Membrane Bioreactors for Wastewater

Treatment.

MBR is the combination of a membrane process like microfiltrationor ultra filtration with a suspended growth bioreactor, and is now widely used for municipal and industrial wastewater treatment with plant sizes up to 90,000 population equivalent.

MBR(Membrane Bio Reactor) is an Advanced technology is now widely used for municipal and industrial wastewater treatment plant. Most important aspect of MBR technology is production of very high quality Treated Water consistently.

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Process of MBR:

When used with domestic wastewater, MBR processes can produce effluent of high quality enough to be discharged to coastal, surface or brackish waterways or to be reclaimed for urban irrigation. Other advantages of MBRs over conventional processes include small footprint, easy retrofit and upgrade of old wastewater treatment plants.

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Two MBR configurations exist: internal/submerged, where the membranes are immersed in and integral to the biological reactor; and external/ sidestream, where membranes are a separate unit process requiring an intermediate pumping step.

Internal/submerged : The filtration element is installed in either the main bioreactor vessel or in a separate tank. The membranes can be flat sheet or tubular or combination of both, and can incorporate an online backwash system which reduces membrane surface fouling by pumping membrane permeate back through the membrane.

External/side stream :The filtration elements are installed externally to the reactor, often in a plant room. The biomass is either pumped directly through a number of membrane modules in series and back to the bioreactor, or the biomass is pumped to a bank of modules, from which a second pump circulates the biomass through the modules in series. Cleaning and soaking of the membranes can be undertaken in place with use of an installed cleaning tank, pump and pipe work.

Process of SBR :

Sequential batch reactors (SBR) are a type of activated

sludge process for the treatment of wastewater. SBR

reactors treat wastewater such as sewage or output

from anaerobic digesters or mechanical biological

treatment facilities in batches. Oxygen is bubbled through

the mixture of wastewater and activated sludge to reduce

the organic matter (measured as biochemical oxygen

demand (BOD) and chemical oxygen demand (COD)). The

treated effluent may be suitable for discharge to surface

waters or possibly for use on land.

There are five stages in the treatment process:

Fill

React

Settle

Decant

Idle

The inlet valve opens and the tank is being filled in, while

mixing is provided by mechanical means (no air). This stage

is also called the anoxic stage.During the fourth stage the outlet valve opens and the clean supernatant liquor

exits the tank.

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The various stages in the sequence are as follows:Stage 1:Filling

During this stage the SBR Tank is filled with the influent wastewater. In order to

maintain suitable F/M (food to microorganism) ratios, the wastewater should

be admitted into the tank in a rapid, controlled manner. This method functions

similarly to a selector, which encourages the growth of certain

microorganisms with better settling characteristics.

Stage 2: Reaction

This stage involves the utilization of biochemical oxygen demand (BOD) and

ammonia nitrogen, where applicable, by microorganisms. The length of the

aeration period and the sludge mass determines the degree of treatment. The

length of the aeration period depends on the strength of the wastewater and

the degree of nitrification (conversion of the ammonia to a less toxic form of

nitrate or nitrite) provided for in the treatment.

Stage 3: Settling

During this stage, aeration is stopped and the sludge settles leaving clear,

treated effluent above the sludge blanket. Duration for settling varies from 45

to 60 minutes depending on the number of cycles per day.

Stage 4: Decanting

At this stage of the process effluent is removed from the tank through the

decanter, without disturbing the settled sludge.

Stage 5: Idling

The SBR Tank waits idle until it is time to commence a new cycle with the filling

stage.

Stage 6: Sludge Wasting Excess activated sludge is wasted periodically

during the SBR operation. As with any activated sludge treatment process,

sludge wasting is the main control of the effluent quality and microorganism

population size. This is how the operator exerts control over the effluent quality

by adjusting the mixed liquor suspended solids (MLSS) concentration and the

Mean Cell Residence Time (MCRT).

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