Research On Corrosion, Inspection And Rehabilitation Of Urban Sewer Systems

Urban sewer systems are one of the most important underground infrastructure in municipalities, draining the waste water produced by municipalities and keeping municipalities away from the disasters caused by waste water. But the urban sewer systems demand huge investments, taking a share of 50～70% out of the whole investments for the waste water projects. As the existing sewer pipes available get older and more sewer systems are being built and enlarged, here comes the questions: how to carry out inspection and rehabilitation of the lager amount of pipes which vary in age and how to make a rehabilitation decision. These are important questions dealing with attempting to minimize pipe building and maintenance investments, keeping accidents of contamination at low level, avoiding damage to the systems, and maintaining the function of the pipes.Concrete sewers are easy to be corroded by corrosive elements in the waste water. The study of the corrosion of concrete sewers is stated in other article and is not described here. The corrosion models deduced from that study are quoted here to predict the probability of failure of concrete sewers under certain circumstances. The article analyzes the possible major harm caused by pipe failure (esp. underground water contamination and soil contamination by waste water leakage) and makes the corresponding contamination assessment, evaluates the economic loss caused by the contaminations and then stresses the necessity of inspection and proactive rehabilitation of sewers. Focusing on inspection and rehabilitation of sewers, this article then discusses inspection and the condition assessment technologies and the rehabilitation technologies of sewers, and finally provides a practicable decision support system for rehabilitation of urban sewer systems. In addition, a example of a concrete sewer system is performed in the article.Inspection of sewer pipes consists of direct inspection and indirect inspection. Direct inspection directly determines the loss level of the corroded pipes. The inspection technologies include: man entry, CCTV, ultrasonic inspection(sonar), ground penetrating radar, etc. Among them CCTV is the most popular and widely spread technology. The condition assessment technologies of pipes vary in different countries. But all the assessment technologies evaluate and rate the state of the damaged pipes as a whole. Indirect inspection infers the corrosion level through measuring the proportion of the corrosive element in the waste water being carried in the pipes. Indirect inspection is carried out through field measurement or taking samples into laboratory for analysis. Results suggest that proactive rehabilitation and trenchless rehabilitation generally are more economical than reactive rehabilitation and excavation rehabilitation. Insert lining of pipe,cured-in-place inversion hose lining and spray lining are the categories of trenchless rehabilitation technology, among them cured-in-place inversion hose lining is widely spread. Every technology has its advantages and disadvantages, and is applied in varied range respectively.The steps of decision making for rehabilitation of urban sewer systems are as follows:1) everyday investigation and record.2) possible impact assessment caused by pipe failure.3) monitoring corrosive elements in waste water, inspecting pipes and assessing condition of pipes.4) predicting failure probability of pipe segment using the corrosion model and determining the planned rehabilitation scheme based on the predicted condition for some sewers which have no rehabilitation requirements at present.5) determining the best rehabilitation time of each pipe segment using optimizing model of planned rehabilitation time.6) selecting rehabilitation technology by comparing advantages and disadvantages of application of different technologies available depending on the defect type and severity of pipe segments.In this article, a practical decision-making is performed as to a simple example of sewer system, with the conclu...