Sewage is the wastewater released by residences, businesses and industries in a community. It is 99.94 percent water, with only 0.06 percent of the wastewater dissolved & suspended solid material. Untreated sewage has a BOD5 ranging from 100 mg/1 to 300 mg/1. Biological treatment is a process to remove dissolved organic matter from wastewater. Sewage microorganisms are cultivated and added to the wastewater. The microorganisms absorb organic matter from the sewage as their food supply. Three approaches are used to accomplish this treatment, fixed film, suspended film and lagoon systems. Fixed film systems grow microorganisms on substrates such as rocks, sand or plastic. The wastewater is spread over the substrate, allowing the wastewater to flow past the film of microorganisms fixed to the substrate. As organic matter and nutrients are absorbed from the wastewater, the film of microorganisms grows & thickens. Trickling filters, rotating biological contactors, and sand filters are examples of fixed film systems. Suspended film systems stir and suspend microorganisms in wastewater. As the microorganisms absorb organic matter and nutrients from the wastewater, they grow in size & number. After the microorganisms have been suspended in the wastewater for several hours, they are settled out as a sludge. Lagoon systems are shallow basins that hold the wastewater for several months to allow for the natural degradation of sewage. These systems take advantage of natural aeration and microorganisms in the wastewater to renovate sewage.

Modelling and simulation of biological systems is used as a predictive method to investigate the potential impact of manipulations at the genetic or molecular level. Modelling itself can help to capture the knowledge of biological systems in an unambiguous way to obtain a comprehensive yet precise & detailed overview of any given system. It requires a deep understanding of biological processes, as well as a range of mathematical and computational techniques. The many potential applications in drug development, genetic engineering, as well as applied and basic research in modern biology has led to systems' biology emerging as a key theme in the post-genome research agenda for the biological sciences community. Despite the current enthusiasm for harnessing mathematical & computational methods to formalise & integrate data and process descriptions in the biological sciences, there are several important challenges & unsolved problems that still exist before models of whole organisms can be considered a realistic goal.