The applications of mathematical models to complex biofilm process resembling natural systems are very rare. Most of the reports are based on laboratory-scale pure cultures. Black-box approaches are normally used to evaluate performance of actual systems. The above quoted example by Nkhalambayausi-Chirwa and Wang (2005) was one of the few efforts to mechanistically model a mixed culture system.

The efforts to understand the fundamental nature of biofilm systems are worthwhile since these offer unique solutions to contaminant treatment. For instance, cells growing in biofilm cultures have been observed to perform better than the same species suspended in medium (Semprini and McCarty, 1981). One reason offered for the better performance of cells in the biofilm environment is the effect of shielding from high toxicity levels. It is esti­mated in most biofilm systems that the bulk liquid concentration is much higher than the concentration in the deeper layers of the biofilm. Other complex interactions are also known to exist within the biofilm system. Cultures grown in a cooperative system where some species of microorgan­isms require a product from other species for survival (Nkhalambayausi — Chirwa and Wang, 2005). The mass transport conditions and balanced retention of substrates are required to sustain a culture for a specific purpose.

Biofilm systems have demonstrated the potential to package complex treatment systems into a small space to achieve removal of multiple sub­strates. These systems are especially useful when relatively sensitive species of bacteria are used to treat toxic waste. It is envisaged in this report that biofilm systems will in future form a significant part of the treatment and recovery regime for nuclear and radioactive waste.