Nanofiltration Pretreatment Research Articles

Dissolved air flotation of surface water for spiral-wound module nanofiltration pre-treatment

Bench-scale dissolved air flotation (DAF) of Tagus River surface water (Valadas, Portugal) was investigated as pre-treatment for spiral-wound module nanofiltration (SWNF). Before the DAF, a coagulation/flocculation of the surface water was performed using aluminium sulphate, ferric chloride and chitosan coagulants and several commercial coagulant aids. The coagulation/flocculation experiments were carried with 2 L of surface water in a jar-test equipment at room temperature. The DAF performance to remove colloidal matter and suspended solids was evaluated through the measurement of the silt density index (SDI) and the modified fouling index (MFI) of the treated water. The air saturation pressure was varied between 300 and 600 kPa and the recycle ratio between 5 and 50%. The DAF without coagulation/flocculation was not effective in the reduction of both the SDI and MFI, even at a high DAF recycle ratio of 50%. The use of coagulants improved the DAF efficacy in reducing the fouling indexes and, for the optimal operating conditions, the MFI of the treated water was reduced by more than one order of magnitude. However, it was not possible to obtain treated water with SDI and MFI values below the recommended ones for SWNF. Further filtration of the DAF treated water with a 5 micron capsule filter was necessary to reduce the SDI below 5%/min and the MFI below 10 s/l 2. The integration of DAF with another process that removes the residual suspended solids and colloidal matter is, therefore, necessary to provide a good pretreatment for the SWNF of Tagus River surface water.

Treatment of textile plant effluent by ultrafiltration and/or nanofiltration for water reuse

The textile wastewater treatment by membrane processes presents some limitations such as membrane fouling which causes a rapid flux decline. In fact, the membrane processes efficiency can be affected by membrane pore blocking or/and cake formation. In order to limit the effect of membrane fouling caused by plugging particles in textile effluent, a combination between two membrane processes was studied. The ultrafiltration (UF) was used as pretreatment for nanofiltration (NF) process. A comparison between direct NF and UF/ NF combination performances was carried out. Experimental results showed that the second treatment improved the textile permeate quality by increasing the retention values of the majority of analyzed parameters. In fact, color retention was about 95%, conductivity and total dissolved salts retentions were about 80% and the bivalent ions retention values exceeded 95%. In the case of direct NF, permeate flux remained constant until a volume reduction factor (VRF) reaching 1.35. After coupling UF with NF process, a stable permeate flux was observed until a VRF equal to 2.77. This result showed that using UF process as pretreatment for NF process improved the efficiency of textile effluent treatment by increasing the membrane run-time.

Salt production from coal-mine brine in NF — evaporation — crystallization system

At "Debiensko" Desalination Plant salt crystallizer is supplied with untreated brine concentrated by RCC evaporation method that results in high energy consumption and huge amount of post-crystallization lye. Nanofiltration pre-treatment of coal-mine brine was then considered to improve plant outcome. Desal-5L (Osmonics) nanofiltration membrane was found to be suitable for the above purpose. The rejection coefficients were found as follows (%): Ca 2+ − 68.8, Mg 2+ − 71.6, SO4 2− − 98.0, Cl − − 11.9, that lets us to estimate the composition of NF permeate at 85% recovery to be as follows (g/L): Ca 2+ − 0.242, Mg 2+ − 0.188, SO4 2− − 0.064, Cl − − 32.2. The performance of crystallization step was then estimated based on chemical composition of NF permeate and compared with operational data from "Debiensko" Desalination Plant. The energetic efficiency of the presently operated coal mine brine treatment system was compared with the one involving NF softening as the pretreatment for low energy VC thermal desalination in place of seeded VC (RCC presently applied). This comparison shows that unit energy consumption decreases from ca 970 kWh per 1 t of evaporated salt for untreated brine concentrated by RCC method to ca 450 kWh/t when low energy VC evaporation is applied. At the same time the amount of salt in post-crystallization lye decreases from approx. 110 kg per 1 t of evaporated salt produced to ca 50 kg.

Microfiltration and Ultrafiltration As a Pretreatment For Nanofiltration of Surface Water

The efficiency of pretreatment using microfiltration (MF) and ultrafiltration (UF) was investigated in surface water nanofiltration for drinking water production. A series of pretreatments prior to NF were performed to examine their efficiency to improve NF flux. Based on lab‐scale nanofiltration experiments, the major foulants appeared to be colloidal particles rather than dissolved organic matter for surface water containing hydrophilic organics. During the NF operations, changes in particle size by MF/UF pretreatment were found to be the reason for different pretreatment efficiencies with the pore size of prefilters. NF flux and pretreatment efficiency were quantitatively interpreted using a theoretical approach based on the particle back‐transport model. Considering the energy consumption to produce unit volume of NF permeate, optimum pretreatment conditions were suggested.

NF membrane characteristics and evaluation for sea water processing applications

Nanofiltration (NF) has not been well defined. Many authors define an NF membrane as any RO membrane that is not as highly salt rejecting as the state of the art. The authors of this report define an NF membrane as a membrane that has a sodium chloride permeability that is proportional to the sodium chloride concentration to a power greater than 0.4. The permeabilities of both sodium chloride and potassium bicarbonate are proportional to the solute concentration to the power of about 0.5 for a typical NF membrane. HL-NF membranes reject sulfate ions very well, typically greater than 99%, and divalent cations fairly well, typically 80–95%. Dr. Ata Hassan of SWCC reported the NF pretreatment results from the Umm Lujj SWRO NF pretreatment plant at the IDA conference in Kuwait in March 2002. The nanofiltration pretreatment step reduces total hardness from 7500 to 220 ppm, total dissolved solids from 45,460 to 28,260 ppm, and chloride from 21,587 to 16,438 ppm. With the ionic makeup at the Umm Lujj plant, sulfate was rejected at a rate better than 99%, magnesium at 98%, calcium at 92% and bicarbonate at 44%. The remained of these ions were less than 2 ppm for sulfate, 29 ppm for magnesium, 40 ppm for calcium and 17 ppm for bicarbonate. Without pushing the membrane and the equipment to their flux and pressure limits, the nanofiltration equipment operated at a 65% conversion rate and the reverse osmosis equipment at a 56% conversion rate, providing an overall seawater conversion rate of 36.4%. This compared with the conversion rate or a parallel plant of 28%, this represents a 30% increase in overall recovery.

Using atomic force microscopy towards improvement in nanofiltration membranes properties for desalination pre-treatment: a review

Seawater is characterised by having high degree of hardness, varying turbidity and bacterial contents and high TDS. These properties give rise to major problems such as scaling, fouling, high-energy requirements and the requirement of high quality construction materials. To solve seawater desalination problems and to minimise their effect on productivity and water cost of conventional plants, a new approach using nanofiltration as pre-treatment to both RO and thermal processes has been shown to enhance the production of desalted water and reduce the cost, yet it is environmentally friendly process. Nanofiltration pre-treatment of seawater feed to RO/thermal processes prevents scaling by removal of scale forming hardness ions, prevents membrane fouling in RO processes by removal of turbidity and bacteria and is expected to lower the required pressure to operate RO plant by reducing seawater feed TDS. This will depend on the type of NF membrane and operating conditions. In this paper, we will present a brief review on the potential use of state-of-the-art technique Atomic Force Microscopy (AFM), as a method for surface characterization, to understandthe membrane characteristics toward significant improvement in NF membrane properties. AFM as a tool for surface characterization can be used to basically (i) quantify NF membrane properties such as pore size distribution and surface morphology, (ii) obtaining surface electrical properties, (iii) surface adhesion-membrane fouling behaviour and (vi) correlating membrane characteristics with process behaviour. Information obtained from these studies can be used, in conjunction with mathematical models and permeation data, to develop a novel approach in the prediction of optimised membrane properties for desalination pre-treatment as well as towards improved model for process performance prediction.

Optimization of hybridized seawater desalination process

Saline Water Conversion Corporation (SWCC), Research and Development Center (RDC) came up in recent years with a breakthrough in feed treatment to all desalination processes. Selective ions, biomass and suspended solids rejection characteristic of nanofiltration (NF) membranes were utilized to retard the ingress of hardness forming ions and other foulants into desalination downstream processing steps, e.g. SWRO and MSF. NF pretreatment using brackish water hardness removal membranes were applied to seawater pilot, demonstration and eventually commercial desalination plants. A good number of such membranes were tested in order to select the best. The screening scheme revealed couple of interesting facts related to characteristics of various brands and grades of NF membranes. In an earlier stage of this multi phase project these membranes were categorized as groups A, B and C. These groups in the same order were found to give from high hardness and salinity rejection with low flow and product recovery down to low rejection with high flow and recovery. Based on the above findings, it was decided that the most suitable membranes would be those of group B which could give good flow and recovery with reasonable rejections. Membranes of group B were tested at various plant sizes, production schemes and configurations. This paper will concentrate on 4, 8 and 9 inch membrane elements used in NF or NF/SWRO combination to provide make-up to pilot plant MSF of 20 kl/d distiller. Such hybridized production configurations were optimized by testing overall performance. Few prime parameters were closely analyzed to come up with the best combination of NF membrane brand and their operating parameters along with MSF and/or SWRO—MSF hybrid conditions. In the latter case SWRO reject was used as make-up to the MSF pilot plant distiller. Moreover, the testing program locked into further hybridization, whereby MSF heated reject water was used as feed to NF pretreatment section. This was done to establish the effect of feed temperature rise benefits on NF and/or NF—SWRO energy input, yield(s) and product(s) flow and quality. The MSF operation was monitored and its operating results were analyzed. Main components of such analyses were gain output ratio (GOR), fouling factor (FF), top brine temperature (TBT), chemical consumption and product to make-up yield ratio and its quality.

Electroplating process: U.S. Patent 5,738,776. Apr. 14, 1998 S.W. Florio et al., assignors to Shipley Co. L.L.C., Marlborough, Mass.

This study shows solar photo-Fenton combined with nanofiltration (NF) to treat microcontaminants (MCs) in actual MWTP effluents. Photo-Fenton was operated at circumneutral pH using (S,S)-Ethylenediamine-N,N′-disuccinic acid trisodium salt (EDDS) as the iron complexing agent (Fe:EDDS in a molar ratio of 1:2) and compared with classical photo-Fenton at pH3. Starting H2O2 concentration was 50 mg/L and Fe was 0.1 mM or 0.2 mM. MC degradation was over 90% in all cases and 35 different MCs were monitored by Liquid Chromatography–Mass Spectrometry enabling the two processes to be compared under real conditions. NF pretreatment enabled photocatalysis to be run at lower flow rates and with higher starting concentrations reducing the surface area of solar collectors and reagents needed. Acute and chronic toxicity tests were also carried out before and at the end of each treatment evaluated and it seemed clear that MC degradation did not produce intermediates with any significant effect on toxicity. In addition, a detailed economic assessment was also performed. Operating costs were higher at circumneutral pH (0.76 €/m3) with EDDS than with classical photo-Fenton at pH3 (0.48 €/m3).

Comparison of membrane separation processes in the internal purification of paper mill water

In recent years the paper industry has been studying different technologies to reduce fresh water consumption in the mills. In this study micro-, ultra- and nanofiltration of some paper mill waters both on a laboratory and a pilot scale were compared. The performances of different membranes and modules are reported and some general conclusions are given. The performance of the membranes was evaluated by determining the reduction of certain measurable parameters in the feed solution such as chemical oxygen demand (COD), total organic carbon (TOC), total solids (TS), lignin, sugar, anionic trash (CD), ions, conductivity, turbidity, silt density index (SDI) and brightness decreasing substances. The tested tubular 0.4 μm microfiltration membrane was not suitable as a pretreatment step to nanofiltration. In ultrafiltration the tested cross rotational (CR) filter using flat sheet membranes gave significantly higher fluxes than the tested polymeric or ceramic tubular membranes. It was shown that the relatively low cut-off (30 kD) hydrophilic C30G membrane made from regenerated cellulose had higher fluxes both at neutral and acidic pH. Also the long time behaviour (duration four or six days) of the C30G was very good. It was also shown that the pH of the original feed, the experimental set-up and the membrane cut-off affected fouling the most in ultrafiltration. In nanofiltration the membrane structure and feed pH affected the results the most. Very good results were obtained by all the tested nanofiltration membranes. It was shown that fluxes similar to microfiltration are obtained by tubular ultrafiltration membranes and that the reductions were better. Moreover, the ultrafiltration process was more reliable than microfiltration as a pretreatment for nanofiltration because the obtained silt density index was always below the value 5 with ultrafiltration. However, it was also shown that in nanofiltration better reductions were obtained if no pretreatment was done, if it was not required by the module configuration.