Effects of Activated Sludge on Hydrodynamic Characteristics of a Hollow Fiber Membrane Bioreactor

Mirjana Čurlin*, Helena Korajlija Jakopović, Ivan Mijatović, Želimir Kurtanjek
University of Zagreb, Faculty of Food Technology and Biotechnology, Pierottijeva 6, 10000 Zagreb, Croatia
E-mail: mcurlin@pbf.hr

Abstract
A submerged hollow fiber membrane module is a widely accepted wastewater membrane bioreactor. Membrane bioreactor with submerged membrane (HF MBR) is complex multiphase physico-chemico-biological system with internal interactions between process variables and dynamic changes within gas-liquid-solids phases. A study of hydrodynamics in an operating laboratory bioreactor at realistic working conditions is crucial for design and scale up of a bioreactor for wastewater treatment processes. Accumulation of high concentrations of biomass causes the occurrence of heterogeneous regions in a bioreactor. This paper focuses on the effects of activated sludge on average residence time, which is often quite different from the ideally designed hydraulic residence time (HRT) in a submerged hollow fiber membrane bioreactor. In this work, tracer experiments using Dirac δ impulse of the saturated NaCl solution and monitoring electrical conductivity in the outflow with a flow-through conductivity cell were performed. From experiments at liquid flow rate range of 7.2–16.5 L h–1, air flow rate range of 1.7–5.1 m3 h–1, and activated sludge concentrations MLSS 4.5–7.4 g L–1 probability density of residence time distribution (RTD) functions for hollow fiber membrane bioreactor and average residence times τMBR were calculated The relative residence time defined as the ratio of HF MBR τMBR and CSTR τid were evaluated as function of MLSS concentration. Proposed is a linear regression model of the relative residence time with MLSS concentration. The model has negative proportionality coefficient k = –0.0814 g/L of MLSS due to decrease of the relative residence time with increase of active sludge concentration. Statistical evaluation of the model gives the correlation coefficient of R2 = 0.8374. Decrease of the mean residence time with increase of MLSS is discussed in view of non-uniform membrane fouling.

Keywords: Activated sludge, membrane bioreactor, hydraulic residence time (HRT)