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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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Abstract 7 accumulating organisms (PAO) was not observed and phosphorous content in biomass reached a maximal concentration of 2.8%. This value is probably too low to claim the presence of PAO in activated sludge. A provisional simplified mathematical model for the simulation of the nutrients (COD and nitrogen) removal processes has been developed. It satisfactory simulated the experimental data (real process). This first form will be developed with more detailed differential equations in further studies. Removal of total coliforms was very effective: a 4.7-5.1 log reduction was observed. Escherichia coli was never found in the permeate. The variation of biomass concentration in the bioreactor did not affected the removal efficiency of pathogens The values of the kinetic constants for the MBR process were experimentally determined based on data collected during the five (pseudo) steady-state conditions reached in the five periods, from 1 to 5. The biomass yield, Y, and the decay coefficient kd, were found to be equal to 0.24 gMLVSS/gCODremoved and 0.01 day -1 , respectively. These values are slightly lower than those generally found in conventional activated sludge. Yobs value showed to decrease exponentially with increasing SRTs. The application of high SRTs determined a decrease in the volatile matter (bacteria) content in activated sludge. The MLVSS/MLSS ratio passed from 80% to 53% and the biomass activity accordingly decreased: the ammonium utilisation rate doubled when passing from some 1000 down to 200 days or less of SRT. This means that working with high biomass concentrations in the bioreactor, let say over 15 g/l, determines the oxidation of sludge in the bioreactor and therefore useless expenses for aeration. Moreover, a decrease in biomass viability was observed. Therefore, it is more convenient to operate in an interval of biomass concentration of some 8-12 g/l. This determines also other advantages in treatment process since this biomass concentration makes it much easier to keep the membranes clean, improve the oxygen transfer efficiency and mix and handles the activated sludge. The removal efficiencies and the fate of both metals and organic compounds in the MBR and in a conventional activated sludge process were studied. The removal efficiency from treated wastewaters for the MBR system was generally high for metals (> 75%). This efficiency was generally 10-15% higher than that of a conventional activated sludge process. The increase in metals removal efficiency was due to the capability of the membrane to perfectly retain the suspended solids from the bioreactor effluent. These results were nearly equivalent when considering periods at 9 or 16 – 18 g/l of biomass in the MBR, so it seems sensible to operate at the lower concentration of MLSS to reduce the treatment process expenses (aeration, mixing, and stabilisation). The MBR allows improving the exploiting of the bio-sorption capability of the activated sludge for metals (especially heavy metals) and can be considered a suitable technology for the removal of this class of pollutants from wastewaters. According to the removal efficiencies found in the MBR experimentation, three different groups of metals can be individuated: the easily removable, the metals which showed removal yields > 75%, like Al, Ag, Ba, Cd, Cr, Cu, Fe, Hg, Sn, the metals only partially removable, with removal yields in the range 40 - 60%, like Co, Mn, Ni, Pb, V and Zn and the metals hard removable, like, As, B and Se, which showed removal yields < 25%. Arsenic in particular showed the worst removal efficiency and was the main problem encountered in the MBR experimentation since it exceeded the discharge limits imposed for very sensitive areas like the lagoon of Venice. The metals concentrations in sludge were equal or higher in the MBR experimentation at 9 g/l of biomass compared to the conventional process (CASP), but those concentrations decreased increasing the solid retention time (SRT) of the MBR system, probably because a steady state condition was not really reached during the experimental periods 2, 3 and 4, when very high SRTs were applied to the MBR (i.e., up to 1000 days).

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 2

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.