Water Journal : Water Journal May 2012
water MAY 2012 89 membrane pre-treatment particles bigger than screen aperture pass through the screen. In addition, machine- cleaning systems have been adapted to the very fine apertures. Adequately designed fine screens are equipped with an additional intervallic high-pressure cleaning system to ensure the fine screen apertures are continuously and reliably kept free, even with high contents of grease and oil in the wastewater. Furthermore, when selecting downstream treatment systems, it is also important to consider the specific properties of the separated screenings and the high amount of screenings. As fine screenings contain much more sludge and fine particles (silt) compared to coarse screenings, the requirements on downstream treatment systems are higher. As an option, the fine screenings can be passed to the sludge treatment system, where they reduce specific polymer consumption and lead to increased sludge dewatering degrees being achieved. In this case, the fine screen design would be such that the mix of screenings and spray water is pumped to the sludge treatment system and the need for additional screenings treatment is eliminated. Planning Guidelines Sewer system Washing and load peaks should be avoided to ensure the reliable operation of the fine screen. Sudden load peaks lead to increased blinding of the fine screen, with the result that bigger machines are required. Also, machine running times and wash water demand increase. Details about the sewer system are of great importance for the layout of the preliminary screen. Especially in combined sewer systems, if the sewer network is long and shallow, sudden peaks are likely to occur. Preliminary screens, therefore, must be designed to ensure that big loads of coarse screenings are quickly and reliably removed from the sewer. Special attention must be paid to sewer systems with stormwater tanks, stormwater overflows and sewers with storage capacity and overflow. When such structures are emptied, very high dry substance amounts arrive in the pre-treatment system. Due to their sludgy consistency they pass through the pre-treatment system. The fine screen must, therefore, be able to handle such amounts and within a very short time separate a very high amount of screenings and remove it reliably. Specific conditions must be clarified in advance and taken into account in the layout of the mechanical wastewater treatment system. Preliminary screen Preliminary screens installed upstream of membrane plants must separate as many as possible of the coarse screenings contained within the wastewater in order to reduce as much as possible the solids load in the fine screening system. Sturdy and efficient machines should be selected that achieve convincing results, such as machines with stationary bars with spacings from 3mm, or step screens with spacings from 3mm. Intensive washing of the separated screenings is required to ensure the organic material contained can be returned to the wastewater in liquid form. Screenings free of faeces are very easy to dewater and odour development in the screen room is avoided. Grit and grease trap Aerated grit traps are normally designed for 0.2mm grain diameter. Many grit traps are over-dimensioned today -- i.e., they have the function of a sludge separation plant due to the great variation between dry and stormwater conditions, resulting in problems with grit removal. Tests carried out at HUBER SE have shown Figure 1.2. Compared to one-dimensional wedge wire screens, two-dimensional perforated plate or mesh screens achieve significantly higher separation efficiency. Figure 1.3. A schematic drawing of two fine screen units. The machine on the left combines the process steps of screening, screw conveyor, dewatering and compaction in one unit, whereas the screenings discharged from the machine on the right are pumped to the sludge treatment system. Figure 1.4. Coarse and fine screening on WWTP Glessen, Germany. The coarse screen is a 5mm step screen, while the fine screen is equipped with a 1.0mm mesh. The screenings from the fine screen are dewatered and discharged into a container.
Water Journal July 2012
Water Journal April 2012