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Rimozione di nutrienti e microinquinanti attraverso un processo a fanghi attivi assistito da membrana e carboni attivi

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Nutrients and micropollutants removal by a membrane bioreactor (MBR) and activated carbons 14 CHAPTER 1 MEMBRANES AND MEMBRANE BIOREACTORS 1.1 Introduction Conventional aerobic treatment methods, such as activated sludge processes, are generally used to treat wastewater flows containing oxidizable substances (e.g. organics, reduced nitrogen). A drawback of the conventional activated sludge process is that sludge concentration limits volumetric load, with a sludge concentration of 5-8 kg/m 3 being the maximum (Stephenson et al., 2000). In spite of the fact that wastewater flow is often low, relatively large specific volumes in conventional plant design are required. In some cases the space available is not sufficient, such as for specific industrial wastewaters. In these cases compact biological systems are often applicable. The main biological compact system is the membrane bioreactor. In this chapter the main features of membranes and membranes bioreactors for wastewater treatment are described. 1.2 The membrane A separation membrane is a thin barrier layer through which fluids or solutes are selectively transported under the influence of a pressure gradient, a chemical concentration gradient or a difference in electric potential called the driving force. Separation of a mixture occurs if there is a significant difference in the transport coefficients through the membrane for the components of the mixture. Membranes can be classified by: ‰ Nature of the membrane (i.e. natural versus synthetic); ‰ Structure of the membrane (i.e. porous versus non porous, morphological characteristics); ‰ Application of the membrane (e.g. gaseous phase separation, gas-liquid, liquid-liquid, etc.); ‰ Mechanism of membrane action (i.e. adsorptive versus diffusive, ion exchange, osmotic or non-selective/inert membranes. 1.2.1 Materials The principal objective in membrane manufacture is to produce a material of reasonable mechanical strength and which can maintain a high throughput of a desired permeate with a high degree of selectivity. These last two parameters are mutually counteractive since a high degree of selectivity s normally only achievable using a membrane having small pores and thus an inherently high hydraulic resistance (or low permeability). The permeability increases with increasing density of pores, implying that a high material porosity is desirable. The overall membrane resistance is directly proportional to its thickness. Finally, selectivity will be compromised by a broad pore size distribution. It stands to reason, therefore, that the optimum physical structure for any membrane material is based on a thin layer of material with a narrow range of pore size and a high surface porosity.

Anteprima della Tesi di Laura Innocenti

Anteprima della tesi: Rimozione di nutrienti e microinquinanti attraverso un processo a fanghi attivi assistito da membrana e carboni attivi, Pagina 9

Tesi di Dottorato

Dipartimento: Dipartimento di Scienze Ambientali

Autore: Laura Innocenti Contatta »

Composta da 183 pagine.

 

Questa tesi ha raggiunto 4319 click dal 01/04/2005.

 

Consultata integralmente 4 volte.

Disponibile in PDF, la consultazione è esclusivamente in formato digitale.