1. Technology of Forward Osmosis (FO)

1.1Principle of Forward Osmosis (FO)

The semipermeable membrane separates solvent from solutions that only solvent can pass through but solute molecule cannot. Under the condition of osmotic pressure, the solute molecule spontaneously enters into solution side from solvent side. This is the osmotic phenomenon, which is also called forward osmosis.

1.2Application of Forward Osmosis in Water Treatment

1.2.1 Seawater Desalination

In desalination, FO is one of the most commonly used in its research fields. The early application research was mainly used in patents, but these researches were not mature and also unreliable.

1.2.2 Treatment of Industrial Wastewater

In the early research, it reported that FO membrane could be used in the treatment of heavy metal wastewater. However, because of the serious contamination of RO (Reverse Osmosis) and rapid decline of flux, it cannot be deeply developed.

1.2.3 Treatment of landfill leachate

CoffinButte refuse landfill, locates in Corvallis, Oregon, USA, products (2-4)×104m3 of landfill leachate every year. In order to reach the water quality standard of land utilization, the TDS of water production must reduce to below 100mg/L.

2. Technology of RO Membrane

2.1Principle of Reverse Osmosis (RO)

Taking pressure as impetus, in the process of separation, RO uses pump to put pressure on brine solution or wastewater so that to overcome the resistance of natural osmotic pressure and membrane, and makes the water pass through the reverse osmosis membrane.

2.2Application of RO Membrane in Water Treatment

2.2.1 Routine Application in Water Treatment

Water is necessary for human life a producing activity. Because of the increasing shortage of fresh water resource, the capacity of RO water treatment in the world has reached millions of tons every day.

2.2.2 Application in Municipal Sewage

At present, the application of RO membrane has been attached great importance, especially in the reuse of the second stage of water outlet in sewage treatment plant and reclaimed water recycling.

2.2.3 Application in the Treatment of Heavy Metals Wastewater

Transferring the pollution is a conventional treatment of heavy metals wastewater, which means transfer the heavy metals dissolved in the wastewater into sediment or other ways. The final disposal is backfilling and the second pollution of the heavy metals still harms to the underground water and surface water in a long term.

2.2.4 Application of Oily Wastewater

Oily wastewater is a kind of large and wide wastewater. If it discharges into water body directly, the oil film will be formed in the water surface and stops the oxygen dissolving into the water so that leading to hypoxia of water, biological death, odor and serious pollution to the ecological environment. Take water from the oilfield of 3.5mg/L oil and (16~23) mg/L TOC, and then treat water to reach the quality of boiler water. This is the process of oily wastewater treatment and reuse to the boiler water in power plant.

3. Technology of MF and UF

3.1Fundamental Principles of Ultrafiltration (UF) and Microfiltration (MF)

Both UF and MF are the process that under the impetus of static pressure difference to make liquid phase separation. They have little difference in principle and both are the process of sieve separation. Under the certain pressure, when the mixed solution of containing high polymer solute and low molecule solute passes through the membrane surface, the solvent and low molecule solute (such as inorganic salt) that smaller than membrane pores permeates through membrane, and be collected as seepage; the high polymer solute (such as organic colloid) will be intercepted and collected as concentrated solution. The membrane separation process for the molecular weight of larger than 500 and smaller than 106 is called UF; the membrane separation process for intercepting the larger molecular (usually called dispersed particles) is the MF.

3.2Application of UF Membrane and MF Membrane

UF and MF could effectively remove particulate matters, including microorganism, such as Cryptococcus, giardia, bacteria and virus. They can also reduce disinfection by-products through the ways of reducing concentration and restricting the oxidant demand in the process of disinfection. However the removal rate of organism in water is very low, just under 20%. UF and MF are used widely and used to treat different water quality.

4. Technology of NF

4.1Principle of Nanofiltration (NF)

NF is a new type of molecular membrane separation technology, which is also one of the hotspots in the field of membrane separation. The pore diameter of NF membrane is larger than 1nm, and usually 1-2nm; the rejection performance to solute lies between RO membrane and UF membrane; the RO membrane has a high removal rate to all solutes, but the NF membrane just has a high removal rate to special solutes. NF membrane can remove divalent ion, trivalent ion, organism of Mn≥200, microorganism, colloid, heat source and virus. The main feature of NF membrane is containing charge in the membrane noumenon. This is the reason that it has a high desalination performance under a low pressure (just 0.5MPa) and the membrane that molecular weight cut-off of hundreds could remove inorganic ions, which is also the important reason that NF has a lower operation cost. NF is suitable for different kinds of salt water, and the availability of water is 75%~85%, the seawater desalination rate is 30%~50%, without discharging of acid and alkali wastewater.

4.2Application of NO Membrane in Water Treatment

4.2.1 Application of NO Membrane in Drinking Water

Because of the small operation pressure, NF is a priority process to drinking water treatment and deep purification.

At present, water sources in most cities are contaminated of various degrees, and the conventional treatment process has a low removal rate to the organism in water. While adopting chlorine to sterilize and disinfect, chlorine will produce halogen disinfecting by-product with the organism in water. According to Peltier's four years of follow-up study: utilizing the NF system, DOC will reduce to an average of 0.7mgC/L, the content of residual chlorine will reduce to 0.1mg/L from 0.35mg/L, finally the formation of trihalomethanes (THMs) in the mesh will be reduced by 50% compared to the system that not adopt NF. In addition, because of the decrease of Biological Dissolved Organic Carbon (BCOD), biological stability of water production will be improved.

NF technology can remove most Ca, Mg ions, so the desalination is the field that NF technology can be widely-used. In the field of investing, operating, maintaining and price, the membrane method technology is similar to the process of traditional lime softening and ion exchange, but is has the advantages of no sludge, no need to regenerate, totally removal of suspended matter and organism, easy operation and less land occupied. It has many application examples. NF could be directly used in the softening of underground water, surface water and wastewater, as well as the pretreatment of Reverse Osmosis (RO) and Photo voltaic powered desalination system.

4.2.2 Application of NO Membrane in Seawater Desalination

Seawater desalination refers to desalt the seawater with the salinity of 35000mg/L to the drinking water below 500mg/L.

4.2.3 Application of NO Membrane in Wastewater Treatment

(1)Domestic Sewage

Domestic sewage is generally treated with the combination of biodegradation and chemical oxidation, but it needs large amount of oxidant and has lots of residue. Xue Gang takes a pilot test to the bath water in hotel adopting the process of filtering, UF and NF with micro-flocculent fiber ball. The effluent quality of UF could reach requirements of the reuse of toiler flushing and greening, and the effluent quality of NF could reach the drinking water sanitary standard (GB5749.85), so it could be reused in laundry, bath and other sides which have a higher standard to water in hotel.

(2)Wastewater of Textile and Dyeing

It's difficult to remove the dye in the textile wastewater with the biological method. Hassani has studied the effects of acidity, activity, concentrations of direct and disperse dye solutions, pressures, total dissolved solids and contents of inorganic salts on the retention performance of NF membrane.

(3)Tannery Wastewater

The tannery wastewater contains a high concentration of organism, sulfate and chloride, and the conductance value of wastewater in acid washing process will reach to 75mS/cm. Bes-Pia adopts NF technology to collect tannery wastewater, and the sulfate concentrated water with high concentration will return to the stage of acid washing and the producing water of chloride will be got back to soaking drum.

(4)Electroplating Wastewater

The electroplating plant produces a large amount of waste liquid. Although through the complex process of acidification, harmlessness, sedimentation and separated sludge, the water production contains high salinity and cannot reuse

(5)Papermaking Wastewater

In pulp and paper industry, the process of homogenate, bleaching and paper-making needs large amount of water. The closed recirculation of the water system is the best way to save water source and reduce discharge for pulp plant and paper plant. The water production of the traditional activated sludge process contains parts of colored compound, microorganism, antibody and a small amount of biodegradable and suspended solid, so it just can be used in manufacturing the wrapping paper, and cannot be used in manufacturing a higher level of paper. In addition, this method cannot reduce the contents of inorganic salt. Koyuncu compares the practicability of two treatment processes, that is from water to NF and from papermaking wastewater to activated sludge and then to NF. The test shows that effluent quality of the two processes is similar, but the second process has a better water producing flux and it can be used in manufacturing a higher level of paper. However, the water production of NF still has a certain amount of valent salt and a low pressure RO device needs to be added to remove salt so that to guarantee the quality of water recycling.

5. Dialysis and Electrodialysis

5.1Dialysis

Dialysis (DI for short) means that under its own concentration gradient, is a process of passing from the upstream of the membrane to the downstream.

Dialysis is the first membrane separation technology ever found and researched. But because of its own restriction, it has a slow movement, low efficiency and less selective. Therefore, dialysis is mainly used in remove the low molecular weight fraction from the solution containing multiple solutes, such as hemodialysis, which means using dialysis membrane to replace kidney to remove carbamide, creatinine, phosphate, uric acid and other toxic low molecular weight fraction so that to alleviate the condition of patients who suffer the disease of renal failure and uremia.

5.2Electrodialysis

Under the role of DC electric field, taking the potential difference as impetus, electrodialysis (ED for short) utilizes ion exchange membrane to select the cation and anion in the solution, separates the electrolyte from solution, and so that to realize the concentration, desalination, and purification of solutions.

6. Technology of Bipolar Membrane

6.1Introduction of Bipolar Membrane

Bipolar membrane is a new type of membrane. It's a composite ion exchange membrane composed by anion-exchange column and cation-exchange column. A third layer of substance can be added between anion exchange membrane and cation exchange membrane, becoming the three-layer structure composed by anion exchange layer, cation exchange layer and intermediate responding layer. Under the role of direct-current electric field, bipolar membrane could produce water dissociation, and H+ and OH- are produced on both sides of the membrane.

6.2Application of Bipolar Membrane

6.2.1 Treatment of Waste Liquid Containing Fluorine and Recycling of fluorine with valence

In the manufacturing of fluorocarbon industry and uranium industry (UF6), the mass fraction of fluorine and organic acid in the discharging waster gas and wastewater is 50～500×10-6. Generally it needs to add KOH to remove them, resulting that the generated KF solution contains lots of heavy metals (such as uranium and arsenic) and trace amounts of radioactive substances, so this process needs Ca(OH)2 and KF to regenerate KOH and generate insoluble wastes. This method will lead to the loss of valent fluorine and the problem that how to deal with the radioactive substance of Ca (OH)2. If taking the technology of bipolar membrane, KF will be directly transformed to HF and KOH, which can not only recycle the valuable fluorine but also avoid using lime and reduce the treatment of waste residue.

6.2.2 Bipolar Membrane Used in the Purification and Recycling of Acid and Alkali Wastewater

Industrial manufacture will produce lots of acid and alkali wastewater, such as regenerated waste water of ion exchange resin, pickling liquor, waste liquid of lead battery and wastewater in paper-making plant. In order to reduce pollution to the environment, the waste liquid must be disposed of in a necessary treatment, but the process is complicated and expensive. Bipolar membrane provides a good solution to treat this kind of waste liquid. In 1986, a union of EDI and ion exchange equipment was installed in post and telecommunications plant in Zhejiang Province. This equipment was used for treating copper-containing wastewater. After treatment, the content of cooper is 100mg/L, and the pH value is 6～7, meeting the standard of discharging.

6.2.3 Treatment of Domestic Sewage

Domestic sewage is generally treated with biodegradation/ chemical oxidant, but the amount of oxidant is used too much and remains lots of residue. If adding NF between them, the small molecule (molecular mass<100) degraded by microorganism can pass through, but the big molecule (molecular mass>100) which cannot be degraded by microorganism is intercepted. After the treatment of chemical oxidizer, macromolecular substance degrades. This method makes best use of biodegradation, saves oxidant and activated carbon, and reduces the final residue.

6.2.4 Purification of Drinking Water

With the increase of water pollution, people are increasingly concerned about the quality of drinking water. The test shows that the bipolar membrane could effectively remove toxic by-products in disinfection, trace of herbicide, insecticide, heavy metals, natural organic matter and hardness, sulfate and nitrate. At the same time, it has the advantages of good quality of water, stability, less chemicals, less land occupied, saving energy, easy management and maintenance.

6.2.5 Treatment of Heavy Metals Wastewater

In electro plating and alloy production, large amount of water is needed to flush. This kind of cleaning water contains a high concentration of heavy metals, including nickel, iron, copper and zinc. In order to make this kind of waster to meet the requirement of discharging, the general measure is transforming these heavy metals into hydroxide precipitate and removing. If taking the NF technology, more than 90% of wastewater could be recycled and purified, while the concentration of heavy metal ions concentrated 10 times. After concentration, the heavy metals could be reused.

6.2.6 Treatment of Industrial Wastewater

N-P Type composite bipolar membrane has an obvious effect on the separation of valent and bivalent salt, significant reduce the amount of COD in wastewater and reach the standard of environmental protection.

6.2.7 Prospect of Bipolar Membrane

As a new type of membrane, the bipolar membrane has unique advantages, proving new ideas and solutions to solve the problem which has been in the environmental engineering for a long time. It has a profound significance to continually develop high-performance bipolar membrane, improve the preparation technology of membrane, reduce the cost of membrane, further research on mechanism and mechanism of ion mobility and water transferring, research on the high-quality bipolar membrane materials and preparation, and exploit their application area.