Temperature plays a critical role in performance and stability of anaerobic digestion processes, subject to frequent meteorological fluctuations. However, state-of-the-art modeling and process control approaches for anaerobic digestion often do not consider the temporal dynamics of the temperature, which can influence microbial communities, kinetics, and chemical equilibrium, and consequently, biogas production efficiency. Therefore, to account for anaerobic digesters operating under fluctuating meteorological conditions, the Anaerobic Digestion Model no. 1 (ADM1) is mechanistically extended in this paper to incorporate temporal changes into temperature-dependent parameters by defining inhibition functions for microbial activities using the cardinal temperature model, and accounting for the lag in microbial adaptation to temperature fluctuations using a time-lag adaptation function. Thereafter, given that temperature fluctuations are a significant disturbance, a control framework based on Model Predictive Control (MPC) is developed to regulate the feeding flow rate and to ensure stable production rates despite temperature disturbances without relying on direct temperature control. An adaptive MPC approach is formulated based on a linear input–output model, where the parameters of the linear model are updated online to capture the nonlinear dynamics of the process and frequent changes in the dynamics accurately. In addition, a fuzzy logic system is employed to assign a reference trajectory for the production rate based on the temperature and its rate of change. Integrating this fuzzy logic system with the MPC controller enhances the production rate on warm days and avoids the operational failure in production on cold days. Additionally, to enhance biogas production rates, the feasibility of utilizing a portion of the produced biogas for external heating purposes is also investigated. It is demonstrated that by utilizing the proposed MPC approach, the additional amount of feed for the digester to produce methane required for a self-consumption biogas-fueled heating system can be calculated according to the meteorological variations. This enhances the process performance and stability. Finally, a thermally optimized dome digester semi-buried in the ground, operating under climate conditions of The Netherlands is considered as a case study to validate the extended model in agreement with biological and physicochemical behaviors of real-world applications, and to demonstrate the effectiveness of the proposed control system in handling temperature changes and enhancing performance.

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A small-scale (up to 5 kWe) biogas-solid oxide fuel cell (SOFC) energy system is an envisioned system, which can be used to meet both electrical and thermal energy demand of off-grid settlements. SOFC systems are reported to be more efficient than alternatives like internal combustion engines (ICE). In addition to energy recovery, implementation of biogas-SOFC systems can enhance sanitation among these settlements. However, the capital investment costs and the operation and maintenance costs of a biogas-SOFC energy system are currently higher than the existing alternatives. From previous works, H2S removal by biochar was proposed as a potential local cost-effective alternative. This research demonstrates the techno-economic potential of locally produced biochars made from cow manure, jackfruit leaves, and jack fruit branches in rural Uganda for purifying the biogas prior to SOFC use. Results revealed that the use of biochar from cow manure and jack fruit leaves can reduce H2S to below the desired 1 ppm and substitute alternative biogas treatments like activated carbon. These experimental results were then translated to demonstrate how this biochar would improve the economic feasibility for the implementation of biogas-SOFC systems. It is likely that the operation and maintenance cost of a biogas-SOFC energy system can in the long run be reduced by over 80%. Also, the use of internal reforming as opposed to external reforming can greatly reduce the system capital cost by over 25% and hence further increase the chances of system economic feasibility. By applying the proposed cost reduction strategies coupled with subsidies such as tax reduction or exemption, the biogas-SOFC energy system could become economically competitive with the already existing technologies for off-grid electricity generation, like solar photovoltaic systems.@en

This paper focuses on the development of linear Switched Box–Jenkins (SBJ) models for approximating complex dynamical models of biological wastewater treatment processes. We discuss the adaptation of these processes to the SBJ framework, showing the model's generality and flexibility as a class of switched systems that can offer accurate predictions for complex and nonlinear dynamics. This approach of modeling enables real-time data reconciliation from experiments and allows the design of model-based control strategies. Through the extension of the Outer Bounding Ellipsoids (OBEs) algorithm, the paper introduces an online two-stage parameter identification algorithm that effectively handles bounded disturbances for SBJ models. Using the OBE method relaxes the stochastic assumptions on disturbances, which may not be satisfied in practice, particularly for biological and environmental fluctuations. The proposed decomposed OBE algorithm separately identifies the switching patterns and parameters of linear submodels, conducting parameter identification in two distinct phases for input/output and disturbance/output submodels. The efficacy of this approach is shown via simulation results validated against both ADM1 and PBM models, demonstrating the proposed algorithm's capability to accurately predict outputs from different biological wastewater treatment models.

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High Pressure Hydrogenotrophic Denitrification (HPHD) provided a promising alternative for efficient and clean nitrate removal. In particular, the denitrification rates at low temperature could be compensated by elevated H₂ partial pressure. However, nitrite reduction was strongly inhibited while nitrate reduction was barely affected at low temperature. In this study, the nitrate reduction gradually recovered under long-term low temperature stress, while nitrite accumulation increased from 0.1 to 41.0 mg N/L. The activities of the electron transport system (ETS), nitrate reductase (NAR), and nitrite reductase (NIR) decreased by 45.8 %, 27.3 %, and 39.3 %, respectively, as the temperature dropped from 30 °C to 15 °C. Real time quantitative PCR analysis revealed that the denitrifying gene expression rather than gene abundance regulated nitrogen biotransformation. The substantial nitrite accumulation was attributed to the significant up-regulation by 54.7 % of narG gene expression and down-regulation by 73.7 % of nirS gene expression in hydrogenotrophic denitrifiers. In addition, the nirS-gene-bearing denitrifiers were more sensitive to low temperature compared to those bearing nirK gene. The dominant populations shifted from the genera Paracoccus to Hydrogenophaga under long-term low temperature stress. Overall, this study revealed the microbial mechanism of high nitrite accumulation in hydrogenotrophic denitrification at low temperature.

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Purple Phototrophic Bacteria (PPB) are increasingly being applied in resource recovery from wastewater. Open raceway-pond reactors offer a more cost-effective option, but subject to biological and environmental perturbations. This study proposes a hierarchical control system based on Adaptive Generalized Model Predictive Control (AGMPC) for PPB raceway reactors. The AGMPC uses simple linear models updated adaptively to project the complex process dynamics and capture changes. The hierarchical approach uses the AGMPC controller to optimize PPB growth as the core of the system. The developed supervisory layer adjusts set-points for the core controller based on two operational scenarios: maximizing PPB concentration for quality, or increasing yield for quantity through effluent recycling. Lastly, due to competing PPB and non-PPB bacteria during start-up phase, an override strategy for this transition is investigated through simulation studies. The Purple Bacteria Model (PBM) simulates this process, and simulation results demonstrate the control system's effectiveness and robustness.

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Particle-bubble collisions in dissolved air flotation (DAF) systems play a crucial role in the removal of total suspended solids (TSS). DAF particle-bubble collision models incorporate factors such as particle diameters, charge and density, bubble diameters, and collision factors. The challenge lies in accounting for the wide range of particle and bubble sizes and obtaining complex model inputs. To address this, a simplified model for TSS removal in DAF units was established using low-cost laboratory measurements, including particle size distribution and density. Additionally, microbubble diameter profiles were derived from bubble velocities using particle image velocimetry software (PIV). Six independent variables, encompassing influent particle characteristics (such as particle size distribution and density) and DAF running characteristics (temperature, contact zone detention time, inflow and recycle flows), were employed in the simplified model. The model's accuracy was evaluated using a laboratory-scale DAF system with two different influents: Delft canal water and anaerobic sludge. The predicted TSS removal from the simplified model aligned well with the laboratory-scale DAF results, yielding removal efficiencies of 68 ± 1 % and 77 ± 3 % for Delft canal water and anaerobic sludge, respectively. Furthermore, when the simplified model was applied to two full-scale DAF systems, it successfully identified an underperforming system (DAF2) with a TSS removal efficiency of 91 %, contrasting with the theoretical removal model-predicted efficiency of 98 %. This study highlights the utility of combining bubble size distribution measured by PIVlab and particle size distribution obtained using FIJI-ImageJ as an economical and efficient approach to acquiring the necessary inputs for predicting TSS removal in DAF systems.

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Wastewater resources can be used to produce microbial protein for animal feed or organic fertiliser, conserving food chain resources. This investigation has employed the fermented sewage to photoheterotrophically grown purple non-sulfur bacteria (PNSB) in a 2.5 m³ pilot-scale raceway-pond with infrared light to produce proteinaceous biomass. Fermented sewage with synthetic media consisting of sodium acetate and propionic acids at a surface-to-volume (S/V) ratio of 10 m²/m³ removed 89%, 93%, and 81% of chemical oxygen demand, ammonium nitrogen, and orthophosphate, respectively; whereas respective removal in fermented sewage alone without synthetic media was 73%, 73%, and 72% during batch operation of 120 h. The biomass yield of 0.88–0.95 g COD_biomass /g COD_removed with protein content of 40.3 ± 0.3%–43.9 ± 0.2% w/w was obtained for fermented sewage with synthetic media. The results revealed enhanced possibility of scaling-up the raceway reactor to recover resources from municipal wastewater and enable simultaneous high-rate PNSB single-cell protein production.

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This study reports the effects of microaeration on a laboratory-scale AnMBR (MA-AnMBR) fed with synthetic concentrated domestic sewage. The imposed oxygen load mimics the oxygen load coming from a dissolved air flotation (DAF) unit, establishing an anaerobic digester-DAF (AD-DAF) combination with sludge recycling. Results showed a reduced COD concentration in the MA-AnMBR permeate compared with the AnMBR permeate, from 90 to 74 mgCOD L–1, and a concomitant 27% decrease in biogas production. The MA-AnMBR permeate ammonium (NH4+) concentration increased by 35%, to 740 mgNH4+-N L–1, indicating a rise in the hydrolytic capacity. Furthermore, the MA-AnMBR biomass seemingly adapted to an increased oxygen load, which corresponded to 1% of the influent COD load (approximately 55 mLO2 d–1). Concomitantly, an increase in the superoxide dismutase activity (SOD) of biomass was detected. Meanwhile, negligible changes were observed in the specific methanogenic activity (SMA) of the microaerated biomass that was subjected to an oxygen load equivalent to 3% of the influent COD load in batch tests. The obtained results showed that an AD-DAF system could be a promising technology for treating concentrated domestic wastewater, improving sewage sludge hydrolysis, and overall organic matter removal when compared to an AnMBR.@en

A reliable drinking water supply and appropriate sanitation in the western world are the product of numerous innovations which addressed pressing historical challenges. This paper discusses that to tackle emerging water supply and sanitation challenges in the Global South, the innovation system cannot be a simple translation of the western “sustaining innovation” system. In this chapter, the authors claim that “frugal innovation” is not only an alternative for the Global South but also for societies concerned about resource overexploitation. The chapter provides various examples of incorporating “frugal thinking” into different water treatment and sanitation aspects in resource affluent and resource-constrained societies. The authors present life-support system engineering concepts as a source of inspiration to design frugal circular solutions. Through these solutions, independency (in time and space) from natural resources and waste valorization in all types of societies can be achieved.@en

Film forming amines (FFA) are corrosion inhibitors added to power plant water. The major concern associated with their application is the thermal stability in the high temperature power plant water medium, along with the risk of decomposition into low molecular weight organic acids that can cause corrosive damages in the water/steam cycle. However, there is still a lack of sufficient data on the thermal stability of FFA corrosion inhibitors. This paper presents a comprehensive critical review and state-of-the-art assessment of the results obtained from studying the thermolysis of FFA corrosion inhibitors in power plant water/steam cycle conditions, highlighting the relevance for practical application and research needs. Temperature, exposure time, initial concentration, and alkalizing agents were identified as key factors influencing the thermal stability of FFA in high temperature power plant water. Organic acids are found in concentrations harmless to metal tubes. Advanced scientific background information and additional research are required on this topic.

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Purple phototrophic bacteria (PPB) show an underexplored potential for resource recovery from wastewater. Raceway reactors offer a more affordable full-scale solution on wastewater and enable useful additional aerobic processes. Current mathematical models of PPB systems provide useful mechanistic insights, but do not represent the full metabolic versatility of PPB and thus require further advancement to simulate the process for technology development and control. In this study, a new modelling approach for PPB that integrates the photoheterotrophic, and both anaerobic and aerobic chemoheterotrophic metabolic pathways through an empirical parallel metabolic growth constant was proposed. It aimed the modelling of microbial selection dynamics in competition with aerobic and anaerobic microbial community under different operational scenarios. A sensitivity analysis was carried out to identify the most influential parameters within the model and calibrate them based on experimental data. Process perturbation scenarios were simulated, which showed a good performance of the model.

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This study investigates the effects, conversions, and resistance induction, following the addition of 150 μg·L⁻¹ of two antibiotics, sulfamethoxazole (SMX) and trimethoprim (TMP), in a laboratory-scale micro-aerated anaerobic membrane bioreactor (MA-AnMBR). TMP and SMX were removed at 97 and 86%, indicating that micro-aeration did not hamper their removal. These antibiotics only affected the pH and biogas composition of the process, with a significant change in pH from 7.8 to 7.5, and a decrease in biogas methane content from 84 to 78%. TMP was rapidly adsorbed onto the sludge and subsequently degraded during the long solids retention time of 27 days. SMX adsorption was minimal, but the applied hydraulic retention time of 2.6 days was sufficiently long to biodegrade SMX. The levels of three antibiotic-resistant genes (ARGs) (sul1, sul2, and dfrA1) and one mobile genetic element biomarker (intI1) were analyzed by qPCR. Additions of the antibiotics increased the relative abundances of all ARGs and intI1 in the MA-AnMBR sludge, with the sul2 gene folding 15 times after 310 days of operation. The MA-AnMBR was able to reduce the concentration of antibiotic-resistant bacteria (ARB) in the permeate by 3 log.@en

Dynamical systems and processes that either exhibit non-smooth behaviours (e.g. through logic control or natural phenomena) or work in different modes of operation are usually represented using hybrid systems models, i.e. mathematical models that combine continuous dynamics with discrete-event dynamics. Identification of a hybrid system includes finding switching patterns and identification of model parameters to obtain a data-driven model. This survey paper provides a systematic review of models (how to parameterize the system) and methods (how to identify unknown parameters) proposed for hybrid system identification with an exposition of recent advances and developments, and further research directions.

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Background: Elevated CO₂ partial pressure (pCO₂) has been proposed as a potential steering parameter for selective carboxylate production in mixed culture fermentation. It is anticipated that intermediate product spectrum and production rates, as well as changes in the microbial community, are (in)directly influenced by elevated pCO₂. However, it remains unclear how pCO₂ interacts with other operational conditions, namely substrate specificity, substrate-to-biomass (S/X) ratio and the presence of an additional electron donor, and what effect pCO₂ has on the exact composition of fermentation products. Here, we investigated possible steering effects of elevated pCO₂ combined with (1) mixed substrate (glycerol/glucose) provision; (2) subsequent increments in substrate concentration to increase the S/X ratio; and (3) formate as an additional electron donor. Results: Metabolite predominance, e.g., propionate vs. butyrate/acetate, and cell density, depended on interaction effects between pCO₂–S/X ratio and pCO₂–formate. Individual substrate consumption rates were negatively impacted by the interaction effect between pCO₂–S/X ratio and were not re-established after lowering the S/X ratio and adding formate. The product spectrum was influenced by the microbial community composition, which in turn, was modified by substrate type and the interaction effect between pCO₂–formate. High propionate and butyrate levels strongly correlated with Negativicutes and Clostridia predominance, respectively. After subsequent pressurized fermentation phases, the interaction effect between pCO₂–formate enabled a shift from propionate towards succinate production when mixed substrate was provided. Conclusions: Overall, interaction effects between elevated pCO₂, substrate specificity, high S/X ratio and availability of reducing equivalents from formate, rather than an isolated pCO₂ effect, modified the proportionality of propionate, butyrate and acetate in pressurized mixed substrate fermentations at the expense of reduced consumption rates and increased lag-phases. The interaction effect between elevated pCO₂ and formate was beneficial for succinate production and biomass growth with a glycerol/glucose mixture as the substrate. The positive effect may be attributed to the availability of extra reducing equivalents, likely enhanced carbon fixating activity and hindered propionate conversion due to increased concentration of undissociated carboxylic acids.

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Sugarcane bagasse (SCB) is increasingly considered as a potential source for bioenergy or bulk chemicals production. However, efficient pretreatment techniques need to be developed to make full use of its potential. A central composite design was employed to evaluate the effect of temperature (146.4–213.6 °C), NaOH concentration (0.7–2.3 M) and treatment time (3.2–36.8 min) on the hydrothermal pretreatment of SCB. Glucose was the abundant fermentable sugar released in the hydrolysate (4.0 g/L) and its concentration was significantly (p-value < 0.05) affected by temperature and NaOH concentration. Sugars released in the hydrolysate and the remaining cellulose, hemicellulose, and lignin in the pretreated fiber were anaerobically co-digested in batch thermophilic assays (55 °C). Propionate, one of the most promising platform chemicals, was the main metabolite produced (0.5 g/L) and its concentration was significantly affected by temperature and NaOH concentration. Both NaOH concentration and pretreatment duration significantly affected methane composition in the biogas (p-value <0.05 and 0,10 respectively). Defluviitoga, and Methanothermobacter genera were favored in response to alkaline hydrothermal pretreatment at the central point conditions (180 °C, 1.5 M NaOH, 20 min).

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In the face of the rapidly dwindling carbon budgets, negative emission technologies are widely suggested as required to stabilize the Earth’s climate. However, finding cost-effective, socially acceptable, and politically achievable means to enable such technologies remains a challenge. We propose solutions based on negative emission technologies to facilitate wealth creation for the stakeholders while helping to mitigate climate change. This paper comes up with suggestions and guidelines on significantly increasing carbon sequestration in coffee farms. A coffee and jackfruit agroforestry-based case study is presented along with an array of technical interventions, having a special focus on bioenergy and biochar, potentially leading to “negative emissions at negative cost.” The strategies for integrating food production with soil and water management, fuel production, adoption of renewable energy systems and timber management are outlined. The emphasis is on combining biological and engineering sciences to devise a practically viable niche that is easy to adopt, adapt and scale up for the communities and regions to achieve net negative emissions. The concerns expressed in the recent literature on the implementation of emission reduction and negative emission technologies are briefly presented. The novel opportunities to alleviate these concerns arising from our proposed interventions are then pointed out. Our analysis indicates that 1 ha coffee jackfruit-based agroforestry can additionally sequester around 10 tonnes of CO2-eq and lead to an income enhancement of up to 3,000–4,000 Euros in comparison to unshaded coffee. Finally, the global outlook for an easily adoptable nature-based approach is presented, suggesting an opportunity to implement revenue-generating negative emission technologies on a gigatonne scale. We anticipate that our approach presented in the paper results in increased attention to the development of practically viable science and technology-based interventions in order to support the speeding up of climate change mitigation efforts.@en

Oil palm empty fruit bunch (OPEFB) is an abundant waste that is commonly incinerated, causing environmental pollution. In this study, an alternative waste management approach was investigated to produce value-added syngas from OPEFB using solar steam gasification. The three operating variables were temperature (1100–1300 °C), H₂O/OPEFB molar ratio (1.7–2.9), and OPEFB flowrate (0.8–1.8 g/min). Central composite design (CCD) was conducted to investigate and optimise the effects of these operating variables on H₂/CO molar ratio and solar to fuel energy conversion efficiency (η_{solar to fuel}). The findings revealed that all investigated operating variables were significant. Experimentally, the highest H₂/CO molar ratio (1.6) was obtained at 1300 °C, H₂O/OPEFB molar ratio of 2.9, and OPEFB flowrate of 1.8 g/min, with a high carbon conversion reaching 95.1%. Results from CCD analysis showed that a higher H₂/CO molar ratio (above 1.8) could be reached at 1200 °C, H₂O/OPEFB molar ratio of ≥3.0, and OPEFB flowrate of ≥2.0 g/min. The maximum η_{solar to fuel} of 19.6% was achieved at 1200 °C, H₂O/OPEFB molar ratio of 1.3, and OPEFB flowrate of 1.3 g/min, whereby a favourable energy upgrade factor (1.2) was achieved. The statistical model showed adequacy to predict H₂/CO molar ratio.

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Phenol conversion under saline thermophilic anaerobic conditions requires the development and sustenance of a highly specialized microbial community. In the present research, an anaerobic membrane bioreactor (AnMBR) fed with an influent containing 0.5 g·L⁻¹ phenol and 6.5 gNa⁺·L⁻¹ was operated at 55 °C for 300 days. Phenol degradation was limited when phenol was the sole substrate. However, the phenol removal efficiency significantly (p < 0.001) increased to 80 % corresponding to a conversion rate of 29 mgPhenol·gVSS⁻¹d⁻¹ when acetate (0.5 gCOD·L⁻¹) was simultaneously provided. Isotopic analysis using 1–¹³C labeled acetate and measuring ¹³CH₄ revealed that acetate was first oxidized to hydrogen and CO₂, prior to methanogenesis, resulting in an increased abundance of hydrogenotrophic methanogens. It is hypothesized that the latter is of crucial importance for achieving effective anaerobic oxidation of phenol and its metabolites. Remarkably, the phenol conversion rate in the membrane-associated biomass was three times higher than in the suspended biomass. The observed difference in the conversion rate could be explained by the presence of an increased abundance of hydrogenotrophic methanogens in the membrane-associated biomass confirmed by a microbial community analysis of Archaea. Benzoate was measured in the permeate suggesting that phenol degradation occurred via the benzoyl-CoA pathway. The results of the current study suggest that syntrophic acetate oxidation coupled with hydrogenotrophic methanogenesis, which results in the presence of an abundant electron sink, plays a key role in enhancing thermophilic phenol degradation. The obtained insights widen the application of anaerobic digestion to treat saline phenolic-rich wastewater at high temperatures.

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In the envisaged hydrogen economy, H₂ could be an interesting alternative electron donor for the denitrification of drinking water or wastewater. The main obstacle to engineering the hydrogenotrophic denitrification process is the low solubility of H₂ in water under atmospheric pressure, which limits denitrification rate and nitrogen removal efficiency. In this paper, we demonstrated a novel configuration of hydrogenotrophic denitrification, namely High Pressure Hydrogenotrophic Denitrification (HPHD). Elevated H₂ partial pressure (pH₂) was employed to increase dissolved H₂ concentration and concomitantly enhance denitrification rate. Our results showed that the specific denitrification rate increased from 9.6 mg N/(gVSS·h) at 0.5 bars to 51.0 mg N/(gVSS·h) at 9 bars in HPHD. The denitrification effect could be retained with elevated pH₂ at a low temperature. The specific denitrification rate at 3 bars and 15 °C was 20.5 mg N/(gVSS·h), approximately 1.5 times that at 1 bar and 30 °C, which was quite beneficial for hydrogenotrophic denitrification under cold conditions. Different from nitrite reduction, less impact was observed on nitrate reduction by low temperature, which explained high nitrite accumulation in HPHD at 15 °C. Overall, our investigations shed light on the role of pH₂ in the promising solution for nitrogen removal in HPHD.

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