Membrane Bioreactor

(Adapted from FITZGERALD 2008)

Membrane Bioreactors combine conventional biological treatment (e.g. activated sludge) processes with membrane filtration to provide an advanced level of organic and suspended solids removal. When designed accordingly, these systems can also provide an advanced level of nutrient removal. In an MBR system, the membranes are submerged in an aerated biological reactor. The membranes have porosities ranging from 0.035 microns to 0.4 microns (depending on the manufacturer), which is considered between micro and ultrafiltration. 

This level of filtration allows for high quality effluent to be drawn through the membranes and eliminates the sedimentation and filtration processes typically used for wastewater treatment. Because the need for sedimentation is eliminated, the biological process can operate at a much higher mixed liquor concentration. This dramatically reduces the process tankage required and allows many existing plants to be upgraded without adding new tanks. To provide optimal aeration and scour around the membranes, the mixed liquor is typically kept in the 1.0-1.2% solids range, which is 4 times that of a conventional plant.

Membrane

(Adapted from RADJENOVIC et al. 2008)

During MBR wastewater treatment, solid–liquid separation is achieved by Microfiltration (MF) or Ultrafiltration (UF) membranes. A membrane is simply a two-dimensional material used to separate components of fluids usually on the basis of their relative size or electrical charge. The capability of a membrane to allow transport of only specific compounds is called semi-permeability (sometimes also permselective). This is a physical process, where separated components remain chemically unchanged. Components that pass through membrane pores are called permeate, while rejected ones form concentrate or retentate.
There are five types of membrane configuration which are currently in operation:

• Hollow fibre (HF)
• Spiral-wound
• Plate-and-frame (i.e. flat sheet (FS))
• Pleated filter cartridge
• Tubular

Read more about the several membrane possibilities in RADJENOVIC et al. (2008).

Pre-treatment

To avoid unwanted solids in the waste stream, which enters the membrane tank, fine screening is an essential pre-treatment step. This minimises an accumulation of solids and protects the membrane from damaging debris and particles, extends the membrane life, reduces operating costs and guarantees a higher sludge quality as well as a trouble free operation (GE 2011).

(Adapted from RADJENOVIC et al. 2008)

Although MBR capital and operational costs (membranes, oxygen utilisation, expert design, etc.) exceed the costs of conventional process, it seems that the upgrade of conventional process occurs even in cases when conventional treatment works well. This can be related to increase of water prices and the need for water reuse as well as with more stringent regulations on the effluent quality.
 

Most MBRs employ chemical maintenance cleaning on a weekly basis, which lasts 30–60 min, and recovery cleaning when filtration is no longer durable, which occurs once or twice a year. A deposit that cannot be removed by available methods of cleaning is called “irrecoverable fouling”. This fouling builds up over the years of operation and eventually determines the membrane life-time (RADJENOVIC et al. 2008). All O&M tasks have to be done by skilled workers.

Fouling

(Adapted from RADJENOVIC et al. 2008)

Modern systems (e.g. KUBOTA systems) are maintained with chemicals, i.e. it is not necessary to remove the membranes from the membrane tank. Organic fouling can be cleaned with as sodium hypochlorite and inorganic fouling with oxalic acid (KUBOTA 2010). 

Fouling occurs as a consequence of interactions between the membrane and the mixed liquor, and is one of the principal limitations of the MBR process. Fouling of membranes in MBRs is a very complex phenomenon with diverse interlinkages among its causes, and it is very difficult to localise and define membrane fouling clearly. The main causes of membrane fouling are:

• Adsorption of macromolecular
• Growth of biofilms on the membrane surface
• Precipitation of inorganic matter
• Aging of the membrane

Operation and maintenance of MBR systems is generally carried out by skilled labourers, which should be sufficiently trained regarding any health risks. Sludge from the biological system should be dewatered (e.g. mechanical dewatering or drying bed) and incinerated with the ashes being stored in a controlled landfill.