Greywater recycling in multi storey residential buildings: modelling a decentralized treatment and reuse system for toilet flushing and irrigation

This study focuses on greywater recycling in multi-storey residential buildings. Greywater is a reliable substitution for potable water at tasks such as toilet flushing and landscape irrigation, and it makes up 50-80% of all wastewaters produced in residential buildings, while having comparatively low BOD and COD levels. Greywater can be treated using rotating biological contactor (RBC) and membrane bioreactor (MBR) systems. Results show 28-42% clean water savings, energy savings of 0.32–0.45 kWh/m³ and 25–35% decrease in greenhouse gas emissions. RBC is optimal for 7 or more floored buildings and has a payback period of 6-9 years, while MBR is used for bigger buildings and clustered systems. No crucial hygienic risks are noted. Compared with real data from implementations in Berlin, it’s found that discrepancies are lower than 15%, and real projects would be totally feasible. Importance of 1-2 days of treatment, capture efficiency, maintenance, and real time monitoring are noted.