Effect of biomass features on oxygen transfer in Conventional Activated Sludge and Membrane BioReactor systems

Abstract

The present study reports the results on the comparison between the oxygen transfer efficiency in a conventional activated sludge (CAS) and membrane bioreactor (MBR) system. The oxygen transfer was evaluated by means of the oxygen transfer coefficient (kLa)20 and α-factor calculation, under different operating conditions and with different activated sludge features, including total suspended solids (TSS) concentration, extracellular polymeric substances (EPS) content and composition, sludge viscosity and size of the flocs. The (kLa)20 and α-factor showed an exponential decreasing trends with TSS in the CAS and MBR, even if it was noted that the (kLa)20 dependence on TSS was stronger in the CAS than the MBR. Under the typical operating TSS concentration for CAS (3 gTSS L-1) and MBR (8 gTSS L-1), the obtained results indicated that oxygen transfer coefficient was lower in the MBR (3.6 h-1 vs 20 h-1). Nevertheless, it was noted that the (kLa)20 in the CAS become comparable to that in MBR when the TSS concentration in the CAS was higher than 5 gTSS L-1, which is typical of plants operating under extended aeration. Within a range of TSS between 4 gTSS L-1 and 6 gTSS L-1 the (kLa)20 value resulted significantly higher in the MBR, thereby suggesting that under similar operating conditions the features of the activated sludge flocs in the MBR were more favorable to oxygen diffusion. Operating under high C/N, as a result of nutrients unbalance, the (kLa)20 increased in both the CAS and MBR because of the sludge deflocculation and, overall, a weaker dependence of (kLa)20 with TSS in both the systems was noted. The obtained results indicated that the most important investigated parameters on the oxygen transfer efficiency were in order: the TSS concentration, flocs size, sludge viscosity, PN/PS and EPS content. More precisely, the TSS, the EPS, the viscosity and the flocs size had a negative influence, meaning that an increase in these parameter led to a decrease of the (kLa)20, whereas the only parameter having a positive influence was the PN/PS. The TSS concentration had a greater influence on (kLa)20 in the CAS than the MBR, as well as the EPS. In contrast, the activated sludge viscosity, the flocs size and the PN/PS had a greater influence in the MBR. Based on the influence of the main biomass features affecting the (kLa)20 and considering the typical operating conditions in both systems, those of the MBR appeared to be more favorable to oxygen transfer efficiency compared to CAS.