EVOLUTION OF BIOFOULING ON HEAT TRANSFER SURFACE CAUSED BY SIMULATED TREATED SEWAGE : INFLUENCE OF CALCIUM IONS

Heat pump using treated sewage water as heat source and sink was employed at Beijing Olympic Village to supply heating and cooling, which is an effective way to reuse urban waste heat. However, fouling occurs in plate heat exchangers of the heat pump. In the current study, an experimental system, which mimics the treated sewage water and simulates the relevant fouling formation process, has been developed to investigate the influence of Ca on biofouling properties. Extensive experimentation including process monitoring and microstructure development in biofilm have been conducted and presented in this paper with the view of attempting to reduce fouling in future. A series of quantitative information has been obtained and discussed. The results show that the interactions between the microorganisms and Ca clearly influenced the biofilm development thus impacting on the thermal resistance created. It was found that influence of Ca on biofouing differs at different formation stages. INTRODUCTION Sewage is an important carrier of urban waste energy, which contains large low-grade heat. Treated sewage can be used as a suitable heat source in the heat pump system, which is an effective way to save energy and protect the environment during heating and cooling (Funamizu et al., 2001). Treated sewage source heat pump system has been introduced into Beijing Olympic Village District Heating & Cooling in 2008 (Shi and Zan, 2008). Treated sewage contains some components causing fouling, such as heterotrophic bacteria, microbial nutrients and suspended substance. These could not be completely removed during treatments conducted by wastewater treatment plant. Thus fouling inevitably occurs in heat exchangers of the treated sewage source heat pump system. Fouling on the heat exchanger surface is a composite material consisting of biological and inorganic fouling (Shi et al., 2009; Zan et al., 2010). It is further demonstrated that biofouling is the major part of the fouling, which affect the fouling properties dramatically (Ma et al., 2010). The dynamic behavior of biofouling on the heat transfer surface is essential to character the development of composite fouling caused by treated sewage. The quantitive correlations of the heat transfer resistance and friction resistance produced by biofouling, biofouling mass and morphology is needed to improve the anti-fouling strategies. Ca concentration in treated sewage varies at different locations, and even varies over time in some wastewater treatment plants due to the injection of industrial sewage. It is known that Ca has significant effects on the biological fouling (Characklis, 1990; Geesey et al., 2000; Lattner et al., 2003). Ca can promote both specific and non-specific interactions with protein and polysaccharide adhesion molecules at the cell surface, and influence the attachment processes by acting as important cellular cations and enzyme cofactors; Ca has the effect on electro-static interactions between cell and substrum surface, and impacts the mechanical properties of biofoulings (Fletcher, 1988; Geesey et al., 2000; Sheng et al., 2008). However, few studies are available on the effect of Ca on biofouling that is related to heat transfer efficiency (i.e. the increase of fouling heat transfer resistance and friction resistance) under the conditions close to actual heat exchangers in a real heat pump energy recovery system using treated sewage water. The purpose of the study was to investigate the influence of Ca on biofouling properties. The paper presents the results obtained with biofoulings formed by a treated sewage water analog, which was made of a mixture aqueous suspension of Bacillus subtilis bacteria and Ca flowing under turbulent conditions. The correlations between the heat transfer resistance, friction resistance, mass and morphology of biofouling were discussed. MATERIALS and Methods A comprehensive assessment system was developed to simulate the treated sewage heat exchange system. The schematic diagram is given in Fig.1. The apparatus handles the model fluids, which had the defined components, to flow though a plate heat exchanger (PHE) and two flow cells (rectangular stainless steel channel). These all served as fouling sections. Under different Ca concentrations, the development of the heat transfer resistance caused by biofouling formed in the PHE was continuously recorded. Meanwhile, the flow cell was designed to allow extraction of stainless steel coupons giving direct measurements of the biofouling accumulated on the surfaces, which can be used Proceedings of International Conference on Heat Exchanger Fouling and Cleaning 2011 (Peer-reviewed) June 05 10, 2011, Crete Island, Greece Editors: M.R. Malayeri, A.P. Watkinson and H. Müller-Steinhagen