A Brief Discussion on the Application of Ultrafiltration Membranes in Water Treatment
I. What Are Ultrafiltration Membranes?
Ultrafiltration membranes were among the first polymeric membranes to be developed; they are microporous filtration membranes with a nominal pore size range of 0.001 to 0.02 micrometres. When appropriate pressure is applied to one side of the membrane, the solvent and some of the lower-molecular-weight solutes in the solution pass through the tiny pores of the ultrafiltration membrane to the other side, whilst higher-molecular-weight solutes or certain emulsified micelle clusters are retained, thereby achieving filtration and separation.
In the field of water treatment, compared with other filtration technologies, ultrafiltration membrane technology offers higher impurity removal efficiency, with a filtration precision of up to 99.99 per cent, effectively removing the vast majority of harmful substances from water; furthermore, as it uses little or no chemical agents, it effectively prevents secondary contamination of the water, resulting in superior treated water quality. From an operational perspective, filtration systems based on ultrafiltration membrane technology are highly automated, simple and reliable to operate, requiring only two operations: ‘on’ and ‘off’. As ultrafiltration membrane materials possess strong chemical stability and are resistant to acid and alkali corrosion as well as high temperatures, they can undergo high-temperature sterilisation and disinfection, making them suitable for a wide range of applications.
1. Ultrafiltration Membrane Technology and Its Characteristics
Principle of Ultrafiltration Membrane Technology
Ultrafiltration membrane technology is a membrane permeation separation technique with a filtration capacity lying between nanofiltration and microfiltration. Its operating principle is as follows: when a solution passes through a semi-permeable membrane, under pressure, small-molecule substances in the solvent and solute can pass through the membrane to the other side, whilst large-molecule substances and colloids in the solute are retained because they cannot pass through the membrane pores. As the solution continues to flow, an increasing amount of material is retained on the membrane, Consequently, to achieve ultrafiltration, greater pressure must be applied to the solvent; at the same time, the substances forming on the membrane surface exhibit certain chemical properties, which can also retain and decompose certain pollutants, thereby purifying the water.
As macromolecules continue to accumulate on the membrane surface, the filtration rate gradually decreases, giving rise to the phenomenon of ‘concentration polarisation’. To ensure that ultrafiltration can proceed continuously and effectively, stirred ultrafiltration units are commonly used in practice to counteract ‘concentration polarisation’.
Characteristics of Ultrafiltration Membrane Technology
Compared with other water treatment technologies, ultrafiltration membrane technology offers many unrivalled advantages:
Firstly, ultrafiltration membranes possess high chemical stability and are resistant to high temperatures, acids and alkalis; consequently, they are not particularly demanding in terms of feed water quality and are highly versatile;
Secondly, the principle behind ultrafiltration membrane technology is straightforward, making it easy to automate, thereby saving labour; it is also simple to operate, easy to maintain, and operates safely and stably;
Thirdly, as ultrafiltration is a physical treatment method, no chemical additives are required during the water treatment process, thereby effectively preventing secondary pollution of the water;
Fourthly, ultrafiltration is highly efficient and capable of treating large volumes of water; it demonstrates particularly high treatment efficiency when treating moderately polluted municipal drinking water;
Applications of Ultrafiltration Technology in Environmental Protection Water Treatment
Purification of Urban Drinking Water
With social development, public demands for drinking water safety are becoming increasingly stringent. At the same time, however, pollution at urban water sources in China is becoming increasingly severe, and the quality of water abstracted directly from these sources is increasingly failing to meet drinking water standards; consequently, urban drinking water must be purified.
Urban drinking water is primarily sourced from two types: groundwater and surface water. Whilst the mechanisms of pollution differ between these two sources, the pollutants are mainly asexual organisms, bacteria, fungi, viruses and suspended solids.
Traditional drinking water purification methods can inactivate and remove microorganisms and bacteria, as well as purify water of micron-sized suspended particles; ultrafiltration membrane technology, building on this, can also effectively remove nanoparticles, thereby delivering higher-quality treated water, which is of great significance to the drinking water health of urban residents.
Seawater Desalination
As a non-renewable resource, the Earth’s supply of fresh water available for human consumption is becoming increasingly depleted, and water scarcity has become one of the most pressing issues facing humanity today. Seawater desalination is regarded as an effective means of addressing the drinking water crisis. Currently, the most widely researched desalination technology globally is electrodialysis. Although electrodialysis is considered an effective method for seawater desalination, it entails high operating costs and has a relatively low recovery rate. With technological advancements, ultrafiltration membrane technology has begun to be applied in the field of reverse osmosis seawater desalination; its excellent separation performance and physicochemical properties have further enhanced desalination efficiency whilst significantly reducing energy consumption.
Treatment of Electroplating Wastewater
The electroplating industry generates vast volumes of wastewater containing high concentrations of heavy metals such as hexavalent chromium, copper and nickel. This wastewater is extremely hazardous and has very low biodegradability. In practice, treatment processes such as iron oxidation and electrolysis are commonly employed; however, the iron oxidation method produces large quantities of sludge, which requires further treatment. Whilst electrolysis is effective at treating electroplating effluent, its high operating costs make it unsuitable for widespread implementation. The combined use of ultrafiltration and reverse osmosis technologies is considered an effective method for treating electroplating effluent. By utilising these two membrane technologies, the majority of heavy metals, organic carbon and nitrates can be removed from the effluent; furthermore, the use of ultrafiltration membranes reduces fouling of the reverse osmosis membranes, thereby extending their service life.
Recovery of Wastewater from the Food Industry
In addition to improving the quality of the treated effluent, ultrafiltration membrane technology can also concentrate and recover large quantities of useful solid substances; the most typical application is in the food industry. Wastewater generated by the food industry contains large amounts of fats, proteins, starch and yeast; if these substances are discharged into the external environment, they will not only cause environmental pollution but also result in significant waste. Consequently, utilising ultrafiltration membrane technology to retain useful components from the effluent, whilst simultaneously separating substances such as BOD and COD from the water, and extracting and recovering the separated solid matter, can yield significant economic benefits for enterprises.
Treatment of Oily Wastewater
The primary sources of oily wastewater include crude oil spills, slaughterhouse effluent and domestic wastewater; its main constituents are free oil, dispersed oil, emulsified oil and heavy oil. Oil separators are commonly used for treating oil-containing wastewater; however, as they are unable to treat emulsified oil, the dissolved air flotation (DAF) process is often employed as a supplementary treatment method. As emulsified oil molecules are typically large, ultrafiltration membrane technology can be utilised: by passing the oil-containing wastewater through ultrafiltration membranes under pressure, the emulsified oil and other macromolecular pollutants are retained, resulting in a high removal efficiency.
Urban Sewage Reuse
The reuse of municipal wastewater is a key measure for alleviating pressure on urban water supplies. This involves treating domestic sewage to meet reuse standards, after which it is utilised for urban landscaping irrigation and municipal reclaimed water systems. The use of ultrafiltration membrane technology enables rapid treatment of municipal wastewater to meet regulatory standards. As municipal wastewater is generally readily biodegradable, the Cyclic Activated Sludge System (CASS) is often combined with ultrafiltration membrane technology in practice to improve effluent water quality. Under conditions of a hydraulic retention time of 12 hours, this method achieves a COD removal rate of over 86 per cent and an ammonia-nitrogen removal rate of over 90 per cent, with the effluent pH ranging from 7.25 to 7.89, thereby meeting urban water reuse standards.
Conclusion
In summary, ultrafiltration membrane technology is a key technology in the field of water treatment for environmental protection projects, with extremely broad application prospects in urban sewage treatment, the treatment of various types of industrial effluent, and the recovery of useful substances. Currently, research into ultrafiltration membrane technology is focused, on the one hand, on developing more efficient ultrafiltration units and, on the other hand, on combining the technology with other water treatment techniques based on the characteristics of the influent water quality, in order to improve effluent quality.
