By Pang Liwei, China National Chemical Information Center (CNCIC)
The membrane bio-reactor (MBR) is a new waste water treatment system combining membrane separation technology and biological waste water treatment technology. It can maintain highly-active sludge concentration and increase the organic loading of biological treatment in the bio-reactor, so as to reduce the coverage area of sewage treatment facilities. MBR can deeply purify sewage thanks to its efficient cutoff function, and can also facilitate deep denitrification and dephosphorization, because nitrobacteria can multiply fully in the system.
Membrane biology technology was applied at one time in the microbial fermentation industry, and later was used in water treatment field in the 1960s. Now it has become one of the most potent technologies in the water treatment sector, efficiently treating and recycling sewage as well as killing byproducts during the sterilization of drinking water. Globally, many countries are using, studying, and upgrading this technology.
MBR technology processes
MBR consists of two parts – the biological reactor and the membrane module (microfiltration, ultrafiltration, nanofiltration or osmosis membrane). It can be classified as either reverse membrane bio-reactor (RMBR) or submerged membrane bio-reactor (SMBR), according to the combination modes of the two parts.
RMBR has its membrane module and biological reactor separate, making it easy for the two parts to be cleaned and changed. However, the high-speed rotating pump inactivates some bacteria, and thus they form deposits on the membrane surface that must be treated as pollutants. The flow of water supplied by circulating pump usually consumes a lot of power in order to reduce the accumulated pollutants.
With no circulating pump installed, SMBR occupies a smaller area in comparison with RMBR. However, the membrane segment of the reactor is difficult to dismantle and wash. So far, producers have overcome the difficulty by modifying the properties and structure of the membrane and by adopting a hollow fiber ultrafiltration membrane module.
MBRs can be divided into membrane separation bioreactors, membrane aeration bioreactors, and extractive membrane bioreactors, according to functions of MBRs. A membrane separation bioreactor separates solids and liquids by identifying and holding back solids via a membrane. A membrane aeration bioreactor generates bubble-free aeration that can enhance oxygen transmission efficiency and shorten exposure time for aerobiont reaction. It’s a machine that can be used in treating waste water in a high oxygen room. An extractive membrane bioreactor is widely used in the treatment of priority pollutants in industrial waste water, with a selective permeable membrane selected to extract undetermined pollutants so as to prevent high-pH or biologically-harmful substances from contacting microbes directly.
Applications of MBRs in sanitary sewage treatment
Anaerobic membrane bio-reactors, aerobic membrane bio-reactors, and anoxic membrane bio-reactors are commonly used in the treatment of sanitary sewage. Two other types of reactors – split-flow membrane bio-reactors and reverse membrane bio-reactors – are used for updating and optimizing traditional MBRs in order to comply with biological nitrogen and phosphorus removal technology. The content of nitrogen and phosphorus is low in sanitary sewage, so plants usually control endogenous carbon at the early stage of MBR waste water treatment for the rational application of carbon and enhancement of nitrification efficiency. Meanwhile, they usually adopt chemical phosphorus removal technology, with ozone as the chemical item, to offset the lower-than-expected biological phosphorus removal effect. As for waste like feces, the denitrification method was the first choice for treatment, but the ultrafiltration membrane filtration method, consisting of a receiver module, a highly-efficient nitrogen removal biological reaction device, and a deep treatment & sterilization module, has replaced it by now.
Application of EMBR in Iindustrial waste water treatment
EMBR combines membrane extraction technology and biodegradation technology. A selectively permeable membrane separates the waste water circulating segment and the biodegradation segment thoroughly, and only allows target organic pollutants - with high toxicity and hydrophobicity - to enter the biodegradation segment through the membrane, in order to be degraded by special degrading bacteria. Other inorganic components of the waste water are kept in the circulating segment, with no influence on microbial metabolism. Some waste water contains substances with high-acid content, high-alkali content, high salinity, and biological toxicity, but EMBR can treat it well and keep it away from microbes.
EMBR can also be used in the process of coking waste water treatment. Coking waste water is a kind of high-concentration waste water generated during coal-to-coke, coal gas purification and coking products recycling. It compromises a big amount of organic pollutants such as phenols, pyridine and indole, as well as a number of toxic and hazardous substances like cyanide and fluoride. With the replacement of traditional secondary sedimentation tank with EMBR, the membrane module of EMBR can treat coking waste water resulting in better solid-liquid separation and pollutant removal.
MBR requires a large amount of oxygen and therefore consumes excessive energy. Meanwhile, the accumulation of substances that are hard for microbes to degrade usually harms microbes and pollutes the membrane. In response to these issues, modification and optimization of current technologies are underway. For example, the structure and properties of the membrane can be modified to improve the separation effect and reduce operation costs; the cleaning method of the membrane can be improved to avoid secondary pollution; the membrane module form can be improved for better structure; and the operation mode, including aeration strength, aeration interval, and cleaning cycle, can be improved so as to moderate pressure, temperature and flow speed.