Extending Wastewater Treatment Process Models for Phosphorus Removal and Recovery: A Framework for Plant-Wide Modelling of Phosphorus, Sulfur and Iron

As problems associated with shortage in resource supply arise, wastewater treatment plants turn to innovation to transform themselves into resource recovery facilities. Water groups worldwide recognize that wastewater treatment plants are no longer disposal facilities but rather sources of clean water, energy and nutrients.

One of the most important resources that can be recovered from wastewater treatment plants is phosphorus. Mathematical modelling can be utilised to analyse various operational strategies to recover phosphorus from the wastewater. However, incorporating phosphorus transformation processes in plant-wide models is complex. Firstly, the tri-valence of phosphates suggests non-ideality, which requires the use of a physico-chemical model to account for this non-ideality. Secondly, phosphorus has strong interlinks with sulfur and iron, which necessitates inclusion of their transformations in biological and physico-chemical models. Lastly, consolidating these into a plant-wide model aimed at describing phosphorus removal and/or recovery requires interfacing, modifications to the plant layout, addition of recovery unit processes and development of new control and operational strategies. The research work presented in this thesis addresses the aforementioned challenges.

A physico-chemical model is developed to take into account ion activity corrections, ion pairing effects, aqueous phase chemical equilibria, multiple mineral precipitation and gas stripping/adsorption. The model is then linked with standard approaches used in wastewater engineering, such as the Activated Sludge Model Nos. 1, 2d and 3 (ASM1, 2d, 3) and the Anaerobic Digestion Model No. 1 (ADM1). The extensions of the ASM2d and ADM1 with phosphorus, sulfur and iron-related conversions followed. And finally, the extended models and the physico-chemical model are consolidated into a plant-wide model provided by the Benchmark Simulation Model No. 2. The resulting model is used for simulation-based scenario analysis for finding ways to improve the operation of a wastewater treatment plant aimed at phosphorus removal and recovery.