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Commentary Article

tain components and additives may need to be exported, and these additives generally range from 2 to 30 % of the weight of the cementitious materials.

8 Concluding Remarks Although the application of absorptive cements does not destroy toxic chemicals as incineration would do, it effectively isolates and immobilises these chemicals within the biosphere. In this respect, it is an acceptable method to solve problems arising from toxic chemicals. Nearly 15 years have passed since the first use of absorptive cements and no failure has been reproted so far. We think that many unsolved problems, not only in South Africa, but on the whole continent of Africa, could thus be amended.

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References GESSBERGER, H. L.: Deponie-Ablagerung von Abfiillen. Berlin, EFVerlag 1987, S. 4 1 9 - 4 4 7 , and POJASEK, R. B.: E n v i r o n m e n t a l Science and T e c h n o l o g y 12, 382 (1987) Council Directive E E C , April 1992 D e p a r t m e n t of W a t e r Affairs and Forestry 1994. W a s t e Managem e n t Series. R e q u i r e m e n t s for the H a n d l i n g and Disposal of Haz a r d o u s Wastes SELL, N. J.: Pollution Engineering, A u g u s t 1988, p. 44 PURTHER-CATHCART, N . L.: Investigation of test m e t h o d s for solidified waste characterisation, in: W a s t e M a n a g e m e n t Division, Env i r o n m e n t C a n a d a , O t t o w a 1986, and DENT, C. G.: International Conference o n N e w Frontiers for H a z a r d o u s Waste M a n a g e m e n t , Pittsburgh 1987, p. 2 9 5 Factors affecting stabilisation/solidification of hazardous waste, EPA, USA Cincinnati, 1986

News & Views Cleanable Particulate Filter for Diesel Engines The Karlsruhe Research Center developed, and tested successfully in long-term experiments, metalbased, temperature and corrosion resistant particulate filters for Diesel engines, which can be regenerated electrically. They are able to reduce dearly the emissions of noxious particulates. The most economical thermal engine used in commerdal vehicles is the Diesel engine. One of its characteristics increasingly coming under criticism is the emission of particulates carrying hundreds of different organic compounds. In Germany, vehides equipped with Diesel engines annually emit 7 000 tons of these airborne compounds. According to measurements conducted by the Landesanstalt ffir Immissionsschutz (State Institute for Environmental Impact Control) in Essen, one cubic meter of air may contain up to 42 micrograms of particulates during peak traffic load periods.

The Microelectronic Nose Scientists at the Institute for Radiochemistry of the Karlsrnhe Research Center (the former Karlsrnhe Nudear Research Center, KfK) succeeded in developing a versatile, low-cost multi-sensor microsystem for the simultaneous detection of atmospheric pollutants. In the opinion of Dr. Joachim Goschnick and Dr. Peter Althainz, one out of many possible applications of this dectronlc nose would be in monitoring and controlling the air quality inside vehides or garages. The microsensor can ensure, for instance, that the outside air flow into a car is interrnpted as soon as pollutant concentrations become too high in a traffic congestion. In controlling the ventilation of vehicles, also of garages or tunnels, especially the traffic-induced components, carbon monoxide (CO), sulfur oxides

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The noxious particulates are only a few ten thousandths of a millimeter in size, become airborne, like gas molecules, and are inhaled by people. The sticky compound made up of finely dispersed particulates, organic airborne pollutants, and plant pollen has a considerably stronger allergic effect than pollen without these additions. Thus, particulates in the exhaust emissions of motor vehides activate poUen as a source of hay fever. Effective filters are needed to remove particulates from Diesel engine emissions, as is also evident from the new legal limits on particulate emissions. The Diesel particulate filter developed at the Institute for Technical Chemistry of the Karlsruhe Research Center is able to prevent the release of particulates. A metal fiber material stable up to approx. 1 100 ~ a technological spinoff from nuclear filter devdopments, can be used for this purpose in a so-called filter candle. This filter candle retains the particulate constituents of Diesel exhaust emissions nearly quantitatively. Particulate filters for Diesel engines must be regenerated at regular intervals in order to keep within tolerable limits the exhaust gas back pressure, which rises with increasing particulate loading of the filter. The particulates are remov-

ed from the filter by electric heating under excess air conditions, which burns practically 100 % of the particulates to CO 2. A prototype filter was run through approx. I 000 loading and regeneration cycles on a laboratory test bench. The particulates required for the test were generated by burning a mixture of Diesel fuel and engine oil to simulate the effect of engine oil additives causing ceramic particulate filters to fail in the long run. The cleanable particulate filter candle developed by the Karlsruhe Research Center was tested for ten weeks in a long-term test on an engine test bench. In the test, the filter candle was loaded with particulate-bearing exhaust gas for 120 hours a week, and the particulates were burned off. After the test campaign, no measurable increase in the pressure drop across the filter candle was detected. There was some discoloration, but it had no impact on filter efficiency. "This filter candle can solve the Diesel particulate emission problem in a simple way," conclude Hans-Georg Dillmann and Dr. Jfirgen Furrer, the inventors of the deanable particulate filter. Source: Regine Hedderich Forschungszentrnm Karlsruhe

(SOx), nitrogen oxides (NOw), benzene, and other hydrocarbons, must be measured reliably and distinguished from other constituents of the air. Such gas measurements can be conducted simply by utilizing the dependence on air composition of the conductivity of semiconducting metal oxides. "In our concept of the multi-sensor system," says Dr. Goschnick, "we are using 39 gas sensors of different sensitivities, which control the access of gases to the metal oxide conductivity detector by means of membranes of selective permeability. The membrane at the same time protects the sensitive detector. The detector 'miffs' the type of gas in the same way in which the human nose operates, which is able to distinguish more than 1 000 smells by means of only some ten types of olfactory receptors. Each type of receptor reacts slightly differentyl, and each gas causes a pattern of receptor signals to be generated which allows the gases to be identified in the brain."

contains 39 detector elements, whose sensitivities can be differentiated in various ways. One way is by setting different temperatures of the elements, another way is by varying the membrane properties; both these steps produce a different response behavior of the individual sensor elements. Pattern recognition is performed in a microprocessor, whose program simulates pattern recogmtion in the brain on the basis of synthetic neuronal networks. The actual sensot chip is only 8 x 9 ram 2 in size. When it is integrated into the measurement and evaluation dectronics, the resultant unit is not bigger than a cigar box. Some preliminary laboratory samples were used to demonstrate the ability to distingmsh among various organic vapors. After a training phase of ten minutes per gas, the unit is able to recognize the entire gas combination within a minute.

In accordance with this principle, the microsensor system developed at the Karlsrnhe Research Center

Source: Regine Hedderich Forschungszentrum Karlsruhe

ESPR-Environ. Sci. & Pollut. Res. 2 (3) 1995