Centralized or Decentralized Rainwater Harvesting Systems: A Case Study

World population growth, climate changes, urbanization, and industrialization have all had a negative impact on natural resources, including water resources. Excessive exploitation and pollution have caused more and more regions to have problems with access to fresh water. Rainwater is perceived as a valuable alternative source of water that is most often used in a hybrid system supplementing tap water. Considering the possibilities of designing a rainwater harvesting system as a decentralized or central system, this research was undertaken to determine the hydraulic and financial efficiency of these two systems. The research was carried out for a single-family housing estate located in Poland. For this research, a simulation model was applied to determine the efficiency of water saving and the life cycle cost indicator. In variants where rainwater was only used to flush toilets, the water saving efficiency was 80% and 79% for the decentralized and centralized rainwater harvesting system (RWHS), respectively. The use of rainwater for toilet flushing and watering the garden resulted in a significant reduction in efficiency to 57% (the decentralized system) and 54% (the centralized system). On the other hand, the results of the life cycle cost (LCC) analysis showed that in spite of reducing tap water consumption, both the centralized and the decentralized rainwater harvesting system were not financially viable solutions for the housing estate, and only cofinancing investments at the level of 25% to 50% resulted in a significant improvement in financial efficiency.