Membrane bioreactors (MBRs) are suspended growth bioreactors that also implement a membrane process such as microfiltration or ultrafiltration. MBRs are widely used in wastewater treatment operations and are favored because of their low carbon footprint.
The MBR process utilizes two techniques, membrane filtration and biological treatment. Through biological treatment, microorganisms and air form an activated sludge to break down organic contaminants in wastewaters. Remaining contaminants and residual solids are then filtered out through a semi-permeable membrane.
The main limiting factor in MBR operation is the fouling factor, which is due to deposition of materials onto the filtration membrane. This deposition is mainly due to interactions between the membrane and components of the activated sludge. This fouling can be reduced through air-induced cross flow, intermittent permeation, membrane backwashing, air backwashing, and proprietary anti-fouling products.
Types of MBRs
Membrane bioreactors are classified as either internally configured or externally configured.
- Internal or submerged bioreactors are configured so that the filtration element is installed in the main bioreactor or in a separate but connected tank. The membranes can be flat, tubular, or a combination. The membranes can utilize a backwash system which pumps membrane permeate back through the membrane to reduce fouling. Submerged MBRs use less energy than their sidestream counterparts.
- External or sidestream bioreactors are configured so that the filtration element is installed externally to the reactor, usually in a plant room. Cleaning and soaking of the membranes can be undertaken in place with the use of an installed cleaning system.
When considering an MBR for a wastewater treatment process, the membrane characteristics, sludge characteristics, and operating conditions all effect filtration performance. Designs must be built to specifications that properly address the incoming wastewater stream and its contaminants.
- Membrane characteristics include its pore size, surface roughness, surface charge, and how it interacts with water (hydrophobicity or hydrophilicity). These generally will be altered based on the characteristics of the residual contaminants.
- Sludge characteristics include particle size, biomass concentration, and viscosity. These will affect how the material flows through the process and will determine the requirements for removal.
- Operating conditions include temperature, aeration, sludge retention time, and hydraulic retention time. Temperature and aeration affect the biomass and colloid concentrations. Retention times are dependent on the particle size and structure of the sludge.
Evaluation of MBRs
MBRs offer a number of advantages to traditional bioreactors using secondary clarifiers and sand filters. They exhibit high removal efficiency, better effluent quality, and the ability to remove a number of contaminants such as nitrogen, phosphorus, bacteria, and suspended solids. They also tend to require less space because they operate at higher volumetric loading rates than conventional systems.
Disadvantages of MBRs are higher capital and operating costs due to membrane cleaning and replacement and high energy costs. Furthermore, additional chemicals may be required to increase a waste sludge's settling rate in the system.
Applications of MBRs
Applications of MBRs include municipal, commericial, and industrial water and wastewater treatment. They can be used in small to large scale operations and are particularly favored for reuse applications due to the high quality effluent that is produced.