Closed-loop two-echelon repairable item systems

In this paper we consider closed loop two-echelon repairable item systems with repair facilities both at a number of local service centers (called bases) and at a central location (the depot). The goal of the system is to maintain a number of production facilities (one at each base) in optimal operational condition. Each production facility consists of a number of identical machines which may fail incidentally. Each repair facility may be considered to be a multi-server station, while any transport from the depot to the bases is modeled as an ample server. At all bases as well as at the depot, ready-for-use spare parts (machines) are kept in stock. Once a machine in the production cell of a certain base fails, it is replaced by a ready-for-use machine from that base's stock, if available. The failed machine is either repaired at the base or repaired at the central repair facility. In the case of local repair, the machine is added to the local spare parts stock as a ready-for-use machine after repair. If a repair at the depot is needed, the base orders a machine from the central spare parts stock to replenish its local stock, while the failed machine is added to the central stock after repair. Orders are satisfied on a first-come-first-served basis while any requirement that cannot be satisfied immediately either at the bases or at the depot is backlogged. In case of a backlog at a certain base, that base's production cell performs worse. To determine the steady state probabilities of the system, we develop a slightly aggregated system model and propose a special near-product-form solution that provides excellent approximations of relevant performance measures. The depot repair shop is modeled as a server with state-dependent service rates, of which the parameters follow from an application of Norton's theorem for Closed Queuing Networks. A special adaptation to a general Multi-Class MDA algorithm is proposed, on which the approximations are based. All relevant performance measures can be calculated with errors which are generally less than one percent, when compared to simulation results. \u

Respirometry in activated sludge

The purpose of the study was (1) to develop a respiration meter capable of continuously measuring, using different procedures, the oxygen uptake rate of activated sludge and (2) to expand knowledge about respiration related characteristics of wastewater and activated sludge.A newly-developed respiration meter is described. The meter consists of a closed, completely mixed respiration chamber of 0.5 to 1 litre through which activated sludge is continuously pumped. The characteristic feature of this meter is that the dissolved oxygen concentration in the sludge entering the chamber and in the sludge leaving the chamber is measured with one single probe, located at one opening. This is realised by changing the direction of the flow through the chamber. The respiration rate is calculated from the dissolved oxygen mass balance over the respiration chamber. Because the derivative of the mass balance is included in this calculation, the respiration rate can also be calculated during dynamic conditions. An improved method for calculating the respiration rate is described, which accounts for the time lag of the DO probe. An additional result of this improvement is that it yields the time constant of the probe response, which provides a diagnosis of the probe condition.Experimental research was performed using a continuous pilot activated sludge plant with a completely mixed aeration tank of 0.475 m 3, fed with domestic wastewater, and a batch reactor with an aeration tank of 1.5 to 2 litres.A strategy is described for measuring four types of respiration rate of the same sludge under different conditions: endogenous, instantaneous, actual and maximum respiration rate. Emphasis is given to the actual respiration rate. The actual respiration rate is defined as the oxygen uptake rate of the sludge in the aeration tank. It is demonstrated that this rate is measured if the respiration chamber and the aeration tank are equally loaded with wastewater. An improved method is described which does not involve addition of wastewater to the respiration chamber. Instead, the transient respiration rate during two modes of operation which are alternately executed is used to calculate the actual respiration rate.The measurement of the maximum respiration rate is discussed in some detail with emphasis on the partition of readily biodegradable matter into two components. The maximum respiration rate is measured if wastewater is continuously fed into the respiration chamber so that the loading exceeds a certain critical loading. Batch respirometric tests are used to verify the continuous measurement of this maximum rate. An application of the developed measurement is described in which the effect of the influent flow rate on the maximum respiration rate of nitrifying sludge was investigated.Methods are described for continuous estimation of the short-term biochemical oxygen demand (BOD st ) of influent and effluent by using respirometry. BOD st values of the influent are verified with batch measurements. The BOD st of the examined wastewater appears to be mainly caused by ammonium being oxidized by nitrifiers.Batch respiration measurements have been used for identifying a mathematical nitrification model. The investigation was focused on finding an optimal experimental design and a good model validation method

Bioremoval of humic acid from water by white rot fungi : exploring the removal mechanisms

Altres ajuts: This Research is part of research program "Increasing the utilization of organic waste and low value feeds with help of lignin degrading fungi" with project number 11611, and is supported by the Dutch Technology Foundation STW, which is part of the Netherlands Organisation for Scientific Research (NWO), and which is partly funded by the Ministry of Economic Affairs.Twelve white rot fungi (WRF) strains were screened on agar plates for their ability to bleach humic acid (HA). Four fungal strains were selected and tested in liquid media for removal of HA. Bioremediation was investigated by HA color removal and changes in the concentration and molecular size distribution of HA by size exclusion chromatography. Trametes versicolor and Phanerochaete chrysosporium showed the highest HA removal efficiency, reaching about 80%. Laccase and manganese peroxidase were measured as extracellular enzymes and their relation to the HA removal by WRF was investigated. Results indicated that nitrogen limitation could enhance the WRF extracellular enzyme activity, but did not necessarily increase the HA removal by WRF. The mechanism of bioremediation by WRF was shown to involve biosorption of HA by fungal biomass and degradation of HA to smaller molecules. Also, contradicting previous reports, it was shown that the decolorization of HA by WRF could not necessarily be interpreted as degradation of HA. Biosorption experiments revealed that HA removal by fungal biomass is dependent not only on the amount of biomass as the sorbent, but also on the fungal species. The involvement of cytochrome P450 (CYP) enzymes was confirmed by comparing the HA removal capability of fungi with and without the presence of a CYP inhibitor. The ability of purified laccase from WRF to solely degrade HA was proven and the importance of mediators was also demonstrated

The influence of a serious game's narrative on students' attitudes and learning experiences regarding delirium:an interview study

Background: Delirium is a neuropsychiatric syndrome that affects patients' attention and awareness as a result of a physical condition. In recent years, persistent gaps in delirium education have led to suboptimal delirium care. Still, little is known about what are the most important aspects of effective delirium education. Serious games are both entertainment and an interactive, safe learning environment where players can experiment and create new knowledge. They have the potential to contribute to improved delirium education. We used a video-based serious games' narrative to explore aspects essential to enhance students' attitudes and learning experiences regarding delirium. Methods: We created a semi-structured interview guide and interviewed seven nursing and nine medical students about their attitudes and learning experiences, after they had played the game. A qualitative descriptive design and inductive content analysis with constant comparison were used. Results: The patient's and nurse's perspective, interactivity to experiment, realistic views on care options, and feedback on care actions were important for enhancing students' attitudes and learning experiences regarding delirium. Students felt these aspects encouraged them to get actively involved in and experiment with the study material, which in turn led to enhanced reflection on delirium care and education. Our findings highlight the importance of a more patient-oriented focus to delirium education to drive attitudinal change. Students' learning experiences were further enhanced through their affective responses provoked by the perspectives, interactivity, realism, and feedback. Conclusions: Students considered the characters' perspectives, interactivity, realism, and feedback important aspects of the game to enhance their attitudes towards delirious patients and enrich their learning experiences. A patient-oriented narrative provides a clinically relevant experience in which reflection plays an important role. The serious game also serves as medium to actively experiment with care solutions to create better understanding of how healthcare professionals can influence a delirious patient's experience.</p

Optimization Issues for a Micropulsed Plasma Thruster

Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/76083/1/AIAA-13954-648.pd

Kinetic and stoichiometric characterization of anoxic sulfideoxidation by SO-NR mixed cultures from anoxic biotrickling filters.

Monitoring the biological activity in biotrickling filters is difficult since it implies estimating biomass concentration and its growth yield, which can hardly be measured in immobilized biomass systems. In this study, the characterization of a sulfide-oxidizing nitrate-reducing biomass obtained from an anoxic biotrickling filter was performed through the application of respirometric and titrimetric techniques. Previously, the biomass was maintained in a continuous stirred tank reactor under steady-state conditions resulting in a growth yield of 0.328±0.045 g VSS/g S. To properly assess biological activity in respirometric tests, abiotic assays were conducted to characterize the stripping of CO2 and sulfide. The global mass transfer coefficient for both processes was estimated. Subsequently, different respirometric tests were performed: (1) to solve the stoichiometry related to the autotrophic denitrification of sulfide using either nitrate or nitrite as electron acceptors, (2) to evaluate the inhibition caused by nitrite and sulfide on sulfide oxidation, and (3) to propose, calibrate, and validate a kinetic model considering both electron acceptors in the overall anoxic biodesulfurization process. The kinetic model considered a Haldane-type equation to describe sulfide and nitrite inhibitions, a non-competitive inhibition to reflect the effect of sulfide on the elemental sulfur oxidation besides single-step denitrification since no nitrite was produced during the biological assays

Instrumentation and control of anaerobic digestion processes: a review and some research challenges

The final publication is available at Springer via http://dx.doi.org/10.1007/s11157-015-9382-6[EN] To enhance energy production from methane or resource recovery from digestate, anaerobic digestion processes require advanced instrumentation and control tools. Over the years, research on these topics has evolved and followed the main fields of application of anaerobic digestion processes: from municipal sewage sludge to liquid mainly industrial then municipal organic fraction of solid waste and agricultural residues. Time constants of the processes have also changed with respect to the treated waste from minutes or hours to weeks or months. Since fast closed loop control is needed for short time constant processes, human operator is now included in the loop when taking decisions to optimize anaerobic digestion plants dealing with complex solid waste over a long retention time. Control objectives have also moved from the regulation of key variables measured online to the prediction of overall process perfor- mance based on global off-line measurements to optimize the feeding of the processes. Additionally, the need for more accurate prediction of methane production and organic matter biodegradation has impacted the complexity of instrumentation and should include a more detailed characterization of the waste (e.g., biochemical fractions like proteins, lipids and carbohydrates)andtheirbioaccessibility andbiodegradability characteristics. However, even if in the literature several methodologies have been developed to determine biodegradability based on organic matter characterization, only a few papers deal with bioaccessibility assessment. 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