of the treatment facilities to be constructed under the plant construction contract. The BGD supply wells were located within two distinct wellfields; one to the northeast of the WTP site and one to the west of the WTP site. Although test wells had been constructed in each of these wellfields, water quality was notably different than the quality that was sampled form the test wells and during the membrane pilot testing that was conducted at test well #1 (TW-1). Of greatest differences that could potentially impact the RO treatment process, slightly higher levels dissolved iron and silica were sampled in the production (BGD) wells. Both parameters are potentially limiting parameters for the system recovery and may impact the performance of scale inhibitors (antiscalants) that are dosed for chemical pretreatment of the RO process. After consulting with the chemical suppliers, the production well water quality is not believed to have an adverse impact on the operations of the treatment facility at recoveries ranging from 85-90%. Special care will be taken at startup to confirm that the pretreatment chemicals are providing effective control of scaling. Treatment Process Overview The RO Treatment Plant for the SAWS BGD Program will be designed to initially produce 10.0 MGD of RO permeate. There will be additional raw water that will bypass the RO treatment and be combined with permeate to assist with post-treatment stabilization. This bypass is currently 3

4 estimated as percent of the permeate flow or 1-2 MGD for the initial phase of this project. Expansion of the treatment facilities is projected to occur in two future phases to provide a total RO permeate capacity of 20.0 and 25.0 MGD, respectively. Brackish groundwater for the initial capacity will be supplied by 12 production wells that will be located near the treatment facility site in southern Bexar County. The proposed RO Treatment Plant will be located at SAWS existing ASR site. The presence of detectable levels of iron in the raw water supply wells is a key pretreatment consideration. Pilot testing indicated that if the raw water remains pressurized from the well to the RO system (conveyed in a closed system) and dissolved oxygen is not present in the RO feed, iron can be maintained is a dissolved form (e.g. remain in solution in its reduced form) that will not form an iron precipitate which can lead to potential fouling of the RO membranes. If dissolved oxygen is allowed to enter the raw water feed, additional pretreatment would be required. Based on providing a typical closed system for raw water conveyance, RO pretreatment processes will consist of chemical and physical processes as demonstrated in pilot testing. Sulfuric acid and antiscalant (i.e., scale inhibitor) will be added to the raw water to prevent scale formation in the RO treatment process. Cartridge Filtration, nominally rated for 5-microns or less, will provide a final measure of protection from particulates that are present in the raw water. The RO process will consist of primary RO trains that will recover up to 80% of the raw water within two stages, followed by smaller capacity RO concentrator trains to act as a combined third stage. These concentrator trains will receive concentrate from the primary RO units and provide additional recovery of up to 90% (50% recovery of the primary RO concentrate) in a combined third stage. The initial phase of the project includes constructing 10.0 MGD of RO permeate capacity, but the building and site are planned such that the layout of the facilities at 25.0 MGD of RO permeate capacity will be split into two (2) equal halves, each capable of treating 12.5 MGD of RO permeate. This configuration will facilitate the provision of: 1. Acceptable flow turndown ratios to be maintained in the RO pretreatment; 2. Operational flexibility of the future RO pretreatment and process units to make the best use of future advances in technology; and 3. Reduction in the cost of process piping in the initial phase. Although not a primary reason, another benefit of this approach is improved constructability and plant reliability in future construction phases by providing the ability to shut down one side of the plant without impacting the operations on the remaining side. This approach is continued through the post treatment facilities to allow for symmetrical sizing of the equipment, process piping, and structures. Post-treatment following the RO process will be required to stabilize the product water and to meet SAWS product water goals. A portion of the permeate flow will receive ph adjustment utilizing CO2 addition, and then be fed through parallel calcite (i.e. limestone) contactor units to meet the minimum water quality goals for Calcium Hardness and Total Hardness in the finished water. The calcite contactor effluent water will recombine with the remaining RO permeate flow 4

5 and blend with raw water that bypasses the RO treatment before passing through degasifiers to remove a majority of excess CO2, radon, and any other potential dissolved gasses of concern prior to the chlorine contact basin. The addition of free chlorine for disinfection and sodium hydroxide for final ph adjustment will occur downstream of the degasifiers at the head of the chlorine contact basin. Transfer pumps will convey the RO finished water to the clearwell storage at the ASR facility. Reverse Osmosis Treatment Process Design Considerations Based on the quality and quantity considerations described above, it became evident during the design phase of the project that a typical high recovery RO design utilizing 3 stages within a single treatment unit or train may not provide the preferred level of operational flexibility. To address the unique limitations and operational challenges that were apparent, an alternate design approach was proposed to achieve the high level of raw water recovery targeted within the RO treatment process. In-Land Desalination and High Recovery RO Similar to coastal desalination, in-land desalination facilities require a reliable means of concentrate disposal, however, in-land alternatives for such disposal are typically more limited in both type and capacity. For these reasons, in-land facilities will often seek to maximize recovery rates of raw water in order to minimize disposal, as this is also the case for the SAWS BGD facility. Although not always a driving factor, use of high recovery RO also provides greater conservation of the raw water supply resource by producing more drinking water per gallon of raw water withdrawn from the ground. Impacts of Concentrate Staging Within the Process Achieving higher recovery of raw water supplies in RO systems can be achieved through concentrate staging (additional stages in series). Design practices typically limit 2-stage, 7 element RO systems to recovery rates of 80-85%. Through the addition of a 3 rd stage the recovery rate can be increased to 90% or higher, however design and operation of 3-stage systems for brackish water treatment can be more challenging when compared to conventional 2- stage configurations. The reasons for these challenges relate to attributes of the Reverse Osmosis process itself. As raw water is recovered in each successive stage of the process, the required osmotic pressure to pass permeate through the membrane is increased. For example, in a 2-stage RO system operating at 75% recover, with 50% recovery per stage, the osmotic pressure requirements at the end of the last element in the 1 st stage are almost double the osmotic pressure of the initial feed water to the 1 st stage. Similarly, the osmotic pressure requirements in the last element of the 2 nd and 3 rd stages are almost 4 times and 8 times higher, respectively, than the initial feed water. As this increase in osmotic pressure is occurring, the pressure of the water on the feed side of the membrane is decreasing due to hydraulic friction losses as the water passes through the feed spacer, and interstage piping and valves. The net result of these conditions is a reduction in the net driving pressure (NDP) that is available to pass water through the membrane as the feed water passes through the system. The cumulative impacts of these effects is an imbalance in permeate flux rates, with higher flux rates occurring at the front end of the system and gradually 5

6 decreasing flux rates as water flows towards the tail end of the system. Optimization of the process using interstage pressure boosting, permeate throttling, hybrid membrane elements, or a combination of these can often be used to improve flux balance in the system. Conventional 3-Stage RO Configuration Although not as commonly used in brackish groundwater membrane facilities, additional concentrate staging above and beyond the typical 2-stage system has been the most common form of achieving higher RO recovery rates. Through the incorporation of an additional stage downstream and in series with a 2-stage system, concentrate flow rates can be reduced by as much as 40-65% (less than 2-stage concentrate flows). This can be a cost effective practice for lower TDS brackish supply sources that have relatively low osmotic pressures to overcome (e.g. low energy requirements) and where both the operating pressures and materials of construction are still within industry norms for low pressure RO treatment systems. This main issue with 3 stage systems for RO treatment has typically been related to challenges in achieving a hydraulically balanced system, leading to under-utilized membranes in the 3 rd stage and a tendency for higher flux rates in the 1 st stage. To address this imbalance requires added operational complexity as well as additional capital costs for providing the equipment and controls. While these measures may have previously been considered as unnecessary or prohibitive, the need to apply RO desalination at in-land facilities where concentrate disposal capacity is much more limited will change the costs vs benefits decision to favor higher recovery operations. In such cases, the additional costs and complexities associated with 3-stage systems can often be eclipsed by the capital costs for additional concentrate disposal wells that would be necessary for a facility operating at standard recovery rates. A second issue with 3 stage systems is their operational trade-offs that are associated with operating as a single treatment train. Adequate pre and post flushing of the membranes is more difficult to achieve due to pressure losses through the membranes and hydraulic restrictions in the 3 rd stage. The membrane cleaning process is also more complicated through additional piping and valves. Additionally, for systems where potential for scaling of the final stage in the RO process is the main operational concern, the entire 3-stage treatment unit must be taken out of service in order to perform cleaning of the 3 rd stage membranes, resulting in a significant loss in production capacity. 6

7 Figure 1 Conventional 3 Stage RO Configuration Concentrator RO Process Optimization As an alternative to the 3-stage RO design drawbacks described above, the concept of a separate concentrator RO train to provide the 3 rd stage of RO treatment was investigated for the SAWS BGD project. This concept utilized conventional 2-stage RO trains, designated as Primary RO trains to achieve a recovery of 75-85% recovery, followed by a separate single stage membrane array that can operate at 50% or higher. The separate 3 rd stage, designated as the Concentrator RO train, receives 2 nd stage concentrate as a feed water stream. The incorporation of the Concentrator RO concept provides a practical approach to address the flux balancing and membrane cleaning drawbacks commonly associated with the conventional 3-stage, all-in-one RO treatment units. Additional details for the design and operation of such a system are provided below. For the SAWS BGD project, two (2) separate concentrator RO trains are proposed to further recover concentrate flow from the four (4) proposed Primary RO trains. Each Concentrator RO train will designed to receive the combined concentrate from two (2) Primary RO trains in order to maintain an approximate 4:2:1 pressure vessel ratio in combined 3-stage system. The goal of treatment through the Concentrator RO system is to provide the capability to achieve additional recovery beyond what is achieved in the Primary RO trains. Assuming the raw water supply is held as a constant value, this additional recovery serves to increase the total permeate volume produced as well as reduce the total concentrate volume that is generated during treatment. 7

8 Figure 2 3 rd Stage Concentrator RO Configuration The Concentrator RO trains will require a higher feed pressure ( psi) to recover additional water from the combined concentrate of the Primary RO trains. To make the best use of energy within the treatment system, residual pressure in the 2 nd stage concentrate is carried through a manifold header to the Concentrator RO trains, where it is boosted by an in-line booster pump at each Concentrator RO train. In addition to these features, the proposed Concentrator RO trains at the SAWS BGD facility will incorporate hydraulic turbochargers for energy recovery of excess pressure leaving the system in the 3 rd stage concentrate flow stream. While the flow and feed pressure requirements are unchanged, the use of a turbocharger allows the motor size of the in-line booster pump to be reduced by approximately 40% when compared to a system without energy recovery. 8

9 Figure 3 SAWS BGD Project RO Configuration The Concentrator RO trains will consist of two independently piped pressure vessel arrays that will be mounted to a common frame that is similar in height and width to one of the Primary RO trains. Separate control valves, process piping, and instrumentation will be provided allowing independent operation of each. By dividing the concentrator RO step into two separate units, the proposed 90% recovery can be achieved when only half of the initially installed primary RO trains are in operation, giving the option for 50% turndown of the total RO treatment capacity. Upon expansion beyond 20 MGD, the concentrator RO trains will require additional pressure vessels and membranes to be installed to provide the 25% increase in capacity within the four (4) total units. Energy Recovery and Flux Balancing Since the highest required feed pressure in a three stage system is in the third stage, the implementation of an interstage ERD between the first and second stages is less preferred compared to applying energy recovery to the third stage in the system (e.g. the Concentrator RO train). One reason for this is that excess energy from the second stage concentrate stream is passed along to the suction side of the third stage feed pump Aside from some flux balancing improvement there is no other benefit to recovering concentrate energy, which is still needed in the 3 rd stage. In fact, to do so would simply increase the booster pump requirements to the third stage, and would have higher equipment and overall energy costs, resulting in a longer pay-back period than the third stage turbocharger. As an alternative for flux balancing in the Primary RO units, a permeate throttling valve can be used. Permeate throttling increases the permeate back pressure on only the first stage, reducing 9

10 the net driving pressure (NDP) and permeate flux in the first stage and resulting in an increased NDP and flux in the second stage. Another solution would be to install an interstage booster pump on the first stage concentrate line, which would increase the feed pressure to the second stage resulting in higher permeate flow. The disadvantage of the 2 nd stage booster pump is a higher initial capital cost and an additional pumping unit to control and maintain. In contrast, permeate throttling achieves the same balancing benefit using a more simplistic installation and operation, with lower capital cost and only a modest impact to energy consumption. For these reasons the use of permeate throttling on the first stage of the primary RO units has been incorporated for additional balancing of flux rates of the first and second stages. Based on the proposed three stage system (4:2:1 array configuration) for the proposed RO plant, the RO target overall recovery rate of 90%, and the preliminary pressures estimated for concentrate disposal, the use of a hydraulic turbocharger was found to be beneficial in the 3 rd stage feed. Operation of a turbocharger at this point in the process unit is anticipated to provide a reduction in energy usage and will pay back the capital costs within an acceptable period of time. This feature will benefit process control by providing a boost to the third stage feed pressure for better utilization of the tail end membrane elements. Process Startup and Control Considerations Startup Sequences Startup of RO processes are typically done is a systematic sequence, starting with commands to begin running well pumps and flushing the pretreatment system. As raw water begins flowing, pretreatment chemical systems can be placed in to automatic control and initial instrument readings can be taken and compared with desirable conditions to begin membrane treatment. Once water quality parameters are within acceptable ranges, feed water can be introduced into the first RO unit and a pre-flush countdown can commence. Once satisfied, the RO unit can be ramped up to normal operating conditions and placed into automatic control mode, and the next RO unit in the sequence can begin pre-flushing. Primary RO Unit Control Loops The Primary RO units will include typical 2-stage control features. The total permeate capacity on each unit will be controlled using a flow set point by adjusting the RO feed pump speed as a control variable. The targeted recovery of each unit will be controlled using a flow set point by adjusting the position of a modulating V-port ball valve on the 2 nd stage concentrate line as a control variable. To provide permeate flux balancing between stages, a 1 st stage permeate flow set point will be controlled by adjusting the position of a modulating V-port ball valve on the 1 st stage permeate line for permeate backpressure/throttling. 10

11 Figure 4 Primary RO Skid Control Loops Primary RO Flushing Valve/Header Once the interstage piping header is placed into flow bypass control mode and operating at elevated pressure, the pre and post flushing of subsequent RO units will require a low pressure means (typically 50 psi or lower) of conveyance to concentrate disposal. Each Primary RO unit is equipped with a concentrate flush valve that connects to a low pressure concentrate header in the trench below. Upon completion of flushing, the valve will close and backpressure will begin to accumulate in the skid as concentrate flow is directed to the interstage high pressure header. Concentrator RO Unit Control Loops To meet the higher pressure requirements in the 3 rd stage, a VFD controlled in-line booster pump is utilized to increase pressure to the 3 rd stage, using 3 rd stage permeate flow as the control loop set-point and pump speed as the control variable. The booster pump is connected in series with a hydraulic turbocharger that serves as an energy recovery device (ERD) to recover excess pressure energy from the 3 rd stage. The targeted recovery of each unit will be controlled using a flow set point by adjusting the position of a modulating V-port ball valve on the 3 rd stage concentrate line as a control variable. A modulating V-port ball valve on the ERD bypass line (in parallel with turbine end of ERD) will bypass during pre and post operational flushing, and serves as an optional means to reduce feed flow boost to the 3 rd stage in the event that VFD turndown is limited. 11

12 Figure 5 Concentrator RO Skid Control Loops Interstage High Pressure Header As previously noted above, to minimize energy used in re-pumping, a key element to the concentrator RO 3 rd stage configuration is the interstage high pressure header. This feature allows residual pressure from the 2 nd stage membrane array to be conserved, thereby minimizing the motor size required for the concentrator booster pump by directly connecting the Primary RO and Concentrator RO units. This design also requires additional provisions for startup and shutdown flushing when the RO concentrator units are not in operation, or when an odd number of primary skids are in service. Exhibited in the table below are eight operational modes that can be accommodated by the proposed design in Phase 1 of the facility. For six of the eight possible modes of operation, concentrate flows are maintained within the capabilities of the concentrate disposal wells. The other two modes #4 and #6 are both conditions where more than 2 Primary RO units are running without the operation of the concentrator RO units to reduce the disposal flow, and thus the total concentrate flow exceeds the proposed Phase 1 disposal capacity. Although not necessary for the Phase 1 facility to achieve operational goals, these modes may be desirable modes of operation when future injection wells are constructed, or in the event that expanded well capacity can be demonstrated through full scale operations. The system is designed for flexible operation based on the production and O&M needs of the facility, as illustrated in the following table of Figure 6 below. 12

13 Figure 6 RO Operational Alternatives Oper. Mode Permeate Flow (MGD) Primary RO Units Running Concentrator RO Units Running Primary Concentrate Bypass Flow (MGD) Final Concentrate Flow (MGD) Overall Recovery % % % % % % % % Also shown in the table is the primary concentrate bypass flow at each mode of operation, which is the amount of concentrate produced in the primary RO units that exceeds the feed water needed by operation of the Concentrator RO units. For example, during initial startup mode or when only one Primary RO unit is in operation, the concentrate flow is not sufficient to provide feed flow to the Concentrator RO unit. In lieu of flushing water through this 3 rd stage, the process was equipped with a pair of bypass valves which will provide additional control of flow in tandem with the Primary RO unit concentrate control valves. A butterfly valve will open fully during pre and post operational flushing on the first and last Primary RO unit in the sequence, respectively. A modulating V-port control valve will provide tandem flow control with the concentrate control valve. Bypass flow through the modulating valve when the system is operating in high pressure mode will be controlled using a combination of pressure or flow set points and the bypass valve position. Flow measurement via a magnetic flow meter and line pressure from a transmitter will provide feedback for the control loop. 13

14 Figure 7 Process Overview with Interstage High Pressure Header and Flushing Valves Concentrator RO Flushing Flushing of the Concentrator RO unis will normally be accomplished utilizing RO concentrate from the 2 nd stage of the Primary RO units. Following shutdown of the Concentrator units, control valves are configured to allow post flushing using pretreated raw water. This is preferred to flush out higher TDS water from the membranes prior to resting the unit. Feed control valves on the Concentrator RO units allow for control of which unit is in flushing mode. Concentrate Disposal RO concentrate will be conveyed through a pipeline using residual RO system pressure to injection wells where injection booster pumps will provide an increase of the line pressure to levels required for disposal. Estimated operating pressures in the RO concentrate pipeline range from psi. Controls at the injection booster pumps will use speed control of the pumps (VFDs) to maintain a pressure set point on the suction piping. If startup flushing flows exceed the injection pump capacity, a pressure relief/surge valve will divert excess flow to a lined pond near the injection wells. High concentrate backpressure at the RO process will generate advisory and then shutdown alarm conditions depending on the severity of the high pressure measurement. 14

15 Concluding Thoughts The SAWS BGD program represents a significant investment by the City of San Antonio in securing future water supplies for their growing community. Some of the process design and control challenges that were confronted in design were related to the RO process configuration, while others were related to concentrate disposal constraints from the projected limitations in injection well capacity based on site specific geology. Through the application of innovative concepts including the use of a Concentrator RO skid configuration within the RO treatment process, the proposed program endeavors to raise the bars of sustainability and technological achievement for in-land RO treatment facilities both within Texas, and abroad. References Kinslow, Jarrett K.; Bryant, Duane, Shirazi, Saqib, Timmermann, David, Big Data: Fully Realizing the Benefits of Pilot Testing in Design Development for the SAWS Brackish Groundwater Desalination Project, Presented at the 2016 Membrane Technology Conference (AMTA/AWWA), Orlando, Florida, March Kinslow, Jarrett K.; Overview of the Proposed RO Facilities for the San Antonio Water System Brackish Groundwater Desalination Project, Presented at the AMTA/SCMA Joint Technology Transfer Workshop, South Padre Island, Texas, October Hudkins, Jill M., Kinslow, Jarrett K., Messner, Brett, Roque, Jennifer, The Science and Reality: A Review of Membrane Treatment Concentrate Minimization Strategies and Their Impact on Project Viability as presented in the proceedings of the 2012 FSAWWA Fall Conference, Orlando, Florida, December