The Most Comprehensive Application of Membrane Technology in Water Treatment

1. Forward Osmosis (FO) Technology

1.1 Principle of Forward Osmosis (FO)

A semi-permeable membrane, allowing only the solvent to pass through but not the solute molecules, separates the solvent and solution. Solvent molecules spontaneously permeate from the solvent side to the solution side under osmotic pressure; this is the phenomenon of osmosis, also known as "forward osmosis."

1.2 Applications of Forward Osmosis Membranes in Water Treatment

1.2.1 Seawater Desalination

FO application in seawater desalination is one of its most extensively researched areas. Early application research was mainly found in some patents, but these studies were mostly immature and lacked feasibility.

1.2.2 Industrial Wastewater Treatment

Early studies reported the use of FO membranes for treating low-concentration heavy metal wastewater, but due to severe fouling and rapid flux decline of the RO (reverse osmosis) membranes used, this research was not further developed.

1.2.3 Landfill Leachate Treatment

The Coffin Butte landfill in Corvallis, Oregon, USA, produces (2-4) × 10⁴ m³ of landfill leachate annually. To meet land use water quality standards, the TDS (Total Dissolved Solids) of the effluent must be reduced to below 100 mg/L.

2. Reverse Osmosis Membrane Technology

2.1 Principle of Reverse Osmosis (RO)

Reverse osmosis is a pressure-driven membrane separation process. In operation, a water pump is used to apply pressure to the saline solution or wastewater to overcome natural osmotic pressure and membrane resistance, allowing water to pass through the reverse osmosis membrane. Dissolved salts or pollutants are retained on the other side of the membrane.

2.2 Applications of Reverse Osmosis Membranes in Water Treatment

2.2.1 Conventional Applications in Water Treatment

Water is an essential material condition for human survival and production activities. Due to the increasing scarcity of freshwater resources, the world's reverse osmosis water treatment capacity has reached millions of tons per day.

2.2.2 Application in Urban Wastewater Treatment

Currently, the application of reverse osmosis membranes in advanced urban wastewater treatment, especially in the reuse of secondary effluent from wastewater treatment plants and the reuse of reclaimed water, has received considerable attention.

2.2.3 Application in Heavy Metal Wastewater Treatment

Conventional treatment methods for wastewater containing heavy metal ions are merely a form of pollution transfer, converting dissolved heavy metals in the wastewater into precipitates or more easily treated forms. The final disposal is often landfill, and the secondary pollution of groundwater and surface water by heavy metals remains a long-standing problem.

2.2.4 Application in Oily Wastewater Treatment

Oily wastewater is a large-scale and widespread type of industrial wastewater. If directly discharged into water bodies, it will form an oil film on the surface, hindering oxygen dissolution and leading to oxygen deficiency, biological death, and foul odors, severely polluting the ecological environment. Oilfield produced water with oil concentrations of 3.5 mg/L and total organic carbon (TOC) of (16~23) mg/L was treated to meet boiler feedwater standards, and the treated water was then reused as boiler feedwater in power plants. 

3. Microfiltration and Ultrafiltration Membrane Technologies

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

Ultrafiltration and microfiltration are both liquid-phase separation processes driven by hydrostatic pressure difference. In principle, there is no fundamental difference; both are sieve-based separation processes. Under certain pressure, when a mixed solution containing high-molecular-weight solutes and low-molecular-weight solutes flows across the membrane surface, the solvent and low-molecular-weight solutes smaller than the membrane pores (such as inorganic salts) permeate through the membrane, becoming permeate and being collected; high-molecular-weight solutes larger than the membrane pores (such as organic colloids) are retained by the membrane and recovered as concentrate. Membrane separation processes that can retain molecules with molecular weights greater than 500 and less than 10⁶ are called ultrafiltration; membrane separation processes that can only retain larger molecules (usually referred to as dispersed particles) are called microfiltration.

3.2 Applications of Ultrafiltration and Microfiltration Membranes

Ultrafiltration and microfiltration technologies can effectively remove particulate matter, including microorganisms such as Cryptocystis, Giardia, bacteria, and viruses. Disinfection byproducts can be reduced to some extent by lowering the concentration of disinfection byproduct precursors and limiting the amount of oxidant required during disinfection. However, the removal rate of organic matter in water is very low, only below 20%. Ultrafiltration and microfiltration have a wide range of applications and can be used to treat different water qualities.

4. Nanofiltration Membrane Technology

4.1 Nanofiltration (NF) Principle

Nanofiltration (NF) is a novel molecular-level membrane separation technology and is currently one of the hot topics in the world's membrane separation field. NF membranes have pore sizes above 1 nm, generally between 1-2 nm; their solute retention performance is between that of RO and UF membranes; RO membranes have high removal rates for almost all solutes, but NF membranes only have high removal rates for specific solutes. NF membranes can remove divalent and trivalent ions, organic matter with Mn≥200, as well as microorganisms, colloids, pyrogens, viruses, etc. A key characteristic of nanofiltration (NF) membranes is their charged structure. This is a crucial reason why they maintain high desalination performance even at very low pressures (only 0.5 MPa) and can remove inorganic salts even with a molecular weight cutoff of several hundred. It is also the main reason for the low operating cost of NF. NF is suitable for various saline sources, with a water utilization rate typically between 75% and 85%, and between 30% and 50% for seawater desalination. No acid or alkaline wastewater is discharged.

4.2 Applications of Nanofiltration Membranes in Water Treatment

4.2.1 Applications of Nanofiltration Membranes in Drinking Water

Nanofiltration operates at low pressures, making it the preferred process for drinking water preparation and deep purification.

Currently, most urban water sources are polluted to varying degrees. Conventional treatment processes in waterworks have low removal rates of organic matter in the water. When chlorine is used for disinfection, chlorine reacts with organic matter in the water to form halogenated byproducts. A four-year follow-up study by Peltier et al. showed that after adopting a nanofiltration system, the DOC in the water decreased to an average of 0.7 mgC/L, the residual chlorine content in the effluent decreased from 0.35 mg/L to 0.1 mg/L, and the formation of trihalomethanes (THMs) in the final filter was reduced by 50% compared to when a nanofiltration system was not used. Furthermore, the reduction in biodegradable dissolved organic carbon (BCOD) improved the biological stability of the produced water.

Nanofiltration technology can remove the vast majority of Ca and Mg ions, therefore desalination is the most widely used application area of nanofiltration technology. Membrane water treatment technology is similar to conventional lime softening and ion exchange processes in terms of investment, operation, maintenance, and price, but it has advantages such as no sludge, no need for regeneration, complete removal of suspended solids and organic matter, simple operation, and small footprint, and has many application examples. Nanofiltration can be directly used for softening groundwater, surface water, and wastewater. It can also be used as a pretreatment for reverse osmosis (RO) and solar photovoltaic desalination systems.

4.2.2 Application of Nanofiltration Membranes in Seawater Desalination

Seawater desalination refers to the process of reducing the salinity of seawater from 35,000 mg/L to below 500 mg/L for drinking water.

4.2.3 Application of Nanofiltration Membranes in Wastewater Treatment

(1) Domestic Sewage

Domestic sewage is generally treated using a combination of biodegradation and chemical oxidation methods. However, the amount of oxidant used is too large, resulting in a lot of residue. Xue Gang et al. conducted a small-scale test on hotel bath wastewater using a combined process of micro-flocculation fiber ball filtration, ultrafiltration, and nanofiltration. Ultrafiltration effluent meets the water quality requirements for reuse in hotel toilet flushing and landscaping, while nanofiltration effluent meets the national drinking water standards (GB5749.85) and can be reused in hotel laundry and bathing facilities with higher water quality requirements.

(2) Textile and Dyeing Wastewater

Textile wastewater contains dyes that are difficult to remove biologically. Hassani studied the effects of concentration, pressure, total dissolved solids, and inorganic salt content of acidic, reactive, direct, and disperse dye aqueous solutions on the retention performance of nanofiltration membranes.

(3) Tannery Wastewater

Tannery wastewater contains high concentrations of organic matter, sulfates, and chlorides. The conductivity of the wastewater from the pickling process reaches 75 mS/cm. Bes-Pia used NF technology to recover tannery wastewater. The resulting high-concentration sulfate concentrate was returned to the pickling section, while the chloride-containing wastewater was returned to the cracking reactor.

(4) Electroplating Wastewater

Electroplating plants often generate large amounts of wastewater. Despite complex treatment steps such as acidification, chemical decontamination, sedimentation, and sludge separation, the produced water has a high salt content and cannot be reused.

(5) Papermaking Wastewater

In the pulp and paper industry, processes such as homogenization, bleaching, and papermaking require large amounts of water. Achieving a (semi-)closed-loop water system is the best way for pulp and paper mills to save water resources and reduce emissions. The produced water from the traditional activated sludge process still contains some colored compounds, microorganisms, antibodies, and small amounts of biodegradable products and suspended solids, and can only be used to manufacture packaging paper, not for the production of higher-grade paper. In addition, this method cannot reduce the content of inorganic salts. Koyuncu compared the practicality of two treatment processes: water → nanofiltration and papermaking wastewater → activated sludge → nanofiltration. Experiments showed that the effluent quality of the two methods was similar, but the second method had a better effluent flux, and the effluent could be used for higher-grade paper. However, nanofiltration permeate still contains a certain amount of monovalent salts, requiring the addition of a low-pressure reverse osmosis unit to remove these salts to ensure the quality of the circulating water.

5. Dialysis and Electrodialysis

5.1 Dialysis

Dialysis (D for short) is the process by which a solute is transported from upstream to downstream of a membrane due to its own concentration gradient.

Dialysis was the earliest discovered and studied membrane separation technology. However, due to the limitations of the system itself, the dialysis process is slow, inefficient, and lacks selectivity. Therefore, dialysis is mainly used to remove low molecular weight components from solutions containing multiple solutes, such as in hemodialysis, where a dialysis membrane replaces the kidney to remove toxic low molecular weight components such as urea, creatinine, phosphate, and uric acid to alleviate the condition of patients with kidney failure and uremia.

5.2 Electrodialysis

Electrodialysis (ED) is a process that uses a DC electric field and the potential difference as the driving force to selectively separate electrolytes from a solution by utilizing the ion-exchange membrane's selectivity for anions and cations. This allows for the concentration, desalination, purification, and upgrading of solutions.

6. Bipolar Membrane Technology

6.1 Introduction to Bipolar Membranes

A bipolar membrane (BPM) is a novel type of membrane, typically a composite ion-exchange membrane consisting of anion exchange layers and cation exchange layers. A third layer can also be added between the anion and cation exchange layers to promote water dissociation, creating a three-layer structure consisting of anion exchange layer, cation exchange layer, and intermediate reaction layer. Under the influence of a DC electric field, the bipolar membrane dissociates water, producing H+ and OH- ions on the cation and anion exchange membranes, respectively.

6.2 Applications of Bipolar Membranes

6.2.1 Treatment of Fluorine-Containing Wastewater and Recovery of Valuable Fluorine

In the production processes of fluorocarbons and uranium (UF6), the discharged waste gas and wastewater contain fluorine and organic acids with a mass fraction of 50–500 × 10⁻⁶. These typically require neutralization with KOH for complete removal. The resulting KF solution contains many heavy metals (such as uranium and arsenic) and trace amounts of radioactive substances. Furthermore, Ca(OH)₂ is needed to react with KF to regenerate KOH, generating insoluble waste. This method leads to the loss of valuable fluorine and leaves users with the problem of how to handle the radioactive Ca(OH)₂ waste. If bipolar membrane electrodialysis technology is used, KF can be directly converted into HF and KOH, not only recovering high-value fluorine but also avoiding the use of lime and reducing the amount of waste residue.

6.2.2 Bipolar Membrane for the Purification and Recovery of Acid and Alkali Waste Liquids

Industrial production generates a large amount of acid and alkali waste liquids, such as ion exchange resin regeneration waste liquid, pickling waste liquid, lead-acid battery waste liquid, and paper mill waste liquid. To reduce environmental pollution, these waste liquids must undergo necessary treatment before discharge, but the treatment processes are complex and costly. Bipolar membrane electrodialysis provides a good solution for the treatment of such waste liquids. In 1986, my country installed a combined electrodialysis and ion exchange system at the Zhejiang Provincial Post and Telecommunications Printing Plant to treat copper-containing wastewater. The treated wastewater had a copper content of 100 mg/L and a pH value of 6-7, meeting the permissible discharge standards.

6.2.3 Domestic Sewage Treatment

Domestic sewage is generally treated using a combination of biodegradation and chemical oxidation methods, but the amount of oxidant used is too large, resulting in significant residues. Adding a nanofiltration stage between these processes allows small molecules (relative molecular mass <100) that can be degraded by microorganisms to pass through, while retaining large molecules (relative molecular mass >100) that cannot be degraded by microorganisms. The large molecules are then treated by a chemical oxidizer before biodegradation. This fully utilizes biodegradability, saves on oxidants and activated carbon, and reduces the final residue content.

6.2.4 Drinking Water Purification

With increasing water pollution, people are paying more and more attention to drinking water quality. Experiments have shown that bipolar membrane nanofiltration can remove micro-toxic byproducts, trace amounts of herbicides, pesticides, heavy metals, natural organic matter, hardness, sulfates, and nitrates generated during disinfection. It also has the advantages of good and stable water quality treatment, low chemical dosage, small footprint, energy saving, and ease of management and maintenance.

6.2.5 Treatment of Wastewater Containing Heavy Metals

In electroplating and alloy production, large amounts of water are often used for rinsing. This rinsing water contains relatively high concentrations of heavy metals, including nickel, iron, copper, and zinc. To ensure that wastewater containing heavy metals meets discharge requirements, the common method is to treat the heavy metals into hydroxides for precipitation and removal. However, using nanofiltration membrane technology can not only recover over 90% of the wastewater and purify it, but also concentrate the heavy metal ion content tenfold, making the concentrated heavy metals valuable for recycling.

6.2.6 Food Industry Wastewater Treatment

N-P type composite bipolar nanofiltration membranes have a significant separation effect on monovalent and divalent salts, significantly reducing the COD content in wastewater and meeting environmental protection requirements.

6.2.7 Bipolar Membrane Outlook

As a novel type of membrane, bipolar membranes, with their unique advantages, offer many new ideas and solutions for solving some long-standing technical problems in environmental engineering. Further development of high-performance bipolar membranes, improvement of membrane preparation processes, reduction of membrane production costs, in-depth mechanistic research, study of ion migration and water transfer mechanisms within membranes, research on high-performance bipolar membrane materials and their preparation, and expansion of application areas are of profound significance.