Draw solution

An interesting alternative development is that of forward osmosis. Whereas in reverse osmosis a high pressure is required to oppose the natural tendency of freshwater to move across such a membrane via osmosis to dilute the seawater, in forward osmosis the system takes advantage of this natural tendency. Here, salt water sits on one side of the membrane, but the freshwater on the opposite side is transformed into a high-concentration solution by adding NH3 and CO2. Water naturally flows from the salt water to what is now the draw solution, which can have a solute concentration as high as 10 times that of the salt water. There is no need for an external pressure. The diluted draw solution is then heated to evaporate off the CO2 and NH3 for reuse, leaving behind freshwater. (See Patel-Predd, 2006). 

Make sure that the pipette filler is dry and that any solution previously used does not contaminate your sample. Fit it to the pipette and check that it is working properly i.e. it draws solution into the pipette and the pipette does not leak when held vertically. Do not force the filler onto the pipette in trying to stop a leak - the pipette is likely to break and cause an injury. 

PRO mode Dense selective layer facing draw solution Dense... 

S. Zhao, L. Zou, D. Mulcahy, Brackish water desahnation by a hybrid forward osmosis-nanofiltration system using divalent draw solute. Desalination 2012, 284, 175-181. 

A number of alternatives to reverse osmosis are being considered. Two promising alternatives are membrane distillation [97] and forward osmosis [98]. Membrane distillation relies on vapor pressure differences across a membrane, arising from a temperature difference, to drive water transport. The process utilizes low temperature heat sources and operates at low pressure which can reduce operating costs relative to reverse osmosis. Forward osmosis relies on water permeation across a water selective membrane to a draw solution - the reverse of reverse osmosis. The water must then be separated from the draw solution but this may be less expense than reverse osmosis because the process operates at low pressure. 

Forward osmosis (FO) is a membrane-separation process that uses osmotic pressure difference between a concentrated draw solution and a feed stream to drive water across a semipermeable membrane [63]. The basis of FO is osmosis, a natural and spontaneously occurring process. It is strictly direct osmosis across an RO membrane. A draw solute of high osmotic pressure, e.g., ammonium carbonate passes across one side of the FO membrane, and a high salinity solution, e.g., seawater flows across the other side of the membrane, as shown in Figure 1.17. Water transfers from the seawater to the draw solute side due to osmotic flow. It is then necessary to regenerate the draw solute and recover the water transferred by the FO process, e.g., in a distillation unit. The primary challenge is... 

Figure 1.17 Schematic flow diagram of a forward osmosis system with ammonia carbon dioxide draw solution (https //www.google.com/search q=cellulose+acetate+membrane). Source [63].

Conventional RO membranes are not suitable because FO differs from RO in having salty solutions on both sides of the membrane. The porous support exposed to the draw solution is subject to fording and internal polarisation from the draw solution. This means that the concentration of salt ions inside the membrane is very diC ferent from the bulk solution, resulting in a loss of osmotic pressure driving force such that conventional RO membranes only achieve <50% of their capability in FO. 

Heat energy is required for the separation of the ammonia and CO2 from the diluted draw solution and for the evaporation oflarge amounts of water. A suitable draw solution is required for treating feeds with high osmotic pressure. 

Even with draw solution optimisation and the benefit of reduced fouling in the regeneration step, the FO process is unlikely to approach the energy efficiency ofSWRO. 

In contrast to a neutral membrane, the positively charged forward osmosis membrane provides double electric repulsions to the salt transfer through the membrane in the active layer facing feed water configuration. This results in a reduction of the salt penetration, while in the active layer facing draw solution config-... 

Step 1 - Filtration Water flows by simple diffusion through a semi-permeable membrane from a feed solution (at lower osmotic pressure) into a draw solution (at higher osmotic pressure) contaminants in the feed water, including dissolved ions, are rejected by the membrane, and are concentrated in the feed. The draw solution is diluted by the water passing through the membrane. 

Step 2 - Water Recovery Water is recovered from the draw solution restoring it back to its original condition to be reused in step 1. 

The recovery of water from the draw solution can be done by a variety of means including evaporation of the draw solution. 

In Step 1, the driving force across the membrane is the osmotic pressure gradient caused by the difference in solute concentrations between the feed and draw solutions [1]. The greater the difference in solute concentrations, the higher the membrane flux. In Step 2, an input of energy is required to remove the water from the draw solution and restore its original osmotic pressure. 

Cost-effective draw solutes that can be easily separated to recover the water ... 

The draw solution is a key element in a FO process, and should possess the following characteristics ... 

Significant work has been undertaken to develop optimum draw solutions with the characteristics listed above [6,1, 8, 9, 10], However, no single draw solution seems to be ideal and the application of one instead of another is more related to the nature of the effluent to be treated. 

The primary disadvantage with FO is typically the input energy needed in step 2 to recover the water from the draw solution. The novel aspect of this project is to eliminate step 2 by to using seawater or thermal brine to draw water from the produced water and then directly discharge the diluted seawater/brine into the Arabian Gulf. This is particularly applicable in Qatar since both the desalination plant and the produced water from the gas fields are in close proximity to each other. 

Figure 2 Effect of draw solution concentration on water flux...

To determine the organic rejection by the FO process, the total organic carbon (TOC) concentration was measured. The initial TOC concentration in the feed was 130 mg/L and the final concentration (after approximate 50% volnme reduction) was 235 mg/L. The TOC concentration in the draw solution was less than 1 mg/L this indicates that the FO membrane can effectively reject the organics in the feed water. Table 1 shows the concentration for the anions and cations present in the feed and draw solutions. 

Parameter Units Initial Feed Final Feed Initial Draw Solution Final Draw Solution... 

To evaluate the effectiveness of FO to reduced injection water volumes several process parameters has been study. Results show that the draw solution concentration plays an important role in water production. Higher draw solution concentration yields higher flux due to higher osmotic pressure difference between the feed and draw solution. 

Also, an experiment was conducted using produced/process water from Qatari gas fields as feed, 70 g/L NaCl as draw solution and 50% feed volume reduction. Results show that the FO membrane is effective in removing the organics present in the feed water. 

---