Biological processes defined as membrane bioreactors, MBR, are obtained by the combination of a biological treatment system of activated sludge with a process of membrane filtration, replacing the classic gravity clarifier. This type of solution is more efficient than the classic active sludge and offers the possibility of reusing part of the treated water, thanks to its high chemical, physical and bacteriological quality.
These values allow to feed directly, without additional filtration treatments, RO systems; thus, with a single technology it is possible to replace the units of sedimentation, quartz filter and ultrafiltration or tertiary disinfection, obtaining a plant considerably more compact and simplified compared to a traditional plant.
Therefore the goals that a MBR system can reach are:
- The complete removal of suspended solids and an increased concentration of activated sludge: in this way we obtain a greater degradation of inlet BOD5, a longer sludge age and a significant reduction of extra sludge quantity;
- A higher and constant quality of treated water: it is possible to avoid problems due to phenomena of bulking, producing water with characteristics such that it can be partially reused in the factory services or for internal use, with a further reduction of the process costs.
- Very compact treatment unit: allowing future expansion of the plant, a lower environmental impact, lower maintenance costs, more space available for other uses.
To obtain an optimal work of MBR, it is very important to pay attention to the operating parameters of the biological plant. In fact, with a too high F/M ratio, the characteristics of the biomass can vary and, thus, reduce the specific flux of the membranes and reduce their efficiency and capacity. The recirculation of the separated sludge in the MBR tank follows the same rules of the classic one.
There are three main scheme of plant with MBR:
- MBR-side: is the most used solution. The MBR system is submerged in the water to be treated in a different tank from biological one;
- MBR-submerged: MBR is immersed directly in the oxydation tank;
- MBR-dry: the membranes are installed in a separate area within suitable vessels, where it is separated liquid-bacteria.
The main types of membranes instead are:
Membranes in hollow fibers: are composed of tubes of synthetic material with a diameter less than 0,1 microns. The filtration is approached tangentially, with insufflation of air through diffusers placed at the bottom of the module. The cleaning cycles are constituted by phases of backwashing and chemical cleaning cycles
Membranes in plane modules: each membrane rests on a plane support with a grid of drainage to avoid the crushing of the permeate’s compartment. The modules include a system of air insufflation through special diffusers in the bottom, so as to implement the so-called "cross-flow"tangential filtration, which is necessary to prevent the settling of sludge and the clogging of the filtering surfaces. Even in this case, cleaning cycles are necessary to preserve the proper functioning of the membranes.
Therefore in summary, the possible advantages that are obtained using a MBR system are:
- Reduced sensitivity to load variations. In fact the MBR system is less sensitive to overload, due to the higher concentration of biomass;
- High quality of effluent, almost devoid of suspended solids;
- Reduced volume of the plant and easier management of the process;
- High Age of sludge and independent on the hydraulic retention time;
- Reduced environmental impact: the respect for bacteriological limits of the effluent can be achieved without the use of chemical disinfectants;
- Possibility of reuse the filtered water for irrigation or as service water at the factory or for feeding of RO systems.
Instead, the disadvantages are:
- Fouling and clogging of the membranes, which require periodic cleaning and shrewdness that extend the duration of the operating cycles;
- The cost of the membranes is high and affects the investment costs;
- The energy cost of a MBR plant is still higher compared to a conventional activated sludge process.
In any case, the strictest limits at the discharge that make obsolete the traditional processes, the expansion need of the existing plants with limited available space, the problems related to the sludge disposal, the tendency to reuse the treated effluent, are all factors that are in favor of the affirmation of MBR systems.