SCREEN DESIGN: PRACTICE IN EUROPE

In Europe fine screens are taken for granted at most wastewater treatment plants today. This technology has been in operation for more than fifteen years and most of the design and operational challenges associated with this relatively new technology have been resolved.Countries in Europe have adopted different solutions for fine screens and screening handling systems. Some, like England and Poland, have mainly adopted 6 mm screens, as where as in Sweden and Italy 3 mm step screens are dominating. However, the most common fine screen configuration in Europe corresponds to the different types of step screens, usually ranging from 3 to 6 mm space openings. The other common technology is rotating sieves. In Germany, rotating sieves are commonly used in almost half the smaller plants. Rotating sieves openings typically range between 2 to 6 mm, and can be more effective in removing screenings than step screens. They demand more space and are therefore not always suitable for large treatment plant applications, where a large number of units would be required. Rotating sieves are also a popular technique for separate treatment of bypassed flows during wet-weather flow conditions.With an effective removal of screenings operational problems in the further downstream processes can be minimized. Heat exchanger, mixers in the digesters, sludge pumping systems, and dewatering systems are very sensitive and can operate more stable with an effective screening system. Even today, the demand on higher degree of treatment have resulted in plants installing new biological processes downstream that are increasingly sensitive to the performance of the screening process. Therefore some plants have begun to remove their step screens and install rotating sieves with even smaller opening sizes.To optimize a step screen it's important to achieve a coating mat on the openings. The mat works as a filter, even removing sand and smaller particles. If a mat does not cover the screen it can lead to operational problems as sand can get stuck between the lamellas. The screen should not be standing straight on the bottom. The screen has to be standing on an angle. This is important to avoid sand getting stuck in the bottom of the screen. In front of the screen air diffusers are often used to maintain sand in suspension.Screened material handling systems, such as washers, compactors, and storage systems, are also critical to the overall operation of the process. Reducing the volume and the odor generation potential of screened material has resulted in the incorporation of fairly sophisticated systems capable of producing screenings with up to 40-60 % dry solids. Case studies from wastewater treatment plants in Sweden will be presented in the paper. Construction, operation and maintenance experiences will be described and discussed.