The water cycle, from catchment to discharge, plays a crucial role in ensuring a stable water supply and mitigating environmental impacts. Wastewater treatment plants (WWTPs) are essential for protecting ecosystems, preventing the spread of disease, and providing alternative resources. Their sustainability depends on identifying the most efficient facilities—those that remove pollutants effectively while using minimal resources. High-efficiency WWTPs offer insights into key factors for design and operation. As central components of urban water management, WWTPs support economic development and reduce pressure on freshwater resources by enabling the reuse of water. Although reuse does not cut overall demand, it limits abstraction and environmental harm. Reimagining the urban water cycle through circular water management enhances the value of WWTPs, allowing for the recovery of energy (biogas) and nutrients (nitrogen, phosphorus). WWTPs must adhere to circular economy principles, which involve improving efficiency, reducing inputs, and minimizing waste. Their performance depends on various factors, including technology, size, age, and maintenance. The debate over centralized vs. decentralized systems has driven research on optimal plant sizing. This chapter explores ideal WWTP dimensions to guide sustainable infrastructure planning. By analysing economies of scale, efficiency, and environmental impact through a novel approach, we aim to provide evidence-based recommendations that support informed, sustainable decisions in urban water management, balancing cost and ecological protection.