The efficient use of water in production processes is not only a proper tool to create a greener image. It is a competitive advantage as the service of local wastewater treatment plants becomes more and more expensive. The application of membrane bioreactors could be a promising option in terms of process water re-use.
Water re-use is gaining significance due to the overall water shortage in many countries around the world. It has become almost mandatory to think of ways to re-use or recycle water in certain parts of the world. Sustainability is the new key word and membrane bioreactors (MBRs) help to achieve just that: a relatively simple way to treat the wastewater and make it available for other uses (e.g. tempering). But the increased popularity of MBRs cannot only be attributed to countries with severe water shortage. Even in countries like Germany, MBRs are becoming more and more popular as a way to solve complex wastewater treatment problems.
Conventional wastewater treatment plants (WWTPs) and MBRs both use activated sludge to clean the wastewater. Subsequently, the activated sludge and the clean water need to be separated once the activated sludge has completed its task. In a conventional WWTP the activated sludge settles in the secondary clarifier and the clean water proceeds to the drainage system. This separation process is quite difficult to control and therefore the separation is never perfect. Certain particles, bacteria and other components will enter the discharge system with the clean water and eventually leave the WWTP.
This makes re-use of the clean water somewhat challenging since any bacterial contamination poses a major problem when recycling the water as process water. In turn, this is one of the biggest advantages of an MBR system. The membranes will be either submerged directly into the activated sludge or in a separate filtration chamber and will ensure that no particles or bacteria will enter the discharge system along with the clean water.
Submerged modules for MBRs exist in a variety of different formats. The differentiating criteria are packing density of membrane area, the hydraulic conditions which will influence the modules’ tendency for braiding or silting, as well as the possibility to backwash the system. The module shown above is based on flat sheet membranes that have been converted into laminated sandwich-like structures that not only guarantee high packing density but can be backwashed with permeate.
What is common to all different makes of submerged modules is the use of cross flow aeration. The cross flow aeration cleans the membrane surface continuously. The use of fine bubble aeration is generally a good idea because then a portion of the air injected is available for the activated sludge process.
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