Keywords: water, wastewater, computational fluid dynamics, process optimisation, model coupling.

Climate change, population growth, urbanisation and pollution all mean that society faces an urgent and compelling need to adapt to reductions in natural resource availability and reductions in energy consumption. At the very heart of this is the availability, management and security of water. To facilitate the required adaptations, we need to deepen our understanding of fundamental environmental processes in both the natural and engineered water cycles. Without such knowledge, adaptation measures will have limited success, and so will our ability to survive.

The hydraulic, biological, physical and chemical phenomena occurring in water and wastewater treatment systems and their associated networks are complex, inter-related and non-linear. As our understanding of environmental systems has increased, so we have been able to develop more sophisticated mathematical models to predict the behaviour and response of water under different scenarios. Numerical modelling of the performance of water and wastewater treatment systems offers a means by which designers and operators can assess the likely impact of water or wastewater quality or process modifications on treated water or final effluent quality. This allows academic and industrial communities to benefit from greater insight into the natural and engineered processes with which we work, and helps to provide an indication of the risk of failure to meet quality standards under different scenarios. The use of computational fluid dynamics (CFD) to simulate hydraulic processes has begun to be exploited within the water industry, and CFD has now been successfully used in water and wastewater treatment applications. Examples of hydraulic simulation are described and discussed here.

It is shown that, subject to careful, well-planned use, CFD is capable of facilitating improvements in the hydraulic and process performance of treatment systems. However, limitations in current simulation techniques for various processes are identified and recommendations for further work are made. Furthermore, it is recognised that hydraulic and biophysicochemical modelling of water and wastewater treatment processes are at different stages of maturity. Nevertheless, a visionary approach to water management simulation is proposed which will move the water industry from a sub-discipline, unit process-based, silo mentality, to a fully integrated, coherent trans-disciplinary approach within the wider environmental engineering field. This innovative approach to simulation of the water environment would deliver a step change in the water industry's approach to research and management. It is suggested that without this type of thinking and approach we will be firefighting with inadequate tools and we will fail to deliver the water aspect of the world's water, energy and food agenda.

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