It is well-known that aggregation of bacterial cells and formation of flocs and even more dense aggregates can be enhanced by addition of such flocculants as calcium, aluminum, and iron ions or organic flocculants. These reagents form salt bridges between cell surfaces, adsorb or connect cells due to electrostatic interactions between charges of inorganic or organic flocs and cell surface (Calleja, 1984). However, cell aggregates will be of irregular shape with different sizes and settling velocities. It can also be too expensive a method of cell aggregation for large-scale wastewater treatment. Discharge of flocculant-containing effluent cannot be safe for environment. Decrease of pH to 4-5 can neutralize net charge of cell surface due to neutralization of carboxylic groups. It is facilitating cell aggregation due to decrease or electrostatic repulsion and increase of hydrophobic interactions. However, it can be applied only to enhance concentration of microbial biomass but not the process of microbial cultivation because optima of pH for aggregation and growth are different.
Particle-based biofilm reactors provide the potential to develop compact and high-rate processes. In these reactors, a large biomass content can be maintained (up to 30 g L-1), and the large specific surface area (up to 3000 m-1) ensures that the conversions are not strongly limited by the biofilm liquid mass-transfer rate. Engineered design and control of particle-based biofilm reactors are established, and reliable correlations exist for the estimation of the design parameters. As a result, a new generation of high-load, efficient biofilm reactors are operating throughout the world with several full-scale applications for industrial and municipal wastewater treatment (Nicolella et al., 2000).
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