Abstract: In the urban development process， some existing sewage treatment plants are encountering the challenge of insufficient treatment capacity. In developed urban zones， constructing new sewage treatment plants is often restricted by high land costs， limited space， and long construction periods. Against this backdrop， in-situ expansion technology has emerged as an effective solution. A sewage treatment plant in Guangzhou， originally designed with a capacity of 1.5×10⁵ m³/d， has been operating over-capacity due to a sharp increase in wastewater volume caused by regional development. To address the issue of insufficient treatment capacity， the plant has designed and implemented an in-situ expansion and technological transformation strategy， successfully increasing its treatment capacity from 1.5×10⁵ m³/d to 2×10⁵ m³/d. Its core lies in the detailed expansion strategies for each treatment process segment. One of the key aspects is the upgrading of the process flow. The original process， which consisted of modified AAO （anaerobic-anoxic-oxic） + secondary sedimentation tank + high-efficiency Sedimentation， has been innovatively improved to "modified AAO + secondary sedimentation tank （integrated aeration and sedimentation） + magnetic coagulation sedimentation. This upgraded process not only enhances the overall treatment efficiency but also significantly improves the plant's ability to handle the increased volume of wastewater. In addition to the process upgrade， the plant has also adopted a phased implementation strategy for technological renovation based on actual influent water quality. This approach allows for a more flexible and cost-effective expansion process. By carefully analyzing the actual water quality data and adjusting the renovation plan accordingly， the plant can achieve an optimal balance between capacity expansion and cost control. This phased implementation strategy maximizes the cost-effectiveness by optimizing investment and minimizing operational costs. Compared with traditional new construction methods， in-situ expansion technology offers several significant advantages. Firstly， it conserves land resources by making full use of the existing plant's infrastructure and facilities， thus avoiding the need for additional land acquisition. Secondly， it reduces engineering investment costs， as the renovation and upgrade of existing facilities are generally less expensive than constructing an entirely new plant. Lastly， it lowers operating costs by leveraging the existing operational systems and personnel， thereby achieving scale economies. In conclusion， the in-situ expansion and technological transformation of the Guangzhou sewage treatment plant not only successfully increases its treatment capacity but also provides an economical and efficient solution. This case demonstrates that in-situ expansion technology can be an excellent alternative for addressing the capacity limitations of existing sewage treatment plants in urban areas， especially where land is scarce and financial resources are constrained.