* B. V. Kjellrup, B. H. Olesen, B. Frølund Danish Technological Institute, Dep. of Environment Teknologiparken, DK-8000 A ˚ rhus C (Denmark) P. H. Nielsen Aalborg University, Section of Environmental Engineering, Department of Life Sciences Sohngaardsholmsvej 57, DK-9000 Aalborg (Denmark) Potential of biocorrosion in Danish district heatings sytems B. V. Kjellerup*, B. H. Olesen, B. Frølund and P. H. Nielsen Danish district heating (DH) systems utilise water with unique characteristics that include low conductivity, high pH, nutrient poor and anaerobic (oxygen free) conditions in order to reduce cor- rosion rates. This survey was carried out in order to investigate the potential for biofilm formation and biocorrosion in these systems. Determination of total bacterial numbers in water samples were per- formed in 29 DH systems and showed a range of 10 2 – 10 5 cells  ml 1 . The potential for biofilm growth was further examined in cor- rosion monitoring units located at 6 DH locations. Total bacterial numbers in biofilm on mild steel were found in the range of 10 4 – 10 6 cells  cm 2 . The mild steel coupons were examined for general corrosion rates based on weight loss together with an investigation of pitting corrosion. The general corrosion rates were up to 12 lm  year 1 , while the pitting analysis showed pit depths up to 90 lm for half a year of exposure. Presence of sulphate reducing bacteria (SRB) was found in all systems tested with corrosion monitoring units, whereas sulphide was found to different extents in the biofilms on all mild steel coupons. It was shown that DH systems with the highest number of bac- teria in the biofilm generally had the most pronounced corrosion. The results show that despite the nutrient poor environment in the DH systems the potential for biofilm formation and biocorrosion was present. 1 Introduction District heating (DH) is the dominant source of heating sup- ply in most Central European cities and some Eastern Eur- opean and North American cities. The water quality of DH systems varies within Europe from low quality water originat- ing from rivers in Russia to well-treated DH water complying with strict requirements in Germany [1]. In Denmark more than 350 cities and county districts are supplied by DH using more than 17,000 km of pipes and a large number of installa- tions. The water quality of Danish DH water is good compared to international standards due to intensive treatment. Danish DH systems can be divided into 3 groups according to water quality: High quality water systems (mainly large city sys- tems), clean water systems (small and medium sized systems) and poorly treated water systems (small county systems). Common for all DH systems is pH adjustment with sodium hydroxide or ammonia and deaeration to prevent chemically induced corrosion [2]. The DH water in the major city systems is characterised by anaerobic conditions (< 100 ppb oxygen), pH ranging from 9.3 – 10.0 and temperatures in the range of 40 – 115 8C. In addition, DH water is low in nutrients (< 2 mg  l 1 ), has a conductivity usually below 25 lS· cm 1 , concentrations of fluoride and chloride below 0.5 mg  l 1 and 1.0 mg  l 1 , respectively, and sulphate below 5.0 mg  l 1 . Microbiological problems such as biofouling and biocorrosion have been considered limited due to this high water quality. Previously, monitoring of DH water quality has been based on general indicators of corrosion, while no attention was paid to microbiological parameters. DH systems represent extreme environments with respect to pH, conductivity, nutrient availability and temperature. Corrosion Concepts In dieser Rubrik ko ¨nnen Sie, die Leser, praxisorientierte Bei- tra ¨ge zur Diskussion stellen. Es werden Korrosionsprobleme, aber auch Lo ¨sungswege und praktische Erfahrungen ver- o ¨ffentlicht. Damit soll der Erfahrungsaustausch unter den Lesern dieses Periodikums zu einer sta ¨ndigen Einrichtung werden. Vor allem die Praktiker und die „Senior Scientists“ sind aufgefordert, von ihrem reichhaltigen Wissen abzugeben. Wir bitten Sie um Ihre Mitarbeit in Form von Beitra ¨gen und Leserbriefen. In this forum readers will be able to present practical problems for discussion. It is envisaged that these contributions will include not only discussion of general problems and incidents of corrosion but that suggested remedies will also be presented and discussed. It is hoped that this exchange of knowledge and experience will become a permanent feature of this periodical. We are particularly anxious that both Senior Scientists and those with more practical experience will make use of this forum to exchange information, problems and potential re- medies. Materials and Corrosion 2004, 55, No. 7 Biocorrosion in heating systems 543 F 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim DOI: 10.1002/maco.200303774