Description of MBR Technology
The membrane bioreactor (MBR) concept is a combination of conventional biological wastewater treatment
plant and membrane filtration. The concept is technically similar to that of a traditional wastewater
treatment plant, except for the separation of activated sludge and treated wastewater.
In an MBR installation, this separation is not done by sedimentation in a secondary clarification tank, but by membrane filtration.
· The wastewater enters the wastewater treatment facility and passes through the usual preliminary treatment,
and primary tTreatment 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. (up to 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. 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.
· 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.
· 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!
· 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.
· We 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.)