SOURCE:http://www.wrights-trainingsite.com/WWT%20MBR.htm
Membrane Bioreactor (MBR) in Wastewater Treatment
Description of MBR technology
The Membrane Bioreactor is a simple, but very effective combination of the activated sludge treatment process and the membrane filtration process. Imagine an activated sludge aeration basin, with sets of micro- or ultra-filtration membrane filtration modules submerged in the aeration basins, and you now have MBR.
The wastewater enters the wastewater treatment facility and passes through the usual Preliminary Treatment, and Primary Treatment processes. Some facilities then place fine screens (opening are less than 2mm in diameter) prior to the MBR reactors to remove small suspended particles such as human hair. This step is designed to reduce the potential fouling of the membranes with these fine particles. The dissolved BOD (sugars, starches, carbohydrates, etc) that is in the wastewater is then consumed by the microbes in the aeration basin, and subsequently converted into additional microorganisms, or becomes attached to the biological floc. The Mixed Liquor Suspended Solids (MLSS) is usually fairly high in MBR units, around 10,000 mg/L. (I have seen installations as high as 20,000 mg/L, and). This high MLSS concentration allows for lower hydraulic retention times (HRT) which equates to smaller aeration basins. This also equates to an activated sludge that may be fully nitrifying, as the Mean Cell Residence Time (MCRT) is usually well above 10 days. (I have seen installations with MCRT's up to 45 days. Talk about "extended aeration ashing!") The microbes are larger than the very small "perforations" or "holes" in the membranes. Pumps are attached to the membrane modules, and pull a slight vacuum that pulls water from the tank through the perforations in the membranes leaving the microorganisms behind in the tank.
Most all of the MBR facilities utilize fine bubble aeration in the aeration tanks, except for those areas that will have the MBR modules. These membrane module areas will usually have coarse bubble diffusers installed beneath them.
Some facilities may use the single tank MBR process, or the double tank MBR process. In the single tank the filtration modules are placed near the opposite end from where the primary effluent enters the tank. In a double tank configuration, designers may have an aeration tank without a filtration module in it, followed by an aeration basin with the membrane filtration unit in it. The treatment process goal in both designs is to allow for suitable time for the conversion of BOD/COD into microbial cells or at least be absorbed/flocculated with the cellular masses prior to being placed near the membrane filtration units. (We obviously do not want to have dissolved organics pass through the membranes.)
In other installations, anoxic selectors or even anaerobic selectors may be placed prior to the MBR aeration units to achieve nitrification goals, control filamentous microbes, etc. (See the Selector's, Part 1 & Part 2 in our Operator Notebook).
In "traditional activated sludge facilities" a secondary clarifier(s) follows the aeration basin which allows for the microbes to settle to the bottom of the tank, and a "clarified effluent" to leave the clarifier. The MBR process obviously does NOT use a secondary clarifier, as the effluent is far cleaner than that which would be produced by a secondary clarifier. In fact, the membranes produce an effluent (filtrate) that should be given "disinfection credits!" The membrane filtration process produces an effluent extremely low in suspended solids concentration and turbidity units. The quality of this water, when the process is properly operated, is amazing. Even when it is used in wastewater treatment applications, it rivals the best potable water I have seen!
I have personally inspected in depth one MBR facility that operates its disinfection process just because "it's there, and our discharge treatment permit requires that we do so." This facility routinely samples and tests for coliform bacteria immediately after the MBR process PRIOR to the disinfection process, and it has without fail MET the disinfection requirements PRIOR to disinfection! Of course, it also disinfects the wastewater and samples/tests after the disinfection process, and has always met their disinfection requirements there also. This is an example of how effective the membrane filtration process is!
It appears that the MBR process works best if it is a fully nitrifying process. As such, one of the benefits of nitrification is the lower sludge production that results from keeping the microbes under aeration for a longer period of time, which allows them to consume almost all of the BOD, convert the BOD into microbes, and have the microbes start consuming each other (endogenous respiration.)
The Membrane Bioreactors
In the one membrane bioreactor installation I am familiar with, thousands of long microstrands are bundled together in modules. The strands are set vertically, with the end of end single strand connected either to the top or to the bottom header. Each strand has millions of "pores" (small openings) that open into the hollow center of each strand. The nominal pore size is 0.04 µm.
The fiber diameters are: inside 0.9mm, outside 1.9mm. If you have ever seen fly fishing line, the floating type, then you will have an idea of what these fibers look like. With a nominal pore size of 0.04 µm, it is easy to see how microbes like the "Paramecium" with a size of 200 µm, and a bacterial cell whose size between 0.5 to 1.0 µm is "filtered out" by the membranes. For reference a fine human hair 30 µm. (The symbol for micron or micrometers is "µm."
The pores in the membranes are kept open by installing coarse air bubblers beneath the modules, which help scour the membranes, and by the injection of timed back-blows of air and/or treated water inside the membranes. A routine schedule of backwashing and chemical treatment (usually injection of a chlorine bleach solution) is also incorporated into the routine maintenance of the modules. This is all designed to reduce the potential fouling and plugging of the pores within the strands.
There are also "plates" that are being manufactured that perform in much the same manner.
Module in the tank without Mixed Liquor
In operation with Mixed Liquor
Collection header above the tank and modules
(Please click on the above thumbnails for full-sized pictures. Please use your browser's back arrow to return here.)
DESIGN NOTES: Energy cost's also need to be accurately compared between MBR and the traditional treatment process train when evaluating a particular facility. Is the operations and maintenance staff capable of operating and maintaining this more complex treatment process given the additional training and support that is required for this process? (That is a nice way of saying, is the agency or company willing to support this effort financially?) In summary, one also needs to consider more than "just the cost." What is the "value" of this cost? In other words, "What does the MBR process, in terms of consistent effluent quality, predictable outcomes, etc., that conventional treatment trains do or do not?"
Make sure you have a method of controlling the dissolved oxygen concentration in each cell with an automatic air balance in each aeration cell, so that electrical energy is optimized and not wasted.
Insure that you have a method of controlling the treatment process flow rate between banks or trains by automatically controlling weirs, valves, or gates. Often the loading rates vary among parallel treatment trains.
Insure that the methods of cleaning the MBR membranes to minimize chemical and biological fouling are proven technologies. (This is not the time to be a company's R&D pilot study.)
ADVANTAGES
1) The effluent is of very high quality, very low in BOD (less than 5 mg/l), very low in turbidity and suspended solids. The technology produces some of the most predictable water quality known. It is fairly easy to operate as long as the operation has been properly trained, pays strict attention to the proper operation, corrective maintenance, and preventative maintenance tasks.
2) The "simple filtering action" of the membranes creates a physical disinfection barrier, which significantly reduces the disinfection requirements.
3) The capitol cost is usually less than for comparable treatment trains.
4) The treatment process also allows for a smaller "footprint" as there are no secondary clarifiers nor tertiary filters which would be required to achieve similar water quality results. It also eliminates the need for a tertiary backwash surge tank, a backwash water storage tank, and for the treatment of the backwash water.
5) Generally speaking it produces less waste activated sludge than a simple conventional system.
6) If re-use is a major water quality goal, the MBR process will be a major consideration. This process produces a consistent, high water quality discharge. When followed by a disinfection process, it allows for a wide range of water re-use applications including landscape irrigation, non-root edible crops, highway median strip and golf course irrigation, and cooling water re-charge. When Reverse Osmosis (RO) water quality is required, the MBR process is an excellent candidate for preparing the water for RO treatment.
DISADVANTAGES
1) The membrane modules will need to be replaced somewhere between five (5) and ten (10) years with the current technology. While the costs have decreased over the past several years, these modules can still be classified as expensive. (The membranes "dry out" due to the flexible polymers leaching out, the closing/plugging of the pores, and the membranes becoming somewhat hard or brittle.) These costs are often offset somewhat when life-cycle costs for comparable technologies are examined. If the costs for the membrane replacement task continue to decrease then over time, then this process is even more financially viable.
2) In most sales pitches the MBR technology is stated as an option of replacing the secondary clarifier. Usually these clarifiers are operated with a single, very low horsepower motor, usually less than 2 HP. The electrical cost for this simple motor is significantly less than the filtrate pumps, chemical feed pumps, compressors, etc., of the MBR system. While this energy cost is significantly higher, the MBR system produces a significantly higher quality effluent that most clarifiers could never achieve.
3) Fouling is troublesome, and its prevention is costly. Several papers and research endeavors have concluded that up to two-thirds of the chemical and energy costs in an MBR facility are directly attributable to reducing membrane fouling. While this is costly to be sure, future advances into this area will continue to reduce these costs.
4) There may be cleaning solutions that require special handling, treatment, and disposal activities depending on the manufacturer. These cleaning solutions may be classified as hazardous waste depending on local and state regulations.
REFERENCES:
Crites, R. W. and Tchobanoglous, G, (1998) "Small and Decentralized Wastewater Management Systems," McGraw-Hill Book Company, New York, NY
Daigger, G.T., Crawford, G., Fernandez, A., Lozier, J.C. and Fleischer, E. "WERF Project: Feasibility of Membrane Technology for Biological Wastewater Treatment B Identification of Issues and MBR Technology Assessment Tool." Water Environment Federation 74th Annual Conference & Exposition, Atlanta, GA, CD-ROM, October 13-17, 2001.
Johnson, W.T. "Recent Advances in Microfiltration for Drinking Water Treatment." AWWA Annual Conference, Chicago, IL. June 20-24 1999
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