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A significant source of solids collected from the roof drainage systems originates from the pollution and dust particles deposited by local industries. Areas with heavy industry, such as a coal power plant, have a higher level of solid particle pollution which corresponds to a higher entry of solids into the sewer system [Godeh2002] [ATV96:2007] [Godeh2000]. (Image: Entry of solids through roof drainage) |
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(Video: Erosion behaviour of non-cohesive ground for leaks) Video: Erosion behaviour of non-cohesive ground for leaks with reference to [Jones84a] [Image: S&P GmbH]. This interactive object is only visible in the online version of the module. |
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Extraneous water consists of groundwater infiltration along with unauthorised sources such as falsely connected utilities, e.g. wastewater connection in the storm sewer, seepage from households and construction sites. The amount of extraneous water fluctuates based on the day and season; generally, higher extraneous water flows are experienced in the winter months. (Image: Infiltration) (Image: Soil infiltration and cave-in resulting from a leak in … |
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(Image: Well compacted cohesive soil [Jones84a]) a) Well compacted cohesive soil (Image: Poorly compacted cohesive soil [Jones84]) b) Poorly compacted cohesive soil Image: Erosion of cohesive soils as a result of a leaking gravity sewer located within the ground water zone [Stein00f] [Jones84a]. |
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The entry of solids due to groundwater infiltration or internal stresses at damaged sections (with exposed soil) leads to a build-up of deposits in the sewer channel, this can lead to backups and cave ins of the pipe itself. (Image: Street collapse as a result of a pipe leak) (Video: TV-Befahrung eines Kanals - massive Ablagerungen) |
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Sep 11, 2012 The transport of solids in drainage systems is influenced by a number of interconnected factors. Discussing the most important parts of these processes is done in order to determine, which effects a reduction of transported solids has on the planning of sewer systems and water treatment facilities. |
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Sep 11, 2012 The overall transport of solids in sewers and drains occurs under turbulent flow conditions through:
[SteinR2005] [Glazi1989] (Image: Forms of motion of solids in sewage pipes and channels) |
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Sep 11, 2012 The complex transport processes in a channel can be divided into four transport conditions, which differ in their vertical distribution of the solids in the flow cross section [SteinR2005] [Führb62]. (Image: Concentration profiles of solids in the flow cross-section as a function of the average flow velocity) |
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Sep 11, 2012 The transport conditions are determined with the help of the two measures of concentration, area- or volume concentration (c) and solids- or transport concentration (also known as volume concentration of suspended solids [ATVDVWKA 110:2001]). The average area concentration c is defined as the ratio of solid volume VS to the total volume VG of solids transported in the medium V [Macke82]. Equation 1: (Formula: Konzentrationsverteilung der Feststoffe) |
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Sep 12, 2012 The concentration distribution of the solids through the pipe cross-section is dependent on the characteristics and particularly on the average flow velocity vm. The flow velocity is for example influenced by the following pipe properties:
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Sep 12, 2012 Subsequently, the transport states in relation to the average flow velocity, shown in the figure, are explained further: Transport state a), floating motion is characterized by a high solids concentration. In this phase, the mean flow velocity of the mixture flow (vm) is much greater than that of the minimum required flow velocity (vc) for a deposit free condition. Maintaining this transport condition requires high flow rates [SteinR2005] [Führb62]. |
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Sep 13, 2012 Subsequently, the transport states in relation to the average flow velocity, shown in the figure, are explained further: Transport state b), floating and leaping motion, occurs with the reduction of the average flow velocity, and has a solids concentration profile in the lower flow cross-section, wherein all the solid particles are still in suspension. The resulting irregular or heterogeneous distribution is due to the lower concentration compared … |
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Sep 12, 2012 Subsequently, the transport states in relation to the average flow velocity, shown in the figure, are explained further: Transport state c), floating and sliding motion is characterized by the increased solids concentration at the channel base and the presence of sediment. The solid particles float and slide through the channel without settling [Führb62] [SteinR2005]. (Image: Concentration profiles of solids in the flow cross-section as a function … |
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Sep 12, 2012 Subsequently, the transport states in relation to the average flow velocity, shown in the figure, are explained further: Transport state d), floating and sliding motion over deposits, does not have enough energy to transport all solid particles. The movement of the solids includes the bed load and suspended load, hence forming ripples, dunes and banks on the channel bottom. Deposit formation occurs because the average flow velocity in the channel … |
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Sep 12, 2012 Sediment transport is divided into three basic processes:
[Bollr89] |
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Sep 12, 2012 (Image: Settling speed for a particle) Sedimentation (deposition), dependent on the settling speed of the particles |
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Sep 12, 2012 Erosion and deposition, characterized by motion and rest phases of the particles. Exceeding the "critical" state, characterized by the (marginal) or shear stress τ or also by the critical flow rate Vcrit, results in the beginning of the fluid and sediment transport. The shear stress τ can also be referred to as erosion shear stress. The opposite shear stress brings the moving particles to an idle sate, and is defined by Kreuter ≥ 0,7 * τ, [Bollr89]. … |
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Sep 12, 2012 With increasing turbulence and flow velocity, particles (solids) of varying size and density can be brought into suspension, with a decrease in velocity the particles settle to the bottom of the channel. This approximate relationship can be shown through the FROUDE number of particles equation: Research by [Kress1964] has shown … |
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Sep 12, 2012 Prior to the new edition of the ATV-DVWK-A 110 in 1988, planners assumed that the required shear force of the wastewater is guaranteed if, at partial filling, flow velocity of 0.5 m / s and at full charge of at least 1.0 m / s is achieved. On this basis, and the investigations of Karakassonis, the minimum slope used in practice was determined using the Faust equation Icrit = 1000/DN (Circular pipe with half to full filling τ0=2,5 N/m2) [ATVA110:… |
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Sep 12, 2012 The current approach to the hydraulic dimensioning of partially filled sewers uses a differentiated sediment transport equation: Equation 3: (Formula: hydraulische Dimensionierung von Kanalisationen unter Verwendung einer differenzierten Feststofftransportgleichung in teilgefüllten Rohrleitungen) with: Q = Flow, discharge, inflow, flow rate [m3/s] cT = Solids concentration in the wastewater ρF = Solid density [kg/m3] ρW = Density of the wastewater [… |
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Sep 12, 2012 The settling velocity is centrally important in describing the transport behaviour of undissolved solids as it determines their sedimentation capacity. It is influenced by:
[Glazi1989] |
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Sep 12, 2012 The settling velocity can be described by the Stokes equation of motion for spherical particles. Since the particle differs from a generally spherical shape, the particle diameter used is taken from a sphere with a similar volume [Kalbs97] [Hörni1989] [SteinR2005]. Equation 4: (Formula: Stokes Sinkgeschwindigkeit Bewegungsgleichung für kugelförmige Teilchen) vs = settling velocity [m/s] g = gravitational acceleration [m/s2] ρ = particle density [kg/m3… |
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Sep 12, 2012 According to the study by Macke deposit free states can be reached by a steady flow, when the following conditions at the critical velocity vc are met: Equation 5: (wastewater) (Formula: ablagerungsfreie Zustände bei stationärer Strömung-Schmutzwasser) Equation 6: (combined and rain water) (Formula: ablagerungsfreie Zustände bei stationärer Strömung-Misch- und Regenwasser) [Macke82] The corresponding wall shear stress is determined using the equation: |
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Apr 30, 2018 According to Macke, the wall shear stress should be at least 1.0 N/m2 in order to prevent build up [Macke82]. The design proposal by Macke was adopted by the ATV (today the DWA German Association for Water, Wastewater and Waste) and has been included since 1988 in ATV-A 110 as a table. This also includes the assumptions based on the minimum flow velocity and the minimum gradient depending on the channel diameter [ATVA110:1988]. (Image: Limitations … |
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