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  • Open access
  • 9 Reads
Shear-thinning fluid flow in variable aperture channels

Non-Newtonian fluid flow in fractured media is relevant for hydraulic fracturing operations aimed at the exploitation of oil, gas and thermal reservoirs. Rheologically complex fluids also interact with pre-existing rock fractures during drilling operations, environmental remediation, enhanced oil recovery, and natural phenomena such as lava and sand intrusions. Even Newtonian flow in single natural or artificial fractures is known to be typically tortuous and preferential, as the fracture aperture is characterized by a strong degree of variability, which in turn is modeled either as a 2-D random field or as the space between self-affine fracture walls. Specific challenges and compound effects arise from the interaction between the nonlinearity of the flow and the inherent multiscale heterogeneity characterizing the fractured medium. To capture such interactions and provide a benchmark for numerical models, we adopt a simplified geometric model to describe the aperture variability, consisting of adjacent one-dimensional channels with constant aperture, each drawn from a stochastic aperture distribution of given mean and variance. The flow rate in such a fracture subject to an external pressure gradient is then derived under the lubrication approximation for the two limiting cases of a pressure gradient which is perpendicular/parallel to aperture variation; these parallel/serial arrangements (PA/SA) provide an upper/lower bound to the fracture conductance.

The fluid rheology is described via a Prandtl-Eyring rheological model. Novel closed-form results for the flowrate in the PA/SA cases are then derived and discussed; different distributions and combinations of the parameters describing the fluid rheology and the variability of the aperture field are considered. Results are compared with those valid for a power-law fluid.

  • Open access
  • 10 Reads
Prediction of phytoplankton biomass in small rivers of Central Spain by data mining method of Partial Least-Squares Regression

Abstract:

The Water Framework Directive (WFD, EC, 2000) states that the "good" ecological status of natural water bodies must be based on the chemical and biological features, compared to the reference conditions. To fulfill the protection of surface waters established in the Water Framework Directive is necessary to monitor the ecological and chemical status of waters quality, especially under drastic conditions of floods or droughts, due to the greater epidemiological risks that occur during these periods.

Phytoplankton is one of the five groups suggested for the assessment of the ecological condition of surface waters under the Water Framework Directive. Phytoplankton is considered a good environmental bioindicator and the spatio-temporal variability in the of phytoplankton communities structures plays an important role in the composition and function of aquatic ecosystems. Although Climate Change has a strong impact on phytoplankton communities and water quality, the development of robust techniques to predict and control phytoplankton growth is still in progress.

The aim of this study is to analyze the impact of the different stressors associated with the change in phytoplanktonic communities in small rivers in the center of the Iberian Peninsula. An statistical study on the identification of the limiting variables in the phytoplankton growth and its seasonal variation by Climate Change was carried out. In this study, a method based on the partial least-squares regression technique (PLS) have been used to predict the concentration of phytoplankton and cyanophytes from 22 variables usually monitored in rivers. The predictive models have shown a good agreement using training data sets. The models were useful to reveal differences between rivers and seasons (dry and wet). The phytoplankton biomass of rivers in dry periods showed greatest similarities, being these dry periods the most important factor in the phytoplankton proliferation.

  • Open access
  • 3 Reads
Preliminary Design and Development of a Coupled Water Resources Resiliency Model of the St. Johns River Watershed Florida, USA

The St. Johns River watershed located in Florida, USA is the focus of planning efforts to improve the resiliency and management of water resources infrastructure from current and future flood threats. These threats are driven by intense urbanization of the basin combined with increased frequency and intensity of coastal storms made worse by sea level rise. Research efforts have begun to develop a comprehensive system of coupled numerical simulation models of the entire watershed in order to assess the current and long-term risks from flood inundation. This study provides a discussion of the system-wide model design and preliminary development. The study first outlines the project area and various numerical models to be coupled together. Then, the study discusses preliminary model development efforts and challenges. This initial study revealed the overall complexity and size of the undertaking. Overall, the study’s primary conclusion is that while an integrated system of coupled models is feasible and potentially very useful for long-term planning, the computational challenges for such an undertaking are extensive.

  • Open access
  • 10 Reads
Flood and Flash Flood hazard mapping using the Frequency Ratio, Multilayer Perceptron and their hybrid ensemble

The importance of identifying the areas vulnerable for both floods and flash-floods is an important component of risk management. The assessment of vulnerable areas is a major challenge in the scientific world. The aim of this study is to provide a methodology-oriented study of how to identify the areas vulnerable to floods and flash-floods in the catchment by computing two indices: the Flash-Flood Potential Index (FFPI) for the mountainous and the Sub-Carpathian areas, and the Flood Potential Index (FPI) for the low-altitude areas, using the frequency ratio (FR), a bivariate statistical model, the Multilayer Perceptron Neural Networks (MLP), and the ensemble model MLP–FR. A database containing historical flood locations and the areas with torrentiality was created and used to train and test the models. The resulting models were computed using GIS techniques, thus resulting the flood and flash-flood vulnerability maps. The use of the two indices represents a preliminary step in creating flood vulnerability maps, which could represent an important tool for local authorities and a support for flood risk management policies.

  • Open access
  • 11 Reads
Surface and ground water quality in South African area – Analysis of the most critical pollutants for drinking purposes

According to the recent World Health Organization (WHO) report, the countries which still have a limited access to water for drinking purposes are mainly those in the Sub-Saharan region [1].

In case of small rural communities it may be very expensive to guarantee (including investment and operational and maintenance costs) a safe water by means of decentralized small water works. Pollution of source water may be due to many causes, among them a continuous release of untreated wastewater (generated within the rural communities), land runoff, acid mine drainage.

This paper deals with the quality of surface and ground water in rural areas in South Africa and Mozambique and its variability along the year (mainly due to temperature variations, rainfall and consequent land runoff) in case of their withdrawal for drinking purposes. In order to evaluate how polluted these waters may be (expected), water quality standards for potable use adopted in South Africa, Germany and Italy and the guidelines set by the WHO are provided.

The aims of the paper are (i) to identify the most critical pollutants (organic and inorganic compounds and microorganisms) in surface and ground water and relate them to their potential origin; (ii) to show the importance of the need of interventions both in sanitation and potabilization fields in order to improve the quality of surface water bodies and groundwater, considered potential sources for drinking purposes for small communities in rural and peri-urban areas; (iii) to discuss the role and the need of adequate and reliable pretreatments in order to guarantee the removal of the most critical compounds even in case of a sudden increment in their concentration in the withdrawn water for different reasons.

Acknowledgements. This project has received funding from the European Union’s Horizon 2020 research and innovation program under grant agreement No 669925 (“SafeWaterAfrica” project).

[1] WHO, 2017. Progress on Drinking Water, Sanitation and Hygiene, available at the website: https://apps.who.int/iris/bitstream/handle/10665/258617/9789241512893-eng.pdf;jsessionid=6D639CB62299CA745657A3B448209937?sequence=1 (last access August 21st 2019)

  • Open access
  • 15 Reads
A promising alternative for water flocculation with Moringa oleifera in Ecuador

According to the inter-American development bank, Latin America is known for its high availability of water resources as it concentrates a third of the world's total water resources; but this availability is affected and reduced because reasons such as water resources distribution is not equitable throughout the area, 53% of the regional runoff is concentrated in the Amazon River, countries with large urban centers have low water quality and many others have a natural chemical quality of water that is not good due to the presence of arsenic and boron for example [1]. In Latin America, 34 million of people do not have access to drinking water [1].

Ecuador is a country with one of the highest concentration of rivers per square kilometer in the world; but there is also evidence of the poor quality of its water resources and the pollution problems they have [2].

By 2013, only 34% of its rural population had access to potable water and 25% to sewerage services. This is mainly associated with the lack of infrastructure and the necessary budgets allocated to them [2].

This paper deals with a proposal for water purification that consists of the use of the Moringa olifeira plant to facilitate the flocculation process, considering that its use reduces the costs associated with water treatment and minimizes the concentration of metals in the residual sludge. The influence dosage effects of crushed Ecuadorian Moringa oleifera seeds during coagulation-flocculation processes were explored using Jar tests.

The aims of the paper are (i) to identify the most effective dosage of crushed Ecuadorian Moringa oleifera seeds in the flocculation processes (ii) to show and discuss an alternative for the need of interventions in potabilization fields in order to improve the quality of life in Ecuador due to the lack of its coverage.

References

[1] Interamerican devolpment bank, 2018. Informe del Proceso Regional de las Américas Foro Mundial del Agua, available at the website: https://www.cepal.org/sites/default/files/news/files/informe_regional_america_latina_y_caribe.pdf

[2] CEPAL, 2018. Diagnóstico de las estadísticas del agua en Ecuador, available at the website: https://aplicaciones.senagua.gob.ec/servicios/descargas/archivos/download/Diagnostico%20de%20las%20Estadisticas%20del%20Agua%20Producto%20IIIc%202012-2.pdf

  • Open access
  • 8 Reads
Performance of heterogeneous catalytic ozonation with minerals in degradation of p-chlorobenzoic acid (p-CBA) from aqueous solutions

Water pollution is a critical environmental issue nowadays. One major problem is the pollution of freshwaters by contaminants of low concentrations (ng/L – μg/L), known as micropollutants. The most promising techniques for micropollutants degradation are the Advanced Oxidation Processes (AOPs). Heterogeneous catalytic ozonation is belonging among them and recent studies shown that can be an efficient water treatment technique. The aim of this study is to evaluate the catalytic activity of five minerals (anatase, dolomite, kaolin, talc and zeolite) on ozonation of small concentrations of p-CBA at pH 7 by batch mode experiments. p-CBA was employed as a model compound for the performance evaluation of single and catalytic ozonation, because it cannot be efficiently removed by direct ozonation (kO3 < 0.15 M-1s-1), while having high reactivity with hydroxyl radicals (k•OH = 5*109 M-1s-1). It was found that all applied solid materials can be characterized as catalysts, except kaolin, whose use presented almost the same performance as single ozonation. The best results were obtained by zeolite and dolomite (> 99.4%) within 15 min reaction/oxidation time. These materials were neutral (PZC = 6.8) and positive (PZC = 10.1) charged respectively, during the oxidation process (pH 7) favoring the contact of micropollutant and ozone with the catalysts’ surface. On the other hand the addition of anatase and talc in the ozonation system resulted in 97.5% and 98.5% p-CBA degradation respectively, due to their slightly negative charge throughout the reaction. Conclusively, the experimental results indicated that the performance of heterogeneous catalytic ozonation is strongly depending on the surface charge of the solid materials (catalysts).

  • Open access
  • 33 Reads
Quantifying the log reduction of pathogenic microorganisms by constructed wetlands: A review

Over the last 30 years, constructed wetlands (CWs) have been used as an alternative, cost-efficient way of treating wastewater, mainly in combination with conventional wastewater technologies. When CWs are attached at the end of conventional wastewater treatment plants, they adequately remove pollutants and thus provide a polishing step. However, recent studies have shown that when CWs are used as a main wastewater treatment method for agricultural reuse of effluents, they perform poorly on meeting the accepted limit of microbial contamination. Moreover, CWs are increasingly used within the scope of circular economy and water reuse applications. Therefore, there is a need for a comprehensive exploration of the performance of CWs on pathogen removal. This paper explores relevant case studies regarding pathogen removal from constructed wetlands in order to create a comprehensive dataset that provides a complete overview of CWs performance under various conditions. After performing a systematic literature review, a total of 48 case studies qualified for both qualitative and quantitative analyses. From the dataset, the general performance, optimal conditions, and knowledge gaps were identified. The review confirmed what was already known, that constructed wetlands (as a standalone treatment) cannot meet the accepted limits of pathogen removal. However, they can be a credible choice for wastewater polishing when they are combined with conventional wastewater treatment systems. Regarding the most common indicators that were recorded, the removal of Escherichia coli ranged between 0.01-5.6 log, the removal of total and fecal coliforms was 0.2-5.32 and 0.07-6.08 log respectively while the removal of fecal streptococci was 0.2-5.2 log. The great variability on pathogen removal indicates that the complexity of CWs makes it difficult to draw robust conclusions regarding their removal efficiency. Moreover, the inadequate reporting of extractable data and CW conditions (except hydraulic characteristics) in the reviewed papers, made it difficult to validate any relationship between CW performance and external parameters. However, the spatial distribution of case studies in the climatic zones around the world indicated a potential statistical correlation between temperature and removal of pathogens. Finally, the dataset can be used as a benchmark of CWs performance as a barrier against the spreading of pathogens in the environment. The knowledge gaps identified in this review can provide direction for further research. A potential meta-analysis of the dataset using statistical analysis can pave the way for a better understanding of the design and operational parameters of CWs in order to fine-tune and quantify the factors that influence the performance of these systems.

  • Open access
  • 10 Reads
PID tuning of a multivariable activated sludge process with nitrification and denitrification with multi-objective optimization

Wastewater treatment plants (WWTPs) are responsible of attenuating the environmental impact that waste in effluent discharged to receiving waters has. As a consequence of this, new techniques for an effective control are valuable, not just for minimising this impact, but also for minimising operational costs using efficiently the energy usage. Such kind of problems, with several objectives to fulfil (and usually in conflict) are termed as multi-objective problems.

Within this context, multi-objective optimisation techniques have shown to be a valuable tool in the control engineering field to tune different kinds of controllers for complex systems. To accomplish this, a simultaneous optimisation approach is carried on, in order to approximate a set of Pareto-optimal solutions. Such solutions differ in the level of trade-off exhibited in two (or more) conflicting objectives.

The multi-objective approach for controller tuning in one-input/one-output processess is well documented in the literature. Nevertheless, that is not the case of multivariable control. This fact is mainly due to the quantity of design objectives required to evaluate the multi-objective performance of several outputs. In this work we elaborate a proposal to handle multi-objective problems for multivariable processes. Performance evaluation is performed (via simulation) in a multivariable benchmark for the PID control of an activated sludge process with nitrification and denitrification.

  • Open access
  • 12 Reads
OPTIMIZATION OF HETEROTROPHIC DENITRIFICATION USING GLYCEROL AS A SUSTAINABLE EXTERNAL CARBON SUBSTRATE

Nutrients removal from residential and industrial wastewaters are essential for environmental and public health protection. Removal of nutrients from wastewater can be achieved chemically or biologically. Biological nutrient removal (BNR) is a series of anaerobic, anoxic, and aerobic zones to provide conditions for the biomass to uptake the nitrogen and phosphorus species, and comes in different configurations such as A/O, A2O, and 5-stage BardenphoTM. However; BNR systems require a sufficient carbon source which most wastewaters lack. The goal of this study is to use a sustainable carbon source to optimize the 5-stage BardenphoTM biological nutrient removal (BNR) systems and reduce the chemical cost. The experiments were carried out using two pilot-scale 5-stage BardenphoTM biological nutrient removal (BNR) systems coupled with side-stream prefermenters. Glycerol, a biodiesel by-product, was used as a sustainable carbon source by direct addition or after fermentation. The results from both systems were beneficial to the BNR system and resulted in similar effluent quality. Both systems achieved complete denitrification and excellent phosphorus removal (82% - 89%). Co-fermentation of glycerol and primary solids resulted in a significant increase in the volatile fatty acids (VFAs) loading beyond the estimated results, but did not correlate to better behavior between the two pilots since both systems achieved complete denitrification.

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