In forward osmosis (FO), like RO, water diffuses through a semipermeable membrane that is impermeable to salt. However, unlike RO that uses hydraulic pressure as a driving force for water transport through the membrane, FO utilizes an osmotic pressure gradient across the membrane (Fig. 13) . A "draw solution'' having a significantly higher osmotic pressure than the saline feedwater flows on the permeate side of the membrane and water naturally diffuses across the membrane by osmosis. Osmotic driving forces in FO can be significantly greater than hydraulic driving forces in RO, potentially leading to higher water flux rates and recoveries. With the use of a suitable draw solution, very high osmotic pressure driving forces can be generated to achieve high recoveries that can lead to salt precipitation . To yield potable water, the diluted draw solution is treated by another separation process that generate a stream of purified water and a stream of reconcentrated draw solution for reuse in the FO process.
The main advantages of the FO process include the relatively low fouling potential and low energy consumption as no high-pressure pump is required, provided that there is no additional energy cost associated with regeneration of the draw solution and with management of the FO concentrate. Depending upon the water composition of RO concentrate, recent studies showed that FO could further reduce RO concentrate volume by 71—81%, achieving an overall water recovery of 94—97% by combination of RO-FO processes [80,85]. However, scaling/fouling still occurs at high recovery conditions as determined by the saturation indices.
Concentrated Draw Solution
Diluted Draw Solution
Figure 13 Schematic of forward osmosis.
Chemical cleaning was proven as being effective to remove silica and calcium sulfate scaling and 81% of the permeate flux was recovered .
The main challenges remain in the manufacturing of high-performance FO membranes, the need for regeneration of the draw solution, and optimization of process configuration. These are the primary obstacles to the practical applicability of FO for water production and thus large-scale commercialization of the FO process. With the suitable draw solution and appropriate semipermeable membrane, FO process can lead to salt precipitation, that is, ZLD. The technology of FO is still being developed.
4.5.2 Vibratory shear-enhanced process for membrane filtration
The vibratory shear—enhanced proces (VSEP) membrane filter pack consists of leaf elements arrayed as parallel disks and separated by gaskets. The shear waves produced by the membrane vibration cause solids and foulants to be lifted off the membrane surface and remixed with the bulk material flowing through the membrane stack. Membrane fouling and scaling is minimized through the effects ofan axial, vibratory-induced shear force imposed directly on the membrane—liquid interface. A pilot scale testing on highly saline boiler blowdown water yielded a recovery of 95% using VESP RO and a solid waste (brine) with approximately 50-65% dry solids . More studies are required to evaluate this technique for the purpose of concentrate minimization and recovery improvement, and its overall cost-effectiveness.
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