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A comparative study of biological nutrient removal processes with gravity and membrane solids-liquid separation Monti, Alessandro

Abstract

The replacement of the secondary clarifier with membrane filtration technology was explored as possible solution to accommodate increased loading rates in existing enhanced biological phosphorus removal (EBPR) plants. The present research study aimed to understand the impact of membrane solids-liquid separation on bioreactor performance, by (1) comparing a membrane and conventional EBPR process operated in parallel, and by (2) assessing the potential of membrane processes to perform EBPR satisfactorily under challenging operating conditions. The utilization of membrane technology resulted in an overall reduced denitrification activity and a reduced observed sludge yield in the membrane process, due to the larger mass of aerobic sludge held in the system. The kinetics of phosphorus release and uptake, together with the stoichiometric coefficients, were intrinsically unaffected by the presence of membrane filtration. On the other hand, the maximum specific nitrification activity in the membrane sludge was significantly lower than that of the reference conventional sludge, possibly due to more extensive decay of nitrifiers originating from the high shear conditions that prevail in submerged membrane bioreactors. Using ribosomal intergenic spacer analysis, the bacterial community composition of the membrane process was found to be significantly different and less diverse than that of the corresponding conventional system, indicating that membrane solids-liquid separation per se is sufficient to shift the dynamics and composition of the microbial population. A last fundamental difference brought about by the employment of membrane filtration was the regular formation of a significant amount of foam on the surface of the anoxic bioreactor zone. With sufficient volatile fatty acids (VFA) concentrations in the influent, the membrane-assisted process could maintain satisfactory nitrification and phosphorus removal performance under high rate conditions, with the lowest hydraulic retention time (HRT) tested being five hours. A low phosphorus concentration in the effluent could be maintained when the solids retention time (SRT) of the process was extended from 12 to 20 days. However, this resulted in increased VFA utilization per unit mass of phosphorus removed, with the SRT having a significantly larger impact than the HRT. An innovative and sustainable membrane-assisted process was proposed with phosphorus removal and recovery merged in one single system.

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