Longantai Sewage Water Treatment Plant

MBR need a shear force over the membrane surface in order to avoid membrane fouling from the wastewater contents and is critical in maintaining a desired permeate flux.

When an air/ liquid stream flows parallel to the membrane surface it creates a shear force which helps limiting the degree of fouling on the latter.

Immersed processes are using the aeration in the bioreactor for this very reason but sidestream MBRs need to employ pumping, as with most other membrane processes. This difference in energy demands explains the iMBR configuration market dominance.

Also the fouling in the sMBRs is more higher due to the pumping of Activated Sludge which increases the shear stress to microbial flocs, causing them to break-up, which leads to a decrease in particle size and the release of foulant material.

The MBR configurations have three principal membrane configurations currently employed inpractice,

1. flat sheet (FS)
2. hollow fiber (HF)
3. multi-tube or multi-channel (MT/MC)

Although sMBRs are more energy intensive than iMBRs, they offer a number of advantages:
1. reduced membrane area requirement, from higher flux operation.  
2. operational flexibility for operation & cleaning cycle; unlike iMBR in-situ chemical cleaning of the membranes can be performed without any chemical risk to the biomass.  
3. maintenance and plant downtime costs, particularly for membrane module replacement, are generally slightly lower; the modules are readily accessible and so can be replaced in a much shorter time than for the immersed membranes.  
4. the membrane modules can be brought on- and off-line according to hydraulic loading.  
5. operation at higher solids concentrations is possible.  
6. operation at a lower energy demand is possible if the pressure and flow rates are reduced or if the membranes are configured for air-lift operation, though this then demands more membrane area.

For small flows of hard to treat effluents, the sMBR is often predominant due to its simple operation, smaller footprint and simpler maintenance especially when it comes to membrane replacement. For very large plants, the iMBR is always selected with the HF membranes since the OPEX is usually lower.

For intermediate flows, we usually choose an HF or FS iMBR. The FS is more simple to operate but both the membrane cost/ m² area and the energy demand are slightly higher than the HF configuration.

Finally, although most of the sMBRs use the classical pump configuration, if the membranes are arranged in series of a serpentine pattern, they can operate in air-lift mode which has an energy demand almost the same like the iMBRs and so can be considered for municipal wastewater applications with the advantage of lower footprint and effective rag removal. The membranes though are usually more costly than the immersed ones.