Efficiently combining water reuse and desalination through Forward Osmosis-Reverse Osmosis (FO-RO) hybrids: a critical review

Forward osmosis (FO) is a promising membrane technology to combine seawater desalination and water reuse. More specifically, in a FO-reverse osmosis (RO) hybrid process, high quality water recovered from the wastewater stream is used to dilute seawater before RO treatment. As such, lower desalination energy needs and/or water augmentation can be obtained while delivering safe water for direct potable reuse thanks to the double dense membrane barrier protection. Typically, FO-RO hybrid can be a credible alternative to new desalination facilities or to implementation of stand-alone water reuse schemes. However, apart from the societal (public perception of water reuse for potable application) and water management challenges (proximity of wastewater and desalination plants), FO-RO hybrid has to overcome technical limitation such as low FO permeation flux to become economically attractive. Recent developments (i.e., improved FO membranes, use of pressure assisted osmosis, PAO) demonstrated significant improvement in water flux. However, flux improvement is associated with drawbacks, such as increased fouling behaviour, lower rejection of trace organic compounds (TrOCs) in PAO operation, and limitation in FO membrane mechanical resistance, which need to be better considered. To support successful implementation of FO-RO hybrid in the industry, further work is required regarding up-scaling to apprehend full-scale challenges in term of mass transfer limitation, pressure drop, fouling and cleaning strategies on a module scale. In addition, refined economics assessment is expected to integrate fouling and other maintenance costs/savings of the FO/PAO-RO hybrid systems, as well as cost savings from any treatment step avoided in the water recycling

Rejection of organic micropollutants by high pressure membranes (NF/RO)

The presence of numerous organic micropollutants, such as pharmaceutically active compounds (PhACs) and hormones, but also pesticides and industrial pollutants, in the sources for the drinking water supply, are a big concern for drinking water utilities. Even though not all pollutants are harmful to human health, less is known about the consumption of drinking water containing a cocktail of all these solutes. To prevent these pollutants from entering the drinking water, a solid drinking water treatment is necessary. This thesis investigates the removal of, amongst others, pharmaceuticals and hormones by nanofiltration and reverse osmosis membranes. These membranes are currently among the most advanced techniques for removal of organic micropollutants. In the thesis it is shown that even reverse osmosis membranes (the membranes with the smallest pore size) are not able to completely remove all pollutants. Combination of nanofiltration/reverse osmosis with other water treatment techniques (such as activated carbon filtration) is therefore necessary. This is no problem for the Dutch drinking water sector, since the principle of "multiple barrier treatment" is used in the Netherlands. For other countries, where the treatment plants are less advanced, the presence of pharmaceuticals and hormones may become a problem. Therefore, the policy towards pesticides, hormones and pharmaceuticals has to be changed, to prevent too many of these pollutants from entering the drinking water.Civil Engineering and Geoscience

A novel hybrid process of reverse electrodialysis and reverse osmosis for low energy seawater desalination and brine management

This paper introduces a novel concept for a hybrid desalination system that combines reverse electrodialysis (RED) and reverse osmosis (RO) processes. In this hybrid process the RED unit harvests the energy in the form of electricity from the salinity gradient between a highly concentrated solution (e.g., seawater or concentrated brine) and a low salinity solution (e.g., biologically treated secondary effluent or impaired water). The RED-treated high salinity solution has a lower salt concentration and serves as the feed solution for the RO unit to reduce the pump work. The concentrated RO brine provides the RED unit a better high salinity source for the energy recovery compared to seawater. In addition, the concentration of the discharged brine can be controlled by the RED unit for improving the water recovery and minimizing the impact on the environment. Different configurations of the hybrid RED–RO processes are presented for a comparative study on the basis of mathematical modeling. Specifically, various operating conditions for the RED unit are investigated for better adaptation to the hybrid system. The variations of the total specific energy consumption and the discharge brine concentration for various hybrid modes are simulated to verify the conceptual designs. The modeling results indicate that the RED–RO hybrid processes could substantially reduce the specific energy consumption and provide a better control of the discharge brine concentration in comparison to conventional seawater desalination RO processe

Drinking water treatment technologies in Europe: State of the art - vulnerabilities - research needs

Eureau is the European Federation of National Associations of Water and Wastewater Services. At the request of Eureau Commission 1, dealing with drinking water, a survey was made focusing on raw drinking water sources and drinking water treatment technologies applied in Europe. Raw water sources concerned groundwater, surface water, surface water with artificial recharge and river bank filtration. Treatment schemes concerned no treatment, conventional treatment, advanced treatment and conventional plus advanced treatment. The response covered 73% of the population to which drinking water is supplied by the utilities joint in Eureau. Groundwater and surface water are the major raw water sources (>90%). In total, 59% of the drinking water supply concerns nottreated drinking water or drinking water treated with only conventional technologies, while 12% of the drinking water is not disinfected. Vulnerabilities of the European drinking water supply are the contamination of raw water sources with emerging substances, the absence of disinfection and the potential formation of disinfection by-products. Based on this, research needs are the development of quantitative structure activity relationships (QSARs) to better understand and predict the removal rates of treatment technologies for emerging contaminants, the introduction of Water Safety Plans to prevent hygienic contamination of drinking water, and the optimization of disinfection processes and strategies.Water ManagementCivil Engineering and Geoscience

A predictive multi-linear regression model for organic micropollutants, based on a laboratory-scale column study simulating the river bank filtration process

This study investigated relationships between OMP biodegradation rates and the functional groups present in the chemical structure of a mixture of 31 OMPs. OMP biodegradation rates were determined from lab-scale columns filled with soil from RBF site Engelse Werk of the drinking water company Vitens in The Netherlands. A statistically significant relationship was found between OMP biodegradation rates and the functional groups of the molecular structures of OMPs in the mixture. The OMP biodegradation rate increased in the presence of carboxylic acids, hydroxyl groups, and carbonyl groups, but decreased in the presence of ethers, halogens, aliphatic ethers, methyl groups and ring structures in the chemical structure of the OMPs. The predictive model obtained from the lab-scale soil column experiment gave an accurate qualitative prediction of biodegradability for approximately 70% of the OMPs monitored in the field (80% excluding the glymes). The model was found to be less reliable for the more persistent OMPs (OMPs with predicted biodegradation rates lower or around the standard erro

Sorption and biodegradation of organic micropollutants during river bank filtration: A laboratory column study

This study investigated sorption and biodegradation behaviour of 14 organic micropollutants (OMP) in soil columns representative of the first metre (oxic conditions) of the river bank filtration (RBF) process. Breakthrough curves were modelled to differentiate between OMP sorption and biodegradation. The main objective of this study was to investigate if the OMP biodegradation rate could be related to the physico-chemical properties (charge, hydrophobicity and molecular weight) or functional groups of the OMPs. Although trends were observed between charge or hydrophobicity and the biodegradation rate for charged compounds, a statistically significant linear relationship for the complete OMP mixture could not be obtained using these physico-chemical properties. However, a statistically significant relationship was obtained between biological degradation rates and the OMP functional groups. The presence of ethers and carbonyl groups will increase biodegradability, while the presence of amines, ring structures, aliphatic ethers and sulphur will decrease biodegradability. This predictive model based on functional groups can be used by drinking water companies to make a first estimate whether a newly detected compound will be biodegraded during the first metre of RBF or that additional treatment is required.In addition, the influence of active and inactive biomass (biosorption), sand grains and the water matrix on OMP sorption was found to be negligible under the conditions investigated in this study. Retardation factors for most compounds were close to 1, indicating mobile behaviour of these compounds during soil passage. Adaptation of the biomass towards the dosed OMPs was not observed for a 6 month period, implying that new developed RBF sites might not be able to biodegrade compounds such as atrazine and sulfamethoxazole in the first few months of operation

Impact of backwash water composition on ultrafiltration fouling control

This paper investigates the impacts of different components in backwash water on ultrafiltration (UF) fouling control. Natural surface water was used as feed water, different backwash waters with different Ca2+ and Na+ concentrations were prepared by dosing CaCl2 and NaCl into demineralized water. Furthermore, UF permeate containing mainly natural organic matter (NOM) with minimal cations (Ca2+ and Na+) was produced by dialysis and used for backwashing as well. Thus, the efficiency of different backwash waters on UF fouling control was evaluated. Results show that the presence of both divalent (Ca2+) and monovalent cations (Na+) in backwash water reduces the fouling control efficiency. Since the negative charges of UF membranes and NOM compounds are screened by the cations during filtration, NOM can easily deposit on the surface of the UF membrane, causing fouling. When the mono- and divalent cations are absent in the backwash water, the charge-screening effect around the negatively charged UF membrane and NOM is reduced, increasing the repulsion force between them. In addition to the charge-screening effect, the absence of calcium in backwash water can also reduce the calcium-bridging effect between the membrane and NOM, increasing the fouling control efficiency of the backwash. Measurements of the streaming potential indicate that backwashing with demineralized water can maintain the negative charge of the membrane. Organic compounds in the backwash water do not influence the fouling control efficiency of the backwash

Pressure assisted osmosis (PAO) to enhance forward osmosis (FO) performance

Water ManagementCivil Engineering and Geoscience