Owing to this knowledge, many authors ( [SteinR08] [Artie1988] [Sande1994] [Bromb1993] [Grott1991] [Giesl1997] [Kraut2000] [Stotz1998] [Somme2004a] [Somme2004] [Fraen2005] [Hilli2005]) recommend the replacement of conventional catch basins with new structures that have an improved cleaning function or solid retention. This improved separation of solids from the flow reduces operational requirements by increasing cleaning intervals.

Requirements concerning improved cleaning function are met by recently developed special catch basin systems described in the following tables.

Table: Structure, function and applications of various systems for decentralized solid retention [Somme2009]

Table: Structure, function and applications of various systems for decentralized solid retention [Somme2009] (Continued)

Table: Structure, function and applications of various systems for decentralized solid retention [Somme2009] (Continued)

The first separation step iconsists of an improved grate design as part of the cast iron frame and grate system “Multitop 500 PF” by the company ACO [FI-ACO].

It is constructed in accordance with DIN EN 124, and includes a shock absorbing riser which disperses traffic energy into the subsoil thus preventing localised settlement.

The second separation step is provided by the use of a waste bucket located just below the catch basin grate in accordance with DIN 4052-4. It has the same function as the bucket for a catch basin with invert outlet, that is, it acts as a sieve with predetermined slits and retains particles greater than 16 mm [DIN4052-4:2006].

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.

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.

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:

• constructive measures to prevent sedimentation;
• procedural principles of these constructive measures.

Pressure lines, that is “lines for the transport of sewage under pressure” are cleaned using

• physical and

• mechanical

methods.

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.

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.

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 …

The following methods are used to remove or to avoid the formation of these deposits:

• High pressure cleaning with ejector nozzles
• Scraper methods
  

Furthermore, there is the possibility to influence the flow velocity by constructive measures, such as:

• Flushing pump systems, and
• Air-cushion inverted siphons.

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)

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.

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%.