Impact of energy fluctuation on permeate quality in autonomous and directly coupled renewable energy powered nanofiltration and reverse osmosis systems

Autonomous membrane systems provide a unique opportunity to overcome challenges of lacking or dysfunctional water supply, sewage and electricity infrastructure which is the case in many rural areas worldwide1. Membrane technology provides a unique advantage where water is available yet through predominantly dissolved contaminants such as TDS, fluoride, arsenic, uranium, nitrate and many other inorganic as well as organic contaminants not usable. Coupling membrane processes directly to renewable energies such as wind or photovoltaics is important to realise robust and decentralised systems for remote areas. However this poses particular challenges in terms of system operation, maintenance, as well as water quality2. Following several years of laboratory studies as well as field work with real waters the impact of such fluctuation has been studied for short term operation with a unique system3,4. To do so, the nature of fluctuations for both wind and solar resources was investigated to understand the impact on the membrane system5,6. This information was then transferred into suitable experimental protocols to study the amplitude, frequency and intermittency of fluctuations in a systematic manner7. In the process the resulting operation – and the safe operating window – was determined as a function of minimum power requirements2. Short term energy buffering was investigated via super-capacitor banks8. Please click Additional Files below to see the full abstract

Renewable energy powered membrane technology:Experimental investigation of system performance with variable module size and fluctuating energy

Integration of renewable energy and membrane filtration technologies such as nanofiltration (NF) and reverse osmosis (RO) can provide drinking water in places where freshwater is scarce and grid electrical connections are unavailable. This study investigated a directly-connected photovoltaic-powered membrane system under fluctuating solar conditions. Specifically, two configurations of NF/RO membranes with the same membrane area were investigated: a) 1 × 4″ module, which contained one 4″ NF/RO element; and b) 3 × 2.5″ module, which contained three 2.5″ NF/RO elements in series. A high fluoride brackish water ([F − ] = 56.2 mg/L, total dissolved solids [TDS] = 4076 mg/L) collected from northern Tanzania was treated by different membranes in the two configurations. Performance indicators such as flux, specific energy consumption, and permeate F − concentration were monitored over a 60-min period of energy fluctuation that are part of a typical solar day. The results showed that the overall performance of the 1 × 4″ module was superior to that of the 3 × 2.5″ module. This is because the performance of a 3 × 2.5″ module degraded significantly from the first element to the third element due to the increased feed concentration and the decreased net driving pressure. Three 1 × 4″ modules (BW30, BW30LE and NF90) and one 3 × 2.5″ module (BW30) were able to meet the drinking water guideline for fluoride. During cloud periods, the transient permeate F − concentration exceeded the guideline value due to insufficient power, however the cumulative permeate F − concentration was always well below the guideline. The photovoltaic-powered membrane system equipped with the above modules provides a promising solution for addressing drinking water problems in remote and rural areas. </p

Synthesis and characterization of carbon nanotube membranes for water treatment

This work presents the synthesis and characterization of carbon nanotube (CNT) incorporated polyethersulfone (PES) membranes. Firstly, CNTs were prepared via a nebulized spray pyrolysis of toluene (carbon source) and ferrocene (catalyst) mixture at a temperature of 850 0C. The CNTs produced were then purified and functionalized by acid treatment to aid their interaction with the solvent and polymer during membrane preparation. Characterization techniques used for CNTs include scanning electron microscopy (SEM) analysis, Raman spectroscopy analysis, thermogravimetric analysis and X-ray photoelectron spectroscopy analysis. The outer diameters of CNTs measured from SEM micrographs using Image J software were in the range of 10 – 14 nm. TGA analysis revealed that the CNTs undergo complete thermal degradation after acid treatment; i.e. no catalyst particle residues were detected after 600 0C. Please click Additional Files below to see the full abstract

Renewable Energy Powered Membrane Technology: Electrical Energy Storage Options for a Photovoltaic-Powered Brackish Water Desalination System

The potential for lithium-ion (Li-ion) batteries and supercapacitors (SCs) to overcome long-term (one day) and short-term (a few minutes) solar irradiance fluctuations with high-temporal-resolution (one s) on a photovoltaic-powered reverse osmosis membrane (PV-membrane) system was investigated. Experiments were conducted using synthetic brackish water (5-g/L sodium chloride) with varied battery capacities (100, 70, 50, 40, 30 and 20 Ah) to evaluate the effect of decreasing the energy storage capacities. A comparison was made between SCs and batteries to determine system performance on a “partly cloudyday”. With fully charged batteries, clean drinking water was produced at an average specific energy consumption (SEC) of 4 kWh/m3. The daily water production improved from 663 L to 767 L (16% increase) and average electrical conductivity decreased from 310 µS/cm to 274 μS/cm (12% improvement), compared to the battery-less system. Enhanced water production occurred when the initial battery capacity was >50 Ah. On a “sunny” and “very cloudy” day with fully charged batteries, water production increased by 15% and 80%, while water quality improved by 18% and 21%, respectively. The SCs enabled a 9% increase in water production and 13% improvement in the average SEC on the “partly cloudy day” when compared to the reference system performance (without SCs)

Regeneration of β-Cyclodextrin Based Membrane by Photodynamic Disulfide Exchange — Steroid Hormone Removal from Water

The occurrence of steroid hormones in water and their serious impact on human and ecosystem demand high performance materials for efficient removal of such micropollutants. Here, an affinity membrane is developed for hormone removal with regenerable binding sites. By using photodynamic disulfides as a linker, UV induced detachment of β‐CD ligands from the membrane surface is demonstrated. The macroporous base membrane is first fabricated via a polymerization induced phase separation method using 2‐hydroxyethyl methacrylate (HEMA) and ethylene dimethylacrylate (EDMA) monomers. Then the affinity membranes are prepared by immobilizing β‐CD ligands to the poly(HEMA‐co‐EDMA) base membrane through the 2‐carboxyethyl disulfide linker. The β‐CD functionalized affinity membrane shows a 30% increase of E2 hormone uptake compared with the base membrane, attributed to the formation of CD‐hormone host–guest inclusion complexes. The photodynamic disulfide linkers enable UV‐induced detachment of blocked β‐CD ligands from and reattachment of fresh β‐CD ligands to the membrane surface after each adsorption cycle, thus conferring the affinity membrane with excellent regenerative properties. It is anticipated that the use of dynamic covalent bonds for binding ligands will be of interest for developing smart affinity membranes with regenerable and readjustable surface properties

Interplay of the forces governing steroid hormone micropollutant adsorption in vertically-aligned carbon nanotube membrane nanopores

Vertically-aligned carbon nanotube (VaCNT) membranes allow water to conduct rapidly at low pressures and open up the possibility for water purification and desalination, although the ultralow viscous stress in hydrophobic and low-tortuosity nanopores prevents surface interactions with contaminants. In this experimental investigation, steroid hormone micropollutant adsorption by VaCNT membranes is quantified and explained via the interplay of the hydrodynamic drag and friction forces acting on the hormone, and the adhesive and repulsive forces between the hormone and the inner carbon nanotube wall. It is concluded that a drag force above 2.2 × 10$^{−3}$ pN overcomes the friction force resulting in insignificant adsorption, whereas lowering the drag force from 2.2 × 10$^{−3}$ to 4.3 × 10$^{−4}$ pN increases the adsorbed mass of hormones from zero to 0.4 ng cm$^{−2}$. At a low drag force of 1.6 × 10$^{−3}$ pN, the adsorbed mass of four hormones is correlated with the hormone−wall adhesive (van der Waals) force. These findings explain micropollutant adsorption in nanopores via the forces acting on the micropollutant along and perpendicular to the flow, which can be exploited for selectivity

Critical risk points of nanofiltration and reverse osmosis processes in water recycling applications

Presented at the International Conference on Integrated Concepts on Water Recycling, Wollongong, NSW Australia, 14–17 February 2005.NF/RO membrane filtration processes have been recognized as an important technology to facilitate water recycling. Those processes are well-proven technologies, which can be used to remove a wide range of contaminants including trace contaminants that are of particular concern in water recycling. However, risk implications in association with brine or concentrate and membrane cleaning wastewater disposal have to date not been adequately understood. This study examines the adsorption and release process of several endocrine-disrupting chemicals (EDCs) during NF/RO filtration processes. Results reported here indicate that the membrane can serve as a large reservoir for EDCs and their release may be possible during membrane cleaning or erratic pH variation during operation. Treatment of membrane cleaning solution should be carefully considered when EDCs are amongst the target contaminants in NF/RO membrane filtration

Desalination using electrodialysis as a function of voltage and salt concentration

Presented at EuroMed 2006 conference on Desalination Strategies in South Mediterranean Countries: Cooperation between Mediterranean Countries of Europe and the Southern Rim of the Mediterranean. Sponsored by the European Desalination Society and the University of Montpellier II, Montpellier, France, 21–25 May 2006.Electrodialysis is a process that competes with reverse osmosis for desalination and the removal of specific inorganic contaminants. Desalination experiments were carried out on aqueous solutions containing 5 and 10 g/L NaCl to determine the optimum operating conditions of an electrodialysis (ED) system. Further desalination of aqueous solutions containing 1, 5, 10, 20, 25 and 35 g/L NaCl at an optimum applied voltage of 12 V was conducted to determine the influence of initial salt concentration on the desalination process. The possibility of removing fluoride and nitrate from a groundwater containing about 4.3 g/L NaCl, as well as 2.8 and 31.3 mg/L of fluoride and nitrate respectively, as a function of applied voltage was also investigated. A laboratory electrodialysis stack containing seven cation-exchange membranes and six anion-exchange membranes of 56 cm2 effective area was used. From these studies it is demonstrated that electrodialysis is an effective method for the removal of fluoride and nitrate from brackish groundwater and that real groundwater showed a slower desalination behaviour. Fouling of the membranes was observed