Ragging in MBRs is almost certainly caused by cotton wool or similar cellulosic textile fibres. Only very short lengths of tiny filaments must break through the 1–3 mm fine screens, which suggests that they are originally dispersed. But they soon get together again to form those long rags.
The MBR Blog
It's their compactness that is one of the beauties of MBRs and often leads to their selection as the preferred technology. Crucially, MBRs permit a higher MLSS concentration than a conventional CAS process, and this means proportionately smaller biological process tanks.
There’s plenty of tales of the impact of poor screening on downstream unit operations in MBRs. While there is little actual proper research into the performance of screens published in the peer-reviewed literature, there are some useful guidelines produced by practitioners.
We’ve spent a few months now here at Qatar University looking at clogging in membrane channels, with our single-channel bench-scale plant. We're taking a few litres of sludge from a nearby MBR and running tests at relatively high flux and slightly diminished air scour rates.
Sharp-eyed readers of this blog may have received the impression that I am not a huge fan of MBR research publications based on fouling. It could be argued that there is far too much effort dedicated to this single area and there is at least some fuel for this particular fire.