SWT - MBR Membrane Bioreactors

In fact, there are many existing purification plants that need to be upgraded to be within the new discharge limits or because they are overloaded or for both reasons. Especially where the available space is limited, the choice must be oriented towards technologies that allow to obtain high purification yields with limited dimensions, as in the case of MBR ( Membrane Biological Reactor ) membrane bioreactor technologies : these are systems made up in synthesis, from `` coupling of a biological activated sludge system with a membrane filtration system, for the separation of biomass. The high filtering efficiency of the membranes allows the maintenance of sludge concentrations considerably higher than the values possible in traditional systems, thus increasing the purification capacity of the system with the increasing age of the sludge and consequent reduction in the quantity of sludge to be disposed of. It also allows the respect of bacteriological limits without having to resort to disinfection treatments.

In general, the MBR implant is particularly suitable when:

• a high quality of the effluent is required (e.g. discharge in sensitive areas),
• a low SST content is required,
• we intend to reuse purified water (for example for irrigation purposes),
• there is the availability of a limited space.

The MBR system can be easily integrated into a traditional plant scheme to replace the sedimentation and disinfection phase. The standard treatment cycle takes place through various sectors such as: primary sedimentation, aeration, sewage filtration. The automated operation of the various electromechanical equipment is managed by a local control panel.

MBRs are nowadays increasingly used in the treatment of civil and industrial wastewater, thanks also to the progressive reduction of membrane costs, but the design of full-scale plants remains rather empirical, due to the complexity in interpreting the interactions between biomass and the filter membranes. This inevitably translates into higher investment and operating costs compared to conventional activated sludge plants, mainly due to the need to provide for the containment of membrane fouling .

The advantages of MBR technology compared to traditional activated sludge systems can be summarized as follows:

1. improved quality of the final effluent , as the membrane acts as a barrier to suspended solids and microorganisms, eliminating the need for further filtration and disinfection of the final effluent;
2. being able to reach and maintain biomass concentrations in reactors that are much higher (up to 20 g / L) than those normally possible with conventional treatments. This substantially reduces the volume of the biological reactor and improves the efficiency of the treatment, since a higher concentration of biomass and a higher SRT (Solids Retention Time) determines a more rapid and complete degradation of nutrients and organic substrates;
3. manage the biological process in a totally independent way from hydraulic load fluctuations within the maximum flow admitted by the membrane
4. higher sludge age implies a lower biomass production , given that the growth of microorganisms is inversely proportional to the age of the sludge (production of sludge in the order of 0.04-0.1 kg for each kg COD removed) while the same parameter for the traditional activated sludge process varies in the range 0.6-1.0 kgSS / kgCOD;
5. lower surface commitments for the treatment of wastewater , given the possibility of developing the biological unit in height and the elimination of the secondary sedimenter, normally of considerable size;
6. separation of solids on the membrane does not affect excess filamentous bacteria (bulking), sludge rising phenomena (bulking) and other sedimentation problems.

The disadvantages of MBR technology compared to traditional biological treatment consist of :

1. higher investment costs and higher energy consumption mainly due to the need to aerate the membranes (for the submerged configuration) and to recycle the retained material (for the side-stream configuration). The difference is much less significant for the same quality of the final effluent, ie if the MBR process is compared with a traditional secondary and tertiary treatment (both stages are in fact replaced by the MBR system);
2. need to grate the incoming wastewater with 1-2 mm micro sieves to avoid clogging of the membranes by fibrous material, hair, etc .;
3. marked decrease in the oxygen exchange factor between the gas and liquid phases due to the high concentration of solids in the tank. If we consider that the aeration system used to clean the submerged membranes provides coarse air bubbles, whose transfer efficiency is very low, it is evident that the energy consumption is further increased;
4. reduced relative knowledge on the life of the membrane (7-10 years), and therefore of the costs deriving from its periodic replacement;
5. loss of membrane efficiency with filtration time with increase in resistance due to fouling phenomena . This makes it necessary to periodically clean the membranes, also through the use of chemical reagents, with a consequent increase in operating and management costs. Other problems can derive from biological fouling, i.e. uncontrolled bacterial growth on the filtering surface, which not only temporarily reduces the performance of the membrane, but also contributes to its degradation over time.