text: Although biological filters have simple superficial structures, their internal hydrodynamics and the microorganisms' biology and ecology are complex and variable. These characteristics confer robustness to the process. In other words, the process has the capacity to maintain its performance or rapidly return to initial levels following a period of no flow, of intense use, toxic shocks, media backwash (high rate biofiltration processes), etc.
  The structure of the biofilm protects microorganisms from difficult environmental conditions and retains the biomass inside the process, even when conditions are not optimal for its growth. Biofiltration processes offer the following advantages: (Rittmann et al., 1988):
  * Since microorganisms are retained within the biofilm, biofiltration allows the development of microorganisms with relatively low specific growth rates;
  * Biofilters are less subject to variable or intermittent loading and to hydraulic shock;
  * Operational costs are usually lower than for activated sludge; 
  * The final treatment result is less influenced by biomass separation since the biomass concentration at the effluent is much lower than for suspended biomass processes;
  * The attached biomass becomes more specialized (higher concentration of relevant organisms) at a given point in the process train because there is no biomass return.