Summary of Issues Strategies Benefits & Costs Key Uncertainties Additional Resources

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1 Summary of Issues Strategies Benefits & Costs Key Uncertainties Additional Resources KEY POINT: High recovery processes, including zero liquid discharge (ZLD) technologies, have high capital costs and are energy or chemical intensive. It has not yet been implemented in a municipal desal setting. SUMMARY OF ISSUES ZLD is a subcategory of high recovery processing where no liquid concentrate leaves the plant boundary. Due to the relative newness of considering ZLD for municipal systems it has been cited as a concentrate management option rather than, more correctly, a desal processing option. The term ZLD is frequently used (incorrectly) to imply use of thermal desal equipment. Historical ZLD technologies involve brine concentrators, crystallizers, and spray dryers that convert concentrate to highly purified water and either a concentrated brine to be disposed of in an evaporation pond or solid dry mixed salt product suitable for landfill disposal. More recent ZLD systems include an RO (or EDR) step to reduce the volume going to the thermal processing steps or in place of them as some membrane-based ZLD systems do not contain any thermal (brine concentrators, crystallizer, spray dryer) processing steps. Brine concentrators are single-effect thermal evaporator systems that are based on vapor compression technology. Most operating brine concentrators consist of a vertical tube and falling film evaporators that use a calcium sulfate-seeded slurry process to prevent scaling. Brine reject from a concentrator typically ranges between two to ten percent of the feedwater flow, with TDS concentrations ranging from 160,000 to as high as 360,000 mg/l depending on feedwater composition (Mickley 2006, Mickley 2008). Thermal evaporation uses a large amount of energy more than 18.5 kwh/m3 of feedwater (Bond and Veerapaneni 2007). Thermal evaporators have been used in industrial RO applications and are known to be a viable and reliable technology. Crystallizers can be used to further reduce concentrate to a transportable solid that is suitable for landfill disposal (ZLD). Like thermal evaporators, crystallizers are also driven by vapor compression. In a crystallizer, the brine enters the vapor body at an angle and swirls into a vortex. As water evaporates, the salt crystals are separated using a centrifuge or filter. The mineral cake removed from the brine concentrate contains 85% solids. Crystallizers are more energy intensive than brine concentrators requiring on the order of 225 kwh/kgal. In situations where evaporation ponds are not practical, the use 1

2 of a crystallizer can provide a technical ZLD solution. The capital and operating costs, however, are prohibitive for municipal settings. Spray dryers provide an alternative to crystallizers for concentration of wastewater brines to dryness. With this method, the concentrated salt solution is reduced to a fine spray and mixed with a stream of hot gas, which provides the heat for evaporation and carries off the moisture released from the concentrate. The resultant dry salt powder is collected in a bag filter. Although energy intensive like crystallizers, spray dryers are generally more cost effective for smaller feed flows of less than mgd (53 m 3 /d) (Mickley 2006). Membrane systems used in high recovery processing may be brackish or seawater types depending on the feedwater salinity. Due to osmotic pressure forces, pressure limitations on commercial seawater RO equipment systems are generally limited to approximately 70,000 mg/l for treating seawater. Most brackish waters being less NaCl dominated than seawater can, in the absence of scaling and fouling limitations, concentrate to higher levels but rarely to more than 100,000 mg/l. The more recent approaches to ZLD that utilize membrane steps before or in place of the thermal steps reduce capital costs in most situations. However, high energy costs associated with the thermal only systems are replaced to a large degree by chemical costs (particularly for high hardness water) and increased costs associated with solids disposal (Mickley 2008). ZLD (and high recovery processing, in general) is being considered in extreme conditions where no other concentrate disposal method is available. The process has very high capital costs, high energy consumption, and potential high costs related to final brine or salt disposal. To date, ZLD has not been used in municipal settings. STRATEGIES For high recovery applications to become more viable, improvements are needed to reduce capital costs and operating costs. Alternative and emerging technologies that may improve certain aspects of high recovery processes are under various stages of development. Example technologies include forward osmosis, dewvaporation, and membrane distillation. Concentrate volume minimization Recently, there have been several studies related to concentrate volume minimization for inland desal. These studies have demonstrated that improving water recovery and reducing the volume 2

3 of concentrate could significantly reduce the costs associated with high recovery (including ZLD) processing. AwwaRF project #3030 Desalination Product Water Recovery and Concentrate Volume Minimization was recently completed by Carollo Engineers and Colorado School of Mines (Sethi et al. 2008). The project included a two-phase study to assess the state of the science and advance desal technologies for enhancement of system recovery and minimization of concentrate volume. Phase I focused on reviewing the state of the science and performing a technical assessment of promising and emerging desal configurations or technologies for recovery enhancement, as well as conceptualizing an innovative desal configuration for increasing recovery and minimizing concentrate. Phase II focused on the advancement of desal technologies via further development and testing of a hybrid approach using intermediate precipitation and ED/EDR to treat primary RO concentrate. The costs of the hybrid configuration were compared to those for a conventional RO system coupled with a brine concentrator. Results indicated that costs associated with the hybrid configuration can be 40 percent lower than costs for a conventional RO and brine concentrator system with equal overall recovery rates. AwwaRF project 3010 and Volume Minimization for Inland Desalination was conducted by Black & Veatch in 2007 (Bond and Veerapaneni 2007). As part of this project, researchers developed a process train involving two RO passes, intermediate concentrate treatment, and a brine concentrator, followed by an evaporation pond. Results showed that energy use associated with ZLD decreased significantly with the use of this technology. WRF project WRF a "Survey of Volume Reduction and Technologies for Water Utilities was recently completed by Mickley & Associates (Mickley 2008). The project developed detailed costs for treating 12 different concentrate streams varying in salinity, composition, and volume by five different commercial high recovery processing schemes. Performance and costs per volume treated of individual processing steps and the total system were highly dependent on salinity, composition, and volume. Eastern Municipal Water District (EMWD) in California and Carollo Engineers evaluated five promising technologies that, individually or in combination, could act as an intermediate brine treatment step to further concentrate existing brine and recover more potable water at a lower cost. This study involved desktop modeling and bench-scale testing to evaluate individual technologies, and combinations of technologies, from which the most appropriate treatment combination could be selected by EMWD for potential testing (Juby et al. 2008). Beneficial recovery of minerals from concentrate. Beneficial recovery of minerals from concentrate can be used for management of concentrate from both brackish water and seawater desal plants. Development of beneficial salt reuse options 3

4 and specific salt separation methods can help offset overall process costs. Beneficial reuse also has the potential to decrease the volume and cost of transporting concentrate. There are a variety of technologies to extract salts from concentrate using fractional crystallization or precipitation. Fractional precipitation is achieved by adding a precipitating chemical agent to selectively remove a target mineral from the concentrate solution. For example, the patented SAL-PROC TM process uses sequential or selective extraction to recover commercial grade sodium, magnesium and calcium salts from concentrate for market and potentially for production of value-added materials from these salts. The patented ZDD technology has been applied to abstract useful salts and chemicals from seawater. Kumar et al. (2006) employed a series of innovative tests utilizing ion exchange, bipolar electrodialysis and electrochlorination technologies to recover useful products from RO concentrate that can be utilized at the treatment facility. The ion exchange experiments focused on recovering phosphate from RO concentrate using a chelating ion exchange resin and converting the phosphate rich regenerant into struvite, a commercially viable fertilizer. Bipolar electrodialysis was used for generating mixed acids and bases from the RO concentrate solution after suitable softening pretreatment. BENEFITS & COSTS Benefits Thermal ZLD can reduce environmental impacts on surface water, groundwater aquifers, soils and vegetation, which are commonly associated with other disposal methods. ZLD can avoid a lengthy and tedious permitting process for concentrate disposal. Thermal ZLD process can improve water efficiency and recover high quality product water from concentrate. Potential beneficial recovery of minerals from concentrate can offset the high costs of thermal processes, and reduce the cost for transport and disposal solids. Costs Thermal ZLD processes have significant capital and operating costs, which can sometimes exceed the cost of the desalting facility (NAS 2008). Additionally, landfilling costs for solids waste disposal can be significant. 4

5 The energy requirement for concentrate evaporation and crystallization is high (100 to 250 kwh/kgal, or 26.4 to 66.1 kwh/m3) (Mickley 2006). KEY UNCERTAINTIES Because of the high costs and intensive energy requirements, ZLD concentrate management approaches are typically not considered for municipal drinking water applications at the present time. Nevertheless, thermal evaporation processes can be a viable option for inland applications where the concentrate flows are small and other methods of concentrate management are not feasible. The positive attribute of salt solidification includes the recovery of salts and potential for revenue generation through resale. The economics and market of products, however, require further investigation. ADDITIONAL RESOURCES AMTA Disposal of Desalting By-Product. Publication FS-4, February Bond, R. and Veerapaneni, S.V for Inland Desalination. AwwaRF Project Published by American Water Works Association Research Foundation, Denver, CO. Jordahl, J Beneficial and Nontraditional Uses of Concentrate. Alexandria, VA.: WateReuse Foundation. Juby, G., A. Zacheis, et al Evaluation and Selection of Available Processes for a Zero- Liquid Discharge System for the Perris, California, Ground Water Basin. Desalination and Water Purification Research and Development Program Report No Denver, Colo.: U.S. Bureau of Reclamation. Kumar M., J. Oppenheimer, S. Adham, L. Webb, and R. Kottensette Innovative Technologies for Beneficial Reuse of RO Concentrate, ACE2006. Denver, Colo.:AWWA Membrane Residuals Management Subcommittee. Mickley, M Review of Options. Available at: < 5

6 Mickley, M Membrane Concentrate Disposal: Practices and Regulation. USBR Desalination and Water Purification Research and Development Program Report No. 123, 2 nd Ed. April Available at: < Mickley, M Survey of Volume Reduction and ZLD Technologies for Water Utilities. Alexandria, VA.: WateReuse Foundation. NRC (National Research Council) Desalination: A National Perspective. Washington, D.C.: National Academy Press. Available: < Sethi, S., Walker, S., Xu, P. and Drewes, J.E Desalination Product Water Recovery and Concentrate Minimization. AwwaRF Project 3030 Final Report. Published by American Water Works Association Research Foundation, Denver, CO., Denver, Colorado. WHO (World Health Organization) Desalination for Safe Water Supply: Guidance for the Health and Environmental Aspects Applicable to Desalination, World Health Organization, Public Health and the Environment. Xu, P., Cath, T, Wang, G., Drewes, J.E. and Dolnicar, S Critical assessment of implementing desalination technology. AwwaRF Project 4006 Final Report. Published by American Water Works Association Research Foundation, Denver, CO. 6