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Abstract The storage of heat in aquifers, also referred to as Aquifer Thermal Energy Storage (ATES), bears a high potential to bridge the seasonal gap between periods of highest thermal energy demand and supply. With storage temperatures higher than 50 °C, High-Temperature (HT) ATES is capable to facilitate the integration of (non-)renewable heat sources into complex energy systems. While the complexity of ATES technology is positively correlated to the required storage temperature, HT-ATES faces multidisciplinary challenges and risks impeding a rapid market uptake worldwide. Therefore, the aim of this study is to provide an overview and analysis of these risks of HT-ATES to facilitate global technology adoption. Risk are identified considering experiences of past HT-ATES projects and analyzed by ATES and geothermal energy experts. An online survey among 38 international experts revealed that technical risks are expected to be less critical than legal, social and organizational risks. This is confirmed by the lessons learned from past HT-ATES projects, where high heat recovery values were achieved, and technical feasibility was demonstrated. Although HT-ATES is less flexible than competing technologies such as pits or buffer tanks, the main problems encountered are attributed to a loss of the heat source and fluctuating or decreasing heating demands. Considering that a HT-ATES system has a lifetime of more than 30 years, it is crucial to develop energy concepts which take into account the conditions both for heat sources and heat sinks. Finally, a site-specific risk analysis for HT-ATES in the city of Hamburg revealed that some risks strongly depend on local boundary conditions. A project-specific risk management is therefore indispensable and should be addressed in future research and project developments.

Both eutrophication and SO4 pollution can lead to higher availability of nutrients and potentially toxic compounds in wetlands. To unravel the interaction between the level of eutrophication and toxicity at species and community level, effects of SO4 were tested in nutrient-poor and nutrient-rich fen mesocosms. Biomass production of aquatic and semi-aquatic macrophytes and colonization of the water layer increased after fertilization, leading to dominance of highly competitive species. SO4 addition increased alkalinity and sulphide concentrations, leading to decomposition and additional eutrophication. SO4 pollution and concomitant sulphide production considerably reduced biomass production and colonization, but macrophytes were less vulnerable in fertilized conditions. The experiment shows that competition between species, vegetation succession and terrestrialization are not only influenced by nutrient availability, but also by toxicity, which strongly interacts with the level of eutrophication. This implies that previously neutralized toxicity effects in eutrophied fens may appear after nutrient reduction measures have been taken.

This paper reviews the long-standing bulking sludge problem in activated sludge systems. Despite the extensive amount of research that has been done on bulking sludge, it still occurs world-wide and a comprehensive solution does not seem to be available. Bulking sludge can be approached as a microbiological problem (occurrence of a specific filamentous bacterium) or as an engineering problem (growth of bacteria with a filamentous morphology). In the first case species-specific solutions should be found, whereas in the latter case, a generic approach might be available. Since bulking sludge is caused by a group of bacteria with a specific morphology, but not a specific physiology we believe that a generic approach would be feasible. Several theories for bulking sludge are discussed. Based on these theories the application and associated problems with the use of biological selectors are critically evaluated. Finally, a set of open research questions is identified.

Abstract This paper concerns a new method of sewage sludge treatment that contributes more than traditional methods to the sustainable technology by achieving a higher rational efficiency of sludge processing. This is obtained by preserving the chemical exergy present in the sludge and transforming it into a chemical one––methanol. The proposed method combines a sludge gasification process and a modified methanol plant. The sludge gasification produces a synthetic gas mainly containing CO and H2, which gas is next used as a reactant to make methanol. The plant, with a capacity of 50 000 tons (dry) solids per year, is simulated by a computer model using the flow-sheeting program Aspen Plus (Aspen Technology). The exergy analysis is performed for various operating conditions and the optimal values of these conditions are found. The irreversibilities of different plant segments: thermal dryer, gasifier, gas cleaning, compressors, methanol reactor, distillation column and purge gas combustion are evaluated. It is shown that the rational efficiency of the overall process is much higher than that of traditional plants of thermal sludge treatment.

A feasibility study was carried out to assess the cultivation of Anammox bacteria in lab-scale closed sponge-bed trickling filter (CSTF) reactors, namely: CSTF-1 at 20°C and CSTF-2 at 30°C. Stable conditions were reached from day 66 in CSTF-2 and from day 104 in CSTF-1. The early stability of CSTF-2 is attributable to the influence of temperature; nevertheless, by day 405, the nitrogen removal performed by CSTF-1 increased up to similar values of CSTF-2. The maximum total nitrogen removal efficiency was 82% in CSTF-1 and 84% in CSTF-2. After more than 400 days of operation, CSTF-1 and CSTF-2 were capable to attain a total nitrogen removal efficiency of 74±5% and 78±4% with a total nitrogen conversion rate of 1.52 and 1.60kg-N/m(sponge)(3)d, respectively. The proposed technology could be a suitable alternative for mainstream nitrogen removal in post-treatment units via the Anammox conversion pathway.

We investigate the dynamic adsorption of anionic surfactant C14 - 16 alpha olefin sulfonate on Berea sandstone cores with different surface wettability and redox states under high temperature that represents reservoir conditions. Surfactant adsorption levels are determined by analyzing the effluent history data with a dynamic adsorption model assuming Langmuir isotherm. A variety of analyses, including surface chemistry, ionic composition, and chromatography, is performed. It is found that the surfactant breakthrough in the neutral-wet core is delayed more compared to that in the water-wet core because the deposited crude oil components on the rock surface increase the surfactant adsorption via hydrophobic interactions. As the surfactant adsorption is satisfied, the crude oil components are solubilized by surfactant micelles and some of the adsorbed surfactants are released from the rock surface. The released surfactant dissolves in the flowing surfactant solution, thereby resulting in an overshoot of the produced surfactant concentration with respect to the injection value. Furthermore, under water-wet conditions, changing the surface redox potential from an oxidized to a reduced state decreases the surfactant adsorption level by 40%. We find that the decrease in surfactant adsorption is caused not only by removing the iron oxide but also by changing the calcium concentration after the core restoration process (calcite dissolution and ion exchange as a result of using EDTA). Findings from this study suggest that laboratory surfactant adsorption tests need to be conducted by considering the wettability and redox state of the rock surface while recognizing how core restoration methods could significantly alter the ionic composition during surfactant flooding.

A partial least-squares calibration model, relating mid-infrared spectral features with fructose, ethanol, acetate, gluconacetan, phosphate and ammonium concentrations has been designed to monitor and control cultivations of Gluconacetobacter xylinus and production of gluconacetan, a food grade exopolysaccharide (EPS). Only synthetic solutions containing a mixture of the major components of culture media have been used to calibrate the spectrometer. A factorial design has been applied to determine the composition and concentration in the calibration matrix. This approach guarantees a complete and intelligent scan of the calibration space using only 55 standards. This calibration model allowed standard errors of validation (SEV) for fructose, ethanol, acetate, gluconacetan, ammonium and phosphate concentrations of 1.16 g/l, 0.36 g/l, 0.22 g/l, 1.54 g/l, 0.24 g/l and 0.18 g/l, respectively. With G. xylinus, ethanol is directly oxidized to acetate, which is subsequently metabolized to form biomass. However, residual ethanol in the culture medium prevents bacterial growth. On-line spectroscopic data were implemented in a closed-loop control strategy for fed-batch fermentation. Acetate concentration was controlled at a constant value by feeding ethanol into the bioreactor. The designed fed-batch process allowed biomass production on ethanol. This was not possible in a batch process due to ethanol inhibition of bacterial growth. In this way, the productivity of gluconacetan was increased from 1.8 x 10(-3) [C-mol/C-mol substrate/h] in the batch process to 2.9 x 10(-3) [C-mol/C-mol substrate/h] in the fed-batch process described in this study.

A modular pore-scale model is developed to assess the response of wellbore cement to geological storage of CO2. Numerical formulations for modeling of solute transport are presented and a methodology for coupling with geochemical processes is discussed, which includes: (1) advective and diffusive fluxes of solutes within the pore space, (2) aqueous phase speciation, (3) mineral dissolution–precipitation kinetics, and (4) the subsequent changes in pore space geometry. A Complex Pore Network Model (CPNM) is used to discretize the continuum porous structure as a network of pore bodies and pore throats, both with finite volumes. CPNM allows for a distribution of pore coordination numbers ranging between 1 and 26. This topological property, together with a geometrical distribution of pore sizes, enables the microstructure of porous media to be mimicked. For each pore element, transport of solute is calculated by solving the governing mass balance equations. Chemical reaction of the fluid phase with the main reactive solid components (portlandite and calcite) is incorporated through coupling with a geochemical reactive simulator. Average values and properties are obtained by integration over a large number of pores. Using this approach, we investigate how chemical reaction between water-bearing wellbore cement and supercritical CO2 can create a distribution of porosity in a direction parallel to the CO2 concentration gradient and transport path, at 50 ◦C. The dynamics of this process involve interaction between diffusion dominated mass transport and the kinetics of dissolution and precipitation of portlandite and/or calcite. Simulation of unconfined chemical degradation, in a fluid of constant composition, shows development of different regions: (1) a zone adjacent to the inlet face, which is characterized by an increase in porosity due to extensive dissolution, (2) a carbonation zone with decreased porosity, (3) the carbonation front which made a thin layer with the lowest porosity due to calcium carbonate precipitation, and (4) dissolution zone. These results are in agreement with laboratory observations under similar conditions. This provides confidence that this pore-scale approach can ultimately be applied to model the progress of coupled CO2 transport and cement degradation at critical points along the length of cemented wellbore sections at CO2 storage sites.