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.     

Erosion of Sediment Beds in Sewers: Model Development

The problems created by sediment deposits in combined sewer systems (sanitary and storm) are internationally recognized. The loss in conveyance due to these deposits contributes to hydraulic overloading, leading to flooding and premature operation of combined sewer overflows. The washout of sediments through combined sewer overflows into urban water courses during times of storm and the associated pollution caused by this phenomenon may be a factor affecting many urban ecosystems. Based on field observations, coupled with sampling and analysis of combined sewer sediment deposits, it has been found that in the invert of pipes there is often coarse, loose, granular, predominantly mineral material overlain by a mobile, fine-grained deposit. The erosion of the latter type of deposit is considered to be the source of the “first foul flush” of pollution, which is observed in many sewerage systems in response to storm events. This paper describes an experimental laboratory investigation of the erosion and subsequent suspended sediment transport of an in-pipe, fine-grained, organic, cohesive-like sediment deposit analogous to those found in sewers. The development of the laboratory system, the test program, the results of the study, and the development of a new approach to model the erosion and transport of cohesive-like sediments in pipes are described. Conclusions regarding the importance of the structure of the bed and its erosional behavior under a wide range of time-varying hydraulic conditions are presented.     

European Research into Sewer Sediments and Associated Pollutants and Processes

Research investigating all aspects of solids in sewer systems has been underway in Europe for nearly two decades. Due to the early development of European sewer systems, originally as part of the industrialization process more than 100 years ago, urbanization has caused the original sewer networks to become overloaded and unable to function efficiently. Operational problems of interest include loss of ability to convey (designed) flows and the performance of “overflows” to relieve the high flows discharging directly into rivers and other watercourses. Research has characterized the nature of the solids getting into sewer systems, how they behave in terms of transport, and some of the main aspects of their effects. It has been possible to demonstrate that much of the pollutants found in suspension during storms, and likely to be discharged from overflows, originate from the predominantly organic “near bed solids” which accumulate in systems during dry weather. New ideas for the way in which the sediments are transported and the importance of the transformation processes, are leading toward the development of a unified and integrated understanding of the way in which sewer solids behave and the associated biochemical transformation processes.     

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.     

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.     

Real-Time Detection of Sanitary Sewer Overflows Using Neural Networks and Time Series Analysis

Sanitary sewer overflows (SSOs) are becoming of increasing concern as a health risk. Utilities and regulators have taken preventive measures but many overflows still occur and are not identifiable, especially in access-challenged locations. Several mathematical approaches are presented for detecting if a disruption in the system is impending or occurring based on measurements at one or more locations in the system. Time series analysis and neural networks are used as prediction tools for expected depths and flows for single measurement locations and a neural network is developed for a multiple monitor system. Control limit theory is applied in all cases for identifying significant deviations of measured values from the expected values that suggest a SSO is occurring. Data from Pima County Wastewater Management’s monitoring system are used in two case studies.     

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.     

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.     

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.     

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.     

Global Predictive Real-Time Control of Sewers Allowing Surcharged Flows

A global predictive real-time control strategy minimizing overflow volumes from combined sewers during rainfalls is presented. For an optimal use of controlled sewer transport and storage capacities, the proposed strategy allows surcharged flows. Flows and piezometric heads in the sewer are computed according to flow inputs by a hydraulic simulation model. The optimal operation of the regulators controlling these flow inputs is determined on a finite control horizon using the generalized reduced gradient optimization algorithm. The control strategy was applied to the 23 rain events that occurred during the summer of 1989 on the urban area drained by the Marigot interceptor in Laval, Canada. In this application, the admitted intensity of surcharges was varied to assess this parameter impact on total overflow volumes. A comparison between performances of the proposed strategy and a local reactive control was also carried out. Results obtained indicate that the global predictive control can reduce overflow volumes during rainstorms and that this reduction may be improved by allowing surcharged flows.     

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.     

Power Series Approach to Holistic Sewer System Modeling

Fast and stable mathematical models for the computation of sewer system outflow are essential for real-time control of urban storm drainage systems. This paper presents a general method to design such models based on the reservoir approach. The unknown inflow-outflow function is developed into a power series. The a priori assumptions made for this are much weaker than the ones for the well-known linear models. The resulting models are nonlinear, but they subsume some of the linear models, depending on the selection of model parameters. Thus, a more general approach to validate the frequently made linearity postulation of the outflow process is provided. The new model is tested with some subcatchments of a larger urban storm drainage network.     

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.     

Bacteria Load Estimator Spreadsheet Tool for Modeling Spatial Escherichia coli Loads to an Urban Bayou

The model developed in this paper, the bacteria loading estimator spreadsheet tool (BLEST), was designed as an easy to use indicator bacteria model that can overcome the shortcomings of many of the simpler total maximum daily load (TMDL) modeling approaches by integrating spatial variation into load estimates. BLEST was applied to the Buffalo Bayou watershed in Houston, Texas and incorporated loading from point and nonpoint sources, such as wastewater treatment plants, sanitary sewer overflows, septic systems, storm sewer leaks, runoff, bed sediment resuspension, and direct deposition. The dry weather Escherichia coli load in Buffalo Bayou was estimated using BLEST to be 244 billion MPN/day and would require an overall 48% reduction to meet the contact recreation standard, while wet weather loads would need to be reduced by 99.7%. Dry weather loads were primarily caused by animal direct deposition, septic systems and discharges from storm sewers under dry weather conditions, while wet weather loads were mostly attributable to runoff and resuspension from sediment. Unlike most simple TMDL load allocation strategies, BLEST can be used to evaluate spatially variable load reduction strategies. For example, septic system load reductions implemented in less than 10% of the subwatersheds resulted in a decrease in bayou loading of more than 20%.     

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.     

Automated Sewer and Drainage Flushing Systems in Cambridge, Massachusetts

This paper summarizes the design of passive automatic flushing systems installed in the City of Cambridge’s storm and sanitary sewer system tributary to the Alewife Brook as part of a $75 million sewer separation program. Grit and debris deposition is severe in the existing combined sewers, storm drains, and sanitary trunk sewers due to the flat topography of the area. This condition is exacerbated by hydraulic constraints imposed on the system’s outlet by the Alewife Brook (shallow stream) and downstream sanitary siphons (again because of the Alewife Brook). The use of pumps to lift flows from sewers and drains to permit self-scouring velocities is prohibitively expensive. To overcome this problem, five automated flushing systems using quick opening (hydraulic operated) gates discharging collected stormwater were constructed in conjunction with downstream collector grit pits covering a distance of 1,604 m for storm drain pipes ranging from 1.4 m circular to 1.2 m by 1.8 m rectangular. New 450 and 600 mm sanitary trunk sewers, 561 m long, will be flushed daily by two flushing systems using spent filtrate water from Cambridge’s water treatment plant recently constructed nearby. The flushing systems are sized to achieve wave velocity of 1 m/s at the end of the flushing segment. The flush vault volumes range from 11 to 40?m3 for the storm drain systems and 6?m3 for the sanitary system. Construction was completed in May 2002 and functional testing of the flushing systems is in progress. Partial test results are reported.     

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.     

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.