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ePaper created 2013-09-24, 11:56:35 | version 1.25.20
Initial situation There is only a limited supply of drinking water available as a resource. At the same time, since the beginning of industri- alization the worldwide level of water consumption has in- creased on a massive scale and will continue to grow further along with the tremendous technological development in Asia. 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 manage- ment and, especially, efficient methods for treating wastewa- ter are becoming more important. Conventional wastewater treatment 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 biode- gradable 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 re- move these compounds, physical-chemical methods such as advanced oxidation processes (AOPs) with ozonation, UV ir- radiation or the addition of iron salts can be used in combina- tion with hydrogen peroxide. However, these purification pro- cesses generally require chemical additives that are classified as hazardous substances and that have to be disposed of as special waste. Plasma processes for water purification The use of atmospheric pressure plasma processes could pro- vide an environmentally compatible and cost-effective alter- native. Ions, highly reactive radicals and short-wave radia- tion, that degrade the contents of the wastewater are formed simply by applying a high voltage, which is igniting a plasma discharge in ambient air or oxygen. This renders the use of chemicals and their subsequent disposal unnecessary. The aim of the EU-funded “WaterPlasma” project was therefore to de- velop a plasma process for purifying water and a suitable plas- ma reactor as a prototype. New type of plasma reactor 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 (Fig. 1). 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 wa- ter. By means of their high oxidation potential these radicals and short wavelength UV radiation break down the dissolved contaminants until they are mineralized (Fig. 3). A decisive advantage of the plasma decontamination method is its durability. The water surface is constantly renewed and, unlike conventional UV treatment, is not soiled with contami- nants, which normally reduces the efficiency within a short time. With the plasma water treatment there is no need for elaborate cleansing of surfaces to remove biofilms and other surface contaminants. 10 6 PLASMA-CHEMICAL DEGRADATION OF ORGANIC CONTAMINANTS IN WATER Dr. rer. nat. Michael Müller ENVIRONMENT DBD plasma discharge Thin film of falling water Grounded electrode (tube of stainless steel) Recirculating con- taminated water Inflow of gas Outflow of gas Gear pump Dielectric barrier (glas) HV electrode (mesh of copper) ˙OH N N N S + CH3 CH3 CH3H3C N N NH2 S OCH3 CH3 CH3H3C = 1 2 3