Water Journal : Water Journal May 2012
88 MAY 2012 water technical features membrane pre-treatment Abstract With the introduction of membrane technology solutions for the treatment of municipal wastewater, the requirements on mechanical wastewater treatment systems changed fundamentally. Conventional screening systems with bar spacings or perforations from 6mm turned out to be not sufficient to guarantee the stable and low-maintenance operation of downstream membrane plants. In particular, fibres and hairs hinder the operation of membrane plants. These materials tend to entangle and may lead to a loss of membrane surface. Hairs and fibres, therefore, need to be removed reliably under any inflow conditions. Consequently, technical systems for the mechanical pre-treatment of wastewater needed to be adapted accordingly in order to meet the requirements. This paper describes the history of pre-treatment systems for membrane plants, with particular emphasis on the importance and influence of operational experience. Finally, planners are provided with guidelines on how to design mechanical pre-treatment systems for membrane wastewater treatment plants. Keywords: Pre-treatment, hairs and fibres removal, membrane protection, fine screens, high removal screens, screen element mesh. Pre-Treatment The efficiency of conventional screens with 6mm to 10mm bar spacing or perforation is insufficient for the membrane plants recently available on the market. Screens with higher separation efficiencies are necessary to ensure the reliable operation of membrane plants without excessive maintenance requirements. Which separation efficiency exactly is necessary depends on the specific requirements of the type of membrane system used (whether hollow fibre modules fixed on one or both sides, or plate modules). Hollow fibre modules need a very fine preliminary screening, as practical experience has shown that hairs and fibres strongly influence operating stability. Due to their flat surface, plate modules have fewer tendencies to clog. Manufacturers, therefore, often bring forward the argument of wider bar spacings and larger perforations. Meanwhile, a certain amount of experience with preliminary screens is available. This experience has been gathered mainly on small and medium- sized, but also on some large, sewage treatment plants. A summary of this experience with fine screening is provided in the following, along with additional information to serve as planning support. Selection of Screens for Membrane Processes As more and more membrane plants were put into operation, it became apparent that great importance needs to be attached to the reliable removal of hairs and fibres from the inlet to membrane bioreactors. Already from 2002 to 2004, the first membrane plants were equipped with preceding fine screens (WWTP Schilde, Monheim, Nordkanal, etc.). The majority of these fine screens were wedge wire screens with apertures from 0.5mm to 1.5mm. However, in the following years it was experienced that hairs and fibres caused problems with the membrane plants, even if functionally efficient fine screens were used as pre-treatment units. The efficiency of one-dimensional screens, such as wedge wire screens, turned out to be insufficient to guarantee the safe operation of membrane plants. It became necessary to develop a new type of two- dimensional mesh or perforated plate screen to improve the retention of fibres and hairs. In 2004, on WWTP Schilde, the first fine screen was changed from wedge wire to mesh. As the apertures of the screen's mesh or perforated plate are defined in two dimensions, the screen is able to ensure the maximum retention of hairs and fibres. Therefore, conventional mechanical preliminary screening systems used upstream of membrane plants today are generally completed with a fine screen equipped with a two-dimensional mesh or perforated plate to remove hairs and fibres. Two-dimensional screens have proven that they achieve a solids removal rate that is by the factor 2 to 4 higher. Due to the much finer apertures and the operating experience, sealing and cleaning systems have become increasingly important. Sealing systems have been further developed to ensure no S Reber, C Frommann A history and guidelines on designing mechanical pre-treatment systems for membrane wastewater treatment plants PRE-TREATMENT FOR MEMBRANE PLANTS Figure 1.1. The fine screen on WWTP Schilde in Belgium was the first that was refitted and equipped with a two- dimensional mesh in 2004.
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