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Increasingly, wastewater treatment facilities are adopting membrane bioreactors (MBRs), mostly made of submerged membrane modules. While submerged MBRs offer significant advantages, there is growing demand for more environmentally friendly cleaning of the modules.
The need for improved cleaning is the driving force behind the development of the new Bio-Cel modules, which permit chemical-free cleaning through a mechanical cleaning process that uses a beaded granulate and achieves a significant enhancement of flux. The Bio-Cel solution has been successfully tested in a pilot unit in collaboration with Darmstadt University of Technology.
Effluent Quality Reigns
MBRs combine classic biological wastewater treatment with membrane technology. The biomass is separated from the cleaned wastewater by means of membranes, usually ultrafiltration or microfiltration. This method provides a significant advantage: The membrane acts as a barrier to biomass, particles and bacteria, which leads to substantial improvement in effluent quality when compared to conventional, purely biological techniques in terms of solids and hygienic parameters.
In addition, combining biological and membrane technology increases the biomass concentration in the activated sludge tank. As a result, capacity can be increased and footprint decreased by converting a traditional activated-sludge plant to an MBR. Additional space is gained because the final sedimentation stage can be eliminated. Furthermore, the discharge from an MBR plant is essentially free of solids, making MBR ideal for supplementary treatment steps with reverse osmosis or nanofiltration as well as ultraviolet or ozone.
Overcoming Braiding & Silting
The Bio-Cel module, developed by Microdyn-Nadir GmbH, combines the benefits of flat-sheet membranes and hollow-fiber modules. This space-saving module, based on a back-flushable flat-sheet membrane, is insensitive to braiding and sludge deposits. It requires only a simple pretreatment of the feed flow and facilitates extended operation between cleaning cycles. Yet, even with this design, periodic intensive cleaning with chemicals is still needed.
Cleaning of the membrane modules is required to remove the deposits—otherwise they will reduce the membrane’s permeability during operation. There are two types of deposits—scaling and fouling—each requiring suitable cleaning methods. Scaling refers to inorganic deposits, usually calcium carbonate or ferric salts. Cleaning with organic acids (e.g., citric, formic or acetic) removes these. The organic acids are not objectionable environmentally because the spent cleaning solutions can be disposed of with the help of membrane biology.
In contrast, fouling is caused by organic deposits, which is the reason it is often called biofouling. Fouling can be eliminated with oxidants like hydrogen peroxide or sodium hypochlorite (NaOCl). In MBR plants, chlorine has become the norm for removing fouling due to its excellent cleaning efficiency. Unfortunately, it is used in high concentrations—up to 2,000 mg/L in some cases—that cause the formation of large quantities of AOX compounds. AOX is a sum parameter for so-called “adsorbable organic halogen” compounds, which are classified as environmentally harmful substances.
Although MBR technology is, in principle, a very eco-friendly process as far as membrane cleaning is concerned, it does have this ecological disadvantage.
Clean & Green
In a quest to eliminate this ecological drawback, a study was conducted to determine the Bio-Cel module’s suitability for chemical-free cleaning in activated sludge without impairing the effluent quality of the membrane. Previous trials confirmed the module could be in use for more than one year without requiring intensive cleaning in a chemical bath. Chemical back-flushing in an activated-sludge tank is adequate; however, the new study revealed that permeability is lost despite back-flushing with chemicals and intensive cleaning is unavoidable sooner or later.
The investigation into chemical-free cleaning of Bio-Cel modules was based on the premise that, by mechanical means, incipient deposits on the membrane can be prevented from forming or deposits already present can be continuously removed. To produce the mechanical cleaning action, plastic granulate was selected. The granulate was to flow upward between the membrane pockets, propelled by the aeration essential to operate the modules. This required the granulate density to be slightly heavier than the activated sludge and thus able to settle outside the module; it could not be so heavy, however, that it could not be whirled up again and conveyed to the top by the aeration underneath the module.
Mechanical vs. Chemical
The study utilized two Bio-Cel modules, each with a membrane area of 10 sq meters, installed in two parallel filtration chambers on a pilot plant scale and under identical conditions. One of the Bio-Cel modules, based on a back-flushable flat-sheet membrane module, was operated conventionally in activated sludge without adding granulate, while the other Bio-Cel module was required to clean a mixture of sludge and granulate.
Both modules were set to a flux of 15 L/sq meter/hr for the first three months. After around 70 days in service, the permeability of the reference module had fallen to approximately 40% of the initial value, whereas the mechanical cleaning process (MCP) module had not yet lost any of its permeability. The module without granulate was then maintained at a constant permeability by introducing intermediate cleaning (back-flushing with NaOCl).
In the next trial phase, the flux of the module with granulate was increased to 40 L/sq m/hr. This high flux eventually triggered a reduction in permeability. The rate was then kept constant at 30 L/sq m/hr for a period of several weeks. The Bio-Cel MCP module has been cleaning without chemicals throughout the trial (700 days and counting).
In a subsequent test, granulate was added to the activated sludge in the reference module. After seven days, this Bio-Cel module had likewise regained its original permeability. This shows that even subsequent cleaning with granulate has the desired effect on the modules.
The study findings demonstrate that the Bio-Cel module can be cleaned mechanically by adding granulate to the activated sludge; therefore, chemical-free cleaning is possible. The use of a Bio-Cel MCP keeps the permeability—and consequently the performance—of the module at a constant level while increasing the hydraulic flux. Furthermore, the MCP is suitable for external module cleaning if the membrane is already fouled.
In addition, the mechanical cleaning process with Bio-Cel modules enhances the environmental compatibility of MBR plants. Problems with AOX compounds will also be minimized. The Bio-Cel MCP will also improve MBR plant availability and pave the way for the construction of even more compact and efficient facilities due to the higher average flux operation.