This process idea was originally developed by Toto Ltd. of Japan in late 1980’s during the Aqua Renaissance 90 project (Imasaka, 1989). As shown in Fig. 1, anaerobic digester was placed above ceramic membrane module to generate static pressure. The head space gas of the digester was recycled underneath the ceramic membrane module using a gas compressor. When gas rises through the pipe and the membrane module, liquid is also pumped up due to so called “gas lift pump” effect. The membrane fouling was effectively controlled by the two phase flow. However, due to the high ceramic membrane price at the time and the high energy costs for gas recycle, the process was not commercially successful.
Fig. 1. Membrane coupled anaerobic digester using gas lift pump effect to circulate digester broth (Imasaka, 1989).
More than 10 years later from the Aqua Renaissance 90 project, similar concept was adapted to aerobic MBR and commercialized as AirliftTM by Norit. A number of different features are found comparing to the old process: 1) AirliftTM uses circulation pump, 2) permeate is withdrawn by vacuum not by static pressure, 3) bioreactor (or aeration tank) is placed in the same level with membrane module, and 4) polymeric membranes are used instead of ceramic membranes. Fig. 2 shows the process diagram and the picture of AirliftTM module. Fig. 3 shows the operating mechanism of AirliftTM.
A 3 m long 0.2 m diameter module containing 620 of 5.2 mm tubes provides 30 m2 membrane area (38PR or 38GR module ). Modules with 700 tubes (34 m2) are also available. Average flux is higher than other hollow fiber modules and ranges 30-60 LMH. Mixed liquor circulation rate and aeration rate vary depending on mixed liquor quality and other operating conditions, but they range 20-25 m3/hr/module and 10-15 m3/hr/module, respectively (X-Flow slide).
Liquid circulation and air blowing are the two contributors for specific energy demand (SED). While static pressures in inlet and outlet sides of the circulation pump closely match each other, the pump must overcome the dynamic pressure loss in the membrane tube. Since the buoyancy caused by the air bubbles in membrane tube partially compensates the pressure loss, the net pressure liquid circulation pump must compensate is not high or it can be even negative (Ratkovich, 2011). In an audit performed in a full scale MBR plant in Ootmarsum, the Netherlands, showed that the specific energy demand (SED) was around 0.35 kWh/m3 and 0.28 kWh/m3 under dry and wet weather conditions including mixed liquor circulation and air scouring in field condition as summarized in Table 1 (STOWA Report, 2009).
a) Process diagram b) Norit AirliftTM tubular module
Fig. 2. Process diagram of Norit AirliftTM membrane.
Table 1. Specific energy demand (SED) of AirliftTM module in Ootmarsum, The Netherlands (STOWA Report, 2009)
|kWh/m3||Design||After optimization (2008)|
|Permeate suction/ backflushing||0.01||0.01||0.01||0.02|
Fig. 3. Operating mechanism of AirliftTM .
The other set of data suggests much lower liquid and air circulation rates as summarized in Table 1. Due to the lower liquid velocity and the higher fluxes than those shown above, recovery of liquid as permeate in one pass is estimated as high as around 10%. This is much higher than those for traditional tubular membrane processes, e.g. 1-4%. SADp is also calculated very low at around 3-6, which is lower than those for immersed membranes. As a result, SED of 0.25 kWh/m3 has been claimed excluding the energy for bio-air (Sparks, 2011).
Table 1. Summary of AirliftTM membrane’s spec and operating parameters (modified from Ratkovich, 2011, but the data originally from Pentair)
|Liquid flow rate||m3/hr/module||12||20|
|Linear liquid velocity||cm/s||22||37.7|
|Air flow rate||m3/hr/module||5||10|
|Linear gas velocity||cm/s||9.4||18.8|
|Recovery in one pass||%||~10|
|# of tubes||ea||700|
© Seong Hoon Yoon