Development of methods for prevention and control of bio-corrosion

The reliability of the cultivation methods is poor, as only a very little fraction of the occuring bacteria can be detected by these methods. On this background the aim this project has been to establish new methods for fast and reliable detection and identfication of possibly corrosive bacteria in district heating systems. We have created a toolbox that makes it possible to identify the majority of the occuring bacteria (by Polymerase Chain Reaction, PCR and phylogenetic analysis) and to identify known bacteria and bacterial groups in a fast and reliable way (by Fluorescence In Sity Hybridisation, FISH), and to get an overview of the predominant mechanisms of metabilism within the bacterial population (by MicroAutoRadiography, MAR). By PCR and phylogenetic analysis a large variety of bacteria has been demontrated in district heating systems. As the conditions in a district heating system, from a microbiologically point of view, can be regarded as being extreme it is somewhat surprising to find such a diverse flora of bacteria. This diversity combined with other results, indictes that district heating systems might not be thta low in nutrients and oxygen as expected. We have succeeded in adapting the FISH techniques to the complicated conditions in samples from district heating systems, and it is now possible to perform quantitative detection of a variety of bacterial groups. As was the case for the PCR approach, a number of different bacterial groups have been detected by the FISH techniques. Investigation of bacterial activity (by MAR) reveals a bacterial community with vrious metabolisms. Aerobic as well as anaerobic bacteria such as sulphate rducing, methane producing and fermenting bacteria have been detected. These differnt metabilic pathways can alone, or in combination with each other, influence MIC. The variety of metabilic and corrosion mechanisms and the diversity of bacteria in district heating systems indicates it might be difficult to identify one single organism or group of bacteria as an unambiguous indicator for MIC. Application of these new techniques has resulted in a more complex but alsomore adequate picture of the microbiology and the mechanisms of MIC in district heating systems. Practical Codntrol of MIC: Apart from developing new techniques, we have been working with practical control of MIC on a peak load unit and on a distribution system. Mechnaical and chemical cleaning of these systems have resulted in major improvements concerning local corrosion caused by bacteria. As a result of these improvements in the MIC has been reduced and corrosion caused by oxygen is now exposed as the main corrosion process. The experiences are accumulated in a series of recommendations for the best available practise for prevention, control and monitoring of MIC. The main poins are listed below. Monitoring: 1) Registration of relevant operation parameters (e.g. frequency of filter change, changes in transmission ability of heat exchangers). 2) Observance of regulations of water quality. 3) Direct monitoring of deposits and corrosion by use of, for instance, corrosion units. Control: 1) Mechanical cleaning (e.g. pressure washing). 2) Chemical cleaning (e.g. by use of hot sodium hydroxide or cold acid). 3) Thermic control (e.g. by alternating temperatures). 4) Chemical control (e.g. by use of biocides or inhibitors). Prevention: 1) Good housekeeping (implementation of appropriate procedures for opertaion, maintenance and repair). 2) Reduction of organic matter to a minimum