A hybrid review of sewer inspection tools and automated CCTV image analysis techniques

Abstract

Maintaining the integrity of sewage networks is crucial for sustainable urban development. Despite extensive research on inspection tools, machine learning applications, and condition assessment for sewer defects, a holistic review of these elements remains absent. This paper addresses this gap by presenting a comprehensive review within a unified framework, employing a mixed-method approach that includes bibliometric, scientometric, and systematic analyses. Our findings reveal that integrating in-pipe and out-pipe inspection methods enhances outcomes. The current literature identifies modified RegNet, dilation segmentation with conditional random field (DilaSeg-CRF), you only look once (YOLO) models, and faster region-based convolutional neural network (Faster R-CNN) as effective algorithms for classification, segmentation, and object detection, both on-site and off-site, respectively. However, machine learning is an evolving field, and future algorithms may surpass these models. Identifying key challenges, we propose recommendations aimed at advancing research and enhancing replicability: notably, the expansion of international research collaborations, particularly in underrepresented regions such as the Middle East, Africa, Asia, and South America; applying the latest version of YOLOv11 in object detection; and investigating defect patterns in polyvinyl chloride (PVC) sewer and rehabilitated pipes using advanced diagnostic methods. This review anticipates aiding policymakers in adopting informed strategies, thereby contributing to the development of smarter, more sustainable cities.