Optimization model of large scale rainwater harvesting system

Abstract

Rapid urbanization, population growth, and industrialization are contributing to the large-scale increase of water demand in Malaysia. The present scheme of inter-state water transfer to meet the increasing demand is unsustainable. It is expected that Johor, Malaysia will face water shortages after 2025. Hence, a new source of water needs to be identified to meet the water demand. Many studies have been done to tackle the issue of water shortage using small scale rainwater harvesting. However, there should be a study on large scale rainwater harvesting systems (LSRWHS) that could provide better economic and environmental benefits because of the ability to mitigate both water scarcity and flood issues in urban areas. An optimization model for systematic design and planning to determine the optimal sizing and network design for LSRWHS to meet the regional supply-demand distribution at minimum cost has been proposed in this research for variation of water supply and demand. A case study of Nusajaya, Johor with the rainfall daily data for 20 years collected from 200 houses rooftop was employed in this research. This thesis introduces three main contributions. (1) A methodology to determine an optimal water storage tank and total outsourced water utility supply required using a new numerical technique (graphical technique) named as Rain Water Harvesting Pinch Analysis (RAGPA). (2) A mathematical model approach to determine an optimal water storage tank to satisfy consistent water demand. The results in the first and second objectives were then used to validate the effectiveness of both methodologies. (3) A spatial planning and scheduling mathematical model for LSRWHS to satisfy inconsistent water demand and determine an optimal location for the construction of LSRWHS. It was found that the optimal size of storage tank based on 20 years rainfall data was 617 m3 using a numerical method and as proposed in the first methodology. This finding shows a very good agreement using the mathematical method with less than 2% error. For third methodology result, it was found that the minimum cost system was RM 475 437 with the optimal size of storage tank 727 m3.The optimizer tend to selects location with the shortest distance of pipe length and larger area as the optimal location for LSRWHS. Sensitivity analysis was further performing by modifying the water demand, roof area collection and water utility cost. The result generated shows area of rooftop and water demand play an important role in the economic analysis. The application of all new methodologies has demonstrated the applicability of the model to design an optimal rainwater harvesting system to face water scarcity problem in future and provide relatively clean, reliable water to people in need.