Key facets to take into account when selecting a screen for your MBR design include the screenings capture ratio, screen apertures, screening flow velocity, ancillary processes and coarse suspended solids, and screen maintenance. The screenings capture ratio (SCR) represents the amount of solids removed by the screen as a proportion of the total coarse solids load, and as such is analogous to membrane rejection.
Features - Membrane operation
The importance of screening upstream of a membrane bioreactor remains a key concern for MBR practitioners. Screening protects the membrane from mechanical damage from sharp or abrasive particles and from clogging from hair and matted cotton fibre. While any screen is better than no screen at all, it is critically important to have a properly designed and sized screening system.
While most scientific articles about MBR systems suggest membrane surface fouling as being the main operational limitation for the technology, it is widely recognised by practitioners that clogging phenomena – possibly related to inefficient pre-treatment – are at least as important. It is also recognised that clogging takes different forms...
There is little doubt that, of the challenges faced in MBR operation, recovery of permeability is arguably the most critical. When the routine maintenance membrane clean fails to achieve the required recovery and more aggressive recovery cleans are also found wanting, then ex-situ cleaning becomes necessary. This invariably incurs extensive manual intervention, not always with guaranteed success.
It is widely recognised by MBR practitioners that membrane bioreactors, and specifically those treating municipal wastewater, are subject to what is sometimes referred to as ‘ragging’. When the membrane module is removed, characteristic thick strands of material can be seen hanging from the bottom of the module.
Fouling in MBRs arises when materials either form a layer on the surface of the membrane or plug its pores. Studies are, however, generally unable to distinguish between coating of the membrane surface and phenomena relating to the agglomeration of solids. The latter has received little or no attention from the scientific community, and yet such irreversible deposition of solids clearly impairs MBR operation.