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What would happen in Mediterranean rivers and streams if warming but not drying occurred? We examined whether the delivery of environmental flows within a warming climate can maintain suitable macroinvertebrate habitats despite warming. A two-dimensional ecohydraulic model was used to (1) simulate the influence of water temperature and flow on macroinvertebrates by calculating habitat suitability for 12 climate change scenarios and (2) identify the mechanism by which macroinvertebrate assemblages respond to warming. The results suggest that not all watersheds will be equally influenced by warming. The impact of warming depends on the habitat conditions before warming occurs. Watersheds can, thus, be categorized as losing (those in which warming will degrade current optimal thermal habitat conditions) and winning ones (those in which warming will optimize current sub-optimal thermal habitat conditions, until a given thermal limit). Our models indicate that in losing watersheds, the delivery of environmental flows can maintain suitable habitats (and, thus, healthy macroinvertebrate assemblages) for up to 1.8–2.5 °C of warming. In winning watersheds, environmental flows can maintain suitable habitats when thermal conditions are optimal. Environmental flows could, thus, be used as a proactive strategy/tool to mitigate the ecological impacts of warming before more expensive reactive measures within a changing climate become necessary.

We propose a systematic procedure that combines computational fluid dynamics (CFD) modeling and experimental work to answer two research questions that are usually posed by researchers and managers of water treatment plants: ‘Is the effect of wind on settling tanks important?’ and ‘How can we determine this effect in our settling tanks?’ We apply this procedure in the water treatment plant of Aharnes, Athens to derive the following conclusions. (1) The effect of wind increases with increasing co-current wind velocity, increasing settling velocity and decreasing flow rate. (2) In windy steady-state flow conditions, the degree of complexity and three-dimensionality of the flow field that is observed in calm conditions is reduced and the removal efficiency decreases from 85.1 in calm conditions to 82.0%. Predicted efficiencies for constant and variable inlet solids' concentrations compare favorably with measurements. (3) In windy, transient flow conditions, field data show that the effect of wind on the tank's efficiency can be very pronounced and within the first half hour of the windy period the efficiency decreases to approximately 55%; the present model does not capture this effect, because it cannot simulate the sludge layer and the subsequent re-suspension of the settled solids.

This paper presents a methodology for the optimization of the operation of an alternating oxidation ditch system, performing carbon oxidation–nitrification, denitrification and settling, through the use of a mathematical model and pilot testing. Measurements have been made in a pilot plant located in the Sanitary Engineering Research and Development Center of E.Y.D.A.P at Metamorphosis-Athens. Also, a mathematical model consisting of 1-dimensional mass balance (convection–dispersion) equations, based on the IAWPRC activated sludge model No. 1 was used. After the calibration and verification of the model with the experimental data, the optimization of the system’s performance was achieved by testing operational cycles with different time lengths. Since carbon removal is way under the 91/271 E.U. directive’s upper limits and since the ditch system has not been designed for phosphorous removal, the limits for nitrogen removal have been used for comparison. In this work’s example, from the eight oper...

Complex spatio-temporal environmental problems call for an integrated assessment of environmental systems due to their interdisciplinary nature. Within this context, the approach of integrated water resource management (IWRM) at the catchment scale, which is at the heart of advanced European environmental policy such as the water framework directive (WFD), argues for integrated modelling tools and approaches. This paper examines recent technological developments that facilitate integrated modelling and focuses on a novel approach, the open modelling interface (OpenMI), which allows models built by different developers to communicate at runtime. The individual models, when made compliant with the OpenMI standard, can be considered as ‘components' of an integrated model and seamlessly exchange data. Two OpenMI-enabled integrated modelling case studies are presented and their results critically discussed. The paper concludes with opportunities and challenges towards the design of flexible, component-based models supporting integrated environmental policy in general and the WFD implementation in particular.

A computational fluid dynamics (CFD) model is presented and applied in the design of the secondary settling tanks of Psyttalia Wastewater Treatment Plant in Athens, Europe’s largest sewage treatment facility. The tanks are of the Gould Type II consisting of the following regions: an inlet-flocculation chamber with an inlet baffle, two zones of settling separated by an intermediate baffle, an outlet region, and a sludge collection region. The number of tanks and their dimensions were determined with an empirical design procedure. Then, theoretical considerations, information from similar existing tanks, and preliminary CFD calculations were combined to determine the dimensions of the main regions and the positions of the baffles. Finally, detailed CFD calculations were performed to examine the performance of the tanks for various design conditions. Computations showed that the flow in the inlet-flocculation region was completely mixed; while in the settling regions a “three-layer” structure with relatively...

Thermal comfort conditions in the amphitheater of the MPC (Main Press Center), which was used in the Olympic Games of Athens 2004, are evaluated with Computational Fluid Dynamics (CFD) modeling. A CFD code is applied to calculate three-dimensional flow velocities and temperatures in the amphitheater for various values of temperatures of conditioned inlet air. Flow velocities and temperatures are used to determine the values of the comfort indices: PMV (Predicted Mean Vote) and PPD (Predicted Percentage of Dissatisfied). Calculated PMV and PPD values show that thermal conditions in the amphitheater of the MPC are very satisfactory for temperature of conditioned inlet air equal to 16.5 °C; only a small percentage (less than 8.5%) of the occupants is expected to be slightly uncomfortable.

A hydrodynamic model was applied to calculate the flow field and the renewal time in Salman Bay, a semi-enclosed coastal water body of particular environmental significance in the region of Jeddah, Saudi Arabia. Initially the model was calibrated with field data. It was then applied to assess the effect of a series of engineering modifications of Salman Bay, involving enlarging of the cross-section of a link channel to the Red Sea, dredging of the bay, and reclamation of islands inside the bay, on the flow characteristics and the renewal of waters. Calculations showed that the channel upgrading would significantly improve the renewal of the bay waters. For tidal calculations the renewal time was reduced from 48·6 days to 12·7 days, whereas for a wind-driven regime it was reduced from extremely large values to 13–34 days.

A numerical model for predicting the flow in clarifiers is presented which accounts for density-affected and turbulent flow as well as for the settling of activated sludge. The reliability of the two-dimensional model is demonstrated through verification by means of a laboratory experiment which was specially designed for this purpose. The numerical model is then applied for an evaluation of inlet and outlet arrangements. The inlet is positioned at the bottom and the inlet aperture is varied. An outlet at the top of the end wall is compared with outlets longitudinally extended at the surface. The alternatives are assessed by analysis of streamline patterns, layer charactersitics and the entrainment into the bottom current, i.e. the increase of its flow rate. The study demonstrates how numerical modelling can be used for systematic evaluation of the effect of geometrical configurations on the flow in secondary clarifiers.

A mathematical model is presented to predict the concentrations of the active heterotrophic biomass, the readily biodegradable substrate (soluble COD) and the dissolved oxygen (DO) in a completely aerobic oxidation ditch. The model involves the one-dimensional convection-dispersion equations for biomass, COD and DO. Hydrodynamic effects are represented in the model by the values of the average flow velocity and the dispersion coefficient. Biological processes are described in the model according to the IA WPRC activated sludge model, using typical values for the model parameters at 10°C. The equations are solved with the finite volume method. The application of the model leads to the following conclusions: (i) Steady state biomass concentrations are almost constant throughout the ditch. (ii) Steady state COD concentrations in the ditch are very low, and COD removal efficiency is practically independent of the values of the flow velocity and the dispersion coefficient. The distribution of the COD concentration in the ditch is less uniform, when small values of the dispersion coefficient are used. (iii) The distribution of the DO concentration in the ditch is very sensitive to the values of the flow velocity, the dispersion coefficient and to the capacity of the rotors. DO concentrations increase when the dispersion coefficient decreases or the flow velocity increases. (v) Daily sludge production, oxygen requirements and sludge age are calculated equal to 0.44 g (g COD removed)‒1, 0.56 g (g incoming COD)‒1 and 6.3 days, respectively.

A river water quality model is presented for the prediction of the concentration fields of the water quality variables in an oxidation ditch performing carbon oxidation, nitrification and denitrification. The model involves the one-dimensional convection-dispersion equations for all the variables, which are described in the IAWPRC Activated Sludge Model No 1. The model is applied to a typical oxidation ditch to show that all variables, with the exception of DO, are uniformly distributed in the ditch; thus permitting the characterisation of the oxidation ditch as a completely mixed reactor. The DO concentration, which controls the reaction rates of almost all the processes, shows a plug flow behaviour. The potential of the present model to be used to improve the DO control system of the ditch and to perform real time predictions is also demonstrated. All calculations have been performed with a PC.