Combined management of water and energy to reduce the environmental footprint in an urban context

© 2015 Dr. Sudeep P SAs potable water scarcity is one of the main challenges countries are facing worldwide, reliance on new and alternative water supply sources is a possible alternative. Strategies based on ‘fit-for-purpose’ water supply in which demand for water of specific quality standards are matched to types of water available of variable quality are increasingly being adopted. This transformation makes urban water systems increasingly decentralised, complex and more energy intensive. A comprehensive review of available studies reveal that the dynamics of water-energy nexus associated with centralized and decentralized water systems supply water on a ‘fit for purpose’ basis has received little attention. This study attempts to bridge this gap by developing a generic modelling framework that integrates the modelling of water and fixed and operational energy use flows in an urban water system using a simulation-optimization approach in conjunction with Life Cycle Energy Assessment (LCEA). The first component of this study is the assessment of energy consumption using a holistic LCEA approach. The water supply systems considered in this research are i) imported water from dam or reservoir, ii) desalinated water, iii) recycled water from a sewage treatment plant (STP), iv) recycled greywater, v) water harvested from a rooftop runoff and vi) harvested stormwater. The system boundary encompasses water extraction through to disposal and recycling. Both operating and embodied energy per unit quantity of water are estimated for the whole water supply system. The energy use variables per unit quantity of water (gigajoule/megaliter) obtained from LCEA software Simapro® are then incorporated in the System Dynamics model built in Matlab/Simulink® software. The second component is based on a stock and flow system dynamic model that tracks the dynamics of water, energy and carbon emission flows. Reservoirs, tanks and water treatment plants represent the stocks while the flow of water, energy, carbon emissions represents the system flows. Each stock encompasses the various processes that occur inside a typical unit operation such as water treatment whereby flows entering the stocks are subjected to variations in terms of gains and losses. The third element consists of linking the dynamic model to an optimization algorithm based on Genetic Algorithm (GA) to determine the minimum energy pathway while attempting to satisfy all the water demands. The model simulates the dynamics of water supply from each source over a period in response to various urban water policy scenarios. The optimization is solved using GA written in Matlab®. The simulation-optimization framework is applied to the Aurora urban development, a new urban precinct in Melbourne consisting of 10,000 households. The scenarios are simulated on daily and monthly time steps to inform various stakeholders about the contribution of water from various water sources considered in this study. It is assumed that for all scenarios reservoir and desalination plant can supply potable and non-potable water while all other sources can supply only non-potable water. At first, simulations are run for a wet month to observe the behaviour of the water supply system and energy use pattern on a daily basis. Similarly, a typical dry month is also run on a daily time step. In order to investigate the long-term effects of water availalability and supply on energy use, the model is also run on monthly time steps. At first, a 20 months perod is run followed by a 120 month period to inform stakeholders about the effect of a decade long drought in Melbourne. Two subcases are also analysed for each scenario to investigate the effect of restriction on reservoir water supply on water supply from alternative water sources and energy use implications. The simulation-optimization results show that during rainfall events, stormwater proves to be the cheapest option in terms of energy use for non-potable purposes. If potable water savings is not the objective the best choice is conventional centralized water supply from reservoir. However, when water availability is constrained due to dry weather conditions and rainfall variability, STP is the next best available option to supply non-potable water. The use of STP provides water managers with dual advantages of potable water saving and less energy use when compared with other alterative supply systems. Furthermore, the results obtained in this study justify the implementation of wastewater treatment plant in Aurora. The results showed the capability of the model to help decision makers in improving the management of urban water systems in terms of environmental sustainability