Membrane biological systems (MBR)

The Membrane BioReactor is a biological water filtration plant combining a traditional cleaning process on activated sludge with a membrane separation system (generally microfiltration or ultrafiltration), replacing a normal secondary separator. The system is extremely useful as it reaches high activated sludge concentrations in the biological reactor (10-15 kgSS/m3), which can’t be reached in traditional systems. The use of membranes instead of a separators avoids possible sludge leakages, extremely frequent in industrial plants, often leading to many problems (variable immediate capacities with the consequent increase of the surface hydraulic load, presence of light sludge, filament bacteria bulking, etc).

The MBR can by set with a two different installation configurations:

• with external filtration (MBR side-stream);
• with submersed filtration (MBR submerged membrane), where the membranes are immersed in the same activated sludge vessel, in direct and constant contact with the waste.

The waste, following pre-treatments, such as solid and grease removal and thin grilling, is conveyed to the activated sludge vessel where, as in the traditional separators, the organic substance is degraded. In the case of external/sidestream filtration, the waste is conveyed to the external filtration process while in the case of the internal/submersed configuration, the waste is sucked through filtrating membranes. The permeate (the filtered liquid crossing the membranes) is then pumped to an external disinfection system, to a reverse osmosis treatment or directly to the outlet, according to the specific operating scopes.

During the filtration process, air intermittently flows inside the lower unit part where the membranes are located, preventing any sludge build-up and thus avoiding the membrane fouling. As to keep the membrane perfectly operating in time, a counter-washing is forecast (washing under counter-current), leading to the complete removal of any material accumulation on their surfaces. The accumulated sludge during the membrane filtration process is well stabilized thanks to the sludge aging and the lower organic load concentrations, depending on the consistently high active biomass concentrations and it can be directly pumped to be disposed without any biological stabilization treatments.

• Possibility to widen existing plants, with a limited intervention on the wall structures;
• Reduced installation size, including the possibility to build covered units with no environmental impact;
• Absence of the final sedimentation unit;
• High cleaning rates and yields (refer to the following table);
• As for the industrial waste water treatment, it is possible to reach very high quantity thus recycling cleaned water inside any production cycle;
• Available in units and therefore the installation size can be modified at ease.
• The high sludge aging lead to a lower excess sludge production, thus cutting disposal costs.

However MBR systems at present lead to general investment slight higher than traditional system thus increasing the power consumption during operation.

Applications:
MBRs are suitable in case of widening of existing treatment plants, with activated sludge, old-fashioned or undersized after an increase of the waste inlet.
MBR units are suitable for the domestic and industrial waste treatments, from AGRICULTURE AND FOOD INDUSTRY (dairy sector, meat processing, wine processing, pasta companies), TEXTILE AND CLEANING INDUSTRY, PAPER INDUSTRY, DRUG AND COSMETIC INDUSTRY.
Concentrations of the main chemical and biological parameters which can be reached with an MBR systems.

Analytical parameters	Values
BOD5	< 2 mg/L
SST	< 0.5 mg/L
NH3-N	< 0.5 mg/L
N-tot	< 3 mg/L
P-tot	< 0.05 mg/L
Fecal coliforms	< 10 CFU/100 mL

STA offers MBR customised systems, but a wide range of prefab units systems are available as well sized according to the specific pollutant load, in a traditional concrete structure or profiting from the latest up-to-date solutions available on the market with structured plastics (PE, PRFV, etc) and steel as well.