Two-stage Reverse Osmosis Process Research Articles

Reverse Osmosis with Intermediate Chemical Demineralization: Scale Inhibitor Selection, Degradation, and Seeded Precipitation.

Two-stage reverse osmosis (RO) processes with intermediate concentrate demineralization (ICD) provide an efficient strategy to treat brines with high CaSO4 contents and reduce concentrate discharge. In this paper, an SRO concentrate is treated using ICD to remove CaSO4 and then mixed with a PRO concentrate for further desalination in SRO, thereby reducing the discharge of the concentrate. We investigate the selection and degradation of scale inhibitors, as well as seeded precipitation in the two-stage RO process with ICD, to achieve a high water recovery rate. A scale inhibitor is added to restrain CaSO4 crystallization on the membrane surface, and the optimized scale inhibitor, RO-400, is found to inhibit calcium sulfate scaling effectively across a wide range of the saturation index of gypsum (SIg) from 2.3 to 6. Under the optimized parameters of 40 W UV light and 70 mg/L H2O2, UV/H2O2 can degrade RO-400 completely in 15 min to destroy the scale inhibitor in the SRO concentrate. After scale inhibitor degradation, the SRO concentrate is desaturated by seeded precipitation, and the reaction degree of CaSO4 reaches 97.12%, leading to a concentrate with a low SIg (1.07) for cyclic desalination. Three UVD-GSP cycle tests show that the reused gypsum seeds can also ensure the effect of the CaSO4 precipitation process. This paper provides a combined UVD-GSP strategy in two-stage RO processes to improve the water recovery rate for CaSO4-contained concentrate.

Forward osmosis (FO)-reverse osmosis (RO) hybrid process incorporated with hollow fiber FO

Currently, desalination is limited by high energy consumption and high operational and maintenance costs. In this study, a new concept of a hollow fiber forward osmosis (HFFO)-based infinity desalination process with minor environmental impacts (free-energy intake and no pretreatment or brine discharge) is suggested. To evaluate the concept, an element-scale HFFO was conducted in both conventional FO and pressure-assisted FO modes, simulating a submerged HFFO operation. In the HFFO test, the impacts of several operating conditions on the performance of the HFFO were investigated to select the best case. Based on these results, the energy costs were calculated and compared with those of a hybrid FO–seawater reverse osmosis (SWRO) process. The HFFO showed a high dilution rate of the draw solution (up to approximately 400%), allowing the downstream SWRO process to operate at 25 bar with the same permeate volume production (recovery rate of 60%). Consequently, the HFFO-based infinity desalination process has an annual energy revenue of 183.83 million USD, compared with a stand-alone two-stage RO process based on a 100,000 m3/day plant.

Influence of osmotic mediation on permeation of water in reverse osmosis: Experimental and numerical analysis

Osmotically mediated reverse osmosis (RO), whose concept is to lower the osmotic pressure difference (Δπ) across the membrane by supplying a “mediate” solution of low or equal concentration (relative to feed solution concentration) in the permeate side, has been recently suggested as a viable way to treat hypersaline brines. However, most of the studies on the osmotically mediated RO process have focused on numerical analyses or simulation. Accordingly, we experimentally delved deeper into the permeation of water through the membrane and numerically analyzed this permeation for the process. When Δπ across the membrane was mediated in the bulk phase for the feed solution of 0.2 to 1.2 M, the water flux significantly decreased with the feed solution concentration. Specifically, for the feed solutions of 0.6 and 1.2 M, the water fluxes of 22.1 and 5.76 L m-2 h-1 were obtained at ΔP of 40 bar and Δπ of 0 bar in the bulk phase, respectively. Numerical results showed that even when Δπ was mediated in the bulk phase, effective osmotic pressure difference (Δπeff) for each concentration increased with ΔP increase and increased further with the feed solution concentration. Nevertheless, the osmotically mediated RO process can be regarded as hydraulic dilution process. By virtue of this process, the high-pressure single stage RO process may be replaced by a moderate-pressure two-stage RO process for low-energy process.

Effect of Stream Mixing on RO Energy Cost Minimization

Various mixing operations between the feed, retentate and permeate streams are studied in this work to determine their effectiveness in decreasing the specific energy consumption (SEC) of single-stage (single-pass), two-pass and two-stage reverse osmosis (RO) processes operated at the limit of the thermodynamic restriction. The results show that in a single-stage RO process, partial retentate recycling to the feed stream does not change the SEC, while partial permeate recycling to the feed stream increases the SEC if targeting the same overall water recovery. Energy optimization of two-pass membrane desalination, with second-pass retentate recycling to the first-pass feed stream and operated at the limit imposed by the thermodynamic restriction, revealed the existence of a critical water recovery. When desalting is accomplished at recoveries above the critical water recovery, two-pass desalination with recycling is always less efficient than single-pass desalination. When desalting is accomplished at recoveries below the critical water recovery, an operational sub-domain exists in which the SEC for a two-pass process with recycling can be lower than for a single-pass counterpart, when the latter is not operated at its globally optimal state. For the two-stage RO process, diverting part of the raw feed to the second stage, in order to dilute the feed to the second-stage RO, does not decrease the minimal achievable SEC of a two-stage RO process. The various mixing approaches, while may provide certain operational or system design advantages (e.g., with respect to achieving target salt rejection for certain solutes or flux balancing), do not provide an advantage from an energy usage perspective.

Energy consumption for sugar manufacturing. Part I: Evaporation versus reverse osmosis

Abstract Removing water from various feeds is usually carried out using evaporation process especially in food industry. Due to the high latent heat of water, this unit operation results in consumption of unacceptable amount of energy. Finding low energy consuming processes which could be replaced with this process is still a challenge. The processes with no phase inversion may be considered for concentration purposes with reasonable energy consumption in comparison with the other various separation procedures. Reverse osmosis and most of the other membrane technologies are separation techniques without any change in the phase and therefore consume low amount of energy. Concentrating the sugar thin juice in the classical sugar manufacturing procedure is carried out using conventional evaporation. Reverse osmosis membranes may be used as a pre-concentration step to partially separate water from the sugar thin juice in combination with this part of the plant. Final concentration and thick juice preparation for crystallization may be carried out in the evaporation unit. In this study, membranes were employed for sugar thin juice concentration using a two-stage reverse osmosis process in two different arrangements. The energy consumption was calculated and compared for conventional evaporation versus reverse osmosis combined with evaporation. The results indicate that the employment of revers osmosis membranes for concentrating the sugar thin juice leads to sensibly lower energy requirements. Furthermore, there is no thermal loss of sugar in the membrane process.

Feasibility study of treatment of amoxillin wastewater with a combination of extraction, Fenton oxidation and reverse osmosis

Wastewater from the synthesizing of amoxillin is characterized by its very high TOC and low biodegradability and the presence of organic solvents and dissolved salts. These hamper direct biological or membrane-based treatments. A combined process of extraction (EX), Fenton oxidation (FO) and a two-stage reverse osmosis (RO) was proposed to treat such wastewater. EX and FO were performed in a serial approach to function as the pretreatments to remove the solvents and other organics before RO treatment. It was noted the appropriate phase ratio (A/O) was 1:1 and the contact time was 3 h. Appropriate dosages of FeSO 4 and H 2O 2 were 10 g/l and 2 g/l, respectively. Under the appropriate conditions, TOC can be reduced by 50.6% and 37.8% in the EX and FO units, respectively. TOC can be further reduced 10.1% and 1.2% in the first and second RO processes, respectively. There was consequently an overall TOC removal efficiency of 99.7% after the whole process of EX–FO–RO–RO. Dissolved salts were also greatly removed during the two-stage RO process. The observations by using SEM showed that the RO membrane can be protected well by the pretreatments of EX and FO.

Ultrapure Water Production by Continuous Electrodeionization Method: Technology and Economy

Electrodeionization (EDI) and mixed bed deionization (MBDI) processes usually occupy the same position in the technological scheme of ultrapure water production, being used for the additional purification (conditioning) of reverse osmosis (RO) permeate (see, [1, 2]). A comparative analysis of the main differences of EDI and MBDI allows these methods to be more expediently used in water conditioning technology. The MBDI process has been used on a commercial scale for more than three decades, mostly in combination with a two-stage RO process. For this method, the content of active chlorine in the initial water flow is maintained within 0.05 – 0.08 mg liter in order to provide for a 3 – 4 month cellulose acetate regeneration cycle in the RO membranes. The ion exchanger is regenerated by treatment with strong acid and alkali solutions, which yields considerable volumes of highly mineralized waste waters implying the need for thoroughly monitoring the pH level. The EDI technology does not involve chemicals and does not require scrubbers for the flow neutralization after acid and alkali regeneration, reservoirs for the storage of concentrated acids and alkalis, the corresponding pumping equipment and valves, and double pipelines of corrosion-resistant steel, which significantly reduces both capital equipment and operating costs.

日本膜学会膜学研究奨励賞（２００２年）受賞論文 濃縮水昇圧２段法海水淡水化逆浸透プロセス・解析技術の確立

Many large seawater desalination plants, using reverse osmosis (RO) membranes, are now running in the world, and large efforts have been conducted in order to reduce the production cost.As one of the solutions for cost reduction, a new scheme called a brine conversion two-stage RO process was developed. In this process, pre-treated seawater is fed to the first stage and approximately 40 % of permeate is obtained from seawater, and concentrate of the first stage is boosted up its pressure again and more permeate is obtained at the second stage RO, in which 60 % can be totally recovered. In this two-stage process, high pressure is applied in order to obtain the effective pressure for the concentrated seawater at the second stage.This process became possible through the development of a new composite membrane and a new element, which can be operated at high pressure and high concentration.In this study, we developed a method to analyze and design the brine conversion two-stage RO process, and it was verified for each stage by comparing simulated result with the actual plant data. Then, we found the relationship between the permeability of TDS (total dissolved salt) and that of boron, and established the technique to estimate the concentration of boron in the permeate.

Design of a 1.4 mgd desalination plant based on MSF and RO processes for an arid area in India

Although India receives an average rainfall of 100 cm, some parts of the country get only scanty rains and are subject to chronic water shortages. Due to the occasional failure of the monsoons, a drinking water problem in these areas becomes quite acute. Water has to be brought in tankers and railway wagons to meet the drinking water needs of the people. With the advent of desalination technology based on both thermal and membrane processes, it is being considered as an alternate source to provide/augment the drinking water sources in the coastal water scarce areas. R & D work carried out at the Bhabha Atomic Research Centre (BARC) on desalination in the last 10–15 y has resulted in the development of technology for brackish water and seawater desalination in the country. Operation of pilot plants based on both membrane and thermal desalination has provided useful data for the design of large-scale desalination plants. This paper gives the details of a 1.4 mgd (6300 m 3/d) desalination plant to be installed in a coastal area for augmenting the drinking water supply. This consists of a 1.0 mgd multi-stage flash (MSF) and a 0.4 mgd two-stage reverse osmosis (RO) plant based on an indigenously available cellulosic membrane. A two-stage RO process is required in view of moderate salt rejection of cellulose acetate membranes for seawater feed. These membranes, however, possess better chlorine tolerance and fouling resistance compared to TFC membranes. As such this system is more appropriate under Indian conditions. The steam for the MSF plant will be available from a thermal power station where the plant is to be set up. Part of the product water obtained from the MSF plant will be used for entire boiler feed make up of the power station (in lieu of obtaining steam and power for the desalination plant). The remaining product water from the MSF plant (around 0.6 mgd) and RO plant (0.4 mgd) will be blended to get potable water of 250 ppm TDS. The MSF and RO plants are based on indigenous technology using most of the materials and equipment made locally. The detail design aspects and the cost estimates are presented in the paper.

Design of a 1.4 mgd desalination plant based on MSF and RO processes for an arid area in India

Although India receives an average rainfall of 100 cm, some parts of the country get only scanty rains and are subject to chronic water shortages. Due to the occasional failure of the monsoons, a drinking water problem in these areas becomes quite acute. Water has to be brought in tankers and railway wagons to meet the drinking water needs of the people. With the advent of desalination technology based on both thermal and membrane processes, it is being considered as an alternate source to provide/augment the drinking water sources in the coastal water scarce areas. R & D work carried out at the Bhabha Atomic Research Centre (BARC) on desalination in the last 10–15 y has resulted in the development of technology for brackish water and seawater desalination in the country. Operation of pilot plants based on both membrane and thermal desalination has provided useful data for the design of large-scale desalination plants. This paper gives the details of a 1.4 mgd (6300 m 3 /d) desalination plant to be installed in a coastal area for augmenting the drinking water supply. This consists of a 1.0 mgd multi-stage flash (MSF) and a 0.4 mgd two-stage reverse osmosis (RO) plant based on an indigenously available cellulosic membrane. A two-stage RO process is required in view of moderate salt rejection of cellulose acetate membranes for seawater feed. These membranes, however, possess better chlorine tolerance and fouling resistance compared to TFC membranes. As such this system is more appropriate under Indian conditions. The steam for the MSF plant will be available from a thermal power station where the plant is to be set up. Part of the product water obtained from the MSF plant will be used for entire boiler feed make up of the power station (in lieu of obtaining steam and power for the desalination plant). The remaining product water from the MSF plant (around 0.6 mgd) and RO plant (0.4 mgd) will be blended to get potable water of 250 ppm TDS. The MSF and RO plants are based on indigenous technology using most of the materials and equipment made locally. The detail design aspects and the cost estimates are presented in the paper.

Treatment of Tar Sand Wastewaters with Activated Carbon, Ozone and Reverse Osmosis

Tar sand deposits offer long-term potential for satisfying future energy needs. Two large field-scale extraction experiments near Vernal, Utah, employed either a sequenced reverse-forward combustion technique (TS-2C) or a steam flooding procedure (TS-1 S) to recover in-place bitumen. This report documents the evaluation of activated carbon, ozone and reverse osmosis in treating the two types of tar sand wastewaters. Substrates included untreated and pretreated (filtration, foam fractionation, chemical treatment with ferric chloride) TS-1S and TS-2C wastewaters. Activated carbon, at a dose of 1,000 mg/L, reduced the Total Organic Carbon (TOC) of TS-1 S wastewater after treatment by 350 mg/L of ferric chloride to 2.3 mg/L TOC, a reduction of 91.6%. At the same adsorbent dos, activated carbon reduced the TOC in TS-2C wastewater from 678 mg/L to 546 mg/L. A study of the six commercially available adsorbents yielded little difference in adsorption capability for either wastewater. Ozonation of TS-lS wastewater after pretreatment by either foam fractionation or chemical treatment with ferric chloride then activated carbon yielded residual TOC levels of less than 1.2 mg/L. Reverse osmosis (RO) studies included investigation of four membrane types (cellulose acetate, poly-ether amide, poly-ether urea and a noncellulosic on a poly-sulfone base), three applied pressure levels (250, 400 and 550 psig) and two solution pH levels (5.5 and 7.8) with 5.0 µ (micron) filtered TS-2C wastewater as the substrate. A two-stage RO process achieved maximum organic and inorganic rejections of 98% and 97%, respectively.

Desalination of sea water by an electrodialysis-reverse osmosis hybrid system

Regarding desalination, the two-stage reverse-osmosis process is compared with the process where reverse osmosis is a first stage and electrodialysis is a second stage. Theoretical consideration and experimental results from plants with a reverse-osmosis system and an electrodialysis unit are discussed, particularly with respect to energy consumption and cost factors. 3 refs.