If the sludge blanket rises to close to the tubes inlet then the sludge blanket may enter the tubes, and result in fouling and subsequent effluent failure.

In addition, because lamella units are used to provide a smaller footprint, you may find that you cannot much of a similar benefit for activated sludge systems because of the higher solids load - you still need to ensure that you have sufficient area to handle the limiting load. With common UK operating conditions the operational point is well below the limiting flux - i.e. there is scope for a reduction in area. With common US operating conditions this is not the case. You should ensure that a mass flux analysis is carried out for a range of operating conditions - flow, MLSS, SSVI and RAS flow.

The tubes can enhance the sludge thickening, but the percieved risk of biofouling means that I have never seen the tubes extend below the anticipated clarification zone.

The only operational lamella settler I have seen with activated sludge had poor performance; the tubes had a lower life than anticipated, and here were fixing problems, so that floating tubes were common. But we are not usually asked to visit sites where there are no problems