Sara Todeschini

Hydrologic and Environmental Impacts of Imperviousness in an Industrial Catchment of Northern Italy

AbstractThe increase of imperviousness due to urbanization creates adverse effects on the hydrological and environmental processes in a catchment. The increase in impervious areas leads to a higher runoff peak flow and stormwater pollution, even for a short duration of low-intensity rainfall. The present study focuses on the Bivio Vela catchment, an urban catchment of Pavia in northern Italy. This paper emphasizes both experimental and modeling approaches to the rainfall-runoff process and pollutant dynamics on the catchment surface and in the combined sewer system, considering significant increases in impervious area. Recent and future catchment developments caused the conversion of approximately 33% of the total area from nonurban pervious area to impervious area. The study also deals with the performance assessment of the wet-weather control system of the catchment, introducing simple indicators that account for the requirements of the urban drainage system and treatment plant. Imperviousness associate...

Impact assessment of urban wet-weather sewer discharges on the Vernavola river (Northern Italy)

The research concerned the ecological impact assessment of urban wet-weather sewer discharges on the Vernavola river (Pavia, Northern Italy) focusing both on the sewer system and on the receiving natural environment. The complexity of the urban drainage system (combined sewer networks, pumping stations, stormwater storage tanks, etc.) was characterised through in situ inspections, measurements and numerical modelling. Various dry- and wet-weather monitoring campaigns on the Vernavola river provided the water quality tracers according to Italian standards: dissolved oxygen, biochemical oxygen demand, chemical oxygen demand, ammonium, nitrate and Escherichia coli. The quality analyses were also devoted to the study of the macro-benthic community. Both the chemical and biological river quality appeared significantly affected by sewer overflow contaminants. During storm events the receiver ecosystem was severely loaded with organic pollution (wet-weather discharges were equivalent to a civil sewer discharge of about 68,000 population equivalent (PE)) and the biodiversity of the benthonic organisms populating the stream was reduced by the physical modification of the habitat. These sampling and modelling studies provided the priority support information for promoting solutions and management strategies on the entire chain ‘sewer system-treatment plant-receiving natural environment’ necessary to reach the stream quality objectives.

Evaluation of the Areal Reduction Factor in an Urban Area through Rainfall Records of Limited Length: A Case Study

AbstractThis paper addresses the determination of the areal reduction factor (ARF) as a function of area and duration. The analysis is carried out using the data recorded in a 10-year period at the rainfall gauge network of the city of Milan (Italy). Four types of probability distributions [exponential, the extreme value type 1 (EV1), the generalized extreme value (GEV), and generalized Pareto], two parameter estimation methods (probability-weighted moments and partial probability-weighted moments), and four different regression models of ARF on area and duration are considered. A sensitivity analysis is carried out to outline the effect exerted on ARF by the choice of probability distribution and the parameter estimation method and by that of the model. The effect of model choice is more important than the choice of the distribution and estimation method. The models that fit the data best are the newly developed ones. The ARF model that has the best fit presents a root-mean-square error (RMSE) equal to 0...

Laboratory Experiments and Numerical Modelling of the Scouring Effects of Flushing Waves on Sediment Beds

Abstract This paper presents an investigation on the scouring effects of flushing waves on sewer sediment deposits in order to derive guidelines on the design and set-up of flushing devices in sewer systems and in stormwater storage tanks. The investigation comprises both laboratory experiments and numerical modelling. The experimental campaign was carried out on a channel at the laboratory of the Department of Hydraulic and Environmental Engineering of the University of Pavia adopting a simple flushing device. The first set of experiments is carried out without sediment and the water levels during the flushing processes are monitored by a digital video-camera. The second set of tests is carried out with uniform layers of sediment at the bottom of the channel and the effects on sediment beds produced by flushes are analysed. The numerical model solves the De Saint Venant equations describing unsteady flow in open channel and the continuity equation of Exner for the conservation of the sediment mass. For this purpose, the MacCormack explicit finite difference scheme is introduced. A sediment erosion relationship linking the rate of erosion to an excess of shear stress at the bottom is included in the model. The comparison between the experimental and computed water depths shows good agreement for different flushing conditions and longitudinal slopes of the channel. The tests with different sediment beds allow the calibration of the parameters that globally describe the erosion process produced by flushes.

Placement Strategies and Cumulative Effects of Wet-weather Control Practices for Intermunicipal Sewerage Systems

This study investigates the rainfall-runoff process and the pollutant dynamics on theoretical catchments and intermunicipal sewer systems in order to characterize the hydraulic and environmental performance of possible types of sewerage system and to compare the effectiveness of different wet-weather control schemes for intermunicipal sewerage systems. A comprehensive investigation on the placement strategies and the cumulative effects of wet-weather control practices is carried out over a broad watershed as a key preliminary step in addressing surface water safeguard requirements in developing and urban areas. The analysis shows that the different types of intermunicipal sewer system exhibit slightly different performance. Placing wet-weather detention tanks at different spatial levels affects the overall effectiveness of the wet-weather control system. In the examined case study schemes, the insertion of a wet-weather detention tank is better on final than on local flow regulators. The performance control also proves to be easier adopting the solution with intermunicipal tank since, for this scheme, the design criteria have a more important influence than rainfall characteristics on the environmental performance. These results represent an issue of primary focus for the implementation of environmental policies and mitigation strategies against surface water impairment in urban and developing areas.

Rheology and Microbiology of Sludge from a Thermophilic Aerobic Membrane Reactor

A thermophilic aerobic membrane reactor (TAMR) treating high-strength COD liquid wastes was submitted to an integrated investigation, with the aim of characterizing the biomass and its rheological behaviour. These processes are still scarcely adopted, also because the knowledge of their biology as well as of the physical-chemical properties of the sludge needs to be improved. In this paper, samples of mixed liquor were taken from a TAMR and submitted to fluorescent in situ hybridization for the identification and quantification of main bacterial groups. Measurements were also targeted at flocs features, filamentous bacteria, and microfauna, in order to characterize the sludge. The studied rheological properties were selected as they influence significantly the performances of membrane bioreactors (MBR) and, in particular, of the TAMR systems that operate under thermophilic conditions (i.e., around 50°C) with high MLSS concentrations (up to 200 gTS L−1). The proper description of the rheological behaviour of sludge represents a useful and fundamental aspect that allows characterizing the hydrodynamics of sludge suspension devoted to the optimization of the related processes. Therefore, in this study, the effects on the sludge rheology produced by the biomass concentration, pH, temperature, and aeration were analysed.

Modularity-Based Procedure for Partitioning Water Distribution Systems into Independent Districts

A proper division of a Water Distribution System (WDS) into District Metered Areas (DMAs) provides important management benefits particularly with regard to leakage detection through water balances, control and optimization of pressure so as to reduce leakage, implementation of monitoring, warning and emergency acting systems against accidental or intentional water contamination. This paper presents a new methodology that combines a suitable modularity-based algorithm for the automated creation of DMA boundaries and convincing practical criteria for the DMA design. A further plus of the proposed methodology is its ability to identify many technically feasible solutions that can be subsequently economically assessed. The successful applications of the proposed methodology to a real case study, already tested by other authors, has proven its effectiveness for the DMA design in existing water distribution systems.

Integrated RTD − CFD Hydrodynamic Analysis for Performance Assessment of Activated Sludge Reactors

This work shows an integrated analysis method for hydrodynamic investigation of biological reactors for wastewater treatment in order to detect the amount and position of possible defects, such as bypass and dead volume, influencing process efficiency. To reach such a goal, the proposed methodology integrates Residence Time Distribution (RTD) analysis, providing global hydrodynamic information, with Computational Fluid Dynamics (CFD) analysis, showing local flow conditions. RTD analysis was performed through a time-discretized analytical model of in-series mixed-flow reactors with dead volumes and bypass. CFD analysis was carried out with a 3D finite volume model allowing the numerical solution of turbulent incompressible isothermal flow. The method was tested on a scale activated sludge pilot plant with pre-denitrification scheme made of two in-series tanks. Hydrodynamic tests were performed carrying out the stimulus-response experiment using clear water inside the reactor and lithium chloride as a tracer. Two operating conditions of practical interest were investigated: (i) no mixing and (ii) upstream mixing. The RTD analysis of the outflow curve of lithium concentration from the experiment allowed detecting: a) no bypass for both operating conditions, and b) 5% dead volume for condition (i). These results were confirmed by the CFD analysis that allowed localizing the position of the dead volume. The integrated analysis proved to be effective for detection of both types and position of hydrodynamic defects. Therefore the proposed method can be adopted for performance assessment of activated sludge reactors and subsequent improvement of their efficiency.

The effects of water management on annual maximum floods of Lake Como and River Adda at Lecco (Italy)

This paper presents the results of an analysis aimed to assess the impact exerted on the flood regime of Lake Como and its emissary by different kinds of waterworks in the Adda basin (Italy). This includes construction of reservoirs in the Alps, increase in flow capacity of the exutory and regulation of Lake Como. The assessment of the management effect on peak floods is of particular interest as flood damages could be particularly heavy in densely populated areas along the lake banks and in the river flood plain downstream of the lake. The analysis is carried out by comparing the annual maxima of Lake Como levels and the river Adda discharges at Lecco, at the lake exutory, in different real and hypothetical situations. Both the construction of alpine reservoirs and the improvement of flow capacity of the exutory decrease lake levels and flood discharges, while lake regulation (for irrigation and power production) increases them. The overall effect is a generalised reduction of the flood levels and a slight increase in the discharges with return periods lower than 10 years. The discharges with higher return periods, instead, remain unchanged. The difference between peak and mean daily discharges negligible in all cases disappears altogether for return periods higher than 10 years.