|【Reduction technology in activated sludge process Photochemical smog】|
Reduction technology in activated sludge process Photochemical smog
Activated sludge process has a high cleaning ability and operation costs are low. So it has been used widely as a purification method of organic sewage. In this process microbes incept organic contaminants in sewage as their nutrient source. However, because such a process results in the increase of microbes, excess sludge is produced as by-product. Resource and disposal for excess sludge is very high cost, so sludge reduction at the generation site is greatly needed.
Sludge reduction technology
High concentration organic wastewater is disposed by using aerobic microorganisms after most of the organic matter is removed by an anaerobic microorganism treatment. Domestic wastewater and low concentration industrial effluent are purified by using aerobic microorganisms.
Activated sludge process is the typical treatment using aerobic microorganisms. Microorganisms in the aeration tank (activated sludge) are transferred to the sedimentation tank for solid-liquid separation. The increased activated sludge is removed as excess sludge and the remainder is transferred to the aeration tank. This excess sludge moisture is 99.5 - 99% for sedimentation. It is concentrated up to around 98.5% moisture. Then it is dehydrated and the volume is reduced to a cake of sludge with 65 - 85% moisture. In treatment plants for large scale sewage and industrial effluent, sludge treatment equipment, such as sludge dehydration equipment is provided. However, in a small scale treatment plant, because the volume of sludge disposal is low, the sludge disposal costs including amortization and maintenance for dehydration equipment is comparatively high.
As contrasted with the conventional activated sludge process, sludge reduction (or sludge substrate) activated sludge process was developed as a method to reduce sludge disposal costs by controlling the excess sludge volume. This is the process that reduces sludge by degradation and digestion of microorganisms after a portion of returned sludge was disposed into the substrate (to change sludge into biodegradable form) and this substrate sludge was returned to the aeration tank.
Principle and classification of sludge reduction process
The zero emission concept should be considered to reduce wastewater and then to reuse it as resource. Also, it should be disposed of properly. Controlling wastewater itself becomes the real zero emission. The process was developed as a biological wastewater treatment process with zero emission in order to dispose of excess sludge using the activated sludge microorganisms after a portion of the returned sludge is biodegraded by some processes and then returned to the aeration tank. In the sludge reduction system, which has already been put to practical use, the reduction rate of excess sludge is 70 - 80% allowing stable operation in the disposal system and comprehensive economic efficiency.
Sludge substrate which has already been developed and proposed from the viewpoint of the cell level of a microbe is divided into the following three classifications: 1) eliminated cells; 2) crushed cells; 3) cells with low molecular weight. There are also some substrate processes which fall on the borderline of the said classifications.
Dead cells and cell-wall materials are degraded and solubilized by enzyme action of microorganisms in the aeration tank. If the whole sludge substrate process including degradation and reduction of cells to a low molecular weight are considered, the sludge substrate process can be also called solubilization.
The major sludge substrate technologies are as follows: the biological method by aerobic microorganisms; chemical procedure using oxidants; the physical technique using ultraviolet, heating, microwave and ultrasonic waves; the physicochemical method by ozone; and alkali heating.