How a Simple Bench-Scale Test Greatly Improved the Primary Treatment Performance of Fine Mesh Sieves
In Norway fine mesh sieves are frequently used for primary treatment or as the only treatment before wastewater is discharged to coastal waters. The reasons for this are the intensive product development of fine mesh sieves taking place in Norway, and the significantly reduced investment costs and space requirements compared to other primary treatment processes. Historically the design of these sieves was not very sophisticated. The goal of this R & D was to develop a fairly simple test procedure that can be used to characterize wastewater, establish design criteria for fine mesh sieves and predict removal efficiencies for full-scale plants under different operating conditions. The bench-scale test procedure was verified at several full-scale municipal wastewater treatment plants that used different types of fine mesh sieves (rotating belt, rotating disc, rotating drum, stationary), by comparing the bench-scale test results to the fullscale results. To achieve high removal efficiencies it was crucial to operate the sieves with a filter mat. Rotating belt sieves performed best in the full-scale tests. Properly operated the rotating belt sieves consistently removed more than 50 % SS and 20 % BOD5, as required by the European Union for primary treatment. Fine mesh sieves with pumped influent should have frequency controlled pumps to avoid on/off operation. At plants with several sieves in parallel, all sieves should be running even at low water flows. This will enable operation with thick filter mats and high removal efficiencies. Simple screw presses dewatered the sludge from the sieves to typically 25 – 30 % total solids. Using fine mesh sieves with < 500 microns openings was found to normally be the most economical process for primary treatment.