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A sump serves as the third separation step. All settleable solids that were not retained in the first two steps are retained here, even in the event of a heavy storm. (Image: Separation catch basin - 3rd separation step) (Image: Separation catch basin - sump) |
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The core component of the separation catch basin is an insert used to funnel the surface water between separation steps 2 and 3 and for a controlled reduction of energy in the silt trap. The insert funnels the incoming water and discharges it through a downpipe into the sump (silt trap). In the process, the water stream is circularly dispersed by a deflection plate which minimises the disturbance of the solids that have already settled in the sump. |
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In a performance comparison with other catch basins, the separation catch basin achieved the best solids retention although its maximum performance limit was not yet met. (Image: Relative solid retention [%] of the catch basin types tested in-situ, assuming one cleaning measure per year) |
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Jul 23, 2012 This module deals with the topics of semi-centralized and decentralized sedimentation of solids. One focus is on decentralized sedimentation using road catch basins. The different designs and modes of operation of these structures are explained and their performance is demonstrated. After completing this module, you will have knowledge regarding:
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Pressure lines, that is “lines for the transport of sewage under pressure” are cleaned using
methods. |
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Physical methods aim at an increase of the sewage flow velocity for cleaning purposes by the addition of pressurised air. Flushing is performed two to four times a day, or more often if required, by blowing air into the pressure pipe system. Thus, the flow cross section in the pressure pipelines is decreased. Due to the increased sewage velocity, deposits are stirred up and transported further downstream. |
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(Image: Pipe scrapers used for cleaning purposes) Mechanical cleaning of pressurised lines uses available methods and technology from the gas and water sectors, for example, the so.called "pipe pigs" [Führb1980] [Arsco1979] [Jenki1968] [Jordan] [Walsk1984]. |
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The pipe pig closes the cross section of the line and is either forced through the line to be cleaned by the sewer flow, or through the addition of pressure (water or pressurised air). Inserting and removing the pipe pig is performed using locks (pig traps). This procedure requires fittings, such as ball valves and isolating valves that can entirely open and close the cross section of the pipe. (Image: Operational principle of a pipe pig) (Image: Operational … |
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According to DIN EN 752, an inverted siphon is a section of a depressed sewer or gravity sewer, which operates under pressure as it is installed lower than the upstream, and the downstream sections of the pipe. It is a system used to pass under obstacles such as streams or structures [EN752:2008]. When inverted siphons discharge wastewater and/or stormwater, they usually cannot be operated entirely free of agglomerations and deposits [Loren2003]. The … |
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The following methods are used to remove or to avoid the formation of these deposits:
Furthermore, there is the possibility to influence the flow velocity by constructive measures, such as:
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The principle of the slush pump system is based on a regular increase of the flow velocity to mobilise deposits. According to the physical law of communicating vessels, the water head for an inverted siphon in a gravity line without flow is equal on both sides of the siphon, i.e. the associated pressure or energy curve is horizontal. (Video: Slush pump system - Inverted siphon) |
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The air cushion inverted siphon [DWAA112:2007] differs from a conventional one owing to an effective cross section within the inverted siphon (filling height or partial filling level) that is reduced by an air cushion. Thus, the flow velocity can be increased [Hosan1998]. This air cushion is formed by a controlled feed of air into the line. To prevent a discharge of this air, a siphon is placed both in the inlet and outlet area of the inverted siphon. |
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Currently, high pressure cleaning is the most commonly used method to clean sewers. According to a survey by Stein in 2000, the use of high pressure cleaning in North Rhine-Westfalia (NRW), Germany, amounted to more than 95% of overall cleaning measures [SteinD00a]. Throughout Germany, this method was used in about 90% of all cases [Geib2002]. Hence the use of other cleaning methods in Germany is not more than 10%. |
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(Table: Recommended fields of application for cleaning methods in accordance with [Baren2006][Dettm2005a]) |
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Jul 23, 2012 Special conditions apply to the cleaning of pressure pipes. This module is therefore explicitly dedicated to the procedures and methods used for cleaning this type of pipe. After completing this module, you will have knowledge of:
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There exist numerous methods for the cleaning of drainage systems. They are applied based on the cleaning objective and the structure specific, local, structural and hydraulic limiting conditions. (Image: Overview of the drain and sewer cleaning methods as per [Führb1980]) |
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Selection of cleaning measures must be in accordance with [DIN EN 14654-1] and should also take into account the technical feasibility and economic viability, along with the following boundary conditions:
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Flushing is considered to be one of the oldest sewer cleaning methods [Frühl10] [Gürsc21] and is divided into:
(Image: Schematic representation of a flush gate) (Image: Flushing shield) Both methods can be used to remove loose unconsolidated deposits in sewer lines with an unobstructed free flow. The flushing systems can either be installed permanently or temporarily. |
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According to [EN752:2008] and [DIN EN 14654-1] surge flushing is defined as “the creation of a temporarily increased discharge to remove any obstacles or deposits in drains or sewers channels.” (Image: Tiber Creek Sewer Flush Gates) |
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The surge flushing process involves temporarily backing up sewage or fresh water upstream from the section to be cleaned using specially developed flushing tanks or flushing manholes (figure). When the backed up flow volume is sufficient, the permanent or temporary flushing devices are activated and the flushing gates are abruptly opened. The wave front that is formed by the abrupt release of water stirs up the unconsolidated deposits and transports … |
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From a physical point of view, potential energy (by backing up the sewage) is transformed into kinetic energy (by a sudden release of the backwater) during surge flushing. During that energy transformation high turbulent flows and shear stresses can occur. When the surge wave has lost the required kinetic energy (transport energy), the solids will settle again [Haußm99]. (Image: Flushing chamber schematic - initial stage (Biogest-System)) (Image: Retention … |
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The flow conditions in a surge wave and their transport effect were researched by Brombach (figure). They are characterised by a distinctive secondary flow with a steady roll at the core of the wave, and heavy turbulences at the waves front and back. (Image: Secondary flows in a surge wave according to [Brombach]) |
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Dettmar characterises the wave family created during surge flushing into so-called bed load and suspension waves [Dettm2005a].
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The efficiency of surge flushing is influenced by the following factors:
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The effective range of surge flushing can be several hundred meters depending on the local conditions and the desired cleaning effect [Frühl1910]. Under favourable conditions, as in a straight section of the line with insignificant ponding, flushing lengths of more than 1,000 m can be achieved [Dettm2004a].
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