The major symptoms of fouling are a decline in flux over operating time, increasing transmembrane pressure, sludge cake formation and changes to the retention coefficient Rt (see Eq. 12.75). Stationary filtration behavior is not possible. Fouling occurs if a critical flux is exceeded. Chang et al. (2002) defined the critical flux as the highest flux for which the transmembrane pressure remains constant based on data from a step-by-step increase in flux, depending on the type of membrane material, MLSS and cross-flow velocity. However, there is a transition between concentration polarization and stagnant cake formation (Chen et al. 1997). For a relatively high c3, polymerization and precipitation can occur, resulting in solid cakes. To a certain extent, nearly every feed component leads to membrane fouling.
The formation of surface deposits on membranes is influenced by feed composition, flow conditions, the chemical nature of the membrane and interactions between components and the membrane. The type of fouling depends on the nature of deposit components (Fleming 1995). We distinguish:
• Scaling or mineral fouling is the deposit of inorganic material with crystal structures like salts.
• Organic fouling is the deposit of organic material like grease, oil, surfactants, proteins and humic substances.
• Colloidal fouling is the deposit of particles like clay and metal-oxides or hydroxides.
• Bio-fouling is the formation of bio-films by microorganisms captured and growing at the surface. A secondary phenomenon is the excretion of enzymes and extracellular polymeric substances (EPS) which influence the gel layer.
Deposits which can be detached by cleaning processes, like back-flushing and mechanical cleaning, are called reversible fouling and cause a reversible gel layer formation. Deposits which lead to an irreversible gel layer formation can only be removed by chemical cleaning procedures and are called irreversible fouling (Rautenbach and Albrecht 1989). Therefore, fouling can be controlled only to a certain degree by hydrodynamics.
Different fouling mechanisms are shown in Fig. 12.9. An irreversible sludge cake layer is formed by particles, contaminants and agglomerates of contaminants which are bigger than the pore size of the membrane.
Due to the heterogeneous nature of bioreactors, mixed liquor fouling is difficult to predict and control in a MBR. Factors affecting fouling are (Chang et al. 2002; Lee et al. 2003; Rosenberger 2003; Shon et al. 2004):
• The membrane material hydrophobicity, porosity and pore size and distribution.
• The mass of microorganism MLSS and of extra-cellular polymeric substances EPS, floc structure, dissolved matter and floc size.
• The operating conditions e.g. configuration, cross flow velocity, aeration, hydraulic and solid retention time and trans-membrane pressure.
Was this article helpful?