The Application of Membrane Separation Process to Domestic and Industrial Wastewater Treatments
When considering advanced sewage treatment methods from a global perspective, the activated sludge method using aerobic microorganisms, which was developed more than a century ago, is the most widespread. The basic components include aeration and sedimentation tanks. However, an additional sedimentation tank is often installed for pre-treatment, depending on the property and the state of sewage water and wastewater. Therefore, the more general structure includes a first sedimentation tank, an aeration tank, and a second sedimentation tank.
Using a membrane separation technique means, needless to say, the introduction of the membrane separation process instead of the sedimentation separation process. The two properties of membrane separation that the sedimentation separation process lacks are the ability to separate small particles and dissolved matter at high rates, and the use of a smaller space when installed. Thereby, the processing capability becomes more advanced, which is particularly evident with the use of the second sedimentation tank.
In the first sedimentation separation tank, the separation technique is provided to materials only contained in raw water, which dramatically increases the separation efficiency. However, in the case where sludge generated in this process includes much organic matter, if it is possible to combine with the anaerobic bio treatment technology that recovers fuel gas such as methane gas in the process, it can bring more advantages.
In the second sedimentation separation tank, activated sludge, which is the bacterial population grown in the aeration tank, is separated. At this time, the dramatic increase of the separation efficiency has two significances: One is that separated water, that is, the very quality of treated water gets better. The other is that it is possible to operate the tank with several times higher concentrations of organisms in the aeration tank than the conventional activated sludge process because the concentration of activated sludge organisms naturally increases. Since it is usually possible to make the installation space of the aeration tank itself smaller, this process provides a big advantage with not only the physical aspect of separation but also the aspect of biological response.
The membrane separation process, however, has many problems as well. There is much to consider, for instance, membrane lifetime and cost, an oxygen supplying method suitable for microbial concentration, and the difficulty level of maintenance and management.
In Japan, the membrane separation process has been utilized from a relatively early period for the sewage and effluent treatments which require only a small quantity of water including human waste. However, the technology has not yet achieved satisfactory results for large-scale facilities treating municipal sewage. For the current state and level of introduction, the “Journal of Environmental Conservation Engineering” (Vol.42, No.8, 2013) features four reports as a special edition, “technological trends in sewage treatment by the membrane bioreactor ” . The four reports are “Application Trend of Membrane Bioreactor for Wastewater Treatment”, “Microbiological Properties of Membrane Bioreactors”, “Introduction and Operation of the Largest Membrane Bioreactor (MBR) in Japan”, and “Technical Review of Membrane Bioreactors”.