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Semi-arid regions throughout the world face water scarcity and the need for more efficient and alternative sources of drinking water supply. Inland regions in the Arabian Peninsula have the alternate option of coastal seawater desalination and long-distance conveyance, often with lift to substantial elevation. In several aquifers of this region, naturally occurring radium in groundwater is above acceptable standards and must be reduced. We analyzed the energy footprint of a modular process employing a combination of pellet reactor for radium and hardness minimization, reverse osmosis with intermediate precipitation, and concentrated brine crystallization to achieve high recovery with zero liquid discharge (ZLD). Pilot tests demonstrate technical viability of the selected processes to achieve high recovery, radium and hardness reduction, and over 95% salinity reduction with zero liquid discharge. The results indicate that the energy usage per unit volume of water produced from groundwater is consistently lower than coastal seawater desalination, regardless of the conveyance distance. The substantial reduction of energy, higher recovery and minimized residual discharge of this process are also beneficial to the environment when compared to conventional processes currently being used. The results may be applicable and beneficial to other regions with similar conditions.

This study evaluated intermediate chemical demineralization (ICD) as a technique for overcoming mineral salt solubility limits that constrain the water recovery of reverse osmosis (RO) desalting. An empirical approach was developed to determine theoretical cation removal set points for ICD based on predetermined process design and operation guidelines to facilitate increasing the overall water recovery of Colorado River water desalting from 85% (i.e., in the primary RO unit) to 95% via a secondary RO (SRO) unit. Additionally, the study evaluated whether microfiltration (MF) or dual-media filtration (DMF) could serve as the filtration step prior to the secondary reverse osmosis (SRO) unit. The model suggested ranges for ICD influent total-carbonate-to-calcium ratios and ICD effluent pH to simultaneously mitigate barium sulfate, calcium sulfate, and silica scaling. MF was preferred over DMF as the filtration step prior to SRO based on significantly lower filter effluent turbidity and silt density indices. The overall PRO-ICD-SRO system operated for over 550 h at 95% total RO water recovery without evidence of scaling of the terminal RO elements, validating the cation removal set points developed through the ICD model and demonstrating the utility of ICD.

The potential impact of coagulants on mineral scaling in reverse osmosis (RO) feed treatment of brackish water was assessed experimentally, with respect to calcium sulfate and barium sulfate scalin...

A two-stage reverse osmosis (RO) process for high water recovery (up to 95%) desalination of Colorado River water was evaluated and demonstrated at the pilot scale. The two-stage process involved intermediate chemical demineralization (ICD) of the concentrate stream from a primary RO (PRO) process followed by secondary RO (SRO) desalting. Using alkaline-induced precipitation in a pilot-scale solids contact reactor (SCR), the membrane scaling propensity of the PRO desalting step was reduced, allowing for further recovery of product water via SRO of the demineralized PRO concentrate stream. When operating the SCR effluent above pH 10, the removal of Ca2+, Ba2+, Sr2+, and silica – the principal scale-forming precursors – were attained via ICD at levels upwards of 94%, 97%, 88%, and 67%, respectively. Despite significant process variations, 95% overall water recovery desalination was demonstrated to be feasible in the pilot scale via the PRO–ICD–SRO approach, with SRO desalting operated at specific permeate flux varying by ≤7% and at stable normalized salt passage of 2.9 ± 0.7% over extended periods of time. This study suggests that long-term implementation of the PRO–ICD–SRO process to achieve overall water recovery ≥95% would require robust on-line pH control strategy to attain consistent SCR effluent pH, along with appropriate selection and use of make-up antiscalants for the secondary RO desalting step, primarily to control membrane scaling by silicates and gypsum.

The integration of membrane RO desalting and accelerated precipitation softening (APS) was evaluated for achieving high product water recovery (>95%) from desalting of mildly brackish surface water. A systematic approach that included laboratory RO demonstrations, membrane scaling diagnostics, and mineral solubility analysis was undertaken to develop an effective physical/chemical APS strategy for reducing the concentration of scale-forming ions in the primary RO (PRO) concentrate. APS demineralization, as an inter-stage process, between the PRO and secondary RO (SRO) desalting, involved alkaline pH adjustment and calcite crystal seeding of the PRO concentrate, followed by microfiltration and pH reduction by acid dosing to avoid calcite scaling in the SRO stage. Colorado River water desalting case study demonstrated that, at 90% PRO recovery, the PRO concentrate stream contained barite, calcite, gypsum, and silica above their solubility limits by factors of 122, 47, 1.1, and 0.8, respectively. Further desalting was not feasible with traditional scale-control strategies (e.g., pH reduction and antiscalant addition). However, APS demineralization enabled significant concentration reduction of calcium (>90%), barium (>95%), and strontium (∼78%) and moderate reduction (10–20%) of magnesium and silica. APS kinetics were favorable even in the presence of antiscalant carryover from the PRO stage. The study demonstrated that high recovery desalination of up to 98% is feasible with the PRO-APS-SRO sequence, which reduces the limitations imposed by membrane scaling.

An approach to ranking antiscalant effectiveness for gypsum scale suppression using combined bulk crystallization and membrane scaling diagnostic tests was explored with a focus on the adverse impact of residual aluminum in dissolved (Al3+) and as colloidal species on gypsum scale suppression. Five commercial antiscalants were ranked based on the crystallization induction time as determined in a well-mixed crystallization vessel with a back-light scattering turbidity meter to monitor gypsum formation. The presence of aluminum, even at trace levels (up to 100 μg/L), significantly reduced the crystallization induction time, thereby reducing antiscalant effectiveness. For a given antiscalant dose, the crystallization induction time decreased with increasing total aluminum concentration according to a log-linear relationship. Although the above qualitative behavior was similar for the tested commercial anti-scalants, the adverse impact of aluminum on antiscalants effectiveness differed markedly among the different antiscalants. Diagnostic scaling tests in a plate-and-frame RO module demonstrated antiscalant performance ranking of the same order as that obtained based on bulk crystallization induction time measurements.

The Metropolitan Water District of Southern California carried out a study to evaluate manganese (Mn) desorption from filter media during biological filtration startup. Mn contamination of the filter media was a by-product of prior use of ferric chloride (FeC13), which contains Mn, for coagulation and free chlorine exposure. During pilot- and full-scale testing at pH 6.5, FeCl 3 coagulation (without chlorine) released soluble Mn at nearly four times the level released by aluminum sulfate (alum; 105 μg/L for FeCl 3 and 27 μg/L for alum). Pilot-scale experiments demonstrated that Mn was present in the filter column in three valence states [Mn(ll), Mn(lll), and Mn(IV)]. Mn(ll) in the filter influent was from the FeCl 3 . Mn on the anthracite media was hypothesized to be Mn(II)/Mn(IV) and first physically displaced by ferric iron, aluminum, and hydrogen, and then catalytically oxidized to Mn(IV) by silica sand.

Soluble aluminum (Al3+) may react with both ambient silica and antiscalant components to form colloidal foulants during reverse osmosis (RO) treatment. Whereas conventional treatment (coagulation/filtration/sedimentation/dual-media filtration) was being used prior to RO, aluminum sulfate (alum) and polyaluminum chloride (PACl) coagulants were evaluated at ambient pH (pH 7.8–7.9) and suppressed pH (pH 6.7) in an effort to lower the total aluminum to below 50 μg/L — a level previously observed to prevent RO membrane fouling. Additional tests were conducted with 5 mg/L citric acid added to the RO influent to chelate the soluble aluminum fraction. All tests were conducted with 1.5–2.5 mg/L chloramines present. Testing of a RO process fed with optimized alum- or PACl-coagulated water showed that PACl outperformed alum regardless of pH. Alum coagulation at ambient pH resulted in 184–273 μg/L total aluminum passing through the filtration process. Only by lowering the mean influent water pH to 6.7 was the mean soluble aluminum residual (45 μg/L) for alum coagulation reduced to below the 50 μg/L aluminum goal. Regardless of pH, for alum-coagulated waters, the higher aluminum carryover resulted in severe RO membrane fouling within 500 h of operation. Only when a chelating agent (citric acid) was added to the RO feed was the loss in productivity and selectivity arrested. However, PACl consistently met the 50-μg/L goal for both total and soluble aluminum for all pH levels tested, which resulted in more stable membrane performance over time. Further research on the compatibility of PACl and polyamide membranes in the presence of chloramines is needed as data from this project suggest PACl coagulation may facilitate membrane oxidation.

The use of aluminum sulfate (alum) coagulation prior to reverse osmosis (RO) treatment has been shown to be problematic. Membrane fouling was theorized to occur through soluble aluminum (Al 3+ ) reacting with ambient silica (H 4 SiO 2 ) to form kaolinite (Al 2 Si 2 O 5 (OH) 4 ) within the RO unit. Chelating agents (citrate at 34 mg/L and ethylenediaminetetraacetic acid [EDTA] at 16 mg/L) were tested for their efficacy in controlling aluminum silicate fouling. The results of bench-scale testing demonstrated that both citrate and EDTA did control aluminum silicate formation, citrate more so than EDTA. Additional aluminum-based fouling was encountered when a commercial, phosphonate-based antiscalant — used to control barium sulfate scaling — reacted with the excess aluminum despite the presence of either citrate or EDTA.

Reverse osmosis testing using monochloramine (NH 2 Cl) and free chlorine (HOCl) in the presence of ferrous iron [Fe(II)] resulted in accelerated chlorination of polyamide membranes. Similar effects were not observed when using ferric iron. Membrane damage was detected through irreversible increases in salt passage and the presence of chloride on the membrane surface using energy-dispersive spectroscopy. A mechanistic study suggested that the formation of an amidogen radical ( NH 2 ) during NH 2 Cl decomposition with Fe(II) led to the reduction of the activation energy for the chlorination reaction to proceed using NH 2 Cl. Enhanced oxidation using HOCl and Fe(II) may have resulted from the formation of OH radicals, which also lowered the chlorination-activation energy. Both pH suppression (from 8.0 to 6.0) and dechlorination (7.0 mg/L sodium sulfite) successfully halted the chlorination reaction.

Cationic polymers used in traditional water and wastewater treatment, in theory, will bind to the partially negatively charged polyamide membrane during reverse osmosis (RO) treatment. This study tested a variety of cationic, anionic, and nonionic polymers in terms of (1) aiding in turbidity removal when used in conjunction with alum coagulation [10 mg/L as Al2(SO4)3·14H2O], sedimentation, and dual-media filtration; and (2) their binding affinity toward a polyamide membrane. Binding affinity was tested by exposing polyamide RO membranes to 50 mg/L of each polymer for 48 h and then conducting attenuated total reflectance Fourier transform infrared (FTIR) spectroscopy on the membrane samples and principal-component analysis (PCA) of the FTIR spectra. Results from the turbidity removal tests showed that cationic polymers provided the lowest median effluent turbidity (0.03–0.11 NTU), followed by anionic polymers (0.06–0.08 NTU), and finally nonionic polymers (0.11–0.17 NTU). Although PCA of the FTIR spectra did detect differences in polymer-exposed membranes vs. a control, no discernable difference in RO membrane performance was observed. Therefore, under the conditions studied, the issue of “membrane poisoning ” caused by organic polymer absorption onto polyamide membranes was minimal. © 2005 American Institute of Chemical Engineers Environ Prog, 2005

A new technique has been developed to quickly monitor the changes in polarity of aqueous natural organic matter (NOM) using solid-phase extraction (SPE) cartridges. This paper introduces the NOM polarity rapid assessment method (PRAM). The PRAM technique characterizes changes in NOM polarity by monitoring the breakthrough curves from different SPE cartridges at UV254. The SPE cartridges used in this study include a wide range of polarity from non-polar C-18 materials to anion exchangers. Each individual cartridge run takes 10 minutes and requires about 15 ml of sample. The collected water sample matrix is not changed, i.e. all PRAM analyses were done under ambient conditions on the original sample. Polarity evaluation is completed without the sample being exposed to changes in sample conditions, such as pH, solvent extraction, sequential evaporations or freeze-drying. This technique was able to monitor the weekly changes in NOM polarity entering a water treatment plant and compares the effects of different water treatment processes on this material.

Parallel experiments using a blend of surface waters were conducted to evaluate differential fouling rates among reverse osmosis (RO) membranes when operated under pilot- vs. full-scale conditions. Testing was conducted using a 230 L/min conventional (rapid mix/flocculation/sedimentation/filtration) package plant (CPP) and a 2,000 ML/d fullscale treatment plant (FTP) as pretreatment to separate RO membrane test units. Coagulation consisted of 10 mg/L alum (as Al2(SO4)3-14H2O) and 2.0 mg/L cationic polymer. A2.5-3.0 mg/L free-chlorine residual was maintained at the filter effluent and converted to chloramines through ammonium sulfate addition (3:1 chlorine-to-ammonia w/w ratio). Membrane performance was based on normalized flux and salt rejection data. Membrane surface analyses included scanning electron microscopy, energy-dispersive spectroscopy, and attenuated total reflectance Fourier transform infrared spectroscopy. Microbial activity and community analyses were conducted through (a) fluorescence staining with 4′,6′-diamidino-2-phenylindole, (b) polymerase-chain reaction amplification of isolated bacterial DNA, and (c) microscopic taxonomic identification. Results indicated that the RO membrane fed by the CPP fouled at least three times faster than the RO membrane fed by the FTP. The differential fouling between the two process streams was determined to be from lack of maintenance in the CPP influent piping that led to the establishment of biological communities consisting of algae, microbes, and, potentially, freshwater clams. These communities produced low levels of natural polymers, which when presented to the polyamide RO membrane surface, resulted in rapid fouling.

In order to meet the growing needs of its consumers, the Metropolitan Water District of Southern California is looking for ways to desalt water from the relatively hard Colorado River. This study evaluated conventional treatment as the pretreatment step for reverse osmosis (RO) desalting. Other pretreatment options studied include microfiltration (MF) and conventional treatment with ozone and biologically active filters. Each of three pretreatment scenarios produced effluent waters generally considered appropriate for use with RO [median turbidity of less than 0.1 Nephelometric turbidity unit (NTU) and median silt density index of less than 3]. Both microfiltered and ozonated/ biofiltered waters gave steady RO performance over 3 months of testing. However, conventional treatment left the RO system vulnerable to organic and biological fouling. Despite maintaining a 2-2.5 mg/L chloramine residual, pretreatment using conventional treatment required more frequent cleanings than either MF or ozonation/biofiltration (O3/BF). The good performance for biofiltered water may have resulted from the stabilization of the natural organic matter through the O3/BF process. Microfiltration, with its superior particle and bacteriological removal characteristics, provided the best RO pretreatment technology.

Pretreatment may constitute up to one-fourth of the total costs of a membrane desalting facility. By using preexisting conventional filtration plants for pretreatment, significant cost savings may be realized. However, coagulant residuals from the pretreatment process may negatively affect reverse osmosis (RO) membrane performance. Various RO membranes were tested at three different treatment plants in southern California, using either aluminum sulfate (alum) or ferric chloride coagulants and chloramines. Repeated testing using alum with multiple RO elements revealed rapid deterioration in specific flux (up to 60% over 100 h of operation), as well as progressive reductions in salt rejection (typically 3–4% over 500 h of operation). Microscopic analysis of the fouled membranes revealed that the foulants were primarily aluminum hydroxide and aluminum silicate materials. In contrast to the RO data for alum coagulation, which showed declining membrane flux, the specific flux data using ferric chloride and chloramines increased over time for all membranes. Salt rejection decreased significantly during testing of each membrane. These data suggest that the RO membranes were physically degrading over time. The RO membranes may have been degraded by residual iron catalyzing a chlorine-amide reaction on the membrane surface, despite the fact that chlorine was present as chloramines.

Capacitive deionization (CDI) with carbon aerogels has been shown to remove various inorganic species from aqueous solutions, though no studies have shown the electrosorption behavior of multisolute systems in which ions compete for limited surface area. Several experiments were conducted to determine the ion removal capacity and selectivity of carbon aerogel electrodes, using both laboratory and natural waters. Although carbon aerogel electrodes have been treated as electrical double-layer capacitors, this study showed that ion sorption followed a Langmuir isotherm, indicating monolayer adsorption. The sorption capacity of carbon aerogel electrodes was approximately 1.0-2.0 x 10(-4) equiv/g aerogel, with ion selectivity being based on ionic hydrated radius. Monovalent ions (e.g., sodium) with smaller hydrated radii were preferentially removed from solution over multivalent ions (e.g., calcium) on a percent or molar basis. Because of the relatively small average pore size (4-9 nm) of the carbon aerogel material, only 14-42 m2/g aerogel surface area was available for ion sorption. Natural organic matter may foul the aerogel surface and limit CDI effectiveness in treating natural waters.

Pore water dissolved organic matter (DOM) plays an important role in the distribution, mobility, and bioavailability of hydrophobic organic chemicals (HOCs) in sediment environments. The effect of aeration on the partitioning of 2,2{prime},4,4{prime}-tetrachlorobiphenyl (TeCB) to anoxic pore water DOM from three estuarine sites was investigated. Pore water DOM was fractionated into molecular size and polarity fractions by ultrafiltration and XAD-8 resin chromatography. Total organic carbon analysis was utilized to determine shifts in molecular size and polarity distributions. Changes in functional groups and aromaticity were evaluated for whole and fractionated pore waters by specific UV absorbance at 254 nm (SUVA{sub 254}). The solubility enhancement method was used to determine the partitioning of TeCB to whole and fractionated pore water DOM. At two sites, the overall TeCB-DOM distribution coefficient decreased by an order of magnitude after aeration. The higher molecular size and all polarity fractions exhibited a decrease in partitioning behavior upon aeration. The aromaticity and TeCB-DOM distribution coefficient of the lowest molecular size fraction decreased upon aeration. The highest and lowest molecular size fractions contributed the most to overall partitioning. The observed aeration effects in anoxic estuarine sediment pore waters differed significantly from those previously reported in freshwater systems.

Inland desalination of brackish water can provide an important source of potable water in many parts of the world. Reverse osmosis (RO) and nanofiltration (NF) have become the technology of choice for many of these applications [T. Pankratz, The 19th IDA Worldwide Desalting Plant Inventory, Global Water Intelligence, Houston, TX, 2006]. However, large-scale deployment of RO/NF in inland locations would necessitate operation at relatively high product water recovery in order to maximize water resource utilization and minimize treatment costs and environmental challenges associated with disposal of the concentrate. Concentrate disposal is generally limited to one or two options for any given area and is directly related to land cost, energy costs, regulations, and the type and quantity of salts in the concentrate stream [M. Mickley, Concentrate management, in: M. Wilf (Ed.), The Guidebook to Membrane Desalination Technology: Reverese Osmosis, Nanofiltration and Hybrid Systems Process, Design, Applications and Economics, Balaban Desalination Publications, L'Aquila, Italy, 2007, p. 524; B. van der Bruggen, L. Lejon, C. Vandecasteele, Reuse, treatment, and discharge of the concentrate of pressure-driven membrane processes, Environ. Sci. Technol. 37 (2003) 3733–3738.]. Inland concentrate disposal options include reuse of concentrates, surface water discharge, sewer disposal, deep well injection, land applications, and evaporation ponds (followed by land filling of solids) [M. Mickley, Concentrate management, in: M. Wilf (Ed.), The Guidebook to Membrane Desalination Technology: Reverese Osmosis, Nanofiltration and Hybrid Systems Process, Design, Applications and Economics, Balaban Desalination Publications, L'Aquila, Italy, 2007, p. 524.]. This chapter provides an overview of post-RO/NF concentrate minimization technologies with the goal of minimizing the concentrate volume such that ultimate disposal options are more feasible. Examples of several of these technologies are also provided.