The ongoing paradigm shift in sewage management from pollution control to resource recovery is an integral part of a circular economy. Efforts have traditionally concentrated on the recovery of organic carbon as bioenergy, and nitrogen and phosphorus as nutrients. In the past decade, technologies have been developed to recover high-value commodities. Simultaneously, innovative processes have been proposed to produce chemical agents that can be immediately reused in the sewage system to facilitate sewage management. This Review evaluates these latest developments and the underpinning scientific discoveries, identifies the key challenges, and outlines a roadmap towards greener and more sustainable sewage management.

This study deals with the potential and the limitations of dynamic models for describing and predicting nitrous oxide (N₂O) emissions associated with biological nitrogen removal from wastewater. The results of a three-week monitoring campaign on a full-scale partial nitritation reactor were reproduced through a state-of-the-art model including different biological N₂O formation pathways. The partial nitritation reactor under study was a SHARON reactor treating the effluent from a municipal wastewater treatment plant sludge digester. A qualitative and quantitative comparison between experimental data and simulation results was performed to identify N₂O formation pathways as well as for model identification. Heterotrophic denitrifying bacteria and ammonium oxidizing bacteria (AOB) were responsible for N₂O formation under anoxic conditions, whereas under aerated conditions the AOB were the most important N₂O producers. Relative to previously proposed models, hydroxylamine (NH₂OH) had to be included as a state variable in the AOB conversions in order to describe potential N₂O formation by AOB under anoxic conditions. An oxygen inhibition term in the corresponding reaction kinetics was required to fairly represent the relative contribution of the different AOB pathways for N₂O production. Nevertheless, quantitative prediction of N₂O emissions with models remains a challenge, which is discussed.

Traditional wastewater treatment plants (WWTPs) are increasingly regarded as water resource recovery facilities (WRRFs), reflecting the value of water, nutrients, energy and other resources, besides ensuring the required effluent quality. Resource recovery techniques involve biochemical, physical and physico-chemical processes, and even previously unexploited biological conversions. Biopolymer and bioplastic production also reveal the remarkable potential present in our microbial cultures. Models have demonstrated their usefulness to optimize WWTP operation to achieve better effluent quality at lower costs; they also constitute a useful tool to support the transition of WWTPs into WRRFs that maximize the valorization of products recovered from wastewater. In this paper, the extent to which the new techniques and unit processes applied for resource recovery could be modelled with conventional activated sludge models (ASMs) and additional modelling challenges being faced are discussed while providing recommendations of potential approaches to address current modelling research gaps.