As a representative form of clarification treatment of wastewater, there is microbial degradation treatment, including, for example, the activated sludge method. However, wastewater also contains substances that are difficult to decompose with microorganisms, and to remove such contaminants, the wastewater is required to be separately treated, physically or chemically.
Such physical or chemical treatment methods include, for example, the absorption method, the photooxidation method, and the combustion method, but the absorption method and the photooxidation method are only effective when the wastewater is a low-concentration wastewater, and the combustion method is only effective when the wastewater is a very-high-concentration wastewater. Hence, in the case of a medium-concentration wastewater, which cannot effectively be treated by these methods, the wastewater is evaporated first by a drum dryer, to be concentrated, and then it is treated by the combustion method. Since this method is, however, high in running cost of the evaporation and concentration treatment, there is a demand for development of a method and apparatus that can treat medium-concentration wastewater directly.
Further, in the case of a method wherein pollutants in wastewater are removed by decomposition, there is a possibility of different pollution by secondary formation of chemical substances, and therefore it is desired to develop a wastewater treatment method free from any risk of secondary pollution.
If pollutants can be separated from wastewater without decomposing them, secondary pollution due to the treatment does not occur. As a solvent that can separate chemical substances from a medium-concentration aqueous solution, there are supercritical fluids used for the separation of water-alcohol in the field of biomass, but there are very few instances of their use in wastewater treatment, and they are accompanied by the problem that they are poor in treatment efficiency. Namely, there is a method in which a supercritical fluid is fed from below, and wastewater is fed from above in a countercurrent contact tower 31, sectioned by sieve trays 32, as shown in FIG. 3(a), or a countercurrent contact tower 33, loaded with Dixon packings 34, as shown in FIG. 3(b), but the treatment efficiency in either method of the method is low, and the method is not suitable for treating a large quantity of wastewater.