In this article we will present the aspects of membrane bioreactors (MBRs) that make them a good alternative to the conventional systems of biological purification using activated sludge for treating waste water. This system consists of a modification of the conventional sludge system, as the secondary settling tanks, typical of the conventional system, are replaced by membrane units.
The main advantages of the MBR system compared to activated sludge is that it consists of relatively small installations that enable high quality effluent to be obtained with low production of slurry. This system is suitable for treating both urban waste water and biodegradable industrial waste water. The majority of the applications of this system are found in the food, pharmaceutical and cosmetics sectors and in dumpsites.
Moreover the MBRs system presents a disadvantage with respect to activated sludge: the cost, as the progressive soiling of the membrane leads to higher maintenance costs than other systems.
Membrane bioreactors are the combination of a bioreactor, in which a concentrated suspension of microorganisms degrades the contamination present in the water, and a membrane filter unit (0.01-0.04 µm) that separates the biomass from the purified water.
As mentioned above, the main advantage of this innovative system is the possibility of obtaining better quality effluent, occupying little space, as it works with high biomass concentration, which generates less slurry. Another advantage is that it provides the freedom to control the process, as the hydraulic retention times and the biomass can be manipulated. As it does not function with sediment, the installation can be small and achieve a greater separation than the previous systems due to membrane technology. Although, at the same time, it generates more soiling than other systems as the medium is subject to a greater agitation compared to the conventional sludge system, which causes a greater production of EPS. Therefore, despite the series of positive characteristics mentioned, this technology involves high costs related to the prevention and disposal of the soiling in the membrane that guarantees it functions optimally.
The permeability of the membrane is influenced by the characteristics of the slurry. The cells, present in the slurry, can form a layer around the membrane during filtration, and form a biofilm that lessens the permeability, a problem that can be increased by the depositing of particles and the adsorption of colloidal materials. Biological factors, such as the presence of nutrients, the age of the slurry and the level of agitation affect the production of extracellular polymeric substances that are responsible for the soiling.
The soiling of the bioreactor can be reversible, in other words, it can be eliminated by physical cleaning, or irreversible, in other words, requiring a chemical clean. This depends on how firmly the soiling binds to the membrane. However, there is also soiling that cannot be reversed, that cannot be removed by any means, which defines the useful life of the membrane. All of these types of soiling occur at different stages in time.
During filtering, the increase in trans-membrane pressure is principally due to the formation of the filter cake that obstructs the membrane. This may be eliminated with physical cleaning, but in the long term this cleaning is not able to resist the initial trans-membrane pressure. When the pressure after the physical cleaning exceeds a certain amount, a chemical clean is performed. Nevertheless as a consequence of the non-reversible soiling, the chemical clean does not prevent the gradual increase in the trans-membrane pressure over a period of years.
Membrane bioreactors consist of several membranes and a bioreactor. The filter module may be external or internal to the bioreactor. The difference is that the energy consumption in the external MBR system is ten times higher than the internal energy consumption. Even so, external MBRs do have some advantages compared to internal MBRs, such as the possibility of cleaning the membrane in situ, easy access to the modules, the possibility of modifying the number of modules and optimizing the aeration of the bioreactor to obtain maximum oxygen transfer coefficients.
There is also an new configuration of MBRs called the Air lift MBR that consists of an external MBR, which provides easy access to the membrane and works with large flows. But, at the same time, the internal system also has its advantages in that it has a low energy consumption (0.5 kwh/m3).
In short, the membrane bioreactor system is a technology capable of competing with the conventional sludge systems, as it enables high quality effluent to be obtained and presents a high level of design versatility. Even so, it is necessary to know the mechanisms and components of the soiling for the correct use and maintenance of this technology.