Side stream biofiltration for improved biofouling control in cooling water systems

Abstract

Biological activity in open recirculating cooling water systems may lead to Microbial Induced Corrosion (MIC) and lower heat exchange efficiencies. The use of oxidizing biocides as a tool to control excessive biofilm formation in a cooling water system can be characterised as fighting the symptoms, because it does not take away the main cause of biofilm growth: the availability of nutrients. Due to frequent use of oxidizing biocides the microbial growth rate in the cooling water system studied (1–2 d−1) was even higher than the dilution rate (0.33 d−1). By means of an ASTRASAND®continuous sand filter used as a side stream biofilter it is possible to achieve considerable reductions in nutrients and in suspended biomass contents. The content of nutrients in the filter influent and effluent was expressed in terms of growth rates and was determined by batch experiments. Based on the assumptions of plug flow characteristics and first order reaction kinetics, the kinetic coefficients for nutrient removal were determined at k1=6 h−1 for fine (0.8–1.25 mm) and at k1=4.5 h−1 for coarse (1.4–2.0 mm) filter material. Due to the first order kinetics of nutrient removal and due to the improved filtration efficiencies at increasing influent biomass contents, it is concluded that a side stream biofilter introduces a self regulating process element in the cooling water system, leading to a faster recovery of the cooling water quality after upset conditions. Considerable savings in the use of oxidizing agents can be established.