Integrated membrane tank vs Separate membrane tank in iMBR

There are two different configurations in iMBR depending on the existence of separate membrane tank. In early days, membranes were immersed directly in the aeration tank (Fig. 1a). However, it had been realized that this configuration makes membrane maintenance complicated. If membrane cleaning, coarse air diffuser cleaning, membrane replacement, or any other maintenance issue occurs regarding membrane separation unit, entire aeration tank must be drained or membrane cassettes must be hoisted out from the basin. Therefore, the integrated membrane tank design has been gradually replaced by the separated membrane tank design (Fig. 1b). In this configuration, separate membrane tank is used with mixed liquor recirculation between aeration tank and membrane tank.

In the separated membrane tank configuration (Fig. 1b), all the above listed drawbacks of integrated membrane tank are partially or fully eliminated. In addition, the chance of leaking untreated contaminants due to the nature of continuous stirred tank reactor (CSTR) can decrease significantly. Most importantly, untreated NH4-N in aeration tank has a second chance to be nitrified in membrane tank. However, disadvantage of the separated membrane tank includes the following: 1) larger footprint, 2) higher capital costs, 3) higher power costs due to the necessity of mixed liquor recirculation, and 4) higher power costs due to the necessity of more biological aeration in aeration tank as discussed in the last paragraph of this page.

The separate membrane tank design again evolved to Fig. 1c for two major reasons: 1) pumping energy savings by reducing the flow to be handled along with a potential saving in capital cost by using smaller recycle pumps and 2) minimizing the chance of small particles contacting membranes, which are broken by the shear stress of circulation pump.

Fig 1

Fig 1c
Fig. 1. Configurations of iMBR.

In the separate membrane tank design, biosolids in the mixed liquor are concentrated in the membrane tank to some extent depending on the ratio of recycle flow (Qr) over feed flow (Q). The level of MLSS in the membrane tank must be controlled properly to prevent excessive membrane fouling and clogging by high MLSS. The steady-state MLSS in the membrane tank can be calculated from the following mass balance around the tank.

Biosolids input = Biosolids output + Biosolids removal
image006                   ——————- (1)

image008                                 ——————–(2)

where
Q   = feed flow rate (m3/min)
Qr  = mixed liquor recycle flow rate (m3/min)
Qx = excess sludge removal (m3/min)
X0 = MLSS in aeration tank (g/L)
Xm= MLSS in membrane tank (g/L)

The equation (2) can be modified to the following equation.

image020                                       ——————-(3)
In this equation Qx can be neglected since it is much less than 1% of Q in most situations, thus the ration of the MLSS in the two tanks becomes a function of mixed liquor recycle ratio, Qr/Q (or inverse of Q/Qr). If the mixed liquor recycle ratio is 2.5, the concentration factor (Xm/X0) in membrane tank will be 1.67, which means 67% higher MLSS in membrane tank than in aeration tank. Since concentration factor does not decrease quickly once recycle ratio exceeds ~4, it is not worth to raise recycle ratio above ~4 as shown in Fig. 2. Therefore, recycle ratio is typically controlled at 2.5-4, where Qr/Q stays between 1.33 and 1.67.

Fig 2

Fig. 2. Relation between mixed liquor recycle ratio and concentration factor in membrane tank.

As discussed above, one disadvantage of the separated membrane tank is the higher energy consumption for biological aeration than its counterpart. In the integrated membrane tank, the oxygen dissolved during the membrane scouring can transfer to the surrounding area quickly and used to treat incoming COD. Thus DO near the membrane module stays relatively low and the driving force for oxygen dissolution is maintained relatively high. In separate membrane tank, on the other hand, oxygen is required only for endogenous respiration in the small membrane tank. The dissolved oxygen is transported to the aeration tank when mixed liquor is recycled, but the amount is not as much as the dissolved oxygen transportation occurring internally in integrated membrane tank. As a result, dissolved oxygen in separated membrane tank is high, which reduces oxygen dissolution rate.

The biological air demand in the integrated membrane tank is variable depending on where the membrane cassettes are placed in the tank and how much scouring air is used, but it can reduce biological air demand significantly. For example, the ration of biological air to scouring air was only 0.15 in an energy audit of a full scale MBR system with integrated membrane tanks (Fenu, 2010). On the other hand, it was found at 0.59 in an MBR with separate membrane tanks (Williams, 2008).

 

© Seong Hoon Yoon