The biological treatment of water/wastewater is generally divided into a suspension growth method and a fixed-film growth method. Such a classification is based on the form by which the microorganism is grown for that purpose. Both the suspension growth method and the fixed-film growth method are widely used nowadays; nevertheless the latter has advantages over the former under the circumstances that the sludge retention time (SRT) is relatively longer, and that the hydraulic retention time (HRT) is relatively shorter. In addition, the fixed-film growth method is generally suggested for a water/wastewater treatment which involves the removal of a special contaminant from the water/wastewater due to its longer SRT. In terms of the microbial growth and the decomposition of contaminants, there are two kinds of carriers: immobilized carrier and fixed-film carrier. The immobilized carrier is a biological carrier in which a pre-incubated microorganism is made immobile in the carrier; it is used only in certain special circumstances in light of its high cost. The fixed-film carrier is relatively cost-effective and is widely used in environmental engineering. The early fixed-film carrier is formed of a corrugated board of a plastic material and is used widely due to its physical stability and its accessibility. However, the usage of the early fixed-film carrier calls for the construction of a relatively large reactor, because the plastic corrugated board has a small specific surface area ranging from 100 to 200 m2/m3. Needless to say, the construction of the large reactor results in a substantial increase in the overall cost of the water/wastewater treatment. In addition, a relatively long start-up period of the early fixed-film carrier gives an added cost to the water/wastewater treatment. The long start-up period is resulted from the fact that the fixed growth of the microorganism on the early fixed-film carrier takes place at a slow pace.
In view of the deficiencies of the conventional fixed-film carrier described above, a fixed-film growth of microorganism on a porous carrier was introduced. The porous carrier has a specific surface area of 1000 m2/m3 or greater. As a result, the reactor can be greatly reduced in volume. In addition, the porous carrier is capable of trapping and intercepting the microorganism efficiently, so as to shorten the start-up period of the water/wastewater treatment. However, the porous carrier is generally hydrophobic in nature and has tendency to float in water/wastewater. Moreover, the porous carrier is incapable of effective adsorption of certain contaminants, such as compounds containing an ammonia, as well as certain contaminants which cause color of wastewater. As a result, the microbiological decomposition of the contaminants can not be effectively carried out in the reactor.