Purifying Water with Plasma
Importance of water
There is only a limited supply of drinking water available as a resource. At the same time, since the beginning of industrialization, the worldwide level of water consumption has increased on a massive scale and will continue to grow further along with the tremendous technological development. Today, the provision of water in potable quality, above all to supply a growing world population with clean water, is a global challenge. For this reason, improved water management and, especially, efficient methods for treating wastewater are becoming more important. With the growing population and depleting natural resources, we now face an unprecedented global water crisis. Water scarcity and water pollution have become critical long-term problems that increasingly are severely impacting industry and the quality of human health around the globe. As a result, industry and water suppliers are facing an enormous challenge to replace existing complicated and costly treatments with simple and cost-effective new technologies. In particular, the energy intensiveness, cost, and ineffectiveness of current water treatment technologies present increasing technical and economic challenges to the food and beverage and pharmaceutical industries.
Scarcity of clean water
The pollutants contained in wastewater are conventionally removed in central treatment plants after separation of the solid content by biological and chemical means. However, to an increasing extent the substances that are not easily biodegradable remain issues of concern. These include, for example, medications in the wastewater from clinics, hospitals and old people’s homes, organic halogen compounds and cyanides from industrial plants or pesticides used in agriculture. To remove these compounds, physical-chemical methods such as advanced oxidation processes (AOPs) with ozonation, UV irradiation or the addition of iron salts can be used in combination with hydrogen peroxide. However, these purification processes generally require chemical additives that are classified as hazardous substances and that have to be disposed of as special waste. Scientists and engineers are working on a method to purify water with the fourth state of matter - plasma. The new technology, which produces reactive radicals that can attack organic contaminants such as pesticides and pharmaceuticals, will help solve a problem not currently being addressed in conventional treatment methods that rely on filtration and chlorine. The technology, originally envisioned as a point-of-use system for underdeveloped countries, could be scaled up to a larger mechanism that would be implemented as a stage in the conventional treatment process. The study paves the way for the next generation of portable water purification devices, which could provide relief to the 780 million people around the world who face every day without access to a clean water supply.
The large industrialized purification plants are just not practical – they consume a large amount of energy and have high labour costs, making them very expensive to run. Small portable purification devices are increasingly recognized as the best way to meet the needs of clean water and sanitation in developing countries and in remote locations, minimizing the risk of many serious diseases. Some smaller portable devices do already exist; however, because they rely on reverse osmosis and thermal processes, they are able to remove salt ions but are unable to filter out organic contaminants from the briny water found in some river and lake systems. For people in remote locations, briny water can sometimes be the only available water source, that's why it's important to not only be able to remove salts from water, but to also be able to put it through a process of purification. The other downside of existing portable devices is that they require a continuous power supply to operate their thermal processes. On the other hand, the new membranes could be operated as a rechargeable device.
Plasma processes for water purification
Plasma-based sources can emit intense beams of UV & X ray radiation or electron beams for a variety of environmental applications. For water sterilization, intense UV emission disables the DNA of microorganisms in the water, which then cannot replicate. There is no effect on taste or smell of the water and the technique only takes about 12 seconds. This plasma-based UV method is effective against all water-born bacteria and viruses. Intense UV water purification systems are especially relevant to the needs of developing countries because they can be made simple to use and have low maintenance, high throughput and low cost. Plasma-based UV water treatment systems use about 20,000 times less energy than boiling water. Innovative water treatment system is capable of destroying waterborne microorganisms by transforming a continuous stream of polluted water into non-thermal plasma. This system achieves total elimination of pathogens at low energy consumption. The microorganisms contained in the water are affected by a number of plasma state phenomena, such as electric fields, ultraviolet and infrared radiation, shock waves and high temperatures. This results in the destruction of all microorganisms by thermal effect of plasma, ionization and irreversible electroporation, among others.
The use of atmospheric pressure plasma processes could provide an environmentally compatible and cost-effective alternative. Simply applying a high voltage, which is igniting a plasma discharge in ambient air or oxygen, forms ions, highly reactive radicals and short-wave radiation that degrade the contents of the wastewater. This renders the use of chemicals and their subsequent disposal unnecessary. The aim was therefore to develop a plasma process for purifying water and a suitable plasma reactor as a prototype. The special design of the plasma reactor insures an effective transmission rate of the highly reactive species formed in the plasma to the contaminated water. This is achieved by forming the plasma in direct contact with a flowing water film. The water to be purified falls through the plasma zone by the force of gravity, directly onto the outer surface of a grounded electrode (stainless steel cylinder). Hydroxyl radicals, among others, are created in the plasma and transmitted to the water. By means of their high oxidation potential these radicals and short wavelength UV radiation break down the dissolved contaminants until they are mineralized.
Electro-plasma water treatment
Ozonation, or electro-plasma wastewater treatment, is designed to disinfect and purify natural waters and wastewater. Electro-plasma treatment removes radionuclides, oil, surfactants, fats, dyes, heavy metals, and other compounds, both of organic and inorganic origin, from the treated waters. Plants currently in operation in Ukraine have a throughput of 500 m3/day, and are designed to be expanded by the addition of further 500 m3/day units. The units currently in use have an areal requirement of 8 m2 for the wastewater treatment unit, and an additional 4 m2 for the drinking water unit. The wastewater treatment units have a power demand of 0.4 to 1.0 kW/m3, and a mass of 1000 kg. The drinking water module has a mass of 500 kg. The electro-plasma wastewater treatment systems used comprise an impulse electromagnetic activator; a counter-turbine ejector; an electro-hydro-gas-impulse reactor; an electro-gas-ionic stabilizer; and a control station. Water (or wastewater) undergoes primary mechanical treatment and comes to the impulse electromagnetic activator (EMA), where it undergoes further treatment by pulse electromagnetic field. This treatment increases the solubility of gases, reduces the scaling capacity, and increases the sorption capacity of suspended matter, increasing coagulation rates by up to 45%. The effluent then flows into the counter-turbine ejector, where, rotating around its axis, the flow pattern changes from laminar to turbulent flow. Simultaneously, the effluent is injected, through the ejector, with ozone, which oxidizes organic compounds and bacteria. The gas-liquid effluent is in a state of slow cavitation (about 7 W/cm2 intensity) when it enters the electro-hydro-gas-impulse reactor (EHGIR). There, the effluent undergoes treatment with pulsed electric discharges which, as a result of the impact of short shock waves (1 to 50 ms at pulse pressure about 20 000 kgf/cm2), increases the solubility of ozone-enriched air by more than 30 times, forming a suspended matter flocculant that is not less than 0.2 m in diameter. The effluent is also subjected to UV-irradiation to remove bacteria and other pathogens. The flocculants are removed by electro-coagulation and flotation in the electro-gas-ionic stabilizer (EGIS), decreasing the COD, and removing oil and grease. Chloride ions are transformed into chlorine during this stage of the treatment process, providing a further element of pathogen protection prior to the discharge of the treated effluent.
Another team of researchers has shown that water purification membranes enhanced by plasma-treated carbon nanotubes are ideal for removing contaminants and brine from water. According to researchers, these membranes could be integrated into portable water purification devices the size of a tea pot that would be rechargeable, inexpensive and more effective than many existing filtration methods. Contaminated water would go in one end, and clean drinkable water would come out the other. The study showed that carbon nanotube membranes were able to filter out ions of vastly different sizes – meaning they were able to remove salt, along with other impurities. Scientists attribute the success of the new membranes to the unique properties of plasma treated carbon nanotubes. Firstly, ultralong nanotubes have a very large surface area that is ideal for filtration. Secondly, nanotubes are easy to modify, which allows tailoring their surface properties through localized nanoscale plasma treatment. Now that the researchers have proven the effectiveness of the method, they plan to extend their research to investigate the filtration properties of other nanomaterials. They will begin by looking at graphene, which has similar properties to carbon nanotubes, but could be made considerably denser and stronger.
Electro-plasma treatment technologies are suitable for numerous operations, and can be easily added onto existing treatment systems. The technology is well suited to producing product water that may be recycled on-site or to produce additional final product waters. Ozonation has a further advantage in that it does not require the effluent to be treated with alum, polyacrylamide flocculent aids, lime, chlorination, or other reagents, which require replenishment, preparation, and additional treatment before water consumption. Ozonation is a reagent-free purification method that is up to 100% effective in removing bacteria and other contaminants, including radionuclides, heavy metals, nitrites, and nitrates, with relatively low power consumption rates (0.4 to 1.0 kWh/m³ of wastewater, depending on concentration of contaminants). The technology is well-suited to providing process water for reuse. The process can be highly automated, and outputs can be tailored to specific requirements.
A method for comparing the efficiency of advanced oxidation processes is the measurement of the energy input that is required to decolor methylene blue by one order of magnitude. Using the plasma process 4 g/kWh is achieved. This value is nearly one order of magnitude better than the energy efficiency measured with a UV H2O2 treatment of methylene blue. In another application of the water plasma method, the project showed that cyanide is broken down by 90 percent within 2 minutes. Unlike well-established advanced oxidation processes the plasma process for water decontamination has no barrier between the plasma and the medium that is to be purified. It, therefore, requires almost no maintenance and is characterized by a long life. A very high degree of efficiency is achieved even without the introduction of hazardous substances such as hydrogen peroxide or ozone. As a result of the project, a demonstrator is now available that is suitable for purifying substantial quantities of contaminated water (240 L/h). At the moment further possible applications for the procedure are being examined.
Acknowledgement: The use of information retrieved through various references/sources of internet in this article is highly acknowledged.
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