Membrane bioreactors (MBR) are no longer novel or unusual wastewater secondary treatment techniques and have become widely used for municipal and industrial wastewater treatment plants up to an 80,000-population equivalent. MBR systems combine suspended growth biological reactors with microfiltration or ultrafiltration for solids removal.
An MBR system must be protected from large particulates by fine screens with openings of less than three millimeters. A properly performing MBR will produce substantially less sludge than conventional treatment, reducing the cost of sludge disposal. MBR systems produce high-quality wastewater effluent, and this makes them especially suitable for reuse applications and surface water discharge applications with extensive nutrient removal requirements.
MBR systems do have higher operating and maintenance costs than conventional treatment techniques, such as membrane cleaning and fouling control. The image below from RiversAlliance.org provides an accurate illustration of the MBR process.
Wastewater, by definition, is water that contains undesirable particulates and biological agents, therefore all aspects of the wastewater influent can lead to MBR fouling. Colloidal particles, like clays and flocculates, pathogens, organic compounds and scale-causing compounds, like calcium carbonate, can cause membrane fouling. Poor MBR maintenance can result in an increase in hydraulic resistance, low quality effluent and increased long-term expenses resulting from membrane damage and replacement.
Membrane fouling can be controlled through several methods. All MBR systems use an air scour technique to reduce build-up of material that leads to a fouling layer on the membrane surface. Pressurized air is forced into the permeate side of the membrane and causes fouling to be released on the influent side of the membrane. Another acceptable method is membrane backwashing.
Permeate is pumped back through the membrane pores to dislodge any internal or external fouling compounds. Intermittent permeation, also called back-pulsing, also can remove a fouling layer and occurs at regular time intervals during the filtration process.
This method causes particles deposited on the membrane surface to diffuse back to the reactor for further treatment. And certain chemical applications, such as bleach and citric acid, in accordance with manufacturer maintenance protocols, can reduce membrane fouling.
Different types/intensities of chemical cleaning can be performed depending on the extent of membrane fouling. Chemically enhanced backwashing can occur daily, while maintenance cleaning with a higher chemical concentration might occur weekly. And intensive chemical cleaning may occur only once or twice a year if proper maintenance techniques are being followed.