D10: Impact of Contaminants - Hydromod

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Integrated Water Resource Management for Important Deep European Lakes and their Catchment Areas EUROLAKES D10: Impact of Contaminants FP5_Contract No.: EVK1-CT1999-00004 Version: 4.0 Date: 25/07/01 File: D10-vers.4.0.doc Page 108 of 136 4. Filtration (mostly rapid filtration, formerly slow sand filtration) 5. Polishing processes (ozonisation, activated carbon adsorption of odours and taste) 6. Disinfection These processes are combined according to requirement and have to be completed according to other substances contained in the raw water. In developing countries the processes described above are still being applied as the only treatment for raw water. In Europe modern sea and river water plants are more and more applying ozone and activated carbon as completing and substituting treatment stages to the traditional ones. Ozone • can support coagulation and flocculation by converting large monopolar molecules into smaller polar compounds • can crack organic molecules like phenoles • can oxidise soluble gases as sulphides and mercaptanes, in order to reduce odour and taste • can de-colourise by destroying colloids and colours of humic acids • can rise the BOD/COD ratio what increases the biodegradability of organic substances Ozone is usually used as pre-ozonisation prior to flocculation and filtration and as main ozonisation after flocculation and filtration in order to reduce odour colour and taste and for disinfection. Ozonisation is mostly applied in combination with activated carbon in order to reduce residual ozone in the treated water. [LEITZKE, 1999, a presentation of applied ozone processing from the company Philaqua, Germany] Activated carbon Activated carbon has a broad spectrum of adsorptive activity, as most organic molecules are retained on its surface. The hardest to retain are the molecules which are the most polar and the linear ones with a very low molecular weight.. Molecules which are slightly polar, generating taste and smell, and molecules with a relatively high molecular weight are for various reasons well adsorbed on carbon. Beyond these adsorbent properties activated carbon is also a bacteria support that is capable of breaking down a fraction of adsorbed phase. Thus, a part of the support is continuously being regenerated and capable of freeing sites, allowing new molecules to be retained. (DEGREMONT, Water Treatment Handbook, 1991) The fact that activated carbon is also a bacteria support lead to the development of biological activated carbon, which operates similar to trickling filters, allowing the growth of bacteria which can specialise in metabolising difficult biodegradable substances. Activated carbon is applied in two forms: Powdered activated carbon (PAC) and granular activated carbon.(GAC). PAC takes the form of grains between 10 and 50µm and its use is generally combined with clarification
Integrated Water Resource Management for Important Deep European Lakes and their Catchment Areas EUROLAKES D10: Impact of Contaminants FP5_Contract No.: EVK1-CT1999-00004 Version: 4.0 Date: 25/07/01 File: D10-vers.4.0.doc Page 109 of 136 treatment. It is added continuously to the water together with flocculation reagents, it enters the floc and is then extracted from the water by sedimentation. The advantage of PAC is that adsorption is rapid since large surface area of powder is directly accessible, that it is constantly exchanged by new one and that quantities can be dosed according to requirement. For that reason it can also eliminate difficult to adsorb substances like pesticides. The big disadvantage of PAC is that it can not be regenerated and that it is difficult to remove final traces of impurities without adding excessive amounts of activated carbon. It is therefore used when intermittent or small quantities are required. Granular activated carbon (GAC) is used as filter bed through which the water to be treated passes, leaving behind the impurities which are thus extracted methodically. The water, as it progressively loses its pollutants, encounters zones of activated carbon which are less and less saturated and therefore more and more active. Some examples of applied surface water treatment processes in European cities are shown in Figure 11-6.
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D10: Impact of Contaminants - Hydromod

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