Reliability and feasibility assessment of community rainwater harvesting system

Abstract

A reliability analysis was performed in this study for community rainwater harvesting system followed by a cost-benefit analysis. A site survey was performed in the study area (Paikgacha Pourashava) as a part of data collection. As community rainwater harvesting as a source of piped water supply is a new concept, the purpose of site survey was not only to collect necessary data but also to make local people aware of this concept. To fulfill these objectives, a three-part questionnaire was formulated. The first part was designed to identify the household information, housing, sanitation, health facilities etc. The second part was outlined to identify the existing water supply, bottleneck of the existing system, water demand, present water use rate, quality of water etc. and the third part was to let people know about the advantages of proposed community integrated rainwater harvesting system and take their opinion and suggestion. Collecting necessary information from site survey in addition to other sources a reliability analysis was performed by developing a behavioral model in MATLab environment. Two reliability index- volumetric reliability and time reliability was determined during reliability analysis. Moreover, this model was used to determine other two performance indicators named water utilization and water spillage of the system. A sensitivity analysis was also performed to assess the effect of major parameters on the performance of community rainwater harvesting system. Moreover, monthly demand satisfaction level and failure percentages were also determined for various cases. A Strom water management model named, USEPA SWMM 5.0 was used in this study to model community rainwater harvesting system as a verification of rational formula used in the behavioral model. To perform cost-benefit analysis, every cost and benefit elements of rainwater harvesting systems were identified along with their prices. At first cost analysis was performed varying different design parameter. After that cost benefit analysis was accomplished using net present value (NPV) as a financial indicator. Likewise cost analysis, effect of different parameters on NPV was also assessed. Precipitation variability due to climate change was found to be a significant factor in the performance of rainwater harvesting system while performing sensitivity analysis using behavioral model. Moreover, community size, water demand and time steps appeared to have marked contribution to the performance. The effect of YAS and YBS algorithm on rainwater harvesting system was not found significant. Likewise before, climate change was found to be a significant factor in monthly demand satisfaction and monthly failure percentage. It was found that performance curves developed with the supply matrix of SWMM model have similar trend to the curves generated from the rational formula. However, higher runoff from SWMM model compared to rational formula indicates that SWMM model ought to be used for community rainwater harvesting system to achieve actual runoff and for economical design. In cost analysis, storage system was found to be the most costly part of the community rainwater harvesting system followed by collection system, operation & maintenance system and distribution system. Reliability appeared to be an important parameter for the cost of the system and it was found that to increase reliability of 10%, every household would have to bear a considerably more amout of money and this amount seemed more for larger community sizes. It was found that precipitation variability due to climate change positively affects community rainwater system and significantly less cost is expected to be required for predicted future precipitation scenario. It was also seen that NPV of community rainwater harvesting system was far above than those of individual household based system for the study area. It was observed that NPV shows an increasing trend with increase in community size followed by some fluctuations and after that the rising trend continues. Although this trend of NPV is similar for different reliability and precipitation pattern of different time periods, the community size at which fluctuations end varies. That is why to assemble reliability, climate change, critical community size, NPV and cost of harvested water in the same plot a nomograph was constructed which offers flexibility to the policy makers or end users to choose reliability and precipitation variability of any time period in making decisions about suitable community size for community rainwater harvesting system.