Kintetic study of the mesophilic and thermophilic sewage sludge anaerobic digestion

Wastewater treatment plants generate sewage sludge as a by-product of the physical, chemical and biological processes used during the wastewater treatment. In these plants, anaerobic digestion has been used, for more than a century, to stabilize the sewage sludge produced. At the present time, most anaerobic digesters are operated at mesophilic conditions. However, the need to improve the process feasibility, by means of increasing the biogas yield and reducing the sludge disposal costs, has increased the interest in thermophilic conditions. In this topic, the thermophilic anaerobic digestion offers some potential advantages over conventional mesophilic anaerobic digestion: (i) an increase of the biological and chemical reaction rates, (ii) an increase of the organic matter removal, (iii) a higher solubilization of the particulate organic matter and (iv) an enhancement of the pathogenic organism destruction.

Further Authors:
M. Peces - Department of Chemical Engineering, University of Barcelona
S. Astals - Department of Chemical Engineering, University of Barcelona

However, its industrial implementation have been limited by some drawbacks that are unavoidable: (i) an elevated energy requirement for heating the digester, (ii) a higher risk of process destabilization, (iii) a poorer sludge dewaterability and (iv) a higher odour potential. The aim of this study was to improve the knowledge of the metabolic pathways involved in the mesophilic and thermophilic sewage sludge anaerobic digestion. To achieve this goal, the volatile fatty acids profile in a continuous stirred tank reactor was evaluated during 48 hours after the feeding. , moreover, the performance of mesophilic and thermophilic sludge digestion was compared to give an overview of the process. In the present study, two jacketed semi-continuous stirred tank reactors with a working volume of 2.5-L were used. The operational temperature of the mesophilic digester (37ºC) and the thermophilic digester (55ºC) were ensured by circulating water from a heated water bath, while the digesters medium was continuously mechanically-stirred at 70 r.p.m. The hydraulic retention time of the mesophilic digester was set at 12 and 9 days, whereas the hydraulic retention time of the thermophilic digester was fixed at 9 and 6 days. The digesters were, once a day, manually fed and purged; where the raw sewage sludge, used as food supply, was a mixture of primary sludge (60% in weight basis) and waste activated sludge (40% in weigh basis) adjusted, with deionised water, to obtain a solid concentration of 30 g TS L-1. The results of the study showed the influence of the temperature conditions and hydraulic retention time in process performance. Those results were further understood when the volatile fatty acids profile (analysed at 0, 1, 3, 6, 12, 24, 28, 32 and 48 hours after feeding) was superimposed with the biogas flow rate profile. With regard to the results obtained, the nearly constant specific biogas yields and removal efficiencies, for both temperature conditions, when the hydraulic retention time was decreased; together with the no accumulation of volatile fatty acids, suggested that the anaerobic digestion process was limited by disintegration-hydrolysis step. Furthermore, thermophilic digested sludge exhibited poorer dewaterability properties than mesophilic digested sludge. Finally, the volatile fatty acids behaviour showed clearly differences between mesophilic and thermophilic anaerobic digestion. Mesophilic digestion presented low acid concentrations, with acetate as the predominant acid; in contrast, thermophilic digesters showed higher levels of volatile fatty acids prevailing propionate and i-valerate over the others. Despite these differences, a clear correlation between total volatile fatty acids evolution and biogas flow rate could be observed.



Copyright: © European Compost Network ECN e.V.
Quelle: Orbit 2012 (Juni 2012)
Seiten: 9
Preis inkl. MwSt.: € 9,00
Autor: Prof. Joan Mata-Álvarez

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