Cause of scouring air imbalance

The maldistribution of scouring air is one of the most common problems in MBR. As the number of membrane cassette increases, it becomes trickier to maintain identical air flow rates in all air nozzles underneath the cassettes. A consequence of maldistribution of scouring air is shown in Fig. 2 here, where over aeration in some cassettes causes damages in membrane panels. There are many causes of imbalanced air flows, but following are the most common.

  •   Total head losses from the blower to the air nozzles can be significantly different for each air nozzle. The common causes are excessive pressure head loss in the main pipe as shown in Fig. 1a. Due to the small main air pipe, air pressure in the down stream is substantially lower than that in upstream. In order to balance the air distribution, valves on the upstream branches must be partially closed. Fig. 1b shows well balanced system, where main pipe diameter is around three times bigger (or cross 9 times larger cross-sectional areas). Due to the marginal pressure head loss in the main pipe, air distribution through the branches can be nearly identical.
  •    Different length of branch pipes causes different pressure head loss in the pipe. The amount of air flow through a pipe is inversely proportional to the square root of the pipe length. Therefore, if one branch is twice longer than other branch and the inlet and outlet pressure of the two pipes, the air flow through the longer pipe will be around 71% of the other pipe (=1/sqrt(2))
  •   Random fouling of air nozzles can cause high pressure head loss in some air nozzles. Some extent of imbalanced air scouring occurs among the cassettes, which is virtually not possible to avoid regardless of the membrane types or manufacturer in spite of the ingenious nozzle designs discussed here. In order to monitor actual air flow to the cassettes, air flow meters are desired for all the cassettes, but the number of air flow meters is often compromised due to the limited capital budgets. Fig. 2 shows plugged scouring air pipes by sludge, which is perhaps promoted by fibrous materials that passed through the screen.
  •    As shown in Fig. 3, if screen fails to remove fibrous material to the low enough level, bottom of the cassette can be blocked, which causes low upflow in the area.
  •    Non-uniform air nozzle submergence can cause the maldistribution of scouring air. It is important to place all the air nozzles in an identical submergence. However, as shown here, diffuser depths can be gradually changed according to a measured pattern considering the head losses in the air pipe. 
Scouri4 Scouri5
 a) Air distribution system with issues    b) Well balanced air distribution system

Fig. 1. Scouring air distribution systems in two different MBR sites.

 

Scouri1Fig. 2. Plugging of coarse air nozzles by sludge (Mason, 2010)

Scouri2sdFig. 3. Fibrous materials can block the air-liquid channel, if screen fails regardless of membrane types (Mason, 2010)

 

© Seong Hoon Yoon