ABSTRACT
The treatment of domestic wastewater in the developed world today is taken for granted. However, the impetus for treating this wastewater only arose in the mid 1800 s and was solely related to public health concerns due to the prevalence of water borne diseases like cholera. The contemporary goal for wastewater treatment is to reduce pathogens and to reduce environmental pollutants like nutrients. Most biological wastewater treatment processes rely upon the selective accumulation of pollutant-degrading microorganisms. Pathogens in the wastewater survive poorly in the process, leading to their reduction. The products of the process are more microbes (since they have grown on the pollutants), dissolved compounds of negligible pollutant impact and volatile compounds which are released into the atmosphere. Pollutants in domestic wastewater are typically measured as biochemical oxygen demand (BOD) or chemical oxygen demand (COD) and as some form of nitrogen (e.g. total nitrogen) and phosphorus (e.g. PO4-P). Since the process is aqueous, the microorganisms must be able to be separated from the treated water. Suspensions of individual microorganisms are extremely difficult to separate from aqueous media and would be completely dependent on expensive additional processes like filtration or centrifugation. Therefore, numerous methods which facilitate contact between the microbes and the wastewater and expedite separation of the microbes from the treated water have been devised. Biofilms of the microorganisms growing on either
suspended carriers (moving bed reactor) (e.g. Hem et al., 1994), rotating discs (e.g. Klemetson and Lang, 1984) or solid media like rocks or plastic media (trickling filter reactor) (e.g. Daigger et al., 1993; Henze et al., 1995) facilitate easy separation of the biomass since it is well attached to a support which the water easily flows away from. Membrane reactors (e.g. Chiemchaisri and Yamamoto, 1993; Delanghe et al., 1994) also rely on the growth of microorganisms as biofilms, and can be used in certain specialised situations.
