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.@en

Recovering phosphorus from wastewater in more concentrated forms has potential to sustainably recirculate phosphorus from cities to agriculture. The environmental sustainability of wastewater-based phosphorus recovery processes or wastewater-derived phosphorus products can be evaluated using life cycle assessment (LCA). Many LCA studies used a process perspective to account for the impacts of integrating phosphorus recovery processes at wastewater treatment plants, while some used a product perspective to assess the impacts of producing wastewater-derived phosphorus products. We demonstrated the application of an end-user perspective by assessing life cycle environmental impacts of substituting half of the conventional phosphorus rock-based fertilizers used in three crop production systems with wastewater-derived phosphorus products from six recovery pathways (RPs). The consequential LCA results show that the substitution reduces global warming potential, eutrophication potential, ecotoxicity potential, and acidification potential of the assessed crop production systems in most RPs and scenarios. The end-user perspective introduced in this study can (i) complement with the process perspective and the product perspective to give a more holistic picture of environmental impacts along the “circular economy value chains” of wastewater-based resource recovery, (ii) enable systemwide assessment of wide uptake of wastewater-derived products, and (iii) draw attention to understanding the long-term environmental impacts of using wastewater-derived products.@en

Wastewater treatment processes with granular sludge are compact and are becoming increasingly popular. Interest has been accompanied by the development of mathematical models. This contribution simultaneously reviews available models in the scientific literature for anaerobic, aerobic and partial nitritation-anammox granular sludge reactors because they comprise common phenomena (e.g. liquid, gas and granule transport) and thus pose similar challenges. Many of the publications were found to have no clearly defined goal. The importance of a goal is stressed because it determines the appropriate model complexity and helps other potential users to find a suitable model in the vast amount of literature. Secondly, a wide variety was found in the model features. This review explains the chosen modelling assumptions based on the different reactor types and goals wherever possible, but some assumptions appeared to be habitual within fields of research, without clear reason. We therefore suggest further research to more clearly define the range of operational conditions and goals for which certain simplifying assumptions can be made, e.g. when intragranule solute transport can be lumped in apparent kinetics and when biofilm models are needed, which explicitly calculate substrate concentration gradients inside granules. Furthermore, research is needed to better mechanistically understand detachment, removal of influent particulate matter and changes in the mixing behaviour inside anaerobic systems, before these phenomena can be adequately incorporated in models. Finally, it is suggested to perform full-scale model validation studies for aerobic and anammox reactors. A spreadsheet in the supplementary information provides an overview of the features in the 167 reviewed models.

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During biological wastewater treatment, substrates undergo simultaneous diffusion and reactions inside microbial aggregates, creating microscale spatial substrate gradients and limiting the macroscale reaction rates. For flocculent and anaerobic granular sludge, this rate-limiting effect of diffusion is often lumped in model parameters, like the half-saturation coefficients of Monod kinetics in activated sludge models (ASM). Yet, an explicit description of the reaction-diffusion process with biofilm models is more common for aerobic granular sludge. This work investigates whether apparent half-saturation coefficients could have applications for aerobic granular sludge as well and examines the implications of this simplification. To this end, the macroscopic reaction rates predicted with a one-dimensional biofilm (1D) model were fitted with Monod kinetics. The results showed that the macroscale rates could indeed be described using apparent kinetics, at the very least over a time scale where the microbial population distribution stays fixed. However, the coefficients were sensitive to changes in the microbial population distribution, which can be affected by long-term changes in operating conditions. Also the activity of organisms that compete for the same substrates affect the parameter value. Be that as it may, apparent kinetics also depend on the operating conditions for flocculent and anaerobic granular sludge, but they have still been used successfully for design and optimization. Therefore, the last section of this work illustrates that they may also have applications for aerobic granular sludge. A simple model for ammonium removal using apparent half-saturation coefficients for oxygen and ammonium is applied to a full-scale reactor, taking advantage of the batch-wise operation and on-line monitoring data for regular recalibration.@en

A stepwise experimental design procedure to obtain reliable data from wastewater treatment plants (WWTPs) was developed. The proposed procedure aims at determining sets of additional measurements (besides available ones) that guarantee the identifiability of key process variables, which means that their value can be calculated from other, measured variables, based on available constraints in the form of linear mass balances. Among all solutions, i.e. all possible sets of additional measurements allowing the identifiability of all key process variables, the optimal solutions were found taking into account two objectives, namely the accuracy of the identified key variables and the cost of additional measurements. The results of this multi-objective optimization problem were represented in a Pareto-optimal front. The presented procedure was applied to a full-scale WWTP. Detailed analysis of the relation between measurements allowed the determination of groups of overlapping mass balances. Adding measured variables could only serve in identifying key variables that appear in the same group of mass balances. Besides, the application of the experimental design procedure to these individual groups significantly reduced the computational effort in evaluating available measurements and planning additional monitoring campaigns. The proposed procedure is straightforward and can be applied to other WWTPs with or without prior data collection.

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Anammox bacteria can perform dissimilatory nitrate reduction to ammonium (DNRA) with nitrite as intermediate coupled to the oxidation of volatile fatty acids (VFA). Batch tests with enriched anammox and a co-culture of anammox and heterotrophic bacteria showed the capacity of Candidatus ‘Brocadia fulgida’ to perform the DNRA coupled to the anammox reaction (DNRA-anammox) at a high rate although the culture was not previously adapted to VFA. From thermodynamic calculations it could be stated that low COD/N influent ratios favour the DNRA-anammox transformation over heterotrophic conversions since more free energy is gained. A process scheme is proposed for an innovative nitrogen removal system in which the nitrate produced by nitrite oxidizing bacteria and/or anammox bacteria is converted during DNRA-anammox pathway, resulting in a sustainable nitrogen removal from municipal wastewater while circumventing the troublesome out-selection of nitrite oxidizing bacteria encountered in mainstream applications.

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Aquasim is public-domain and user-friendly software that is widely used for biofilm modeling. The program uses built-in process unit blocks, such as biofilm and mixed compartments. In the literature, often several of these compartments are linked with artificial advective flows to represent a single physical well-mixed system, such as a granular sludge reactor. This work shows that caution should be exercised with this popular approach because numerical errors occur when too high artificial advective flows are used. These errors could stay unnoticed if mass-balances of the simulation results are not checked. A solution to this problem is proposed in this work. A new method based on diffusive links between compartments instead of advective links leads to more reliable and faster simulations and is much easier to implement. Biotechnol. Bioeng. 2017;114: 2131-2136.

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