Optimal Distribution and Control of Storage Tank to Mitigate the Impact of New Developments on Receiving Water Quality

Storage tanks are commonly installed in a combined sewer system to control the discharge of combined sewer overflows that have been identified as a leading source for receiving water pollution. The traditional approach to determine the distribution of storage tank volume in the sewer system is confined to the use of objectives within the system itself due to the limits of separate modeling of urban wastewater systems, consisting of the sewer system, wastewater-treatment plant, and receiving water. The aim of this study is to investigate the optimal distribution and control of storage tanks with an objective to mitigate the impact of new residential development on receiving water quality from an integrated modeling perspective. An integrated urban wastewater model has been used to test three optimization scenarios: optimal flow rate control, storage distribution, and a combination of these two. In addition to the cost of storage tank construction, two receiving water quality indicators, dissolved oxygen and ammonium concentration, are used as optimization objectives. Results show the benefits of direct evaluation of receiving water quality impact in the context of storage distribution optimization. Results indicate that storage allocation should be considered in conjunction with optimal flow rate control to achieve the maximum effectiveness in water pollution mitigation.     

Parsimonious Model for Combined Sewer Overflow Pollution

The reservoir concept for flow modeling has been generalized for the purpose of the parsimonious modeling of combined sewer overflow pollution. Conceptual models have been used for the buildup and washoff of pollutants on the paved surfaces, and the transport of these pollutants in the sewer system (advection, dispersion, sedimentation, and resuspension). For the parsimonious modeling of the water quality of the sewage in the effluent of the combined sewer system, the conceptual submodels of these different processes were lumped into one single model equation. When ancillary structures such as a storage sedimentation tank are present at the combined sewer overflow, the additional effect of advection, dispersion, storage, and sedimentation is considered in a similar parsimonious conceptual way. Such a parsimonious model aims to reduce the model complexity, and therefore the number of calibration parameters. In most practical cases of urban drainage modeling, water quality data are extremely limited and consequently only a small number of parameter values can be identified from the data. The proposed model is tested on the basis of 10-min and hourly concentration measurements for total suspended solids, settleable solids, biochemical oxygen demand, and ammonia at the outlet of the combined sewer system of the village of Dessel (Belgium), which were available in this case only for six overflow events.     

Dynamics of Air Flow in Sewer Conduit Headspace

Pressurization in sanitary sewer conduit atmosphere is modeled using computational fluid dynamics techniques. The modeling approach considers both turbulent and laminar flow regimes. The turbulent model takes into consideration the turbulence-driven secondary currents associated with the sewer headspace and hence the Reynolds equations governing the air flow field are closed with an anisotropic closure model which comprises the use of the eddy viscosity concept for the turbulent shear stresses and semiempirical relations for the turbulent normal stresses. The resulting formulations are numerically integrated. The turbulent model outputs are verified with experimental data reported in the literature. Satisfactory agreement is obtained between numerical simulations and experimental data. Mathematical formulas and curves as functions of longitudinal pressure gradient, wastewater velocity, and sewer headspace geometry are developed for the cross-sectional average streamwise velocity.     

Influence of Wastewater Constituents on Hydrogen Sulfide Emission in Sewer Networks

Transport of wastewater in sewer networks causes potential problems associated with hydrogen sulfide in regard to odor nuisance, health risk, and microbially induced corrosion. To what extent these problems occur depends not only on the rate of sulfide formation but also on the rate of hydrogen sulfide emission into the sewer atmosphere. To gain understanding of the influence of wastewater constituents on the emission process, a number of batch experiments were conducted on domestic wastewater collected from sewer networks. The emission rate of hydrogen sulfide in the wastewater investigated was found to be approximately 60% of that in de-ionized water in terms of the overall mass-transfer coefficient, resulting in a correction factor (alpha) of 0.6. The alpha factor did not change significantly within the turbulence range studied (Froude numbers of 0.04–0.23). The Henry’s law constant for hydrogen sulfide in wastewater was observed to be close to that in de-ionized water, reflecting a correction factor (beta) of 1.0. By taking these results into account, modeling aspects of hydrogen sulfide emission in sewer networks are presented in this paper.     

Validation of Existing Bed Load Transport Formulas Using In-Sewer Sediment

Granular sediment in pipe inverts has been reported in a number of sewer systems in Europe. Given the range of flow conditions and particle characteristics of inorganic sewer sediments the mode of transport may normally be considered as bed load. Current commercial software for modeling the erosion and transport of sediments in sewer pipes still utilizes well-known, or modified versions of transport equations that were derived for transport of noncohesive sediment in alluvial streams. In this paper the performances of the equations of Ackers and White (originally developed for the transport of river sediments) and of May (derived from laboratory pipe experiments) are examined against two separate data sets. One set is from laboratory erosion experiments on sewer sediment obtained in Paris. A second data set has bed load transport rate measurements recorded in a sewer inlet pipe. The formulas were selected because of their widespread use in the prediction of in-sewer sediment transport both in commercial software and in the latest United Kingdom design guidance for new sewers. The results indicated that both the relationships performed poorly, even in such well-controlled conditions. These formulas have significant difficulties in predicting the erosion thresholds and fractional transport rates for non-uniformly sized in-sewer sediments. An empirical formula to adjust the threshold of motion for individual grain size fractions was developed which significantly improved predictions. Although such techniques have been used in gravel bed rivers, the threshold adjustment function for in-sewer deposits was significantly different from these previously published for fluvial gravels, indicating that a direct transfer of fluvial relationships to sewers may be inappropriate without further research.     

Computational and Experimental Study of Surcharged Flow at a 90° Combining Sewer Junction

Combining sewer junctions with a lateral inflow at 90° angle are commonly used in our sewer systems. A computational fluid dynamics (CFD) model based on Ansys CFX 10.0 was established to simulate fully surcharged flow at a 90° combining sewer junction. The model was carefully assessed by comparing its results with the measurements of detailed physical experiments. Good agreement was obtained between results of the computational model and of the laboratory experiments. The computational model was proved to be capable of simulating surcharged combining junction flow in the aspects of water depth, energy losses, velocity distributions, and turbulence. The verified CFD model was also used to investigate air entrainment and effects of the size of the junction chamber on the flow. Such CFD models can be used to optimize the design of sewer junctions and will also be useful in studying sediment transport at sewer junctions.     

Hydraulic Efficiency of Continuous Transverse Grates for Paved Areas

The hydraulic behavior of several continuous transverse gully grates is studied. Their use is common in some urban or impervious areas (squares, airport pavements, parks, pedestrian areas, etc.) where isolated inlets turn out to be ineffective in collecting the whole runoff into the sewer system during a storm event. Normally, manufacturers of such structures provide information about the bearing capacity, but little concerning their hydraulic behavior. Some grates with various widths were tested at the Technical University of Catalonia hydraulic laboratory for different approach flows and a large set of longitudinal slopes. Experimental equations were obtained to relate hydraulic efficiency of these structures to some relevant flow parameters such as the Froude number and the hydraulic depth. The first equation achieved may be used as a first approach to design a surface drainage system considering a constant circulating flow, whereas the second expression does not depend on flow values and could be used in storm-water and wastewater software packages to design surface drainage systems and for dual drainage modeling.     

Dual Multilevel Urban Drainage Model

In urban areas, when heavy rains occur, the discharge capacity of sewers is usually unable to transport the effective rainfall reaching the streets. When the runoff flow rate exceeds the capacity of the storm sewer system, the excess flow is conveyed through the street network as overland flow. A dual model is proposed for modeling the system as a double network, formed by an upper network of open channels (street gutters) and a lower network of closed conduits (sewer pipes). What is new in this model is its capacity to take into account the hydrodynamic relationship between the flows in the upper and lower networks. The model is applied to computing the response of a real monitored basin; the historical flow rates measured during a first rainfall event are used to calibrate the model, which is then validated using the simulation of two other measured events.     

Flow Data, Inflow/Infiltration Ratio, and Autoregressive Error Models

Sanitary sewer overflows (SSOs) are a major environmental issue. One of the major factors causing SSOs is the rain-derived inflow and infiltration (RDII) to a separate sanitary sewer system. If a wastewater collection system is not well maintained, cumulative system-wide RDII could easily cause the wastewater conveyance and treatment capacity to be overwhelmed, and thus lead to SSOs. Monitoring system condition is a key component in system management. The industry’s standard approaches to system monitoring include the practice of collecting and analyzing continuous rainfall and flow data at certain key locations in the system to estimate the level of RDII. However, the writer is of the opinion that the current standard analytical methodologies of the industry can be significantly improved. This paper introduces a basic regression approach with autoregressive errors to support statistical inferences with respect to the level of RDII.     

Supercritical Flow in Bend Manhole

Supercritical flow in a bend manhole for combined sewer systems is considered for a typical relative curvature and deflection angles of 45° and 90°. The typical features of bend flow in a U-shaped channel are determined, including the surface profiles along the inner and the outer bend walls, impact flow at the manhole outlet into the downstream sewer pipe, air entrainment characteristics, and wave development in the downstream sewer. Also, typical velocity distributions along the manhole are presented, and the capacity of the conventional bend structure is determined. To increase the discharge capacity across a bend manhole, the so-called bend cover is introduced. This cover element finished in steel can be removed for maintenance and inspection purposes. It limits the wave height close to the bend outlet to 90% of the sewer diameter and allows air entrainment into the downstream sewer. The performance and capacity of bend flow may significantly be increased, and the novel element may easily be added to existing manholes.     

Modeling In-Sewer Deposit Erosion to Predict Sewer Flow Quality

High levels of suspended solids are typically observed during the initial part of storms. Field evidence suggests that these suspended solids derive from the erosion of in-sewer sediment beds accumulated during dry and previous wet weather periods. Suspended sediment transport rate models within existing sewer network modeling tools have utilized inappropriate transport rate relationships developed mainly in fluvial environments. A process model that can account for the erosion of fine-grained highly organic in-sewer sediment deposits has been formulated. Values of parameters describing the increase in deposit strength with depth are required. These values are obtained using a genetic algorithm based calibration routine that ensures model simulations of suspended sediment concentrations that correspond to field data collected in a discrete length of sewer in Paris under known hydraulic event conditions. These results demonstrate the applicability of this modeling approach in simulating the magnitude and temporal distribution of suspended in-sewer sediment eroded by time varying flow. Further work is developing techniques to enable the application of this type of model at the network level.     

Propagation of Waves and Dissolved Compounds in Sewer

The phenomenon of the surface wave propagating faster than the fluid that induced it is studied experimentally and numerically. The study focuses on the importance of the wave phenomenon for the urban hydrology system, where it can determine the impact of combined sewer overflows on the environment and the operation of a novel urine (anthropogenic nutrient solution) separation system. Urine would be stored decentrally and released during the night hours so that a wave would form in the sewer. The full-scale experiments were carried out in a 2-km section of a main sewer. The wave was induced with the aid of fire hydrant water traced by salt. Five measurements and sampling stations were operated downstream through which the transport of both fluid and compounds were analyzed. Numerical simulations of the results are discussed focusing on the reliability of friction approaches and dispersion prediction. Although difficult to model with commercial tools, it was shown that the wave phenomenon has no adverse effects on the practicability of the urine separation system, but can lead to the release of undiluted wastewater during a rain event.     

Closed-Conduit Bed-Form Initiation and Development

Nonintrusive measurement of closed-conduit erodible-bed development was undertaken for 12 experiments of ranges of flow strengths and sediment (solids) sizes. Analogous to open-channel flows, wavelets on the sediment bed of a closed-conduit are instigated by discontinuities in the bed, with wavelet lengths λ for laminar and turbulent open-channel and closed-conduit flows given by λ = 175d0.75, where λ and sediment size d are in millimeters. For closed-conduit flows, ripples, and dunes grow from these wavelets (at rates increasing with increasing flow strength, and utilizing the mechanisms of bed-form coalescence and throughpassing) to limiting lengths, heights, steepnesses, and bed friction factors that are approximately maintained or possibly decrease thereafter. Limitation of free-surface deformation results in increased rates of bed-wave development for closed-conduit flows in comparison to open-channel flows. Measured results indicate that equilibrium closed-conduit ripple and dune magnitudes can be predicted using relations derived for equivalent open-channel flows. The present findings are of particular relevance for understanding and modeling engineering activities ranging from dredging to transport of solids in stormwater and sewer systems, bed-form transport of solids in closed conduits influencing (potentially markedly) conduit conveyance, rate of solids transport, and system head losses for such flows.     

Improved Simulation of Flow Regime Transition in Sewers: Two-Component Pressure Approach

Operational problems and system damage have been linked to the flow regime transition between free surface and pressurized flow in rapidly filling stormwater and combined sewer systems. In response, emphasis has been placed on the development of numerical models to describe hydraulic bores and other flow phenomena that may occur in these systems. Current numerical models are based on rigid column analyses, shock-fitting techniques, or shock-capturing procedures employing the Preissmann slot concept. The latter approach is appealing due to the comparative simplicity, but suffers from the inability to realistically describe subatmospheric full-pipe flows. A new modeling framework is proposed for describing the flow regime transition utilizing a shock-capturing technique that decouples the hydrostatic pressure from surcharged pressures occurring only in pressurized conditions, effectively overcoming the cited Preissmann slot limitation. This new approach exploits the identity between the unsteady incompressible flow equations for elastic pipe walls and the unsteady open-channel flow equations, and the resulting numerical implementation is straightforward with only minor modifications to standard free surface flow models required. A comparison is made between the model predictions and experimental data; good agreement is achieved.     

Optimization Modeling for Sewer Network Management

Due to their low visibility, sanitary sewers' condition assessment and rehabilitation are frequently neglected until a catastrophic failure occurs. Neglecting regular maintenance of these underground utilities adds to life-cycle costs and liabilities, and in extreme cases causes stoppage or reduction of vital services. A systematic approach for the determination of deterioration of sewer systems and an integrated management system are necessary to fully understand the complete status of this underground infrastructure system. This paper discusses the major aspects of integrated management for sewer systems, namely, the development of network identification, sewer classification and sewer condition rating systems, sewer deterioration mechanisms, prediction modeling, and the use of optimization techniques for maximizing benefit∕cost ratios over a planning horizon. A case study, based on large combined sewers from the city of Indianapolis, has been used to demonstrate the use of the framework of this integrated life-cycle based sewer management system. Deterministic dynamic programming is employed to identify appropriate sewer rehabilitation techniques at different stages during the planning horizon adopted for the sewer systems.     

Bed Shear Stress Boundary Condition for Storage Tank Sedimentation

Computational fluid dynamics-based (CFD) software tools enable engineers to simulate flow patterns and sediment transport in ancillary structures of sewer systems. Lagrangian particle tracking represents a computationally efficient technique for modeling sediment transport. In order to represent the process of sedimentation in storage tanks, careful consideration must be given to the boundary condition at the bottom of the tanks. None of the boundary conditions currently available in the FLUENT CFD software appears to represent the observed behavior of sediment particles, which may become resuspended after first contact with the bed if the local flow velocity is sufficiently high. In this study, a boundary condition based on bed shear stress has been implemented in FLUENT and evaluated against laboratory data. A particle is trapped if the local bed shear stress is below the critical bed shear stress; otherwise, the particle is resuspended. The approach gives satisfactory agreement with measured sedimentation efficiency data, and the simulated spatial distribution is very similar to the sediment distribution observed in a laboratory tank.     

GIS-Based Engineering Management Service Functions: Taking GIS beyond Mapping for Municipal Governments

A geographic information system (GIS) is found to be a platform of choice for meeting the needs of several core engineering functions. However, there is a dearth of GIS usage for engineering management service functions (EMSFs), which typically are specific to individual jurisdictions. This study outlines a simple approach for automating EMSFs on a GIS platform and demonstrates its efficacy. In the first phase of this research, a review of GIS applications in various municipal government functions is conducted. The review indicated that most municipal governments have currently employed GIS for a variety of general administrative services such as revenue collection, data archival, and information dissemination purposes. The proposed approach is demonstrated by developing a GIS model for a sanitary sewer reimbursement (GIS-MSSR) program at a fast-growing urban municipality. Data modeling issues related to representing complex management tasks within the context of a relational database model are discussed. Development of functional requirement specifications for GIS models and their implementation are also discussed. The effectiveness of the approach is verified by highlighting productivity gains resulting from the development of GIS-MSSR. The research concluded that several complex engineering service management functions can be automated on GIS platforms to realize substantial productivity gains. Automating multiple tasks, however small they might be, in an integrated environment can increase the productivity even further.     

Prediction of Sewer Condition Grade Using Support Vector Machines

Assessing the condition of sewer networks is an important asset management approach. However, because of high inspection costs and limited budget, only a small proportion of sewer systems may be inspected. Tools are therefore required to help target inspection efforts and to extract maximum value from the condition data collected. Owing to the difficulty in modeling the complexities of sewer condition deterioration, there has been interest in the application of artificial intelligence-based techniques such as artificial neural networks to develop models that can infer an unknown structural condition based on data from sewers that have been inspected. To this end, this study investigates the use of support vector machine (SVM) models to predict the condition of sewers. The results of model testing showed that the SVM achieves good predictive performance. With access to a representative set of training data, the SVM modeling approach can therefore be used to allocate a condition grade to sewer assets with reasonable confidence and thus identify high risk sewer assets for subsequent inspection.     

Sewer-Sediment Control: Overview of an Environmental Protection Agency Wet-Weather Flow Research Program

This paper presents a historical overview of the sewer sediment control projects conducted by the Wet-Weather Flow Research Program of the United States Environmental Protection Agency. The research presented includes studies of the causes of sewer solids deposition and development/evaluation of control methods that can prevent sewer-sediment accumulation. Discussions focus on the relationship of wastewater characteristics to flow-carrying velocity, abatement of solids deposition and solids resuspension in sewers, and sewerline flushing systems for removal of sewer sediment. Methods for abating sewer sedimentation include steeper sewer slope, pipe bottom shapes that maintain high velocity during low-flow conditions, and periodic sewer flushing. The future research program plan for sewer-sediment control is also presented.