Prospects for flux enhancement in anaerobic membrane bioreactors treating saline wastewater

Abstract

Successful high-rate treatment of wastewaters in bioreactors is largely dependent on effective sludge retention. Despite the availability of sludge granulation techniques, physical retention by membranes remains a good option, especially when good sludge granulation cannot be guaranteed. The granulation of anaerobic sludge is, for example, impeded by the effects of sodium on sludge properties, such as a weakened granule strength, which might be attributed to disruption of bivalent cation linkages between extracellular polymeric substances (EPSs) that play a key role in granular sludge stability. Under such conditions, the use of membranes ensures full sludge retention, providing a suspended solids-free effluent. However, the feasibility of using membranes in wastewater treatment, especially under anaerobic conditions, requires major improvements in attainable membrane fluxes. This study has therefore investigated methods to increase the membrane flux of anaerobic membrane bioreactors that are operated under saline process conditions. Two methods for increasing membrane flux have been tested. The first method involved increasing the shear stress at the surface of the tubular membrane employed, in order to enhance the back transport of foulants from the membrane surface to the bulk solution; slug bubbles and inserts were used to increase the shear stress. The second method involved decreasing the concentration of foulants in the bulk solution through the addition of adsorbents and the use of coagulation. Coagulation was induced by the sodium ions naturally present in saline wastewater and through the direct addition of an aluminum-based coagulant. The applied gas slug appeared to be unable to adequately control fouling, resulting in rapidly increasing trans-membrane pressures (TMP) when operating at a flux in excess of 16 L/m.h, as described in Chapter 2. However, the chemical oxygen demand (COD) removal efficiency did not show any significant deterioration, whereas the specific methanogenic activity (SMA) increased from 0.16 to 0.41 g COD per g volatile suspended solid (VSS) per day. The tubular membrane was subsequently equipped with inert inserts in order to produce locally increased shear stress for enhanced fouling control. Results showed that, following the mounting of the inserts in the membrane tube, the membrane flux increased from 16 L/m.h to 34 L/m.h. However, the pressure drop along the membrane was also greatly increased and it was therefore concluded that the gas slugs were insufficient to increase the membrane flux and the inserts did not offer a practical solution. In order to understand why the bubbles did not effectively increase the membrane flux, the mass transfer by the bubbles was quantified through computational fluid dynamics modeling. The model and its results are presented in Chapter 3. The modeling indicated that the mass transfer capacity at the membrane surface was higher at the noses of gas bubbles than at their tails, which is in contrast to the results when water was used instead of sludge. The filterability of the sludge at a given mass transfer rate was found to have a strong influence on the TMP, at a steady flux. The model also showed that the shear stress within the internal space of the tubular membrane was mainly around 20 Pa, but could be as high as about 40 Pa due to gas bubble movements. Nevertheless, a stable particle size distribution (PSD) for sludge particles was found at these shear stresses. It was, therefore, hypothesized that a high flux would be possible by applying biogas bubbles induced slug flow conditions in