A membrane bioreactor (MBR) is an advanced approach to wastewater treatment. It integrates a biological treatment process, specifically a suspended growth bioreactor, with a membrane filtration system. The purpose of an MBR is to treat wastewater to a high quality, making it suitable for discharge into sensitive environments or for various reuse applications. This combination allows for efficient removal of pollutants.
The MBR Process Explained
The MBR system operates through two main components. The first stage is biological treatment within a bioreactor tank. Here, a dense population of microorganisms, known as activated sludge, breaks down organic pollutants. These microorganisms consume dissolved and suspended organic matter, converting it into carbon dioxide, water, and new microbial biomass. This high concentration of microorganisms enhances pollutant degradation.
Following biological treatment, membrane filtration separates the treated water, known as permeate, from the activated sludge mixture. Membranes, typically made of polymeric materials, serve as a physical barrier. These membranes, which can be microfiltration or ultrafiltration types, have microscopic pores (0.03 to 0.4 micrometers). This pore size effectively retains suspended solids, bacteria, and some viruses, allowing only clean water to pass.
Two primary configurations exist for MBR systems. In a submerged MBR, membrane modules are immersed within the bioreactor tank, drawing treated water by suction. This configuration often benefits from simpler pipework and lower energy consumption. Conversely, a sidestream MBR circulates mixed liquor from the bioreactor through a separate external membrane tank. This allows for easier access to membranes for maintenance and cleaning.
Key Differentiators from Conventional Treatment
MBR technology offers significant advantages over conventional activated sludge (CAS) systems, primarily due to the membrane barrier. This physical separation leads to superior effluent quality, as membranes block suspended solids, bacteria, and pathogens. MBR-treated water typically exhibits very low turbidity (often less than 0.2 NTU) and achieves high removal rates for microorganisms like E. coli and Cryptosporidium. This purification surpasses what is achievable with secondary clarifiers in CAS systems.
The membrane’s ability to retain solids allows for a much higher concentration of microorganisms within the bioreactor, often reaching mixed liquor suspended solids (MLSS) levels of 8,000 to 15,000 mg/L. This elevated biomass concentration translates to a smaller physical footprint for the treatment plant. Unlike CAS systems that rely on large secondary clarifiers, MBRs eliminate these large sedimentation tanks, reducing space requirements by 50% or more.
MBR systems generally produce less excess sludge. The longer sludge retention time (SRT) allows microorganisms to remain in the system for extended periods. This leads to more complete degradation of organic matter and increased endogenous respiration. As a result, waste sludge volume can be reduced by 20% to 50%, lowering disposal costs.
Common Applications and Industries
MBR technology is widely applied across sectors requiring high-quality wastewater treatment. In municipal wastewater treatment, MBRs are valuable in urban areas with limited land, due to their smaller physical footprint. They are also used where stringent discharge limits apply or for direct effluent reuse. Their ability to produce a consistently high-quality effluent makes MBRs a preferred choice for sustainable water management.
Industrial wastewater treatment is another significant application. Industries like food and beverage, pharmaceuticals, and textiles often generate wastewater with high concentrations of organic pollutants. MBRs effectively treat these high-strength organic wastes, consistently meeting discharge regulations. Their compact nature suits industries with limited space.
MBR technology plays a prominent role in water reuse and reclamation projects. The high-quality treated water produced by MBRs is suitable for various non-potable applications, including agricultural irrigation, industrial process water, and replenishing groundwater aquifers. This enables communities and industries to reduce their reliance on fresh water sources. The permeate from MBRs often requires minimal further treatment, such as disinfection, before being safely reused.
Operational Considerations and Membrane Fouling
A primary operational challenge in membrane bioreactor systems is membrane fouling, which refers to the accumulation of particles, microorganisms, and dissolved substances on or within the membrane pores. This accumulation reduces the membrane’s permeability, decreasing permeate flow and increasing energy consumption. Fouling can be categorized into several types, including biofouling (caused by microbial growth), organic fouling (from soluble microbial products and humic substances), and inorganic fouling (due to mineral scaling).
To maintain efficient operation and mitigate fouling, MBR systems employ routine maintenance procedures. Backwashing involves periodically reversing water flow through membranes to dislodge foulants. Relaxation, a simpler method, temporarily stops filtration to allow solids to detach, often facilitated by aeration. These physical cleaning methods are typically performed daily or multiple times a day to prevent severe fouling.
When physical cleaning methods are insufficient, chemical cleaning procedures restore membrane performance. This involves circulating specific cleaning solutions, such as sodium hypochlorite for organic and biological foulants, or citric acid for inorganic scales. These chemical agents dissolve or break down foulants, allowing them to be flushed away. Regular chemical cleaning, performed on a weekly or monthly basis, is necessary to sustain MBR membrane performance over their operational lifespan.