Towards a sustainable framework
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A sustainable wastewater system should be designed and managed to fully contribute to the objectives of society, now and in the future, while maintaining their functions of resource recovery and wastewater assimilation in the environment. Basically, at least four strategies are available as paths towards such a target, namely control and optimization of process operations, upgrading of entire wastewater treatment plants (WWTP), nutrient trading between point and non-point sources in watersheds, and the separation of wastewater fluxes at their sources. In order to evaluate all these strategies from the various different perspectives (WWTP, watershed, and global), a comprehensive set of data was utilized to calibrate Activated Sludge Model No.2d (ASM 2d). Afterwards, an algorithm composed of a computationally inexpensive High Dimensional Model Representation (HDMR) and a simple interpolation scheme is developed to approximate the full simulation model under all possible combinations of process control variables and model parameters, for the purpose of optimizing plant operating performance. A systematic framework is then developed for estimating the costs of removing nutrients in a WWTP in the context of nutrient trading across watersheds. Last, the sustainability of different strategies of nutrient removal and recovery is assessed, using indicators across the economic, environmental and social domains. From this investigation, the optimal solution to wastewater management varies as a function of the scope of analysis (in-plant, across the watershed, and from the global perspective of sustainability). Restricting the scope to the WWTP alone, it is found that the A/A/O (Anaerobic/Anoxic/Oxic) configuration is preferred in respect of the targets of removing more nutrients at a lower operating cost. However, from the perspective of the watershed, it is found that the AS (Activated Sludge) design with alum addition becomes most favorable, given its lowest Total Annual Economic Cost (TAEC). Finally, from the perspective of sustainability, source-separation strategies are indicated as better than current wastewater management strategies.