The ideal flow pattern in submerged flat sheet membrane is a uniform upflow induced by evenly distributed air bubbles. Unfortunately, it has been observed that the actual upflow is not uniform, but it consists of swirling and meandering flows, local down flows, and concentrated air bubbles in both sides of the channel (or column). However, the upflow in flat sheet membrane is still more uniform than in hollow fiber membranes, which gives some advantages to flat sheet membranes when they handle high level suspended solids (Lebegue,2008)
When a flat sheet submerged membrane panel with 0.1 m2 is installed in a slit channel with 7 mm spaces from the wall in both sides of the panel, as soon as the gas leaves the diffuser it moves towards the sides of the column as it rises at gas flow rates of 2 – 8 L/min ( or 20-80 L/m2/min ). The hydrodynamic conditions were simulated using computational fluid dynamics (CFD). The gas divides into two main streams that liquid moves upward near the two sides of the column. In addition, these two main bubble plumes move up in a meandering manner. The degree of meandering was small for the low air flow rate of 2 l/min and was much higher at the higher flow rate of 8 l/min. The flow meandering became more significant with larger bubbles.
As a result of the fast two phase flow in both sides of the channel, liquid in the center of the channel tends to move down in the range of air flow rate used in the experiment shown in Fig. 1. In the both ends of the column, narrow down flow zones also exists. Although the down flow velocity was substantial compared to the upflow liquid velocity, the single phase liquid down flow generated much less shear stress on membrane surface than the two phase upflow. Thus shear stress was much higher in both sides of the column as illustrated in Fig. 2a at an air flow rate of 4L/min. When shear stress was measured with electro-chemical sensors in other experiment, shear stress was found 2-4 times higher in both ends of the column than in the center (Zhang, 2009).
It has been observed that cake layer tends to form in the center of the flat panel membranes rather than in both sides as shown in Fig. 2b. This coincides with the regions of the sheet that appear to be less sheared by bubble flow (Fig. 2a). The two main plumes of upflow explain the typical membrane fouling pattern in flat sheet membranes.
In the same CFD study, the average shear stress on membrane surface increased only about 50% from 0.07 Pa to 0.10 Pa when air flow rate increased from 2 L/min to 8 L/min (Ndinisa, 2006c). This is in line with the observation with hollow fiber membranes, where the efficacy of air scouring was flattened at certain air flow rate.
Fig. 1. Vertical water velocity profile distribution at 30 cm from the bottom (Ndinisa, 2006c)
|a) 4 L/min||b) Used panel|
In order to reduce the uneven membrane scouring by the maldistribution of bubbles, vertical baffles were devised (Ndinisa, 2006c). Multiple thin baffles were vertically placed in between membrane and wall. Once bubbles are caught by the openings of the baffle bottom, bubbles rise through the vertical spaces with 1 cm width among baffles. As a result, bubbles could not gather to form large plumes and shear stress evened out. The spacers enabled achieving the same ‘critical flux’ as an empty channel with half the airflow rate.
© Seong Hoon Yoon