Inorganic particles increase biofilm heterogeneity and enhance permeate flux

Highlights

• We studied the influence of inorganic particles on the biofilm permeability.

• Inorganic particles shaped biofilm structure in low-pressure ultrafiltration.

• Accumulation of small particles lead to homogeneous biofilms and low permeate flux.

• Large particles promoted heterogeneous biofilms with higher permeability.

• Vertical or horizontal membrane orientation determined size of particles accumulated.

Abstract

This study investigated the influence of inorganic particles on the hydraulic resistance of biofilm grown on membrane surface during low-pressure dead-end ultrafiltration. Gravity-driven ultrafiltration membrane systems were operated during several weeks without any flushing or cleaning. Smaller (kaolin d_0.5 = 3.6 μm) or larger (kaolin with diatomaceous earth 50/50%, d_0.5 = 18.1 μm) particles were added to pre-filtered creek water or to unfiltered creek water. It was demonstrated in both experiments that presence of finer particles in the feed water (kaolin) induced formation of compact and homogeneous biofilm structure. On the other hand presence of the larger particles (diatomite) helped to counterbalance the effect of fine particles due to the formation of more heterogeneous and permeable biofilm structure. The hydraulic resistance of biofilms formed with fine particles was significantly higher than the resistance of biofilm formed in (1) absence of any inorganic particles or (2) in presence of the mixed particle population. The membrane orientation (vertical or horizontal) determined which particles were accumulating at the membrane surface, with structural differences shown by Scanning Electron Microscopy (SEM). For vertical membranes, the larger particles were selectively removed due to sedimentation and did not contribute to the biofilm development. Thus the selection of smaller particles due to vertical membrane configuration negatively affected the biofilm structure and permeation rates, and such selective accumulation of fine particles should be avoided.

Introduction

Cake formation due to particle deposition is a major problem in membrane systems. Cake formation is associated with reduced permeate flux. Different strategies are applied to limit the negative effects of particle deposition and to maintain high permeate fluxes, e.g., feed water pre-treatment or high cross-flow during filtration. Gravity-driven membrane (GDM) filtration is applied to the decentralised production of drinking water (point-of-use systems) (Peter-Varbanets et al., 2009). GDM filtration is performed in dead-end mode, without cross-flow, without control of the biofilm formation, and typically without pre-treatment (Peter-Varbanets et al., 2009, Peter-Varbanets et al., 2010). Thus, cake formation in GDM filtration cannot be avoided using conventional approaches. However it is not clear to what extent particle deposition influences membrane flux by influencing specific resistance of the biofilm that forms on the membrane during GDM filtration.

Different strategies can be applied to control particles deposition. One approach is to pre-filter feed water to remove particles using, e.g., bag or cartridge pre-filtration devices (Huang et al., 2009). Such pre-treatment reduces the overall particle loading. But on the other hand it changes the size distribution and increases the fraction of fine particles that deposit on the membrane (Li et al., 1998). Ultimately, a cake with smaller porosity and higher specific cake resistance forms (Carman, 1938, Kim and Ng, 2007). Another approach to control particle deposition is to apply cross-flow conditions. But cross-flow conditions also promote the deposition of small particles on the membrane while large particles are more likely to be removed (Li et al., 1998). Thus cakes formed under cross-flow conditions may have higher specific cake resistances than cakes formed in the dead-end filtration (Le-Clech et al., 2006). It remains unclear how the removal of large particles and in turn the selection of fine particles is beneficial or detrimental for the operation of the system.

Large particles in the feed water can also be beneficial to filtration. Larger particles that accumulate on the membrane surface can act as a secondary membrane layer that captures smaller particles and reduces the fouling potential of the primary membrane (Arora and Davis, 1994, Kuberkar and Davis, 2000). It has in fact been suggested that the addition of inorganic particles into the feed water can help to control membrane fouling in membrane bioreactors (Teychene et al., 2011). The added particles improved performance of the membrane bioreactor during supernatant filtration due to formation of a non-compressible fouling cake. However, previous studies (Arora and Davis, 1994, Kuberkar and Davis, 2000, Teychene et al., 2011) were performed over short term (several hours) and/or under shear conditions, and most importantly: with suppression of biofilm growth on the membrane surface.

During GDM filtration the cake developing on the membrane is not just particles from the feed water but also a biofilm that grows on soluble substrates and products of hydrolysis. The activity of the biofilm in GDM filtration allows for long-term operation (months) of the system at a stable permeate flux. Similarly to permeable bio-barriers (Baveye et al., 1998), the total permeability in GDM results from both the biofilm growth and the particles presence. Accumulation of the particles could have both positive effects (by preventing biofilm densification) as well as negative ones (adding additional hydraulic resistance to water flow). However, the contribution of the inorganic particles to the development of the biofilm structure in low-pressure ultrafiltration is unclear.

The aim of this study was therefore: (1) to evaluate how the presence of inorganic particles in the feed water influence the hydraulic resistance of biofilms developed during gravity-driven membrane ultrafiltration, (2) to understand how the configuration of the membrane (horizontal or vertical) influences the formation of the biofilm and in turn the system performance, and (3) to determine whether it is beneficial to pre-sediment the particles prior to the filtration. For this purpose gravity-driven systems equipped with horizontal (HOR) and vertical (VER) ultrafiltration membranes were operated in dead-end mode (no cross-flow), in the presence or absence of selected inorganic particles. The influent was augmented either with kaolin (=small particles), or with a mixture of kaolin and diatomite (50/50%, by mass), and compared to control (no particle addition). Kaolin represented non-settleable (under experimental conditions) particles, while diatomite represented the settleable particles.