Designing air scouring system is one of the key technical challenges in MBR. Air flow rate must be uniform among nozzles so that the membranes above the nozzles are evenly scoured. Otherwise, a localized membrane fouling occurs where the scouring air is not sufficient. The areas affected by the fouling expand since the flux in unaffected area must increase to compensate the loss in the fouled area. Since the membrane fouling rates are exponentially proportional to the flux as discussed here, membrane fouling can spread quickly across the entire membrane cassettes. Therefore maintaining a uniform aeration underneath the membrane modules is crucial for stable membrane filtration.
However, maintaining uniform air flow rates among nozzles is challenged by the following factors.
- Pressure loss in air pipes makes it hard to equalize the total pressure drop to each nozzle identical especially as the number of cassettes and trains increases.
- Some air nozzles get fouled quicker than others, which develops unequal pressure drop to those nozzles.
- The submergence of air nozzles must be identical, but as the tank size grows it may become tricky to keep the submergence identical.
Numerous different attempts have been made by membrane manufacturers and engineering firms, but not firm solutions are known to date. In addition, little has been published on such practical knowhow.
Cause of nozzle plugging
The major cause of coarse bubble diffuser fouling is known to be the dried sludge inside and outside of the pore. Since the air passing the diffuser pores is heated in the adiabatic compression in blowers, relative humidity is low. Therefore, if there is any sludge in its way, it dries the sludge quickly. A large amount of mixed liquor can intrude coarse bubble air pipes when no aeration is performed. The mixed liquor intruded can be dried for extended period of time and eventually accumulates enough to plug some of the coarse pores.
If intermittent aeration is performed, mixed liquor intruded to the air pipe may not be dried completely until the next pause cycle starts and fresh mixed liquor intrude and wet the partially dried sludge again. But, if continuous aeration is performed, any mixed liquor intruded to air pipe will be dried and accumulates.
Nozzle design and cleaning method
Kubota Corp. of Japan came up with a ingenious solution that allows periodic wetting of air pipes with mixed liquor earlier in the last decade. The detailed design factors are proprietary, but the general mechanisms are illustrated in Fig. 1. This system has two distinguished feature: 1) gradually declining main pipe submergence as air proceeds and 2) air pipe flushing.
As illustrated in Fig. 1a, pressure decreases as air proceeds through the pipe from the left to the right due to the pressure loss. At the same time, the slowing air velocity in the pipe acts as a factor increasing pressure since the kinetic energy of the air converts to pressure according to the Bernoulli’s theorem. In ideal system, the small pressure loss in the pipe can be compensated by the kinetic energy conversion to pressure, thereby air flow rates through the nozzles in different location can be uniform. However, air pressure tends to decrease overall in the main air pipe and this needs to be corrected by reducing the submergence of the main air pipe gradually.
During normal operation, the valve in the end of the main air pipe is closed. If the valve is opened, air can proceed through the vertical pipe (Fig. 1a). Due to the air leak through the valve, the air pressure in the main pipe declines below the static pressure and mixed liquor intrudes through the branch pipes. As a result, air and liquid mixture is pumped up by airlift effect in the vertical pipe and is discharged back to the tank. A periodic exposure of the internal surfaces of diffuser to liquid (or mixed liquor) can prevent the permanent loss of nozzle openings by dried sludge. Fig. 2 illustrates the structure of coarse air diffusers and the diffuser cleaning method used by Kubota engineering.
a) Aeration mode
b) Diffuser cleaning mode
Fig. 1. Schematic of the mechanisms of diffuser cleaning system.
Fig. 2. Coarse air diffuser design (left) and diffuser cleaning method (Kubota, 2010).
Fig. 3 shows the schematic of a coarse bubble diffuser for membrane scouring presented by Econity in 2010. There is no experimental data available yet, but it is claimed that the dried sludge accumulated in the diffuser is wet during aeration pause and is discharged through the holes pointing downward in both ends of the air pipe. Other feature of this diffuser is that the “V” shaped air pipe allows uniform differential pressure between inside and outside along the air pipe in spite of the pressure loss toward the end of the pipe.
Fig. 3. Schematic of the coarse bubble diffuser for membrane scouring (Econity, 2010).
The upflow pattern of two phase flow can change dramatically by subtle differences in air flow distribution, flow rate, dimension and configuration of the upflow channel and even biological conditions affecting sludge rheology. Therefore, the upflow patterns must be studied empirically while theory can be used limitedly. It was found that, if aeration is slightly off-centered as shown in Fig. 4, liquid circulation loop develops in one side of the channel and bubbles rise through the other side of the channel. Since the membrane scouring by liquid is not as efficient as by air-liquid mixture, membrane surface with liquid-flow only is preferentially fouled. Same phenomenon can occur if one nozzle opening in the right side is partially or fully compromised.
Fig. 4. Aeration heterogeneity in sMBR (Nguyen Cong Duc, 2008)
© Seong Hoon Yoon