An investigation of desalination by nanofiltration, reverse osmosis and integrated (hybrid NF/RO) membranes employed in brackish water treatment

Secondary effluent reclamation and reuse has been considered as an alternative for agricultural irrigation water. Whilst all constituents in the reclaimed wastewater could affect plant growth and soil characteristics, the most important parameters for agricultural irrigation are salinity and SAR (Sodium Adsorption Ratio). Salinity affects the availability of crop water and sodium causes clay soils to disperse. Membrane technologies, especially NF (Nano-Filtration) and RO (Reverse Osmosis), have played in a key role reclaiming the secondary effluent. RO can remove monovalent and divalent cations simultaneously. However NF processes reject preferably divalent cations and most monovalent ions are allowed to pass through the NF membranes. This could make them have different SAR values for both NF and RO processes. Therefore the primary objective of this study is to examine if the SAR values of the reclaimed water could be changed while they undergo NF and RO processes. The measured SAR values of the secondary effluent, NF permeate, and RO permeate were 1.78, 4.67, and 0.72 respectively. The SAR value after NF (4.67) increased to more than twice that of the feed solution, whereas the SAR of the RO permeate decreased to 0.72. In general, the higher SAR the water has, the greater risk the soils have. Although the SAR value after NF was within the safe range, this increased SAR value will affect permeability of soil, thus limiting the reclaimed wastewater use for as agricultural irrigation water. Consequently, when the NF system is used for the reclamation of the secondary effluent, SAR has to be examined first because potentially it tends to increase the SAR value.

The salinity of the ground water in Berjand, the capital of the province of South Khoorasan in Iran, has been a matter of concern since the ground water is the only source of drinking water in the city. Firstly, the authors studied the effects of both nano-filtration (NF) and reverse osmosis (RO) for the reduction of total dissolved solids (TDS) in the north and the south of the city. Secondly, we applied the Reverse Osmosis System Analysis simulator to the pilot study under the same circumstances as the pilot study to investigate the suitability of the software as a predictor of water quality. The results of the pilot study showed that the NF and RO processes have a high percentage of TDS removal at all pressures. The NF and RO membranes showed very high efficiency in the reduction of TDS. The RO system needs more energy than the NF system.

This study compares the performance of nanofiltration (NF) and reverse osmosis (RO) for the reclamation of ultrafiltered municipal wastewater for irrigation of food crops. RO and NF technologies were evaluated at different applied pressures; the performance of each technology was evaluated in terms of water flux, recovery rate, specific energy consumption and quality of permeate. It was found that the permeate from the reverse osmosis (RO) process complied with Food and Agriculture Organization (FAO) standards at pressures applied between 10 and 18 bar. At an applied pressure of 20 bar, the permeate quality did not comply with irrigation water standards in terms of chloride, sodium and calcium concentration. It was found that nanofiltration process was not suitable for the reclamation of wastewater as the concentration of chloride, sodium and calcium exceeded the allowable limits at all applied pressures. In the reverse osmosis process, the highest recovery rate was 36%, which was achieved at a pressure of 16 bar. The specific energy consumption at this applied pressure was 0.56 kWh/m3. The lowest specific energy of 0.46 kWh/m3 was achieved at an applied pressure of 12 bar with a water recovery rate of 32.7%.

Distillery wastewater treatment by the membrane-based nanofiltration and reverse osmosis processes

The aim of this study was to investigate the treatment and reuse of laundry wastewater with couple of nanofiltration (NF) and reverse osmosis (RO). In the NF process, optimal values of pH, temperature, transmembrane pressure (TMP) and cross-flow rate were determined using the Taguchi L16 (44) experimental design method. The smaller-the-better signal-to-noise (S/N) ratio was used to analyze the results of experiments. Flux decline caused by fouling was selected as response parameter. A pH of 8.5, temperature of 30°C, TMP of 12bar and cross-flow rate of 2L/min were determined as optimum operating conditions in the NF process. According to analysis of variance (ANOVA), pH was the most effective factor while TMP and cross-flow rate had low effects on the fouling. Membrane fouling was also evaluated with scanning electron microscope (SEM), energy dispersive X-ray spectroscopy (EDX), zeta potential and optical profilometer measurements. In the RO process, the quality of NF permeate obtained from optimum operating conditions was improved by an RO membrane. Although a NF membrane was not effective in reducing chemical oxygen demand (COD), Orto-P and NH4 +-N, these parameters were almost removed in the NF + RO system. These results showed that, the Taguchi method was successfully applied to determine the optimum operating conditions for the treatment of laundry wastewater with an NF process. Water treated with integrated membrane processes (NF + RO) is sufficient for use as laundry washing water.

Ultrafiltration as direct pre-treatment of seawater – a case study

This study investigated the removal characteristics of N-Nitrosamines and their precursors at three pilot-scale water reclamation plants. These plants applies different integrated membrane systems: (1) microfiltration (MF)/nanofiltration (NF)/reverse osmosis (RO) membrane; (2) sand filtration/three-stage RO; and (3) ultrafiltration (UF)/NF and UF/RO. Variable removal of N-Nitrosodimethylamine (NDMA) by the RO processes could be attributed to membrane fouling and the feed water temperature. The effect of membrane fouling on N-Nitrosamine removal was extensively evaluated at one of the plants by conducting one month of operation and chemical cleaning of the RO element. Membrane fouling enhanced N-Nitrosamine removal by the pilot-scale RO process. This finding contributes to better understanding of the variable removal of NDMA by RO processes. This study also investigated the removal characteristics of N-Nitrosamine precursors. The NF and RO processes greatly reduced NDMA formation potential (FP), but the UF process had little effect. The contributions of MF, NF, and RO processes for reducing FPs of NDMA, N-Nitrosopyrrolidine and N-Nitrosodiethylamine were different, suggesting different size distributions of their precursors.

Application of low-pressure nanofiltration coupled with a bicycle pump for the treatment of arsenic-contaminated groundwater

Chapter 21 - Nanofiltration and reverse osmosis processes for the removal of micro-pollutants

The existing body of experimental data in the open literature clearly indicate that reverse osmosis (RO) processes reject ions of identical valence (i.e., homovalent ions) to different degrees. For example, rejections (or relative permeations) of monovalent anions (such as Cl-, NO3-, Br-, CH2ClCOO-, ClO4-, etc.) during RO processes are different under otherwise identical conditions. The same is true for divalent anions (namely, SO42-, HPO42-, SeO42-) or monovalent cations (namely, Li+, Na+, K+, and NH4+). The solution diffusion model with the Nernst−Planck equation is unable to predict the differential permeation behaviors of homovalent ions. It is recognized that hydrated ionic radii data, if available, could be used to compute interdiffusion coefficients (or salt permeability coefficients) of permeating electrolytes. However, a careful scrutiny of the existing body of hydrated ionic radii data in the open literature provide clear evidence that they are unreliable for polyatomic ions such as nitrate, nitrite, selenate, phosphate, chloroacetate, sulfate, etc. Also, ionic diffusivities computed from equivalent conductance data fail to predict the hierarchy of relative permeations of homovalent ions in RO processes. Central to this study is the underlying scientific premise that ion-exchange selectivity can be used as an effective parameter to predict the relative permeability of homovalent ions in a multicomponent system. For two ions of identical valence, ion-exchange selectivity based on Coulombic interaction is dependent only on their relative hydrated ionic radii, which in turn govern the interdiffusion coefficients of permeating electrolytes. The higher the ion-exchange selectivity of a specific ion, the lower is its hydrated ionic radius and, hence, more permeable is the ion. The theoretical relationship between ion-exchange selectivity and permeability can be well explained with the aid of the Stokes−Einstein equation. Experimental results presented in this study with both monovalent and divalent ions show a strong characteristic correlation between ion-exchange selectivity and relative permeation, i.e., a higher ion-exchange selectivity always leads to a greater permeability. One major advantage of this approach is the ease with which ion-exchange selectivity can be determined by ion chromatography and/or batch isotherm technique. A large body of existing experimental data for RO and nanofiltration processes in the open literature, when carefully reviewed, also validate this scientific premise. Type of membrane, solvent dielectric constant, and pH influence the overall solvent and salt permeation fluxes, but the relative permeation of homovalent ions still follows the ion-exchange selectivity sequence.

Modeling the impacts of feed spacer geometry on reverse osmosis and nanofiltration processes

The main objective of this paper is to develop computer simulation program for performance evaluation and cost estimation of a reverse osmosis (RO) and pressure-retarded osmosis (PRO) hybrid process to propose guidelines for its economic competitiveness use in the field. A solution-diffusion model modified with film theory and a simple cost model was applied to the simulation program. Using the simulation program, the effects of various factors, including the Operating conditions, membrane properties, and cost parameters on the RO and RO-PRO hybrid process performance and cost were examined. The simulation results showed that the RO-PRO hybrid process can be economically competitive with the RO process when electricity cost is more than 0.2 $/kWh, the PRO membrane cost is same as RO membrane cost, the power density is more than and PRO recovery is same as 1/(1-RO recovery).

Treatment and reuse of textile effluents based on new ultrafiltration and other membrane technologies

Effects of surfactants, ion valency and solution temperature on PFAS rejection in commercial reverse osmosis (RO) and nanofiltration (NF) processes

Chapter 10 - Removal of Pharmaceuticals by Ultrafiltration (UF), Nanofiltration (NF), and Reverse Osmosis (RO)