The Membrane Bioreactor (MBR) system, which utilizes a submerged membrane filter in an aeration tank, is the method to separate activated sludge and treated wastewater. The method, wherein the flat sheet membranes are dipped into the aeration tank, has been used for a Johkasou since 1990. So far, approximately 2000 KUBOTA Membrane installations have been built in Japan. Overseas, KUBOTA Membrane was first installed at Porlock sewage treatment plant in the UK in February 1998. Since then, the method has been employed by plants in the areas where water quality regulations are strict and for the purpose of reusing treated water. As regards Kubota Submerged Membrane Unit (submerged flat sheet membrane bioreactor system), the Membrane Cartridge, which is a flat membrane plate constructed by ultrasonically welding sheets of chlorinated polyethylene on the front and back of the ABS resin membrane panel, is dipped in the activated sludge. Then, filtration is conducted by using the driving force brought about by the pressure difference on both the front and back of the membrane via the Nozzle. The Membrane Cartridge with its specifications and a view showing the submerged flat sheet membrane bioreactor system are shown in Fig. 1. Its features are as follows. ?) Solid-liquid separation can be carried out perfectly. ?) Even if the operation pressure is low, sufficient amount of penetration is obtained. ?) Since a membrane has physical and chemical durability, an oxidizer like high-concentration sodium hypochlorite can be used for membrane washing. ?) When high-concentration activated sludge is separated, the form of the flat sheet membrane does not cause membrane clogging easily.
2. Sample case of wastewater treatment
The MBR using a submerged membrane filter is mainly for organic wastewater treatment. The method has been employed in various fields, such as washing at electronic industry, etching wastewater treatment, coating effluent from mechanical industry and plating wastes, and food industrial wastewater. The industry-classified adoption of Kubota Submerged Membrane Unit is shown in Table 1.
Sample case 1: Wastewater treatment in small and medium-sized plants
The Ministry of the Environment announced the results of wastewater treatment from soy sauce brewing using the Submerged Membrane Unit in Table 2 as a model business1) (organic wastewater treatment technology field for small-sized plants）in order to demonstrate environmental technology for the promotion of new technology suitable for wastewater treatment in small and medium-sized plants. The disposal amount was 35m3 a day. The report was between September 2004 and February 2005. The results are shown in Table 2.
It was reported that the removal efficiency for BOD and SS was more than 99.9% and more than 97% for COD, n-Hex, T-N, T-P. The amount of surplus sludge was 8.9kg/day in the form of dry matter and sludge yield was approximately 20%.
Sample case 2: Wastewater treatment of starch effluent (potato washing) at HOKKAIDO JA SHIHORO
The operation period of a factory depends on the harvest season of potatoes and the weather (temperature), in which the wastewater treatment plant operates only half a year from the second half of May up to midDecember every year, and is recessed for winter. Therefore, the MBR with submerged membrane filter was applied because the activated sludge process could not maintain constant the microbiological process of the treatment and for a safe solid-liquid separation. The operation has been running smoothly since 2001.
The wastewater from the plant consisted of the semi-solid effluent from the washed potatoes and wastewater with approximately 2% protein separated from the liquefied starch after the potatoes were mashed and the skin was separated. The protein-containing effluent was treated by the UASB process (Upward-flow Anaerobic Sludge Bed) using anaerobic granular methane bacteria. In this method, organic matter was covered by producing methane from gasification. Since high levels of ammonia derived from protein caused active inhibition of methane bacteria, the protein was rendered insoluble in the acid region as part of pretreatment and then removed by floatation separation. Furthermore, the concentrated ammonia left in the UASB wastewater was denitrified by biological nitrification. The amount of treated water was 3000m3 per day. The treated water was re-used for washing potatoes. The processing results are shown in Fig. 2.
The submerged flat sheet membrane bioreactor system produces high quality permeate with non-detectable SS, less than 5mg/L BOD, and less than 0.1 NTU turbidity. In addition, the treated water is often reused because of the absence of bacteria in the treated water, for example, in some of Johkasous, effluent treatment from kitchens, and laundry industry.
When the rate of reusing the treated water is raised to more than 50%, back-penetrating membrane treatment (RO membrane system) is needed to remove concentrated organic matter and the salinity. Many reports indicate that the FI value of the treated water by MBR using submerged membrane filter for the subjected domestic wastewater is stable at approximately 2.5 and is sufficient to supply RO membrane system with feed water 2)3) if it is within the normal operating range (from several days to approx. 300 days with SRT (solid retention time), TMP (the trance membrane pressure) of some kPa).
The cost performance in the reuse of sewage is far lower because no electric expense for the osmotic pressure employed in the method is required as compared to the process of desalination. If psychological antipathy against sewage disposal water disappears, it is thought that the reuse rate will increase significantly. The wastewater treatment will then become one of the useful measures for water resources in the 21st century.
1) Ministry of The Environmental (MOE) of Japan: The Pilot Project of The Environmental Technology Verification in fiscal year 2004.
2) Kawakami et al.: “Basic Investigation for MF Treatment in MBR Application to Aim Water Reuse”, proceedings of 37th Japan Society for Water Environment Annual Meeting.
3) Kobayashi: Eligible and examples as RO influent of the treated water by ＭＢＲ(Membrane Bio-Reactor), The 20th New Membrane Technology Symposium 2003, P5-3-1?P5-3-12