Community rainwater harvesting financial payback analyses - case study in Malaysia

Malaysian water demand is increasing at an alarming rate reaching 27 to 38% higher than the World Health Organisation recommended consumption limit of 165 liters per capita per day. Therefore, the Malaysian water shortage crisis is quite possible in future due to this water demand uptrend. The average annual rainfall of Malaysia is 2,400 mm but large portion of this fresh water resource becomes runoff and lost through our catchments. Urban flash flood is also becoming more frequent due to fast pace of urban development and anthropogenic induced runoff. Malaysia has experienced drought and flooding in different areas and therefore, it is crucial to study the feasibility of alternate water resources in Malaysia to manage and maintain the sustainability of urban township. This study reviewed a past rain water harvesting system (RWHS) case and conducted the financial payback analyses on its proposed system. If there were 177 rain days per year with at least 52 mm of rainfall event depth, the payback period of the proposed RWHS would be 5.8 years when the discount rate (i%) = 2% and 8.2 years if i% = 10%. The payback period became longer when the annual rain days dropped below 106 and 89 rain days per year. If the proposed RWHS only serve the community under this study, it will take 12 days to consume 800 m³ stored water, while any rainfall of consecutive days will not be harvested as the underground storage tank is in full capacity. The proposed RWHS must be filled up at least 38 times per year in order to break even with the proposed annual maintenance cost but will never be able to achieve any payback from its initial investment. Rain water harvesting and full utilisation is the only way to achieve high water cost savings and shorter payback period, and maximise urban excess runoff reduction.