Minimum cost solution to residential energy-water nexus through rainwater harvesting and greywater recycling

Abstract This paper presents an integrated rainwater harvesting and greywater recycling system to tackle the energy-water nexus in residences with unreliable water and electricity supply. The system is made up of the municipal water and energy supplies, a rainwater harvesting system and a greywater recycling system to supply the residential water demand. A combined sizing and operational optimization approach is developed. In particular, a mixed integer linear programming model is formulated to determine the optimal sizes of the water tanks and optimal operation of pumps to reduce potable water consumption and electricity cost under the time-of-use tariff. The model formulated is applied to a practical case study of a single-family house in Durban, Kwa-Zulu Natal province of South Africa. Simulations results with measured rainfall intensity over five years show that the proposed integrated rainwater harvesting and greywater recycling system is beneficial for the household in terms of both water savings and financial cost savings with an acceptable payback period of 4.39 years. To investigate the validity of the results obtained in different applications, a sensitivity analysis was performed concerning uncertainties in water demand, rainfall intensity, the cost of electricity and discount rate. The findings confirm that the model developed is robust against uncertainties in these parameters. It is also concluded that payback period of the project can be even shorter if applied to a case with a higher water demand.