Sustainable Retrofitting of Soviet-Era Heritage Bathhouses: Greywater Reuse and Geothermal Heat Recovery at the Arasan Complex

Water-intensive heritage buildings, such as Soviet-era public bathhouses, face critical challenges in aligning with contemporary sustainability goals due to architectural, operational, and regulatory constraints. This study proposes a structured framework for retrofitting such facilities by integrating greywater reuse and energy optimization technologies, using the Arasan Bath Complex in Kazakhstan as a model case. A modular greywater treatment system—combining ultrafiltration, activated carbon, and ozone disinfection—is designed to recover shower and pool wastewater for non-potable reuse, with simulations indicating a potential 30% reduction in freshwater consumption (~30,000 m³ annually). To improve energy efficiency, a hybrid heating concept integrating geothermal heat pumps and greywater heat exchangers is evaluated, with projected results suggesting a 29% offset in thermal energy consumption. Financial analysis estimates a payback period of approximately five years for the water reuse system and under eight years for the combined heat recovery strategy. The framework emphasizes regulatory compliance and preservation of architectural integrity, making it adaptable for other heritage-protected facilities in resource-constrained contexts. The study contributes a replicable and conservation-sensitive model for sustainable retrofitting of cultural infrastructure in Central Asia and beyond.