The selection of the mesh size or perforation depends on to what extent hairs and fibres must be removed to meet the specific requirements of the type of membrane system used (whether hollow fibre modules fixed on one or both sides, or plate modules). Experience has shown that especially hollow fibre modules need a high removal of hairs and fibres to maintain reliable operation. Fine ROTAMAT® Membrane Screens with 1.0 mm mesh are therefore primarily used to protect hollow fibre membrane modules. As plate modules are less sensitive to blocking due to their plane surfaces, pre-screening with a ROTAMAT® Perforated Plate Screen RPPS with 3.0 mm perforation is sufficient in most cases.
Two-dimensional mesh or perforated plate screens achieve a high removal of hairs and fibres, i.e. two to four times the amount of solids are removed than one-dimensional screens are able to separate. The significantly increased separation efficiency is accompanied by a high level of screen surface blinding and requires larger dimensioning of the fine screens compared to other commonly known screens. The channel system should be operated in a way to ensure that washing and load peaks are avoided as far as possible.
The screenings separated by the fine screen can either be dewatered and discharged into a container or passed on to the sludge treatment system. As fine screenings contain much more sludge and fine particles (silt) compared to coarse screenings, fine screens must be designed to meet the increased requirements of wear resistance and screenings dewatering. If the screenings separated by fine screens are not directly dewatered but passed on to the sludge treatment system, the fine screen can be designed to provide for the possibility to pump the screenings into the sludge storage tank. Fine screenings in the sludge treatment system enhance the dewatering process due to the structural material contained within fine screenings. Improved dewatering results are achieved with even a lower specific polymer consumption compared to dewatering without fine screenings.
Without any exaggeration, we can say that HUBER is a leading supplier in the field of pre-screening for membrane plants. Since 2004, we have installed more than 160 fine screens upstream of membrane plants, almost 50% of them with a screen basket diameter bigger than two metres! The extensive experience gathered from real size installations is reflected in the continuous development and optimisation of our fine screens.