CONCENTRATE MANAGEMENT: CASE STUDIES IN BEST PRACTICES. Abstract

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1 CONCENTRATE MANAGEMENT: CASE STUDIES IN BEST PRACTICES Colin M. Hobbs, Ph.D., P.E., BCEE, CDM Smith, 2301 Maitland Center Pkwy, Suite 300, Maitland, FL 32751, Ph.: Jorge M. Arevalo, Ph.D., P.E., CDM Smith, Maitland, FL Curtis A. Kiefer, P.E., BCEE, CDM Smith, Boca Raton, FL Abstract The management and disposal of concentrate is often considered to be one of the most significant obstacles to the implementation of desalination technologies. While the ionic constituents which comprise the concentrate are the same as those present in the source water (with the exception of chemicals added to the source water during pretreatment), the absolute and relative concentration of those constituents can significantly affect the environmental conditions of the receiving water into which the concentrate is discharged. Therefore, a welldesigned concentrate management strategy is critical to the long-term success of any desalination facility. The optimal concentrate management strategy for a given facility is dependent on numerous factors. As such, the optimal concentrate management strategy identified for one facility may not necessarily be an appropriate strategy for another. For example, the 12 mgd Blue Hills desalination facility in the Bahamas and the 0.6 mgd Sand City desalination facility in California (United States) take advantage of highly permeable, naturally-occurring subsurface formations and discharge the concentrate into seawater injection wells. In contrast, the 25 mgd Tampa Bay desalination facility in Florida (United States) utilizes 1,400 mgd of cooling water from a colocated power plant to dilute the 19 mgd of concentrate prior to its discharge into the Tampa Bay. Another strategy is to utilize natural intense near-shore tidal mixing to disperse and dilute the concentrate, as practiced by the mgd Ashkelon and mgd Hadera desalination facilities in Israel. This paper will present current concentrate management practices for select desalination facilities throughout the world. Benefits and drawbacks of each strategy will be discussed. This paper will also present numerous factors that should be considered when evaluating concentrate management strategies for a new desalination facility, such as environmental regulations (national, state, and local), concentrate characteristics, prevailing oceanic conditions, bathymetric profiles, geography, dispersion and mixing strategies, surrounding infrastructure, costs, etc. Background Since the development of the first functional synthetic reverse osmosis membrane at the University of California, Los Angeles (UCLA) in 1959 [UCLA Engineering (2015)], membranebased processes have transformed the desalination industry. Membrane-based processes are currently utilized in approximately 80 percent of the world s ~14,000 desalination facilities. Together, these membrane-based desalination facilities produce approximately 50 percent of the 1

2 world s ~7 trillion gallons per day desalinated water supply [Frenkel (2011)]. Despite the meteoric rise of membrane-based processes in an industry once dominated by thermal processes, implementation of membrane-based processes for desalination is not without challenge. One of the most significant challenges to the implementation of membrane-based processes relates to the management and disposal of the concentrate. Concentrate The quantity and quality of concentrate generated from a membrane-based desalination process is influenced by many factors including feed water quality, type of membrane process, and recovery rate. For example, concentrate generated from membrane-based desalination facilities utilizing seawater as their source water can range from 35 to 60 percent of the feed stream (i.e. recovery rates of 65 to 40 percent) and can reach total dissolved solids concentrations in excess of 85,000 milligrams per liter (mg/l). In contrast, concentrate generated from membrane-based desalination facilities utilizing fresh water as their source water can range from 5 to 25 percent of the feed stream (i.e. recovery rates of 95 to 75 percent) and can have total dissolved solids concentrations less than 1,000 mg/l. When coupled with relevant site-specific conditions (i.e. capacity of the facility, location of the facility, proximity of the facility to surface water bodies, geologic conditions surrounding the facility, surrounding infrastructure, etc.), the significant range in concentrate quantity and quality emphasizes the need for engineers to develop unique concentrate management strategies that are appropriate for each facility. Concentrate Management Strategies Strategies to manage and dispose of concentrate generally fall within one of six categories: surface water discharge, subsurface discharge, sanitary sewer discharge, evaporation, land application, and zero liquid discharge. However, it is important to note that concentrate management strategies based upon discharge to sanitary sewers will ultimately share elements with other categories (i.e. surface water discharge, subsurface discharge, evaporation, or land application) as the effluent from the wastewater treatment facility will likely be disposed of in one of these manners. Additional information, including benefits and drawbacks, associated with each of the six categories of concentrate management strategies is presented below. Surface Water Discharge Concentrate management strategies based upon the direct discharge of concentrate to a surface water body (i.e. lake, river, ocean, etc.) are considered surface water discharge strategies. Such strategies are commonly utilized for large seawater desalination facilities located in coastal settings. The fact that this concentrate management strategy has been so extensively employed is primarily attributed to the following factors: the ability to manage/accommodate large quantities of concentrate, the ability of the receiving water to dilute the highly saline concentrate, and reduced capital costs when compared with other strategies. However, implementation of such strategies requires significant planning and testing, substantial permitting, and extensive longterm monitoring to ensure native flora and fauna are not adversely affected. A variety of techniques are available to minimize the environmental impacts associated with surface water discharge strategies. Large seawater desalination facilities located in coastal 2

3 settings commonly utilize a series of diffusers to disperse and dilute the concentrate within a defined mixing zone, rely on intense tidal mixing to disperse and dilute the concentrate, and/or pre-dilute the concentrate to reduce the salinity of the stream prior to discharge to reduce the environmental impacts to the receiving water body. The use of these techniques is less common for small inland desalination facilities as many inland surface water bodies could not tolerate the sustained salt load associated with such facilities; however, site-specific conditions may allow surface water discharge for select facilities. Significant benefits and drawbacks associated with the surface water discharge concentrate management strategy are summarized in the bulleted list below. Feasible for large volumes of concentrate Concentrate diluted by the receiving water Often the lowest cost alternative for large seawater desalination facilities Potential for pre-dilution/co-disposal (i.e. blending the concentrate with other process streams prior to discharge, most commonly wastewater effluent or industrial cooling water) Drawbacks: Potential adverse impact to native flora and fauna (which may be mitigated through proper planning, system design, and monitoring) Rate and extent of dilution dependent on local hydrodynamic conditions (i.e. flow, current, velocity, turbulence, temperature, etc.) and/or diffuser array Extensive planning and testing requirements Extensive permitting efforts Extensive and long-term testing and monitoring requirements Subsurface Discharge Subsurface discharge strategies include all strategies in which the concentrate is injected into an appropriate injection zone which can accommodate the volume and quality of concentrate produced by the desalination facility. In the case of deep injection wells, this injection zone should be below and isolated from all underground sources of drinking water. In the case of slant wells, this injection zone should be hydraulically connected to the proximate surface water body such that the subsurface acts as a large and extensive diffuser array. As specific geologic conditions are required for implementation, such strategies are not feasible for many desalination facilities. Furthermore, high costs commonly associated with the implementation of deep injection wells generally limit their applicability to small and medium sized facilities. Significant benefits and drawbacks associated with the subsurface discharge concentrate management strategy are summarized in the bulleted list below. No direct impact to surface water bodies 3

4 Drawbacks: Specific geologic conditions required High cost alternative which may be prohibitive for large volumes of concentrate (i.e. low transmissivity/deep wells) Potential for groundwater contamination Extensive and costly geologic investigations required Extensive and long-term testing and monitoring requirements Sanitary Sewer Discharge Sanitary sewer discharge strategies generally encompass all strategies in which the concentrate is conveyed to a wastewater treatment facility for treatment, blending, and/or pumping prior to discharge. Sanitary sewer discharge strategies are frequently utilized by small and medium sized facilities which desalinate fresh or brackish water supplies. The quantity and quality of concentrate typically generated from seawater desalination facilities generally precludes this strategy for such facilities. The ability of this concentrate management strategy to dilute the concentrate with other waste streams and to leverage existing infrastructure (i.e. collection system piping, wastewater treatment facility, effluent pumping, and effluent disposal) makes this approach attractive to many small and medium sized inland desalination facilities. However, it is imperative that the desalination facility and the wastewater treatment facility work together to ensure both facilities operate effectively and efficiently and within the constraints of all applicable permits. In an effort to minimize capital costs, many desalination facilities which utilize this concentrate management strategy discharge the concentrate directly into a component of the existing collection system. This practice effectively reduces the capacity of all downstream components (i.e. piping, pump stations, wastewater treatment facility, etc.) to convey and/or treat wastewater and reduces the length of time before an expansion project is required to compensate for the lost capacity. Recognizing that this practice may not be prudent in certain circumstances, several desalination facilities pursue an alternate approach to this concentrate management strategy. This alternate approach generally consists of the construction of a dedicated pipeline to convey the concentrate from the desalination facility to the wastewater facility. However, instead of discharging the concentrate at the head of the wastewater facility, the concentrate is discharged downstream of all treatment processes. This alternate approach to the sanitary sewer discharge concentrate management strategy preserves the original capacity of the collection system and the wastewater treatment facility and provides the same degree of dilution as the conventional approach. As this alternate approach introduces concentrate to the wastewater treatment facility downstream of all treatment processes, it is imperative to conduct a thorough evaluation to ensure the blended concentrate/wastewater effluent complies with all applicable water quality requirements at all potential operating conditions. Significant benefits and drawbacks associated with the sanitary sewer discharge concentrate management strategy are summarized in the bulleted list below. Concentrate diluted by other waste streams Leverages existing infrastructure 4

5 Often the lowest cost alternative for small and medium sized desalination facilities Potential for reuse (i.e. irrigation, industrial, etc.) Conventional approach provides full wastewater treatment of concentrate Alternate approach does not consume capacity of collection system or wastewater treatment facility Drawbacks: Conventional approach consumes capacity of collection system and wastewater treatment facility Variations in concentrate and/or wastewater flow may require frequent adjustments to the operation of the wastewater treatment facility Quality of blended concentrate/wastewater effluent must comply with applicable water quality requirements for the given method of discharge (i.e. surface water discharge, reuse, land application, aquifer recharge, etc.) Evaporation Concentrate management strategies based upon the evaporation of concentrate (i.e. solar evaporation, thermal evaporation, mechanical evaporation, etc.) are considered evaporation strategies. With the exception of solar evaporation, most evaporation techniques are generally considered cost prohibitive for most municipal desalination facilities due to high capital and/or energy costs. As a result, solar evaporation strategies are generally limited to small facilities located in arid regions with high net evaporation rates. Evaporation ponds are the nucleus of most solar evaporation strategies; however, a variety of devices are frequently utilized to enhance the evaporation process. These devices generally focus on increasing the surface area of the liquid phase to increase the rate at which concentrate evaporates and most commonly include spray nozzles and mechanical misting equipment. While the use of such devices provide significant benefits (i.e. reduce the area requirements and capital costs) to a solar evaporation system, the benefits do not come without a cost. Spray nozzles and mechanical misting equipment will require frequent cleaning and maintenance to remove accumulated salts and the high salinity mist created by these nozzles can migrate to areas beyond the evaporation pond which can cause adverse environmental impacts. Significant benefits and drawbacks associated with the evaporation concentration management strategy are summarized in the bulleted list below. No direct impact to surface water bodies Potential to commercialize recovered salt Independent of concentrate water quality Drawbacks: Large land requirement Dependent on local environmental conditions Potential for groundwater contamination Requires handling and disposal of solids 5

6 Land Application Land application strategies are based upon the direct application of concentrate to land surfaces and include spray fields, rapid infiltration basins, and percolation ponds. Limitations on the salinity of water applied to land and vegetation generally precludes these concentrate management strategies for facilities which generate high salinity concentrate streams unless significant dilution is provided prior to application. As such, these strategies are most appropriate for facilities that desalinate fresh or brackish water supplies. Furthermore, as these strategies are based upon the application of concentrate to land surfaces, it is imperative that the infiltration capacity of the soils within the application zone exceed the rate at which the concentrate is applied. Significant benefits and drawbacks associated with the land application concentrate management strategy are summarized in the bulleted list below. No direct impact to surface water bodies Improves usage of water resources Drawbacks: Requires salt tolerant vegetation Requires storage and distribution system Potential for groundwater contamination Alternate method of disposal may be required Long term testing and monitoring requirements Zero Liquid Discharge Concentrate management strategies in which no liquid residual is discharged from the desalination facility are described as zero liquid discharge. Such strategies typically utilize multiple treatment processes to reduce the volume of concentrate prior to a final solar or thermal evaporation process. While the concentrate reduction processes are highly dependent on the quality of the initial concentrate stream, processes such as lime softening, ion exchange, secondary reverse osmosis, mechanical evaporation, and brine crystallization have been utilized in facilities which employ zero liquid discharge concentrate management strategy. Due to the complexity and high costs associated with zero liquid discharge strategies, such strategies are infrequently utilized in municipal potable water applications. However, as environmental regulations become more restrictive and as conventional water supplies approach their maximum sustainable limits, the use of zero liquid discharge strategies in these applications will likely increase. Significant benefits and drawbacks associated with the zero liquid discharge concentrate management strategy are summarized in the bulleted list below. No direct impact to surface water bodies or groundwater supplies Potential to commercialize recovered salt Viable solution when no other concentrate management strategies are possible 6

7 Drawbacks: Expensive and complex process Process is highly dependent on quality of initial concentrate Requires handling and disposal of solids Requires highly trained operational staff Select Case Studies The following case studies present concentrate management strategies utilized by select desalination facilities throughout the world. The geographic location of these facilities are shown on Figure 1. Tampa Bay United States The Tampa Bay Seawater Desalination Plant is a 25 mgd facility which desalinates water from the Tampa Bay. Co-located with Tampa Electric s Big Bend Power Station, this facility mixes 19 mgd of concentrate with 1,400 mgd of cooling water prior to discharge. As a result of the significant pre-dilution, the salinity of the concentrate/cooling water stream is only 1.0 to 1.5 percent greater than the salinity of the Tampa Bay and falls within the Bay s normal seasonal salinity fluctuations. Additional dilution provided by seawater in the discharge canal (i.e. the canal between the Big Bend Power Station and the Tampa Bay) reduces the salinity of the discharged water even further until it is approximately equal to that of the Tampa Bay. Extensive studies completed prior to the implementation of this project indicated the facility would not adversely impact the environment of the Tampa Bay and comprehensive hydrobiological monitoring program implemented in 2003 confirms no measurable impacts have been observed since the facility s commissioning in 2007 [Tampa Bay Water (2015)]. Ashkelon and Hadera Israel The Ashkelon and Hadera Desalination Plants desalinate seawater from the Mediterranean Sea and have capacities of mgd and mgd, respectively. Both facilities are co-located with power generation facilities and utilize cooling water to dilute the concentrate. Intense and natural tidal activity provides rapid dispersion and additional dilution of the discharged water and precluded the need for lengthy and costly outfall diffuser systems. A comprehensive evaluation completed prior to the implementation of these projects indicated these facilities would not adversely impact the local marine environment. Studies conducted after the implementation of each project confirmed that the ecosystems in the immediate vicinity of each facility were similar to those in surrounding locations and that no long-term ecological deterioration had occurred [Spier (2007); Einav & Lokiec (2003); WateReuse Association (2011); Spiritos and Lipchin, 2013). Perth Australia The Perth Seawater Desalination Plant is a 38 mgd facility which desalinates water from Cockburn Sound. Despite being co-located with the Kwinana Power Station, the Perth Seawater Desalination Plant discharges its concentrate through a multi-port diffuser system. Designed to achieve a 45:1 dilution factor within a 150 foot radius initial mixing zone, the diffuser system consists of a single 1,500 foot long pipeline equipped with 40 diffusers. Each diffuser is located approximately 30 feet below the water surface, 1.5 feet above the seabed, at a 60 degree angle. 7

8 Figure 1 Select Case Studies 8

9 Data obtained from extensive water quality sampling events indicate the salinity of the receiving water increased less than 1.0 percent and remains within the normal seasonal salinity fluctuations. Furthermore, extensive pre- and post-construction studies, both macro-faunal and benthic in nature, indicate than no adverse impacts can be attributed to the operation of the Perth Seawater Desalination Plant [WateReuse Association (2011); Water Technology (2015); Degremont (2015)]. Blue Hills Bahamas and Sand City United States The Blue Hills Desalination Plant has a capacity of 12 mgd and desalinates seawater from the Atlantic Ocean while the Sand City Desalination Plant has a capacity of 0.6 mgd and desalinates seawater from the Monterey Bay. Both of these facilities utilize a series of wells to withdraw and convey seawater from the subsurface to the respective facility for treatment. In addition, both facilities take advantage of unique hydrogeologic conditions and discharge their concentrate through injection wells. Due to the fact that these facilities utilize the subsurface as both sources of water and as concentrate disposal locations, extensive hydrogeological modelling was required to ensure the source waters and concentrates were hydraulically isolated [Kiefer and Grotke, (2007); Water Technology (2010)]. Ormond Beach United States The Ormond Beach Water Treatment Plant is a 12 mgd facility which is comprised of both a lime softening train (8 mgd) and a low pressure reverse osmosis train (4 mgd). The low pressure reverse osmosis train desalinates brackish groundwater from the Upper Floridan aquifer and discharges its concentrate to the Ormond Beach Wastewater Treatment Plant through a dedicated forcemain. Unlike the conventional approach to sanitary sewer discharges, concentrate from the low pressure reverse osmosis train of the Ormond Beach Water Treatment Plant is directed to either the equalization basin where it is blended with wastewater effluent and ultimately discharged to the Halifax River or to the reclaimed water tanks where it is blended with wastewater and ultimately reused for irrigation purposes. Extensive studies completed prior to the implementation of this project indicated this method of concentrate disposal would not adversely impact the quality of the Halifax River, the vegetation in the reclaimed water service area, or the quality of the surficial aquifer. Operation of the Ormond Beach Water and Wastewater Treatment Plants over the past seven years has demonstrated the success of this concentrate management strategy. Since the low pressure reverse osmosis train was commissioned, the City increased the total available reclaimed water supplies by 920 million gallons and reused approximately 520 million gallons of concentrate for irrigation purposes [Hobbs & Arevalo (2015)]. Palm Coast United States The Palm Coast Desalination Plant has a capacity of mgd which is comprised of a low pressure reverse osmosis train (4.8 mgd), raw water bypass (1.584 mgd), and a new zero liquid discharge concentrate treatment train (1.2 mgd). As a result of recent regulatory developments, 1.2 mgd of concentrate once discharged into a surface water body is now treated and recovered through a new concentrate treatment train. The concentrate treatment train generally consists of lime/soda ash softening, ph adjustment, ultrafiltration, and disinfection. Sludge produced in the softening process is thickened (gravity thickener), dewatered (belt filter press), and ultimately hauled offsite for use as roadway base material. Supernatant from the thickener, filtrate from the 9

10 belt filter press, and residuals from the ultrafiltration process (i.e. spent backwash water, spent cleaning solutions, etc.) are directed to an equalization tank and returned to the head of the zero liquid discharge concentrate treatment train (i.e. the lime/soda ash softening process) for treatment. As a result of implementing this concentrate management strategy, Palm Coast eliminated all surface water discharges associated with the facility and increased its production capacity by 1.2 mgd [Locke, et al. (2015)]. Conclusion As evidenced by the variety of concentrate management strategies utilized by desalination facilities throughout the world, no single strategy is appropriate for all facilities or geographies. Unique features of each project must be thoroughly evaluated in order to identify feasible alternatives for concentrate management. Once identified, each alternative must be further developed and refined through additional data collection and evaluation efforts in order to select the best alternative for implementation. While formal permitting activities will not commence until the best concentrate management alternative is selected, the engagement of the permitting authorities during the alternatives identification and evaluation process can greatly facilitate the permitting process. Once implemented, it is strongly recommended to monitor the impacts of the concentrate management strategy such that the strategy can be modified to mitigate any unintended consequences or optimized to further enhance its performance. 10

11 References Degremont (2015), Perth Seawater Desalination Plant I Degremont Australia, Available at Einav, Rachel & Fredi Lokiec (2003), Environmental Aspects of a Desalination Plant in Ashkelon, Desalination, 156 (1-3), pp Frenkel, Val S. (2011), Seawater Desalination: Trends and Technologies, Desalination, Trends and Technologies. Michael Schor, ed. Available at: Hobbs, Colin & Jorge Arevalo (2015), Sustainable Low Pressure Reverse Osmosis Concentrate Management Seven Years and Counting, 2015 American Water Works Association Water Quality and Technology Conference Kiefer, Curtis & Eric Grotke (2007), Startup of the Blue Hills Seawater RO Plant: Charting the Course of Nassau s Water Supply Future, 2007 American Water Works Association Membrane Technology Conference Locke, Phillip, Fred Greiner, Ryan Popko & Nichole Cooke (2015), ZLD Process Utilizes Ultrafiltration to Achieve 98.5% RO Recovery, 2015 American Water Works Association/American Membrane Technology Association Membrane Technology Conference Spier, Varda (2007), Summary of the Environmental Document for the Desalination Plant in Hadera, Available at: Spiritos, Erica & Clive Lipchin (2013), Water Policy in Israel: Context, Issues and Options, Global Issues in Water Policy, Nir Becker, ed. Tampa Bay Water (2015), Supplying Drinking Water to the Tampa Bay Region, Available at UCLA Engineering (2015), History, Available at Water Technology (2010), Sand City Coastal Desalination Plant, Available at Water Technology (2015), Perth Seawater Desalination Plant, Available at WateReuse Association (2011), Seawater Concentrate Management White Paper, Available at: 11