Question

What are the main advantages of incorporating membrane technology in biological treatment such as ASP? Draw a schematic diagram of activated sludge process (ASP) and membrane bioreactors (MBRs). Label all streams and provide a brief description for mixed liquor suspended solids (MLSS), return activated sludge, wastage sludge, hydraulic retention time, solid retention time, and F/M ratio.

Answer 1

The combination ?of membrane ?filtration and ?a suspended ?growth ?bioreactor ?efficiently ?removes organic ?and suspended ?solids. ?The membrane bioreactor (MBR), ?unlike the ?conventional ?activated ?sludge (CAS) ?system, ?doesn’t ?use a secondary ?clarifier or a ?settling tank ?which would ?allow the ?solids to ?settle and ?separate them ?from the liquid.? Instead, ?membranes are ?used for this ?purpose. This ?results in much ?better water ?quality.

Also, MBRs can be designed at higher MLSS concentrations because they are not concentrations because they are not affected by the affected by the limitations of gravity limitations of gravity sedimentation for solid sedimentation for solid -liquid separation separation.

Below is the schematic of the two processes namely ASP and MBR.

MLSS: The activated sludge process relies on the cultivation of a population of millions of microorganisms of many different types, mostly aerobic and facultative heterotrophic bacteria suspended in the wastewater, as it passes through a reactor (aeration tank). This suspension is called Mixed liquor suspended solids (MLSS).

Return activated sludge: Following an adequate amount of treatment time the mixed liquor flows from the aeration tank to a secondary clarifier where the biomass is allowed to settle out of the wastewater and the effluent passes to the next treatment step. The settled biomass is returned to the treatment process to provide organisms which will continue removing pollutants. This returned biomass is referred to as Return Activated Sludge (RAS).

Wastage sludge: The amount of biomass in the process is controlled by removing (wasting) a portion of it each day. This excess biomass removed from the secondary system is known as Waste Activated Sludge (WAS).

Hydraulic retention time: It is a measure of the average length of time that a soluble compound remains in a constructed bioreactor.

Solid retention time: It is defined as the average length of time in days that an organism remains in the secondary treatment system.

Food to Microorganism Ratio (F/M) is one of the primary controls used in activated sludge plants. This helps the operator to maintain a balance between the quantity of food available, with the quantity of microorganisms in the aeration tanks.