Evaluation of Sediment Removal Efficiency of Flushing Devices Regarding Sewer System Characteristics

Source: ICHE Conference Archive - https://mdi-de.baw.de/icheArchiv

Application of Flushing Devices in Sewer Systems (Toepassing van systemen voor de spoeling van sedimenten in rioleringen)

Prevention of sewer sediment accumulation, which can cause major sewer operational problems, is imperative and has been an immense concern for sewer designers. So far, researchers have mostly attempted to investigate the conditions where sewer systems can operate under self-cleansing condition, this way neglecting the essential attention that has to be addressed to sediment deposition patterns which occur in sewer reaches under various sewer geometrical and hydraulic conditions. To account for the effects of sediment formation in sewer systems, an appropriate sediment transport modelling with the ability to determine the location and depth of sediment deposit is needed. Besides, in order to prescribe proper solutions for the removal of the deposited layers these models need to appropriately analyse the in-sewer sediment transport patterns.In this regards, a validated numerical model incorporating shock-capturing TVD-McCormack scheme, developed in the University of Catania, Italy, is utilized in this research to help determine the sediment accumulation patterns in rectangular sewer channels, and to evaluate the sediment transport capacities that can be reached by implementing flushing devices and in particular the Giehl flushing tank (manufactured in Germany). Initially, the simulations are carried out to obtain the sediment bed deposition along the rectangular flumes (with geometries close to sewers) in stormwater sewer conditions (specifying a particular dry-weather period). This is followed by simulating the sediment heights after erosion by successive flushes in order to inspect the eroded sediment volume from the deposited bed and the cleaned channel lengths achieved by means of the applied flushes. Moreover, the sediment entrainment and bed-load transport due to flushing is also investigated in sewer channels with uniform bed layers under stormwater and combined sewer environments. It is discovered that in channels with widths up to 500mm and sufficient bed slope (e.g. 20 , which might be the case in quite a number of stormwater sewer networks) 20 to 50 flushes by Giehl flushing tanks can lead to the erosion of deposited sediment layers with a length between 50 and 100m. Besides, 50 successive flushes demonstrated sufficient capacity to completely erode up to (and possibly more than) a length of 20m of a 4cm thick uniform bed layer composed of medium grit particles, d50≤0.5mm, in stormwater sewer channels with widths up to 250mm with minimum inverted slope of 8 or higher. Under the same conditions, a bed composed of coarser particles, d50≤0.8mm, only the upper 2.5cm of the 4cm thick bed will erode. Under combined sewer conditions, 50 successive flushes will erode over a length of at least 20m a 5cm uniform bed layer composed of fine combined sewer sediment, d50≤0.08mm, ρs≤1600 kg/m3, in channels with widths up to 500mm with inverted slope of 5 or higher. In channels of 100m length or longer, and considering an inverted slope in the range 2 to 8 , 50 successive flushes seem to be deficient in scouring a 2.5cm thick sediment bed. In general terms it is discovered that Giehl flushing tanks are able to remove over a distance of 50m the uniform sediment layers with depths below 4cm in combined sewers with widths smaller than 250mm.In a subsequent investigation, the InfoWorks CS software (Innovyze Co., UK) is utilized to evaluate the efficiency of the flushes from Giehl flushing tanks in increasing the required sediment scouring forces, particularly the bottom shear stresses, in sewer pipes. Initially, the effect of the flushes in long pipe-series is assessed so that an intuition concerning the propagation of the flushes through long distances together with the corresponding shear stress enhancements can be attained. After that, the increase in bottom shear stresses by the flushes along 20, 50, and 100-m channels is inspected. It is detected that in combined/stormwater sewers with diameters of 250 to 500 mm and inverted slopes smaller than 8 , and considering a sediment-cleansing threshold of 3.385 N/m2, the effective flushing lengths are limited to 20 to 42m (respectively in wider and narrower channels). In sanitary networks (considering a self-cleansing threshold of 2 N/m2), the sediment-cleansing efficiency is more discernible and given an inverted slope of maximum 8 the flushing length varies between 100m in narrow channels down to 54m in wider channels. In brief, Giehl flushing tanks are believed to be suitable devices for generating self-cleansing hydraulic forces up to a distance of 50m in narrow (i.e. width up to 250mm) combined/stormwater sewers, even exceeding 100m in narrow sanitary sewers with moderate inverted slopes of maximum 8 .Subsequently, through a novel attempt the most appropriate position of this flushing device in sewer networks for optimum efficiency in terms of self-cleansing maintenance is investigated with the application to a case study, i.e. sewer network of Erpe-Mere in Flanders, Belgium. In the latter attempt, the sediment deposition and erosion is modelled using InfoWorks CS with the double aim to investigate the effect of the flushes on the scour of deposited sediment beds and to discover the lengths along which a specific self-cleansing criterion can be maintained by the discharged flushes. The possible negative impacts in terms of sewer surcharging or manhole flooding is also inspected. The self-cleansing shear stresses were achieved in 61 sewers or in 69.3% of the pipes suitable for the installation of a flushing tank, covering a total cleansing length of 5788 m, representing 74.4% of total length of the pipes apt to the implementation of flushing tanks. It is deduced that flushing tanks cannot always guarantee self-cleansing prevalence in all parts of a sewer network, even after increasing the number of flushing tanks particularly in locations prone to sedimentation. Moreover, Giehl flushing tanks do not guarantee the removal of sediments from places susceptible for sedimentation. For these locations in order to prevent the accumulation of sediments and once deposited for theirerosion the effectiveness of a Giehl flushing tank has to be assessed separately, using an appropriate sediment transport modelling tool. In this accordance, it is agreed that InfoWorks CS is incapable of properly simulating the sediment bed development (deposition phase) and the change in sediment heights during flushing operations in sewer channels.In the end, through an unexampled attempt the two numerical models, i.e. InfoWorks CS being a commercial software and the Catania model as a specialized research-based numerical model, developed for different type of objectives and circumstances, are compared for identical channels with respect to the simulated hydraulic components and sediment deposition patters. Accordingly, the application of the Giehl flushing tank in this sewer network is evaluated with reference to the sediment deposition results achieved earlier by the Catania numerical model, so that by learning the potential spots with the highest sediment deposition likelihood the implementation of these flushing tanks can be optimized. This would require sediment transport and hydraulic modelling schemes to be integrated.status: publishe

Application of flushing tanks in simple sewer networks for in-sewer sediment erosion and transport

Assessing in-sewer sediment accumulation, which causes problems such as loss of hydraulic capacity of sewers and pollution consequences when re-suspended by peak flows, is becoming more in concern and is important in urban drainage design and maintenance. An evaluation based on simulations carried out with InfoWorks CS regarding application of a flushing tank as a tool for eroding deposited sediments from a simple sewer network is presented. The hydrodynamic modelling is comprised of implementing InfoWorks CS (Wallingford Software, UK) to assess the eroding capability of the generated flush waves regarding sediment removal and transport, applying the model based on the shear stress estimation in the software (the KUL model developed by Bouteligier et al., 2002). The simulations were initially done for a simple network composed of one straight conduit partitioned into 5 pipes with lengths equal to 10 m. Various combinations of pipe diameter, pipe slope, sediment characteristics, and DWF in the network was considered. Regarding the dry weather sediment build-up modelling, it was important to reach an equilibrium condition before any implementation of flushing tanks would be considered. Initially, the effect of flush waves emitted from one flushing tank implemented at the most upstream manhole on sediment removal was analysed. Various flushing events were assumed (i.e. 1 flush, 7 flushes with 5 min intervals, and 10 flushes with 10 min intervals). After the flushing events occurred in the network, their effects on sediment transport were assessed. Whenever the upstream flushing tank was found incapable of generating required criterion for sediment erosion and transport through the network, more flush tanks were proposed to be applied in downstream manholes. Next, having implemented more flushing tanks in potentially effective locations, the results for sediment erosion and transport in the network was analysed and the positive and negative outcomes of such applications were assessed. Afterwards, in the second phase, few more pipes (as branches) were added to the initial network and the same analyses were repeated to evaluate the overall effects of added parts on sediment build-up and transport. The final aim was to evaluate whether by implementing flushing tanks in such a simple network, erosion of the settled particles and removing them out of the system could be reached. Whether a flushing tank was efficient in removing and transporting sediments from a part of the sewer network was related to various parameters such as sediment characteristics and the flush interval. In fact, it was understood that the type of sediments and their characteristics (mainly the particle size and density) and other parameters with relevant effects on the overall hydraulic characteristics of the sewer network need to be carefully considered in sediment transport modelling in order to reach to suitable modelling results. In an overall perspective, the capability of such flushing tanks to produce effective forces for removal of the settled particles in sewer pipes is well accepted.status: publishe

Evaluation of Sediment Removal Efficiency of Flushing Devices Regarding Sewer System Characteristics

Flushing devices are considered to be effective in removing settled particles from urban drainage networks and are becoming more frequently used. Modelling analyses with InfoWorks CS software were carried out to investigate the influence of sewer network design and sediment characteristics on the efficiency of a certain type of flushing device. The simulations were done for a sewer network composed of a series of connected pipes with identical diameters with a flushing device installed at the upstream part of the network and for various combinations of pipe diameters (300 mm & 400 mm), pipe slopes (2 mm/m & 3 mm/m), sediment particles (d50 equal to 0.2 and 0.3 mm), sediment concentration (50 mg/l & 100 mg/l), and dry weather flow (DWF) (0.005 m3/s & 0.008 m3/s). The DWF, sediment particle size and the slope of the sewer pipes have major effect in modifying the sediment bed and in predicting the sediment bed formation. Besides, the effect of flush intervals has proved to be an influencing parameter.status: publishe

Spoelputten als systemen om afzettingen in riolen te vermijden

Afzettingen in riolen kunnen leiden tot allerhande problemen (onder meer vermindering van de hydraulische sectie en verhoging van het vervuilingspotentieel ten gevolge van heropwoeling en uitspoeling van de afzettingen). Vandaar dat afzettingen in het riool best vermeden worden. Het is echter niet altijd mogelijk om vanuit het ontwerp hiermee rekening te houden. Bij de bouw van nieuwe woonwijken, waarbij het regenwater gescheiden gehouden wordt van het afvalwater, kan men gravitaire vuilwaterleidingen vaak slechts met een beperkte helling (kleiner dan diegene vereist opdat de leiding zelfreinigend zou zijn) aanleggen. De inzet van spoelinrichtingen die een gecontroleerde spoelstoot kunnen genereren, zouden een oplossing kunnen bieden om afzettingen tegen te gaan. In voorliggend artikel worden de resultaten toegelicht van twee studies rond de spoelput van het type ‘Giehl’: één uitgevoerd aan de Fachhochschule Münster (Uphoff, 2004) en één uitgevoerd aan het Laboratorium voor Hydraulica van de Katholieke Universiteit Leuven (o.a. Shirazi, 2004). Aan de Fachhochschule Münster werd er experimenteel onderzoek gedaan naar de efficiëntie van de spoelput. De opzet was om na te gaan of er inderdaad een golfbeweging wordt geïnduceerd en om de spoellengte in functie van de helling en het type bezonken materiaal te bepalen. Het uiteindelijke doel was te komen tot een nieuwe, aangepaste dimensioneringsgrafiek. Uit de proeven blijkt dat het spoeleffect sterk afhankelijk is van de zandfractie die inspoelt. Bij vuilwaterstelsels is deze fractie meestal klein en worden er belangrijke spoellengtes en goede spoeleffecten verkregen. Dit werd tevens bevestigd door metingen van het spoeleffect op een bestaande rioolleiding in het Duitse Ochtrup. In tegenstelling tot het onderzoek dat werd uitgevoerd aan de Fachhochschule Münster, werd er bij het onderzoek aan de K.U.Leuven niet gewerkt met vervangingsmaterialen, om dan zo het spoeleffect empirisch te beschrijven. Bij het onderzoek aan de K.U.Leuven werd er in eerste instantie aandacht besteed aan het hydraulisch functioneren van de spoelput (welk spoeldebiet wordt er gerealiseerd?). In een tweede fase werd er met behulp van de InfoWorks CS software (Wallingford Software, Verenigd Koninkrijk) het gevonden spoeldebiet opgelegd aan een leiding, om zo de uit het spoeldebiet resulterende schuifspanningen te berekenen en dit voor verschillende leidingdiameters en bodemhellingen. De modelresultaten tonen aan dat, naast de diameter en de helling van de leiding, ook de tijdsinterval, gedurende hetwelk een bepaalde schuifspanning wordt overschreden, van belang is bij de analyse van het spoeleffect.status: publishe

Modelling the erosive effects of sewer flushing using different sediment transport formulae

A numerical investigation to simulate the cleaning effects of successive flushes over sediment beds in prismatic channels is presented in this paper. The 1D De Saint Venant-Exner equations were used to describe the temporal evolution of the sediment bed after each flush. The predictive capacity of two sediment transport formulae was explored against experimental results from laboratory tests. Results show that the adopted model can successfully describe the evolution of the sediment bed due to the flushes exerted during the experiments, with differences between the used transport formulae depending on the channel invert slope and on the flush energy.status: publishe

Implementation of flushing tanks in combined sewer networks to comply with optimal self-cleansing properties

Combined sewer networks are designed to collect and transport the aqueous and solid wastes originating from domestic, industrial and catchment surfaces for treatment and disposal. One of the problems that arise is sedimentation of the particles within the collected water while flowing throughout such networks. In fact, many sewer pipes in combined sewer systems experience considerable fluctuations in flows, ranging from high flow during short-term storm events to longer periods of much lower dry weather flows. In low flow periods sewers experience a characteristic diurnal flow variation. In such periods when the pipe filling level is very low, minimum critical velocities might not be satisfied (Bertrand-Krajewski, 2002). Thus, deposition generally occurs during these periods and also during decelerating flows when storm runoff is receding. Although the flow of surface runoff into the sewer network generates considerable rates of shear stresses, this does not guarantee proper sediment transport in downstream sewer pipes due to lack of enough strength of the flow to constantly produce the required shear stresses. Hence deposition is likely to occur, which can generate problems such as hydraulic overloading due to a reduction in flow capacity and increase of the risk of surcharging during storm events. This could result in both surficial flooding and unwanted early operation of combined sewer overflows (CSOs). Thus, the issue of designing sewer systems to be self-cleansing becomes important. This is however not always promising, particularly in flat regions, where the necessary slopes for sewer pipes to be self-cleansing are not available (especially in the most upstream parts of the network) due to the costs of required deep excavations and pumping systems. In this regard, the use of flushing devices that generate controlled flush waves into the downstream sewer system once the tank is completely filled with runoff water could be a proper solution. The effect of such devices as external sources to help remove the settled particles from sewer pipes has been investigated by many researchers (Bertrand-Krajewski et al, 2005; Bouteligier et al, 2006; Campisano et al, 2004; Dettmar et al, 2002). In the paper an evaluation of the proper implementation of flushing tanks for eroding sediments from a combined sewer network in the village Erpe-Mere in Flanders, Belgium, is presented based on experiments and numerical simulations carried out at the Hydraulics Laboratory of the Katholieke Universiteit Leuven (K.U.Leuven). Implementation of these flushing tanks in different locations of a sewer network is studied with regard to subsequent modifications of hydrodynamic components of the flow throughout the network (shear stress, flow discharge, flow velocity). The research takes account of the hydraulic characteristics of the flushing tank (released flow rate as a function of time and the specific effects on in-sewer sediment transport). The methodology consists of utilizing version 7.5 of InfoWorks CS (Wallingford Software, UK) in order to calculate the resulting spatially distributed shear stresses as a function of the pipe diameters and slopes to evaluate eroding capabilities of the generated flush waves in an existing combined sewer network. Emphasis is given on verifying whether the simulation-based shear stresses throughout the sewer network satisfy the required self-cleansing conditions reasonably, i.e. generated shear stresses remain high enough for a small time interval. Of particular concern is the proper location of multiple flushing devices all over the combined sewer network with respect to the potential degree of sediment removal and transport, and the possible drawbacks which could occur during such flushing events such as modification of the cross section of the flow and consequent effects on sewer surcharging or flooding. In fact, there are various influencing parameters engaged in the proper installation of these devices such as sewer network characteristics, contributing catchment characteristics, etc. The evaluation results indicate that attaining a minimum shear stress criterion (e.g. 3 N/m2 for combined sewer networks based on the Flemish sewer design requirements) could not be satisfied in all parts of the considered sewer network and even some undesired effects emerged. Nevertheless, regarding the accomplished modelling analyses, the capability of such devices to produce effective forces for removal of the settled particles in combined sewer networks is well accounted for.status: publishe

Preliminary results of investigating proper location of flushing tanks in combined sewer networks for optimum effect

Accumulation of in-sewer sediments often causes problems such as loss of hydraulic capacity of sewers. Designing sewer systems to be self-cleansing is however not always possible due to the prevailing conditions. The use of flushing devices that generate controlled flush waves could be an appropriate solution. In the paper the effect of such devices to remove the settled particles from sewer pipes has been investigated. The research focuses on the hydraulic characteristics of the flushing tank and evaluates eroding capabilities of the generated flush waves utilizing the InfoWorks CS model (Wallingford Software, UK). Emphasis is given to simulation-based assessment of spatially distributed shear stresses throughout the sewer network and the specific effects on in-sewer sediment transport. Appropriate implementation of these flushing devices in various locations of a sewer network and their impact over sediment bed modifications is studied. The sediment characteristics influence the overall effect of flushes in the network. The results indicate that attaining the self-cleansing criterion could not be satisfied in all parts of the considered sewer network. However, the capability of such devices for sediment removal in sewer networks is well accounted for.status: publishe