How well-mixed is well mixed? Hydrodynamic-biokinetic model integration in an aerated tank of a full-scale water resource recovery facility

Hybrid differential equations: Integrating mechanistic and data-driven techniques for modelling of water systems

Mathematical modelling is increasingly used to improve the design, understanding, and operation of water systems. Two modelling paradigms, i.e., mechanistic and data-driven modelling, are dominant in the water sector, both with their advantages and drawbacks. Hybrid modelling aims to combine the strengths of both paradigms. Here, we introduce a novel framework that incorporates a data-driven component into an existing activated sludge model of a water resource recovery facility. In contrast to previous efforts, we tightly integrate both models by incorporating a neural differential equation into an existing mechanistic ODE model. This machine learning component fills in the knowledge gaps of the mechanistic model. We show that this approach improves the predictive capabilities of the mechanistic model and is able to extrapolate to unseen conditions, a problematic task for data-driven models. This approach holds tremendous potential for systems that are difficult to model using the mechanistic paradigm only.

Plant-wide modelling in wastewater treatment: showcasing experiences using the Biological Nutrient Removal Model

Plant-wide modelling can be considered an appropriate approach to represent the current complexity in water resource recovery facilities, reproducing all known phenomena in the different process units. Nonetheless, novel processes and new treatment schemes are still being developed and need to be fully incorporated in these models. This work presents a short chronological overview of some of the most relevant plant-wide models for wastewater treatment, as well as the authors' experience in plant-wide modelling using the general model BNRM (Biological Nutrient Removal Model), illustrating the key role of general models (also known as supermodels) in the field of wastewater treatment, both for engineering and research.

Water and wastewater CFD and validation: are we losing the balance?

A literature screening on computational fluid dynamics (CFD) modelling in water treatment applications showed a vast range of validation ranging from no validation at all, over residence time distribution (RTD) and tracer testing, to velocity field, species concentration and, finally, turbulence properties measurements. The validation level also differs depending on process scale (laboratory, pilot, full) and type of system (rheology, single phase vs. multiphase). Given the fact that CFD is in more widespread use, a discussion on the extent and need of validation needs to be initiated. This paper serves as a discussion starter on the topic.

Full-scale investigation of in-situ iron and alkalinity generation for efficient sulfide control

Hydrogen sulfide induced corrosion of concrete sewer pipes is a major issue for wastewater utilities globally. One of the most commonly used methods to combat hydrogen sulfide is the addition of ferric chloride. While a reliable and effective method, ferric chloride is acidic causing OH&S concerns as well as alkalinity consumption in sewage. This study investigates, under full-scale field conditions, an alternative method for sulfide control by in-situ electrochemical generation of iron ions using sacrificial iron electrodes. This method concomitantly produces alkalinity through cathodic OH^- generation, rather than consumption. The gaseous hydrogen sulfide concentrations at the discharge wet well of a real-life rising main (length: ∼1 km in, diameter: 150 mm) decreased from 173 ppm to 43 ppm (90 percentile of peak values), when a current of 0.86 A/m³ of sewage was applied. The 90 percentile peak H₂S value was further reduced to 6.6 ppm when the applied current was increased to 1.14 A/m³ sewage. Moreover, methane generation was almost completely inhibited from 25.3 ± 1.46 mg COD/L to 0.06 ± 0.04 mg COD/L. The overall cell voltage remained constant throughout the experimental period clearly showing the stability of the process. Detailed characterization of the down-stream sewer pipe biofilm revealed the complexity of the iron chemistry as the in-situ produced iron ions undergo transformation into a variety of iron species. Overall, this study demonstrates that in-situ generation of iron and alkalinity is an effective alternative method for hydrogen sulfide control in sewers.

Next steps in clean water oxygen transfer testing - A critical review of current standards

Current application of clean water aeration test standards for wastewater treatment systems is reviewed with a focus on problematic interpretations and practices of the most commonly and internationally used standards, edited by the ASCE (USA), CEN (EU/EFTA), and DWA (Germany). Remedy is proposed for these practices, with a view to inspire revision of existing standards toward better understanding and achievement of test accuracy with regard to oxygen transfer rate. Themes are (i) the importance of mixing for testing and (ii) the importance of mixing for the relevance of standard oxygen transfer rate for the treatment process, (iii) loop reactor testing and the impact of aeration layout, the circulation time scale and dispersion, (iv) tapered aeration testing, (v) impact of probe response time, (vi) sulfite interference at low temperatures, (vii) the effective depth concept for the dissolved oxygen saturation concentration, and standard corrections to the in situ saturation concentration, (viii) uncertainty estimates on oxygen transfer capacity and efficiency, and (ix) the prospect for simplification and harmonization among existing standards. It is suggested that tests based on an improved understanding of the effects of mixing, probe lag, sulfite oxidation kinetics, and fluctuations, will eventually result in the reduction of costly oversizing of aeration systems and improve the understanding of aeration system design and oxygen transfer rate vis-a-vis energy consumption and substrate conversion in biological wastewater treatment.

Tanks in series versus compartmental model configuration: considering hydrodynamics helps in parameter estimation for an N2O model

The choice of the spatial submodel of a water resource recovery facility (WRRF) model should be one of the primary concerns in WRRF modelling. However, currently used mechanistic models are limited by an over-simplified representation of local conditions. This is illustrated by the general difficulties in calibrating the latest N₂O models and the large variability in parameter values reported in the literature. The use of compartmental model (CM) developed on the basis of accurate hydrodynamic studies using computational fluid dynamics (CFD) can take into account local conditions and recirculation patterns in the activated sludge tanks that are important with respect to the modelling objective. The conventional tanks in series (TIS) configuration does not allow this. The aim of the present work is to compare the capabilities of two model layouts (CM and TIS) in defining a realistic domain of parameter values representing the same full-scale plant. A model performance evaluation method is proposed to identify the good operational domain of each parameter in the two layouts. Already when evaluating for steady state, the CM was found to provide better defined parameter ranges than TIS. Dynamic simulations further confirmed the CM's capability to work in a more realistic parameter domain, avoiding unnecessary calibration to compensate for flaws in the spatial submodel.

The future of WRRF modelling - outlook and challenges

The wastewater industry is currently facing dramatic changes, shifting away from energy-intensive wastewater treatment towards low-energy, sustainable technologies capable of achieving energy positive operation and resource recovery. The latter will shift the focus of the wastewater industry to how one could manage and extract resources from the wastewater, as opposed to the conventional paradigm of treatment. Debatable questions arise: can the more complex models be calibrated, or will additional unknowns be introduced? After almost 30 years using well-known International Water Association (IWA) models, should the community move to other components, processes, or model structures like 'black box' models, computational fluid dynamics techniques, etc.? Can new data sources - e.g. on-line sensor data, chemical and molecular analyses, new analytical techniques, off-gas analysis - keep up with the increasing process complexity? Are different methods for data management, data reconciliation, and fault detection mature enough for coping with such a large amount of information? Are the available calibration techniques able to cope with such complex models? This paper describes the thoughts and opinions collected during the closing session of the 6th IWA/WEF Water Resource Recovery Modelling Seminar 2018. It presents a concerted and collective effort by individuals from many different sectors of the wastewater industry to offer past and present insights, as well as an outlook into the future of wastewater modelling.

Understanding the effects of bulk mixing on the determination of the affinity index: consequences for process operation and design

The main objective of this study is to demonstrate the importance of mixing conditions as a source of inconsistencies between half-saturation indices in comparable systems (e.g. conventional activated sludge, membrane bioreactor) when operated at different conditions or different scales. As proof-of-principle, an exemplary system consisting of the second vessel of a hybrid respirometer has been studied. The system has been modeled both using an integrated computational fluid dynamics (CFD)-biokinetic model (assumed to represent the physical system) and a tanks-in-series, completely stirred tank reactor biokinetic model (representing the applied model). The results show that different mixing conditions cause deviations in the half-saturation indices calculated when matching the applied model to the physical system performance. Additionally, sensor location has been shown to impact the calculation of half-saturation indices in the respirometric system. This will only become more pronounced at larger scales. Thus, mixing conditions clearly affect operation and design of wastewater treatment reactors operated at low substrate concentrations. Both operation and design can be improved with the development and application of integrated CFD-biokinetic or compartmental models.