Water Quality and Financial Analysis of a System Combining Rainwater and Greywater in a House

This study aimed to evaluate the installation of a system that combines rainwater to wash clothes and greywater to flush toilets in a house located in Florianópolis, southern Brazil. The evaluation consisted of determining the potable water savings, performing a financial analysis, and verifying the possibility of using rainwater and reusing greywater according to the local legislation. The reduction in potable water consumption was determined by comparing the monthly water and sewage bill, before and after installing the system, combining rainwater and greywater. The financial analysis was performed considering the financial benefits, operational costs, costs for installing the system, and local water tariffs. After installing the system, there was a decrease of 38.0% in the monthly potable water consumption, which represents a decrease of 7.00 m3 in the monthly potable water consumption. The payback periods for the old and new water tariffs were equivalent to, respectively, ten years and seven months, and eight years and five months. Both rainwater and greywater had parameters with concentrations higher than the limits established by the legislation. Therefore, the rainwater had to go through a device for discarding the first flush and needed disinfection
and the greywater needed to go through a treatment process. The results indicated that the system combining rainwater and greywater shows significant potential for potable water savings, which can be a good alternative to the preservation of potable water resources. It is important to note that current studies on rainwater use and greywater reuse are mainly related to non-potable purposes, but it was shown herein that rainwater can be used for washing clothes.