Application of reverse osmosis to reduce pollutants present in industrial wastewater

Reverse osmosis (RO) is increasingly used as a separation technique in chemical and environmental engineering; desalination, selective separation and wastewater treatment are well established examples. Treatment by RO reduces high levels of dissolved salts but has certain limitations in the removal of organics from chemical industry effluents. We describe a comparative study of four different polyamide membranes that were tested for their ability to reduce the concentrations of pollutants in a synthetic effluent stream containing acrylnitrile and three inorganic species (sulphate, ammonium and cyanide). The pH value of the solution plays an important role in the ionization of the different species and, subsequently, in their rejection. The membranes were checked using a 3×10⁻³ m² flat cell where the permeated and concentrated currents were recirculated to the feed reservoir. In a preliminary set of experiments the pH of the feeding solution was 9.0. The rejection percentage of sulphate ion was high in all the membranes tested (96% to 99.4%) regardless of the working pressure. Ammonium rejection values were between 72.3% and 83.9%, while acrylnitrile rejection was low (10.5% to 28.8%) compared with the results obtained for the other pollutants. Cyanide rejection was negative for all membranes tested except for HR95PP, which produced a rejection percentage of 16.5%. The same membrane also produced higher rejection percentages for cyanide and acrylnitrile than the other membranes. Finally, this membrane was selected to study the influence of the feed stream pH on the rejection of ammonium and cyanide ions. The study concluded that cyanide and ammonium could not be acceptably eliminated in a single step operation when they are simultaneously present in industrial wastewater. The results pointed to the need to carry out several steps at different pH values to reduce the level of both pollutants in the studied wastewater.     

Treatments of stainless steel wastewater containing a high concentration of nitrate using reverse osmosis and nanomembranes

A nanomembrane (NTR 729HF) and three different types of reverse osmosis (RO) membranes (CPA2, LFC1, and ESPA1) were tested for the treatment of high concentration of nitrate wastewater from stainless steel industry. All the tested RO membranes showed higher rejection rates (90–99% at 1000–60 mg/L of NO₃⁻ -N) than the NF membrane. The rejection rate and flux of RO membranes were not highly affected by pH variation and Ca²⁺ as co-existing ion. However, the rejection rate of NF, which was 67% at 60 mg/L of feed concentration, was decreased as pH decreased and Ca²⁺ concentration increased indicating that charge repulsion is one of the major rejection mechanisms. As nitrate concentration increased from 20 to 1000 mg/L in feed water, the removal rate decreased from 67 to 20% in NF membrane. The flux of RO was relatively high and ESPA1 (a low-pressure RO type) showed more than two times higher flux compared to the NF. ESPA1 was successfully tested for a longterm experiment with real stainless steel wastewater for 30 days of experimental period. Current study implicates that RO membranes could be an alternative for the treatment of stainless steel wastewater.     

Pilot study for reclamation of secondary treated sewage effluent

A pilot study for reclamation of secondary treated sewage effluent in Singapore was conducted using a MF/RO system with the capacity of 20 m³/d. A 0.1 μm MF membrane from Asahi and RE-4040-FL RO membrane from Saehan were used in this study. The pilot plant consists of six spiral-wound RO elements. The RO train was configured in single stage. The pilot plant was designed with automatic control system and it was operated continuously (24 h) during the study. Trial runs on various flux rates of the RO membrane at different operating pressures were conducted over 3 months. The pilot results showed that the optimal operation flux rate of the RO membrane ranged from 10 to 15 gal/f²/d (GFD) for this application. The normalized flux after CIP was 97% of the initial one. At a flux rate of 10 GFD and water recovery of 50%, the average operating pressure of 57 psi was noted corresponding to a high normalized flux of 38 L m⁻² h⁻¹ MPa⁻¹ at 25°C. Rejections of the RO membrane in terms of conductivity, TOC, ammonium and nitrate were higher than 96%,97%,90% and 85%, respectively. It was concluded that the RO permeate quality in terms of conductivity, turbidity, TOC, ammonium, nitrate, hardness, total bacteria and total coliform matched the quality of high-grade water (NEWater) for use in the electronics industry.     

Steady state and dynamic modeling of spiral wound wastewater reverse osmosis process

Reverse osmosis (RO) is one of the most important technologies used in wastewater treatment plants due to high contaminant rejection and low utilization of energy in comparison to other treatment procedures. For single-component spiral-wound reverse osmosis membrane process, one dimensional steady state and dynamic mathematical models have been developed based on the solution-diffusion model coupled with the concentration polarization mechanism. The model has been validated against reported data for wastewater treatment from literature at steady state conditions. Detailed simulation using the dynamic model has been carried out in order to gain deeper insight of the process. The effect of feed flow rate, pressure, temperature and concentration of pollutants on the performance of the process measured in terms of salt rejection, recovery ratio and permeate flux has been investigated.     

Possibilities for the use of membrane processes for the pre-treatment of wastewater from the production of dried potato purée

Our research focused on the membrane separation of wastewater resulting from the production of dried potato purée. Our aim was to investigate possibilities for recycling obtained retentate back to the actual production process, and, consequently, for reducing wastewater pollution. This paper describes trials of MF and RO membrane filtration of starch wastewater. The treated water contained starch, in either granulated or gelatinized form, and solids (fine pieces of potato skins). The trials were conducted in either one or two stages. We used a pilot plant equipped with a ceramic membrane with a filtration area of 0.35 m² and pore sizes of 500 and 100 nm. We also tested an organic RO membrane (7410) in the laboratory.High permeate flux above 100 l/(m² h) was measured for the 100 nm membrane, but with considerable fouling. Filtration through this membrane resulted in high COD and BOD₅ rejection (approximately 60%), an effect which was increased by the subsequent RO filtration. The content of soluble carbohydrate, 0.011% in permeate (with 0.44% dry substance), was analyzed using high-performance anion-exchange chromatography coupled with pulsed amperometric detection.     

Optimization of lead removal from aqueous solution by micellar-enhanced ultrafiltration process using Box-Behnken design

The main objective of this research was to use Box-Behnken experimental design (BBD) and response surface methodology (RSM) for optimization of micellar-enhanced ultrafiltration (MEUF) to remove lead ions from synthetic wastewater using spiral-wound ultrafiltration membrane. The critical factors selected for the examination were surfactant concentration, molar ratio of surfactant to metal (S/M) and solution pH. A total of 17 experiments were accomplished towards the construction of a quadratic model for both target variables. The experimental results were fitted with a second-order polynomial equation by a multiple regression analysis, and more than 95%, 93% of the variation could be predicted by the models for lead rejection and permeation flux, respectively. The optimum condition was found by using the obtained mathematical models. Optimization indicated that in C _SDS =2mM, pH=6.57 and S/M= 9.82 maximum flux and rejection efficiency can be achieved, simultaneously.     

Desalination of agricultural drainage return water. Part II: Analysis of the performance of a 13,000 GDP RO unit

The daily performance of a 49 ms³/d RO unit is reported in terms of the water and TDS transport parameters A and B, respectively. The rejections of the ions Na⁺, SO^2-₄, Cl⁻ and NO⁻₃, and alkalinity, boron and selenium are also reported for the begining and end of the testing period. The single stage RO unit consisted of six spiral-wound membrane elements (Model 4040-MSY- CAB3 manupactured by Hydranautics) placed in series.     

COD Removal from Concentrated Wastewater Using Membranes

Membranes can be used for wastewater treatment. The selection of the appropriate membrane depends on a number of factors, such as waste characteristics, nature of materials present in the wastewater, concentration, temperature, pH, etc. If the wastewater contains low molecular weight organics, reverse osmosis (RO) is the best treatment process. RO membranes allow solvent (water) to pass and prevent the transport of organics, either completely or partially. In this study, raw wastewater from an alcohol manufacturing plant was treated using a RO process. The chemical oxygen demand (COD) of the wastewater was between 35000-40000 mg/l due to the presence of organic components. Eight polymeric membranes (e.g. FT30, PVD, DSII, DS, BW30, 37100, 3750 and NF45) were used in total. None of the membranes were able to reduce COD to a desirable level (i.e. less than 200 mg/l) in one step. However a two-step process could be designed for wastewater treatment. Based on data obtained for flux and rejection, the NF45 nanofiltration membrane exhibited the best performance. A high volume of fluid can pass through the NF45 membrane because of its high porosity. The flux of this membrane (i.e. 15 kg/m².h) was higher than the reverse osmosis membranes tested. The NF45 membrane decreased COD to a greater extent than the other membranes tested (52%). While the PVD membrane showed better efficiency compared to the other reverse osmosis membranes, probably because of its material of construction and configuration.     

A pilot scale study of reverse osmosis for the purification of condensate arising from distillery stillage concentration plant

Reverse osmosis (RO) is an interesting process to eliminate small organic solutes (carboxylic acids and alcohols) from distillery condensates before recycling them into the fermentation step. This work investigates the influence of transmembrane pressure, pH and volume reduction factor (VRF) on the efficiency of reverse osmosis treatment of condensate from distillery stillage concentration at pilot scale using three pre-selected membranes (CPA2 and ESPA2 from Hydranautics, BW30 from DOW). Performances were assessed according to permeate flux, solutes rejection and abatement of fermentation inhibition. Transmembrane pressure increase leads to an increase of these three parameters with a plateau for rejections and abatement at 20 bar; however, in order to comply with membranes manufacturer's recommendations and to limit or delay polarization and fouling, it was decided to keep the permeate flux below a value of 30 L h⁻¹ m⁻². This corresponded to a maximum pressure of 10 bar for CPA2 and ESPA2 membranes and 25 bar for BW30 membrane. pH increase leads to a diminution of permeate flux and an increase of carboxylic acids rejection whatever the membrane; nevertheless, no abatement of fermentation inhibition is observed. Increasing VRF provokes a decrease of the permeate flux. Although local rejections are stable, the mean rejection assessed with the raw condensate (feed) and the mean permeate decreases. However, the fermentation inhibition remains under 10% up to a VRF of 8. BW30 membrane exhibits the highest rejections and inhibition abatement. On the basis of the pilot scale results with the BW30 membrane, a preliminary estimation of the membrane area is proposed for an industrial plant with 100 m³ h⁻¹ of condensate flow rate and the optimized parameters (pressure 25 bar, no pH modification, VRF 4 and 8).     

Influence of di-hydrogen phosphate ion on performance of polyamide reverse osmosis membrane for nitrate and nitrite removal

Reverse osmosis process has great potential in treatment of water and wastewater containing undesirable dissolved species. In the real world, the wastewater contains mixtures of ions. Generally, the presence of particular substances may affect the removal of specific ions and harmful substances in wastewater treatment. In this work, a Filmtec TW30HP-4641 RO element (polyamide, thin-film composite) with the capacity of 14.38 m³/d was employed for wastewater treatment in Exir Pharmaceutical Co. (Borojerd, Iran). The rejections of individual nitrite, nitrate, and sulfite ions were around 91, 93, and 95%, respectively. However, the addition of KH₂PO₄ to the solution containing nitrite and nitrate ions improved the rejection up to 99%. Polyamide has electronic lone-pair in amino group that can make a resonance structure with carbonyl segment. The feed solution containing potassium and di-hydrogen phosphate ions may establish binding with membrane. The binding of potassium ions to the electronic lone-pair of membrane hold H₂PO₄ ⁻ ions and provide a negative layer on the surface of membrane. Diffusion of anions through the membrane is minimized by establishment of the proposed negative layer. This improves the ion rejection capability of the membrane.     

Characteristics of thin-film nanofiltration membranes at various pH-values

Salt rejection and ion selectivity of NF-255 and NF-45 nanofiltration (NF) membranes were investigated. The rejection of two cations (Na⁺, Ca²⁺) and two anions (Cl⁻, SO_4²⁻) which are common in natural and in industrial wastewater, were studied as a function of pH at permanent pressure and temperature. The ion rejection of NF membranes were investigated in single salt solutions like NaCl, CaCl₂, Na₂SO₄, CaSO₄, and in multicomponent systems that contained all the previous ions. We found that, there is a minimum rejection of the Na⁺ and Cl⁻ ions between pH 4-5 in NF-255 and between pH 7-8 in NF-45. The rejection of calcium ions were increased in each case at lower pH in both membranes. However the pH value where the ion rejection behaviour of membranes changed, were different: pH 4 in NF-255 and pH 8 in NF-45. In NF-45 the chloride ion has negative rejection which depends on the quality of ions and the pH. We found that below pH values of 4 the selectivity of mono- and multivalent cations considerable increased in NF-255. This phenomena may be used for separation of calcium ions from sodium ions from weakly acidic (hydrochloric and sulfuric acid) solution, e.g. regeneration solution of sodium form softening ion exchangers.     

Dye removal from colored textile wastewater using acrylic grafted nanomembrane

In this paper, the dye removal ability of the acrylic grafted polysulfone nanomembrane using ultraviolet radiation was studied to remove dyes from colored textile wastewater. Acrylic acid was used to modify polysulfone ultrafiltration membrane. The effect of different operating parameters such as pressure, salt concentration and chemical structure of dyes was evaluated. Data indicated that the photografted membrane has acceptable performance both in terms of flux and rejection. The dye rejection and hydraulic permeability were 86–99.9% and 7.6 L m^{− 2} h^{− 1} bar^{− 1}, respectively. It was found that the rejection of dyes decreased with salt concentration due to a decrease of the Donnan effect. Also, the low molecular weight dyes and highly charged dyes were more sensitive in the presence of salts. Addition of 80 mM Na₂SO₄ in dye solution decreased the dye rejection more than 15%. The rejection enhancement for all cases was negligible by increasing driving pressure from 1 to 4 bar. Dyes with low charger were more sensitive to operating pressure than that of dyes with higher charges. All findings supported that acrylic grafted nanomembrane is potentially capable to separate dyes from colored textile effluent.     

Rejection of As(III) and As(V) from arsenic contaminated water via electro-cross-flow negatively charged nanofiltration membrane system

A specially designed electro-cross-flow nanofiltration (NF) membrane system was used for this investigation. To enhance the rejection of arsenic ionic species like H₂AsO₄⁻, a NF membrane having a negative surface charge was fabricated via the interfacial polymerization process. The membrane was characterized by SEM, AFM, surface charge density, molecular weight cut-off (MWCO), total and skin thickness and pure water flux. The parameters that affected the rejections of As(III) and As(V) were studied; they included the initial arsenic concentration, the applied potential, pH of the feed, the cross-flow filtration pressure and the presence of different salts in the feed. Among those parameters, the pH of the feed greatly affected As(V) rejection; As(V) ([As(V)]_o = 1000 ppb) rejection was increased from 72.3 to 98.5% when pH of the feed was changed from 3.0 to 10.0. This might be due to the fact that higher pH enhanced the formation of negative divalent anion like HAsO₄²⁻ which should be rejected more effectively by the negative surface charge of the NF membrane. Beside the effect of the negative surface charge of the membrane, applied potential increased the As(V) rejection by 48.2% when the applied potential was increased from 0 to 2.0 V for a feed containing 1000 ppb initially. For the same change of applied potential rejection of As(III) was increased from 52.3 to 70.4%; this might be the result of the formation of anionic species like H₂AsO₃⁻ from the neutral molecule of H₃AsO₃ by the applied potential.     

Nanofiltration study of the interaction between bicarbonate and nitrate ions

In contrast to other anions, there are not many studies focusing on the effect of bicarbonate ion on the rejection of other anions in nanofiltration. However, bicarbonate is one of the major anions in many inland waters and its concentration remains significant even after acidification. In order to obtain specific information about the interaction of bicarbonate and nitrate ions, nanofiltration experiments with the membrane SU-610 (Toray) were performed using solutions containing sodium, nitrate and bicarbonate ions. The influence of the bicarbonate ion concentration was studied in the range from 4 to 32 mol·m^− 3 of bicarbonate ion and from 1 to 4 mol·m^− 3 of nitrate ion. The experimental data were analysed by means of multiple linear regression. The results showed that nitrate rejection was decreased by bicarbonate ion concentration. This effect was compared with that obtained using sulphate or chloride as co-ions. The regression models obtained for nitrate and bicarbonate ion rejection showed good fitting to the experimental results (r-squared equal to 99.3% and 97.6% respectively). The models were able to predict the evolution of nitrate in a concentration process at constant pH.     

Nanofiltration and reverse osmosis of model process waters fromthe dairy industry to produce water for reuse

In dairy plants the process waters generated during the starting, equilibrating, interrupting and rinsing steps contribute to the production of effluents. They correspond to milk products (milk, whey, cream) diluted with water without chemicals. The treatment of these dairy process waters by nanofiltration (NF) or reverse osmosis (RO) operations was proposed to concentrate dairy matter and to produce purified water for reuse in the dairy plant. The study reports one-stage and two-stage (NF + RO and RO + RO) spiral-wound membrane treatments with five model process waters representative of the main composition variations observed in dairies. Performances (permeate flux, milk components rejection, purified water characteristics) of the different operations were compared. Discussion was focused on the comparison between quality of produced waters and vapour condensates (from product drying and evaporation processes) reused in dairy plants. Accordingly, both total organic carbon (TOC) and conductivity of water treated by a single RO or NF + RO operations were convenient for reuse as heating, cooling, cleaning and boiler feed water. With the two-stage RO + RO process, a more purified water complying with the TOC drinking water limit was achieved.     

Characterization and control of foulants occurring from RO disc-tube-type, membrane treating, fluorine manufacturing, process wastewater

This study was carried out to find a way to limit fouling of disc-tube-type reverse osmosis membranes in the treatment of acid rinse wastewater from the fluorine manufacturing process as well as to pretreat the wastewater before it entered the membrane process. Experiments showed that the scale consisted of Ca²⁺, SO₄⁻² and F⁻. Complex scales were removed in a subsequent procedure where the membrane was rinsed with NaOH followed by citric acid which could consequently recover its flux up to 86%. Cleaning chemicals had to be used regularly for efficient recovery of permeate flux. Ultrasonic cleaning could also improve the recovery of permeate flux up to 83%. Calcium salts were used to remove fluoride ions. CaCl₂ removed fluoride ions up to 11% more than Ca(OH)₂ at 0.5 [Ca²⁺]/[F⁻]. At acidic pH 4-7 or alkaline pH 7 and above, residual fluoride ions increased as Ca²⁺ reacted more efficiently with Cl⁻, OH⁻ and SO₄⁻² rather than F⁻. On the other hand, fluoride ions were best removed at pH 7. Adding Ca²⁺ salt above pH 7 caused an increase of residual Ca²⁺ salt in the effluent, even if fluoride ions can be ideally removed in the form of CaF₂ at a pH over 11.     

Simultaneous removal of organic and inorganic pollutants in tannery wastewater using electrocoagulation technique

Tannery wastewater can cause severe environmental problems related to its high chemical oxygen demand, high biochemical oxygen demand, high total suspended solids, high oil and grease contents together with the elevated chromium concentration and objectionable color. The one-step electrocoagulation process was carried out to simultaneously remove chromium and various pollutants from tannery wastewater at ambient temperature in the laboratory scale. Low-cost commercial iron plates were employed in this study as anodes and cathode materials. Effects of various parameters were investigated including types of electrode configuration, initial pH of wastewater (7–9), current density (15.7–24.6 Am⁻²) and circulating flow rate of wastewater (0–3.67 lmin⁻¹). The optimum condition was found by applying the mono-polar electrode in a parallel connection at the current density of 22.4 Am⁻², flow rate of wastewater of 3.67 lmin⁻¹ and 20 min electrolysis time. The initial pH of wastewater ranging from 7–9 provided the similar removal efficiency. At optimum condition, more than 95% of chromium and pollutants except TKN and TDS were eliminated from the wastewater and the properties of the treated wastewater met the standard and permitted to discharge into the environment. The required energy consumption at optimum condition was less than 0.13 kWhm⁻³ wastewater. In addition, the COD reduction was fit very well with the first-order kinetics model.     

Simultaneous removal of chromium and organic pollutants in tannery wastewater by electroprecipitation technique

The simultaneous removal of chromium and other organic pollutants from tannery wastewater was investigated in a batch electrochemical membrane reactor. This reactor, having a total capacity of 1 liter, was separated into two compartments (anodic and cathodic compartments) by using an anionic membrane. A stainless steel sheet with the square holes having total surface area of 0.0215 m² and a Ti/RuO₂ grid was used as the cathode and anode, respectively. The results indicated that the optimum condition for removal of chromium from tannery wastewater was found at the current density of 60.5 A/m² at initial pH of 4.5. At this condition, more than 98% of chromium was removed within 60 min. Some organic pollutants contained in wastewater such as oil and grease, color and the level of biochemical oxygen demand (BOD), chemical oxygen demand (COD) and total kjeldahl nitrogen (TKN) were also markedly reduced.     

A study on acid reclamation and copper recovery using low pressure nanofiltration membrane

In this study, nanofiltration membrane is used to separate proton (H⁺) and copper ions from a ternary ions mixture (H⁺, Cu²⁺, SO₄^2?). The performance of membrane in separating Cu²⁺ and H⁺ was tested under the effect of pressure, concentration and different acid strength (pH). It was found that the H⁺ rejection is independent of the applied pressure. Permeability of solution decreased linearly with the increase of CuSO₄ concentration. In terms of H⁺ rejection, there is a continuous drop in rejection from 0.1 mM CuSO₄ to 10 mM CuSO₄ solution. H⁺ was poorly retained and concentrated in the permeate stream in corresponding to the electro-neutrality requirements, on the other hand, the rejection of copper ion was almost constant with pH. In overall, optimum acid reclamation and copper recovery can be achieved at higher volume flux. A Three Parameters-Combined Film-Extended Nernst-Planck Equation (CF-ENP) model is successfully applied to predict the performance of nanofiltration membrane in separating the ternary ions.     

Removal of Ni(II) and Zn(II) from an aqueous solutionby reverse osmosis

The removal of Ni⁺² and Zn⁺² from an aqueous solution by reverse osmosis (RO) at different pH, conductivity and EDTA concentrations was investigated. In addition, the removal in the pretreatment units (PU) with filtration (F) and granular activated carbon (GAC) at the same conditions was also determined. It was observed that Zn⁺² rejections did not change much with pH, usually less than 0.88 mg/l, and Ni⁺² rejection was below the detection limits of AAS in the range of pH 4-8. While Ni⁺² and Zn⁺² were removed 23-25% and 25-45%, respectively, by PU, the rejection of Ni⁺² and Zn⁺² was, respectively, determined to be higher than 99.2% and 98.8% by PU+RO at experiments related to the determination of the effect of initial metal concentration on rejection. It was found that the influent conductivity affected metal rejection at an unimportant level, but the increase of conductivity affected the effluent conductivity. The addition of EDTA into the aqueous solution increased Ni⁺² and Zn⁺² rejection from 99.3% to 99.7% and from 98.9% to 99.6% at an EDTA concentration of 240 ppm, respectively.