Water end-uses and financial analysis of rainwater harvesting systems in Florianpolis, Brazil

This paper aims to estimate the water end-uses in a flat and to evaluate the financial feasibility of rainwater harvesting systems in houses and flats in the city of Florianópolis, southern Brazil. The city has an average annual rainfall of 1738 mm/year, well distributed throughout the year, and a humid subtropical climate according to the Köppen-Geiger climate classification. Ten scenarios were evaluated, i.e. five for a single-family house and five for a multi-family building, with variations of ± 5% and ± 10% on the rainwater demand, based on a flat with two residents and potable water demand of 162.4 litres per person per day. The highest daily uses were due to showering ─ 60.6 litres (37.3%), toilet flushing ─ 48.5 litres (29.8%), washing machine ─ 29 litres (17.9%), and dishwashing ─ 14.3 litres (8.8%). Washing hands, brushing teeth, washing the face, cooking and others, represented 10 litres per person per day (6.2%). The rainwater demand, based on the amount of water used for toilet flushing and washing machine, was estimated as 47.7% of the total water demand. The rainwater tank sizing and the financial analysis were carried out using the Netuno computer programme. For the houses, the system was financially feasible in three out of five scenarios. The payback periods ranged from 220 to 238 months and the internal rate of return, from 0.07% to 0.25% per month. The system was financially feasible in all scenarios evaluated for the multi-family building. The payback periods ranged from 96 to 103 months and the internal rates of return, from 1.07% to 1.17% per month.