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A novel approach for resource recovery includes forward osmosis (FO) as a concentration step in municipal wastewater treatment. The current study investigates different pre-treatment strategies including biological treatment with a moving-bed bioreactor (MBBR) at different loading rates and particle removal by filtration and sedimentation. Membrane performance and recovery potential for energy and nutrients were investigated in laboratory-scale FO experiments in batch mode using pre-treated municipal wastewater as feed and 35 g/L NaCl as a draw solution. Initial water fluxes were in the range of 6.3 to 8.0 L/(m2·h). The baseline fluxes were modelled to account for flux decline due to concentration effects and to enable the prediction of flux decline due to membrane fouling. Fouling-related flux decline varied from 0 to 31%. Both organic fouling and precipitation of CaCO3 and CaHPO4 were identified by using SEM–EDS. High-rate flushing resulted in complete flux recovery under most conditions. Scaling could be avoided by lowering the pH. Two operation strategies were tested to achieve this: (1) applying a bioreactor with a low organic loading rate to achieve high nitrification, and (2) adding a strong acid. A low organic loading rate and the use of additional particle removal were efficient measures that reduced organic/particulate fouling. The recovery potentials for COD and phosphorous in FO concentrate were close to 100%.

Pressure retarded osmosis (PRO) is one feasible technology that can be used to exploit the mixing energy from salt gradients which is commonly referred to as salinity gradient power or osmotic power. This paper focuses on the construction of iso-watt diagrams and their application for detailed evaluation of specific power in PRO on basis of membrane characteristics and site specific design parameters. Iso-watt diagrams are demonstrated to be a useful tool for investigation of the effect of variation in the membrane parameters, i.e. the water permeability, the salt permeability and the structure parameter, and how mutual changes in the characteristic parameters will impact the PRO performance. Thus, iso-watt diagrams can be considered an effective aid for goal-oriented membrane development. In addition, iso-watt diagrams can be considered to be useful also with respect to design related aspects, such as e.g. site selection. Further, membrane performance data are reported in the literature for various experimental conditions, which makes direct comparison difficult. The use of iso-watt diagrams enables an efficient way of presenting and comparing the efficiency of different membranes. This is demonstrated in the current paper for ten states of the art membranes.

Pressure retarded osmosis (PRO) can be used to exploit the mixing energy e.g. between river water and sea water. A PRO membrane must be highly permeable for water, whereas salt ions should be retained. Furthermore, the structure parameter of the membrane support and backing structure must be low. This paper summarises an assessment of the pressure dependency of the structure parameter for flat sheet membranes, and a transport model for PRO and procedures for determination of the pressure dependency of the structure parameter are presented. The results from laboratory experiments show that that the structure parameter increases significantly with increasing trans-membrane pressure. The increase in the structure parameter was observed to depend on both characteristics of the membrane and the fresh water spacer. Using a finely textured tricot spacer reduced the pressure dependency on the structure parameter, compared to a coarser spacer. Applying a non-woven backing material between the membrane and the fresh water spacer also reduced the impact of pressure. The results show that membranes suitable for river water/sea water PRO must have a sufficiently low structure parameter and additionally resist severe deformation at relevant operating pressures.

Fouling is one of the major challenges to be solved in order to commercialise osmotic power generation by pressure retarded osmosis (PRO). This paper deals with natural organic matter (NOM) fouling. The impact of i.a. NOM concentration and ionic strength in the freshwater feed have been investigated for five different types of PRO membranes by performing fouling experiments in PRO mode at isobaric conditions. An important finding was that fouling propensity in terms of relative flux decline as a function of accumulated NOM load was independent of the NOM concentration at otherwise identical conditions. Further, it was confirmed that increased ionic strength in the support structure, due to internal concentration polarisation and reverse salt diffusion, aggravates membrane fouling in PRO. The observed variation in fouling propensity between different types of PRO membranes was significant. The CA membrane was less susceptible to NOM fouling than the four TFC types of PRO membranes. It is anticipated that the variation in fouling propensity that were observed for different membranes was related to both material and structural properties of the membranes. The results show the importance of applying fouling propensity as an additional main criterion to membrane performance during future research and development of PRO membranes.

The principle of pressure retarded osmosis is to utilize parts of the free energy of mixing of solutions with different salinities for production of mechanical energy. The mixing process takes place over a semi-permeable membrane as water is transported from the less concentrated salt solution to the more concentrated salt solution due to difference in chemical potential of water. The PRO process will be operated with elevated pressure on the side of the membrane having the highest salt concentration, corresponding to approximately half the difference in osmotic pressures for the two solutions. The net increase in volume on the pressurised side can be utilized to run a turbine. This paper has focused on modelling the power production and pressure drop in hollow fibre modules with different flow configurations, i.e. co-current and counter current longitudinal flows and elements with longitudinal flow inside the fibres and radial flow in the shell. The overall PRO performance is similar for the different flow configurations. The radial flow configuration has a superior pressure drop in the shell side compared to the longitudinal flow configurations, but the total membrane area for a given element size will be larger for the longitudinal flow configurations.

The principle of PRO is to utilise parts of the free energy of mixing when mixing freshwater with seawater for production of electrical energy. The mixing process takes place over a semi permeable membrane that will retain salts, and ideally allow only transport of water. The transport of water will occur due to differences in chemical potential across the membrane. The PRO process will be operated with elevated pressure on the seawater side of the membrane, and the net increase in volume due to osmotic transport of water across the membrane, can be utilised to run a turbine and hence produce energy. At present the biggest challenge in PRO is to develop membranes with sufficient specific power. This challenge involves optimisation of the basic membrane characteristics, i.e., high water permeability (A), low salt permeability (B) and low structure parameter (S). In this paper a transport model for water and salt in hollow fibres in PRO has been developed. Further, a structure parameter equivalent to the structure parameter for flat sheet membranes has been defined. The impact of the membrane parameters (A, B and S) on PRO performance in a hollow fibre element has been demonstrated by applying iso-watt diagrams. In general, for a given structure parameter S, increasing the A value and decreasing the B value will increase the specific power. It was also found that if the salt permeability exceeds a certain level, a further increase in the water permeability will not result in a corresponding increase in the specific power. Similarly, if the water permeability is lower than a certain level the effect of increased salt permeability will be low. Further, the PRO performance of different hollow fibre membranes has been estimated using the iso-watt diagrams. Based upon the reported membrane characteristics the PRO performance for the selected membranes ranges from less than 1 W/m2 to above 4.5 W/m2. The membranes having a specific power around 4.5 W/m2 are promising candidates for future PRO plants, given that these membranes can tolerate the typical operation pressure in a PRO plant of 12–14 bar.