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.     

Experimental Study of Surcharged Flow at Combining Sewer Junctions

The results of a laboratory investigation on surcharged flow in combining sewer junctions are presented in this paper. Experiments were conducted in a 90° model junction and the 25.8° Edworthy model junction. The study confirmed the existence of three flow regimes in sewer junctions with a steep outgoing pipe: Regime I denotes the open-channel flow through the junction chamber; Regime II flow is partially surcharged flow featured by orifice flow into the outlet pipe; and Regime III flow is fully surcharged flow with all connecting pipes running full. The transition flow from Regimes II to III was investigated, and it may be related to the inlet waves at the entrance of the outlet pipe. Criteria for the transition were provided. Theoretical analyses were conducted based on one-dimensional momentum equation. The derived equations are able to estimate the water depth in the junction chamber. Energy loss in Regime III flow was studied and predictions based on the momentum equation were evaluated.     

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.     

Multivariable Regulator Approach to Sewer Network Flow Control

The problem of optimal water distribution to a range of retention reservoirs in an urban sewer network during rainfall events is considered in this paper. The goal of the control actions is the minimization of overflows and eventually the reduction of their polluting impact on receiving waters. A multilayer control structure consisting of an adaptation, an optimization, and a direct control layer, is proposed for the solution of this complex control problem. Several approaches have been proposed with regard to the optimization layer. This paper proposes a linear multivariable feedback regulator that is developed via a systematic design procedure, including a simplified model, a quadratic minimization criterion, and subsequent application of the linear-quadratic optimization method. Inflow predictions are accommodated through feedforward terms in the control law. The control design guidelines facilitate a quick and efficient regulator design for a broad class of sewer network control problems. Simulation tests for a particular large network and various inflow scenarios indicate that significant overflow reductions are achieved by the application of the linear-quadratic regulator.     

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.     

Diameter and Surcharge Effects on Solute Transport across Surcharged Manholes

New data are presented describing the retention time and longitudinal dispersion of a solute tracer across circular surcharged manhole structures of different diameters. The variations with both discharge and surcharge level are described and the relationships quantified. The variation of the longitudinal dispersion coefficient exhibits poorly defined trends, however using an aggregated dead zone technique both the reach time delay and travel time show clear variations. A surcharge threshold level for these parameters is evident at the larger manhole diameters and this is explained in relation to jet theory. The variation of the surcharge mean time delay and postthreshold mean travel time are quantified, while the prethreshold travel times are shown to be dependent on both discharge and surcharge. The relationships allow for inclusion in sewer water quality modeling and provide a method for improving predictive techniques.     

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.     

Sewer Sideweir with Throttling Pipe

Sewer sideweirs in a combined sewer system are used for diversion of excess discharge during rainfalls. Their hydraulic performance depends significantly on the approach Froude number, the relative weir height, and the overflow length. The throttling pipe is a simple device to limit the discharge to treatment facilities. An experimental work was conducted to establish the main flow features of the converging sewer sideweir. The effect of the throttling pipe was determined. It was concluded that Abwassertechnische Vereinigung of Germany or the European regulations for side overflows can only be satisfied if the weir height is larger than about 70% of the approach diameter. Also, the approach Froude number should be smaller than about 0.7. Using the energy equation, relations for the free surface profile were developed that agree with experimental observations. Further results include the spatial discharge and velocity distributions, the lateral outflow characteristics, the excess overflow, the end depth, and equations for the discharge in the throttling pipe. Previous results of Gisonni and Hager for the short sewer sideweir without throttling pipe are confirmed, and photographs illustrate the complex spatial flow patterns.     

Modeling Ventilation Phenomenon in Sanitary Sewer Systems: A System Theoretic Approach

Municipal wastewater collection systems, due to the nature of their functions, carry varying concentrations of odorous gases. The production rate and transport of these gases within and out of sewer systems depend on air flow rate in the system piping. However, municipal sewers are generally designed to only transport sewage flow without giving consideration to the air flow field. As a consequence, the movement of air into, along, and out of collection systems is for the most part uncontrolled. The purpose of this paper therefore is to provide a new design protocol based on system theoretic techniques to be used by municipal engineers and environmentalists involved in odor control and sewer foul air transport studies. The modeling formulation accounts for combined wastewater drag and pressure-induced air flows, and manhole pressurization. The developed framework is applied to both hypothetical and real sewer systems to only illustrate the applicability of the modeling formulation.     

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.     

Statistical Modeling of the Serviceability of Sewage Pumps

Sewage pumping stations represent an element of the sewer system, which is directly responsible for affecting serviceability; i.e., failing pumps may result in combined sewer overflows or flooding. However, failures of sewage pumps are not yet incorporated in sewer assessments due to lack of data. This paper presents the analysis of pump failure data provided by two sewer management authorities in The Netherlands. Pump failures have been studied accounting for the nature of the failures, the operation and maintenance procedures of the management authority, the aging of the pumps, and the changes in the environment of pumps. The analysis shows that sewage pumps fail relatively often due to the composition of sewage and the discontinuous operation of the pumps. The interarrival time and the duration of failures are highly variable and independent of the specific function of the pump. Resulting pump failure characteristics are applied in a Monte Carlo simulation to calculate the impact of failures on combined sewer overflow volumes. The results indicate that the serviceability of sewer systems is significantly affected by failing pumps. Therefore, including pump availability in sewer system assessments should be considered.     

Hydraulics of Stacked Drop Manholes

This paper presents the results of an experimental investigation on flows inside stacked drop manholes (SDM). An SDM consists of two identical rectangular or square manhole chambers stacked together at an elevation difference. SDMs for different conditions were assessed on their ability to dissipate the energy of the approaching flow and their suitability to perform adequately under different flow conditions. Flow regimes were classified based on the inflow conditions and geometry of the structure in the first chamber and downstream outflows in the second chamber. An analysis based on the integral momentum equation was developed to estimate pool depths and energy losses under critical flow conditions. A fully surcharged stage with inflow and outflow pipes running full was also tested and velocity profiles were measured at a horizontal center plane to the opening connecting both shafts. Additionally, air flow rates were measured to assess the air demand into a large-height SDM.     

Integrated Storm-Water Management for Watershed Sustainability

One aspect of integrated watershed management evaluates the impact of development on the local hydrologic cycle and, in particular, drinking water, wastewater, and storm-water infrastructure. Sustainable storm-water management focuses on selecting storm-water controls based on an understanding of the problems in local receiving waters that result from runoff discharges. For example, long-term problems associated with accumulations of pollutants in water bodies include sedimentation in conveyance systems and receiving waters, nuisance algal growths, inedible fish, undrinkable water, and shifts to less sensitive aquatic organisms. Short-term problems associated with high pollutant concentrations or frequent high flows (event-related) include swimming beach closures, water quality violations, property damage from increased flooding, and habitat destruction. A wide variety of individual storm-water controls usually must be combined to form a comprehensive wet weather management strategy. Unfortunately, combinations of controls are difficult to analyze. This will require new modeling techniques that can effectively evaluate a wide variety of control practices and land uses, while at the same time ensure that the flood-control objectives also are met. The results of these new models and novel techniques used for storm-water control then can be incorporated into an evaluation of the urban water cycle for a specific service area to determine whether storm-water controls can provide additional benefits such as reduction of potable water use and reduction of sanitary sewer overflow events.     

Design and Hydraulics of a Combined Storm Overflow Structure

Storm overflow structures in combined sewers are used to separate sewage from storm water. For subcritical approach flow, sideweirs are currently used; however, they often behave hydraulically poor. This study was conducted to explore a combined storm overflow structure able to improve the hydraulic performance of sewer sideweirs. Such a structure consists of a low-crested sideweir and a bottom opening just downstream from the sideweir end. First, the hydraulic design procedure is described, and then, based on laboratory experiments and by using the governing flow equations, the main hydraulic features are highlighted. The results indicate that the proposed device has advantageous characteristics in terms of hydraulic efficiency, reliability, and maintenance.     

Supercritical Flow across Sewer Manholes

The hydraulics of supercritical flow across manholes in sewers is explored using systematic experimentation. Due to the expansion at the manhole entrance an in-manhole wave is generated. Further, at the downstream manhole end, flows with a sufficiently large filling ratio impinge on the wall and lead to a so-called swell. In addition, due to shock wave generation in the downstream sewer, a sewer wave is generated. The heights and locations of these three waves were determined in terms of basic hydraulic quantities. More importantly, the capacity of the manhole and the downstream sewer under wave action was quantified. It was found that in order for free surface flow to be maintained the common design standard for sewers with a supercritical approach flow have to be revised. These implications have to be accounted for in future designs.     

Godunov-Type Solutions for Transient Flows in Sewers

This work is part of a long term project which aims at developing a hydraulic model for real-time simulation of unsteady flows in sewers ranging from gravity flows, to partly gravity–partly surcharged flows to fully surcharged flows. The success of this project hinges on the ability of the hydraulic model to handle a wide range of complex boundaries and to provide accurate solutions with the least central processing unit time. This first paper focuses on the development and assessment of two second-order explicit finite-volume Godunov-type schemes (GTS) for unsteady gravity flows in sewers, but with no surcharging. Traditionally, hydraulic transients have been modeled using the method of characteristics (MOC), which is noted for its ability to handle complex boundary conditions (BCs). The two GTS described herein incorporate BCs in a similar manner to the MOC. The accuracy and efficiency of these GTS schemes are investigated using problems whose solution contains features that are relevant to transient flows in sewers such as shock, expansion, and roll waves. The results show that these GTS schemes are significantly faster to execute than the fixed-grid MOC scheme with space-line interpolation, and in some cases, the accuracy produced by the two GTS schemes cannot be matched by the accuracy of the MOC scheme, even when a Courant number close to one and a large number of grids is used. Furthermore, unlike the MOC solutions, which exhibit increasing numerical dissipation with decreasing Courant numbers, the resolution of the shock fronts was maintained by the GTS schemes even for very low Courant numbers (0.001).     

Ferrous Salt Demand for Sulfide Control in Rising Main Sewers: Tests on a Laboratory-Scale Sewer System

The addition of ferrous salts is a commonly used strategy for sulfide control in sewer networks. The Fe2+ dosing requirement in rising main sewers which takes into account of the effect of anaerobic sewer biofilms on the dosing demand is investigated. A laboratory-scale rising main sewer, consisting of four biofilm reactors in series and fed with real sewage, was operated for over 12 months, during which FeCl2 was dosed at several locations and at various dosing rates. The experimental results consistently revealed that approximately 0.7 mol of Fe2+ was required to precipitate sulfide formed from the reduction of 1 mol of sulfate by anaerobic sewer biofilms. This ratio is significantly lower than the ratio expected from reaction stoichiometry (molar ratio of 1:1), and also the Fe2+ to sulfide ratio (1.07–1.10 mol:1 mol) observed in batch tests conducted with real wastewater in the absence of sewer biofilms. Biofilms adapted to Fe2+ addition were found to contain a substantially higher amount of elemental sulfur than biofilms not receiving Fe2+ dosage. This suggests Fe2+ addition might have altered the final product of sulfate reduction by anaerobic sewer biofilms. The study also showed that the addition of ferrous salts at the inlet of a rising main sewer can effectively control sulfide throughout the whole system despite of the presence of competing anions in wastewater. Phosphate precipitation with ferrous iron in anaerobic rising main sewers is negligible.     

Dynamic Model for Subcritical Combining Flows in Channel Junctions

A one-dimensional theoretical model for subcritical flows in combining open channel junctions is developed. Typical examples of these junctions are encountered in urban water treatment plants, irrigation and drainage canals, and natural river systems. The model is based on applying the momentum principle in the streamwise direction to two control volumes in the junction together with overall mass conservation. Given the inflow discharges and the downstream depth, the proposed model solves for each of the upstream depths. The interfacial shear force between the two control volumes, the boundary friction force, and the separation zone shear force downstream of the lateral channel entrance are included. Predictions based on the proposed approach are shown to compare favorably with existing experimental data, previous theories, and conventional junction modeling approaches. The main advantages of the proposed model are that the proposed model does not assume equal upstream depths and that the dynamic treatment of the junction flow is consistent with that of the channel reaches in a network model.     

Bottom Slot Discharge Outlet for Combined Sewer Diversion Structure

The concept of a bottom slot outlet was developed to serve as a combined sewer overflow diversion structure to convey flow to deep tunnel storage under free discharge conditions and pass the remaining volume once storage capacity is exceeded with minimum backwater effects. In order to test the concept, a 1:19.5 scale model was constructed and tested over a range of discharges in order to determine the required slot length to pass a certain discharge. Different bottom slopes and slot widths were tested. For subcritical approach flow, the flow in the structure passed through critical depth at the upstream end of the slot, eliminating the possibility of backwater effects. The results were found to be nearly independent of the bottom slope for typical small slopes in sewer systems, and a simple dimensionless relation was developed to relate the slot length to other parameters. A mathematical model combining continuity and energy relations for the remaining flow in the structure with an orifice relation for flow through the slot was capable of reproducing the observed experimental results.     

Drop in Combined Sewer Manhole for Supercritical Flow

Manholes often contain small drops for various reasons, the most important being submergence. While this may be appropriate for subcritical flow, its effect was considered doubtful for supercritical flow. This note aims at investigating the effect of a manhole drop on the hydraulics of sewer flow. Based on systematic experimental observations, the flow pattern associated with a manhole drop was established. A distinction was also made between small and intermediate drops. Then, the main wave features were analyzed to result in expressions that contain both the upstream filling ratio and the Froude number of the approach flow. In addition, the discharge capacity was also investigated, and selected photographs show typical drop flow in combined sewer manholes. The result of the present study is evident, based on these observations, and recommendations towards future design of combined sewer manholes are also made.