Evaluation of sewage collection network vulnerability by Analytical Hierarchy Process method (Case study: sewerage collection network implemented in a part of District 5 of Tehran Municipality)

Sewage collection network is one of the largest and most valuable infrastructure assets. Therefore, managing the wastewater collection network faces many challenges. Unfortunately, due to the extent of the damaged wastewater collection network and time and budget constraints, it is not possible in practice to treat and eliminate all wastewater collection system damages at the same time. So, this issue requires management activities in the form of vulnerability rating, to develop a maintenance program for sewage collection system maintenance. The present study evaluated and ranked the vulnerability of the sewerage network implemented in a part of District 5 of Tehran Municipality using a combination of Analytic Hierarchy Process (AHP) and GIS technique. AHP is one of the most efficient multi-criteria decision-making techniques, in this method, the factors are compared with each other in pairs and the highest weight is given to the layer that has the maximum effect on goal setting. In this regard, according to the opinion of experts and literature review, all types of possible damage to the sewage collection network at three levels of manhole, pipeline and branch were investigated and classified. In order to determine the relative importance of the defects, a questionnaire was prepared and distributed amongst experts working in the field of design and maintenance of sewage systems. Using AHP method, pairwise comparison between the relevant factors was performed and the relative weight of each criterion was calculated. After calculating the inconsistency ratio and confirming the meaningfulness of the judgement matrices of effective factors, the final weight of each criterion was applied to the corresponding layers to prepare the zoning map of the area. Next, judgement matrices were transferred to ArcGIS software and raster output was generated from them. The raster layers of different factors in ArcGIS software were homogenized and then combined using Fuzzy Membership method. In fact, the ultimate goal of the GIS technique used in this study was to obtaine the final overlap of all factors affecting the vulnerability of the sewage collection network (at two levels of manhole and pipeline clogging) and provide a zoning map of damages in the study network. It is important to note that the transformed raster maps for each of the effective factors have different units that must be scaled to the fuzzy membership method. As a result, all values scaled between 0 and 1 range. In general, each number in this range indicates the strength of membership in a set, based on a specific fuzzy algorithm. Next, the Fuzzy Membership command was run to obtain fuzzy the classified raster, from the Spatial Analyst Tools toolbox and the Overlay menu. Thus, fuzzy raster of quantitative criteria affecting the vulnerability of the sewage collection network was obtained. Weighted Overlay method was used to correctly apply the resulted weights and present the final model in ArcGIS. Thus, the fuzzy raster was added to the Spatial Analyst Tools toolbox and the Overlay menu, using the Weighted Sum command. Then, the aggregation of the obtained weights revealed the total weight of each of the target factors. Finally, the vulnerability zoning map of the sewage collection network was prepared in two levels of manhole and pipeline. According to the results, the probability of failure of the implemented manholes was very low and more than 70% of the manholes were classified in the category of low and very low damage. One of the possible reasons for this could be the young age and the principled implementation of the sewage collection network. The final overlap of the factors and sub-criteria affecting the vulnerability of the sewerage network indicated that the probability of destruction of pipelines was very low. More than 74% of the sewerage collection pipeline implemented in the case study area was classified in the category of low and medium damage. The results showed that only 30 wastewater transfer pipelines (about 7% of the whole network) were in critical conditions that should be given more attention. It could be considered that the small number of critical pipelines was due to the short life of the implemented pipelines. Based on the results, among the three target factors (manhole, pipeline and branch), the highest vulnerability of the sewerage collection network was related to the pipeline blockage. Also, the defects of the operational defects in all three levels of manhole, pipeline and branch clogging was the most affecting factor and have the highest impact.