Reverse osmosis desalination: Water sources, technology, and today's challenges

Reverse osmosis membrane technology has developed over the past 40 years to a 44% share in world desalting production capacity, and an 80% share in the total number of desalination plants installed worldwide. The use of membrane desalination has increased as materials have improved and costs have decreased. Today, reverse osmosis membranes are the leading technology for new desalination installations, and they are applied to a variety of salt water resources using tailored pretreatment and membrane system design. Two distinct branches of reverse osmosis desalination have emerged: seawater reverse osmosis and brackish water reverse osmosis. Differences between the two water sources, including foulants, salinity, waste brine (concentrate) disposal options, and plant location, have created significant differences in process development, implementation, and key technical problems. Pretreatment options are similar for both types of reverse osmosis and depend on the specific components of the water source. Both brackish water and seawater reverse osmosis (RO) will continue to be used worldwide; new technology in energy recovery and renewable energy, as well as innovative plant design, will allow greater use of desalination for inland and rural communities, while providing more affordable water for large coastal cities. A wide variety of research and general information on RO desalination is available; however, a direct comparison of seawater and brackish water RO systems is necessary to highlight similarities and differences in process development. This article brings to light key parameters of an RO process and process modifications due to feed water characteristics.     

Harmful algae and their potential impacts on desalination operations off southern California

Seawater desalination by reverse osmosis (RO) is a reliable method for augmenting drinking water supplies. In recent years, the number and size of these water projects have increased dramatically. As freshwater resources become limited due to global climate change, rising demand, and exhausted local water supplies, seawater desalination will play an important role in the world's future water supply, reaching far beyond its deep roots in the Middle East. Emerging contaminants have been widely discussed with respect to wastewater and freshwater sources, but also must be considered for seawater desalination facilities to ensure the long-term safety and suitability of this emerging water supply. Harmful algal blooms, frequently referred to as ‘red tides’ due to their vibrant colors, are a concern for desalination plants due to the high biomass of microalgae present in ocean waters during these events, and a variety of substances that some of these algae produce. These compounds range from noxious substances to powerful neurotoxins that constitute significant public health risks if they are not effectively and completely removed by the RO membranes. Algal blooms can cause significant operational issues that result in increased chemical consumption, increased membrane fouling rates, and in extreme cases, a plant to be taken off-line. Early algal bloom detection by desalination facilities is essential so that operational adjustments can be made to ensure that production capacity remains unaffected. This review identifies the toxic substances, their known producers, and our present state of knowledge regarding the causes of toxic episodes, with a special focus on the Southern California Bight.     

海水淡化水进入市政管网需考虑和解决的问题

近几年海水淡化在我国发展较快,规模越来越大,用途也逐渐从工业扩大到市政供水,这意味着海水淡化水进入市政管网已提上议事日程.从饮水安全和保护管网两个方面阐述了海水淡化水与传统水资源的差异,并在此基础上初步提出海水淡化水的后处理应考虑的各项水质指标和要求以及稳定水质的方法,介绍了国际大型海水淡化工程产品水的后处理常用方法.     

Formation of hazardous inorganic by-products during electrolysis of seawater as a disinfection process for desalination

From our previous study, an electrochemical process was determined to be a promising tool for disinfection in a seawater desalination system, but an investigation on the production of several hazardous by-products is still required. In this study, a more intensive exploration of the formation patterns of perchlorate and bromate during the electrolysis of seawater was conducted. In addition, the rejection efficiencies of the targeted by-products by membrane processes (microfiltration and seawater reverse osmosis) were investigated to uncover the concentrations remaining in the final product from a membrane-based seawater desalination system for the production of drinking water. On the electrolysis of seawater, perchlorate did not provoke any problem due to the low concentrations formed, but bromate was produced at a much higher level, resulting in critical limitation in the application of the electrochemical process to the desalination of seawater. Even though the formed bromate was rejected via microfiltration and reverse osmosis during the 1st and 2nd passes, the residual concentration was a few orders of magnitude higher than the USEPA regulation. Consequently, it was concluded that the application of the electrochemical process to seawater desalination cannot be recommended without the control of bromate.     

天津反渗透海水淡化示范工程(1000m~3/d)

介绍了天津反渗透海水淡化示范工程(1 000 m3/d)的项目概况、工艺设计、设备配置、运行状况及造水成本分析.结果表明,针对天津海域海水浊度高、污染重的特点,采用初步预处理/双膜法淡化工艺完全能达到设计产水量和产水水质指标,并实现系统的稳定运行.     

Mineral recovery from inland reverse osmosis concentrate using isothermal evaporation

Mineral recovery from reverse osmosis (RO) concentrate after concentration by a secondary sea water-type RO system with lime–soda pretreatment was the focus of this study. Lime–soda pretreatment removed Ca, Mg and Si allowing for the application of sea water-type RO resulting in a concentrate composed of sodium, potassium, sulfate and chloride. The overall objective was reduction in concentrate volume that will require disposal by evaporation while producing by-products with potential resale value. Thermodynamic phase equilibrium calculations using Pitzer’s correlations for 25 °C, accurately predicted the solubility and evaporation path of the sodium sulfate minerals as potential by-products. Bench-scale evaporation experiments verified the model predictions and indicated that 81–88% of the sodium sulfate by-products were Na₂SO₄.     

Variations in isotopic composition of desalinated water

Variations in isotopic ratios of water samples collected from three plants functioning on two different desalination processes were evaluated by comparing their measured δ¹⁸O, δ¹⁷O and the δ²H isotopic ratios before and after desalination using a system based on tunable off‐axis integrated‐cavity‐output diode laser spectroscopy (OA‐ICOS). The δ¹⁸O, δ¹⁷O and the δ²H isotopic ratio measurements for water samples collected before desalination are compared against their peers of desalinated water from two desalination plants on the Arabian Gulf that are operating under the Multistage Flash (MSF) process. Also, variations in the δ¹⁸O, δ¹⁷O and the δ²H isotopic ratio measurements due to desalination from a Reversal Osmosis (RO) facility in Dhahran, Saudi Arabia were also reported and compared against variations in isotopic composition of water desalinated in two MSF plants.     

反渗透海水淡化脱硼技术研究现状

硼在水中以硼酸的分子形式存在,其浓度过高对农作物和人体会产生不利的影响。由于硼的分子半径小,且不具有电性,因此反渗透海水淡化需要与其他方法相结合才能使硼含量达到饮用水的卫生要求。介绍了目前反渗透海水淡化中的脱硼方法。     

Adsorption combined with ultrafiltration to remove organic matter from seawater

Organic fouling and biofouling are the major severe types of fouling of reverse osmosis (RO) membranes in seawater (SW) desalination. Low pressure membrane filtration such as ultrafiltration (UF) has been developed as a pre-treatment before reverse osmosis. However, UF alone may not be an effective enough pre-treatment because of the existence of low-molecular weight dissolved organic matter in seawater. Therefore, the objective of the present work is to study a hybrid process, powdered activated carbon (PAC) adsorption/UF, with real seawater and to evaluate its performance in terms of organic matter removal and membrane fouling. The effect of different PAC types and concentrations is evaluated. Stream-activated wood-based PAC addition increased marine organic matter removal by up to 70% in some conditions. Moreover, coupling PAC adsorption with UF decreased UF membrane fouling and the fouling occurring during short-term UF was totally reversible. It can be concluded that the hybrid PAC adsorption/UF process performed in crossflow filtration mode is a relevant pre-treatment process before RO desalination, allowing organic matter removal of 75% and showing no flux decline for short-term experiments.     

Bioanalytical and chemical assessment of the disinfection by-product formation potential: Role of organic matter

Disinfection by-products (DBP) formed from natural organic matter and disinfectants like chlorine and chloramine may cause adverse health effects. Here, we evaluate how the quantity and quality of natural organic matter and other precursors influence the formation of DBPs during chlorination and chloramination using a comprehensive approach including chemical analysis of regulated and emerging DBPs, total organic halogen quantification, organic matter characterisation and bioanalytical tools. In vitro bioassays allow us to assess the hazard potential of DBPs early in the chain of cellular events, when the DBPs react with their molecular target(s) and activate stress response and defence mechanisms. Given the reactive properties of known DBPs, a suite of bioassays targeting reactive modes of toxic action including genotoxicity and sensitive early warning endpoints such as protein damage and oxidative stress were evaluated in addition to cytotoxicity. Coagulated surface water was collected from three different drinking water treatment plants, along with reverse osmosis permeate from a desalination plant, and DBP formation potential was assessed after chlorination and chloramination. While effects were low or below the limit of detection before disinfection, the observed effects and DBP levels increased after disinfection and were generally higher after chlorination than after chloramination, indicating that chlorination forms higher concentrations of DBPs or more potent DBPs in the studied waters. Bacterial cytotoxicity, assessed using the bioluminescence inhibition assay, and induction of the oxidative stress response were the most sensitive endpoints, followed by genotoxicity. Source waters with higher dissolved organic carbon levels induced increased DBP formation and caused greater effects in the endpoints related to DNA damage repair, glutathione conjugation/protein damage and the Nrf2 oxidative stress response pathway after disinfection. Fractionation studies indicated that all molecular weight fractions of organic carbon contributed to the DBP formation potential, with the humic rich fractions forming the greatest amount of DBPs, while the low molecular weight fractions formed more brominated DBPs due to the high bromide to organic carbon ratio. The presence of higher bromide concentrations also led to a higher fraction of brominated DBPs as well as proportionally higher effects. This study demonstrates how a suite of analytical and bioanalytical tools can be used to effectively characterise the precursors and formation potential of DBPs.     

Economic evaluation of different scenarios for gas turbine waste heat recovery to produce water and power

In this study, economic analysis of different configurations of combined power and water systems is performed using DEEP software. Three different desalination systems including reverse osmosis (RO), multi-effect desalination (MED) and multi-stage flash (MSF) coupled to a gas turbine power plant with 2?×?165.1?MW capacity are considered and analysed. A specific capacity of 50,000?m³/day is assumed for all desalination systems. The results indicate that the RO system has the lowest simple payback time, while MED has the highest thermal utilisation. In terms of levelised cost of water, the RO system has the lowest amount with 1.03?$/(m³/day), while levelised cost of water for MED and MSF are equal to 1.47 and 1.93?$/(m³/day), respectively. Also sensitivity analysis showed that the fuel cost is the most effective parameter on the levelised cost of water. In general, it can be concluded that the RO system is the best choice in terms of economics.     

海水淡化水在既有管网中的水质变化研究

通过在实验室搭建小型管网,模拟了自来水及海水淡化水在已有市政管网中的输配过程,并选取pH、碱度、Langelier饱和指数和Ryznar指数作为控制指标来评价其化学稳定性.结果表明,海水淡化水的碱度极低,具有不饱和性(Langelier饱和指数为-3.25)和强腐蚀倾向(Ryznar指数为13.92),即化学不稳定性,进入既有管网后易溶解原先积存的管垢,并加速金属管道内壁腐蚀.     

The formation of halogen-specific TOX from chlorination and chloramination of natural organic matter isolates

The formation of disinfection by-products (DBPs) is a public health concern. An important way to evaluate the presence of DBPs is in terms of the total organic halogen (TOX), which can be further specified into total organic chlorine (TOCl), bromine (TOBr), and iodine (TOI). The formation and distribution of halogen-specific TOX during chlorination and chloramination of natural organic matter (NOM) isolates in the presence of bromide and iodide ions were studied. As expected, chloramination produced significantly less TOX than chlorination. TOCl was the dominant species formed in both chlorination and chloramination. TOI was always produced in chloramination, but not in chlorination when high chlorine dose was used, due to the limited presence of HOI in chlorination as a result of the oxidation of iodide to iodate in the presence of excess chlorine. The formation of TOI during chloramination increased as the initial iodide ion concentration increased, with a maximum of ∼60% of the initial iodide ion becoming incorporated into NOM. Iodine incorporation in NOM was consistently higher than bromine incorporation, demonstrating that the competitive reactions between bromine and iodine species in chloramination favoured the formation of HOI and thus TOI, rather than TOBr. Correlations between the aromatic character of the NOM isolates (SUVA₂₅₄ and % aromatic C) and the concentrations of overall TOX and halogen-specific TOX in chloramination were observed. This indicates that the aromatic moieties in NOM, as indicated by SUVA₂₅₄ and % aromatic C, play an important role in the formation of overall TOX and halogen-specific TOX in chloramination. THMs comprised only a fraction of TOX, up to 7% in chloramination and up to 47% in chlorination. Although chloramine produces less TOX than chlorine, it formed proportionally more non-THM DBPs than chlorine. These non-THM DBPs are mostly unknown, corresponding to unknown health risks. Considering the higher potential for formation of iodinated DBPs and unknown DBPs associated with the use of chloramine, water utilities need to carefully balance the risks and benefits of using chloramine as an alternative disinfectant to chlorine in order to satisfy guideline values for THMs.     

海水淡化水与地表水掺混后的水质稳定性研究

以掺混水的朗格利尔指数作为评价标准,对掺混后的水的化学稳定性进行评价。结果表明,单纯依靠掺混无法满足水质稳定性;用Ca（OH）2对掺混水进行调值,可以增加pH值但不显著影响其他指标,且能部分满足水质的稳定性。在此基础上计算出水源水与海水淡化水在不同季节以不同比例进行掺混并调节pH后的水质稳定性,由于地表水水质随季节发生变化,同样的掺混比例获得的掺混水在某些季节是稳定的,在另一些季节可能存在风险。因此,单纯的掺混不是安全使用海水淡化水的可靠方案。     

Long-term impact of water desalination plants on the energy and carbon dioxide balance of Victoria,Australia: a case study from Wonthaggi

In 2007, the state government of Victoria, Australia, announced plans to build a large desalination plant with a capacity of 150 million m³ per year of desalinated water. Currently, the only feasible source for significant expansion of the greenhouse-gas neutral (GGN) energy generation in the state is wind power. A criterion for GGN of a desalination plant has been formulated. In a case of no greenhouse gas contribution from the desalination plant, the criterion is satisfied if the annual growth of the electrical energy generated by GGN sources is around 6–9% for the period 2010–2070. Higher annual growth of 18% for the period 2008–2015, 8% annually for the period 2015–2035 and 6% annual growth thereafter are required if the desalination plant is contributing to the greenhouse-gas balance of the state.     

海水淡化水经矿化调质后引入市政供水系统工程实例

青岛市某海淡水调引工程设计规模为6×104 m3/d。为保障输送管网及用户的水质安全,本工程将海水淡化水先进行矿化处理,再引入常规工艺水厂进行掺混后进入市政管网,出水水质优于生活饮用水水质标准。本工程在充分利用现状海水淡化水的水质条件和现有的工程设施条件的基础上,新建海水淡化水矿化站1座,矿化工艺采用“二氧化碳+矿化滤池”工艺,矿化滤池采用上向流池型。另外,本工程还对现状供水系统进行改造以引入海水淡化水,平衡了供水缺口,改善了供水品质,保障了供水安全。     

Transformation of phenazone-type drugs during chlorination

Chlorination is one of the most popular disinfection steps for water treatment in Europe. However, chlorine can react with pharmaceuticals and other micropollutants leading to either their elimination or by-products being formed. These by-products are frequently not identified and therefore the consequences of chlorination can be underestimated. In this work, the degradation of two analgesics and antipyretics, phenazone (antipyrine) and propyphenazone, during chlorination was investigated by liquid chromatography-mass spectrometry (LC-MS). A quadrupole-time-of-flight (Q-TOF) system was used to follow the time course of the pharmaceuticals, and also used in the identification of the by-products. The degradation kinetics was investigated at different concentrations of chlorine (1-10 mg/L), bromide (0-100 μg/L) and sample pH (5.7-8.3) by means of a Box-Behnken experimental design. Depending on these factors, half-lives were in the ranges: 0.9-295 s for phenazone and 0.4-173 s for propyphenazone. Also, it was observed that chlorine concentration was a significant factor for propyphenazone, resulting in increased degradation rate as it is increased. The transformation path of these drugs consisted mainly of halogenations, hydroxylations and dealkylations. After several days of reaction two derivatives remained stable for phenazone: chloro-hydroxy-phenazone and N-demethyl-chloro-hydroxy-phenazone and two for propyphenazone: N-demethyl-hydroxy-propyphenazone and N-demethyl-chloro-hydroxy-propyphenazone. Moreover, experiments conducted with real water matrices, tap and surface water, showed that reaction, and formation of by-products, can take place both at the emission source point (household) and during drinking water production.     

Potential effects of desalinated water quality on the operation stability of wastewater treatment plants

Desalinated water is expected to become the major source of drinking water in many places in the near future, and thus the major source of wastewater to arrive at wastewater treatment plants. The paper examines the effect of the alkalinity value with which the water is released from the desalination plant on the alkalinity value that would develop within the wastewater treatment process under various nitrification-denitrification operational scenarios. The main hypothesis was that the difference in the alkalinity value between tap water and domestic wastewater is almost exclusively a result of the hydrolysis of urea (NH₂CONH₂, excreted in the human urine) to ammonia (NH₃), regardless of the question what fraction of NH_3(aq) is transformed to NH₄⁺. Results from a field study show that the ratio between the alkalinity added to tap water when raw wastewater is formed (in meq/l units) and the TAN (total ammonia nitrogen, mole/l) concentration in the raw wastewater is almost 1:1 in purely domestic sewage and close to 1:1 in domestic wastewater streams mixed with light industry wastewaters. Having established the relationship between TAN and total alkalinity in raw wastewater the paper examines three theoretical nitrification-denitrification treatment scenarios in the wastewater treatment plant (WWTP). The conclusion is that if low-alkalinity desalinated water constitutes the major water source arriving at the WWTP, external alkalinity will have to be added in order to avoid pH drop and maintain process stability. The results lead to the conclusion that supplying desalinated water with a high alkalinity value (e.g. ≥ 100 mg/l as CaCO₃) would likely prevent the need to add costly basic chemicals in the WWTP, while, in addition, it would improve the chemical and biological stability of the drinking water in the distribution system.     

Comparison of the disinfection by-product formation potential of treated waters exposed to chlorine and monochloramine

The formation of disinfection by-products (DBPs) from chlorination and monochloramination of treated drinking waters was determined. Samples were collected after treatment at 11 water treatment works but before exposure to chlorine or monochloramine. Formation potential tests were carried out to determine the DBPs formed by chlorination and monochloramination. DBPs measured were trihalomethanes (THMs), haloacetic acids (HAAs), halonitromethanes (HNMs), haloacetonitriles (HANs), haloaldehydes (HAs), haloketones (HKs) and iodo-THMs (i-THMs). All waters had the potential to form significant levels of all the DBPs measured. Compared to chlorine, monochloramination generally resulted in lower concentrations of DBPs with the exception of 1,1-dichloropropanone. The concentrations of THMs correlated well with the HAAs formed. The impact of bromine on the speciation of the DBPs was determined. The literature findings that higher bromide levels lead to higher concentrations of brominated DBPS were confirmed.     

Effects of blending of desalinated water with treated surface drinking water on copper and lead release

This study examined effects of desalinated water on the corrosion of and metal release from copper and lead-containing materials. A jar test protocol was employed to examine metal release from copper and lead-tin coupons exposed to water chemistries with varying blending ratios of desalinated water, alkalinities, pHs and orthophosphate levels. Increasing fractions of desalinated water in the blends resulted in non-monotonic changes of copper and lead release, with generally lower metal concentrations in the presence of desalinated water, especially when its contribution increased from 80% to 100%. SEM examination showed that the increased fractions of desalinated water were associated with pronounced changes of the morphology of the corrosion scales, likely due to the influence of natural organic matter. This hypothesis was corroborated by the existence of correlations between changes of the ζ-potential of representative minerals (malachite and hydrocerussite) and metal release. For practical applications, maintaining pH at 7.8 and adding 1 mg/L orthophosphate as PO₄ were concluded to be adequate to decrease copper and lead release. Lower alkalinity of desalinated water was beneficial for blends containing 50% or more desalinated water.