Decentralised solid retention is defined to include “measures for the retention of solids in the place of their formation before the sewage is fed into the sewage system”.

In the opinion of many authors ( [Grott1991] [Giesl1997] [Kraut2000] [Stotz1998] [Somme2004a] [Somme2004] [Fraen2005] [Hilli2005] [SteinR05]) the runoff entering the sewage system from the surface via catch basins is the main source of mineral solids resulting in the formation of …

Catch basins with no sump and a direct invert discharge into the sewer line should use a perforated waste bucket as a sieve retaining coarse debris and floatables, before they enter the sewer system. Any particles that can pass through the waste bucket openings are able to reach the main sewer line via the lateral catch basin line [DIN4052:2006].

Catch basins with a sump retain solids in the sump space located between the bottom of the structure and the lateral invert. The disadvantage of this system is that floatables such as leaves cannot be retained and end up in the sewer system. This is also the case with debris in the sump which gets disturbed during heavy storm events [DIN4052:2006].

Catch basins which are currently in use are not appropriate for the targeted, let alone entire retention of the settleable solids carried along in the surface water. As shown in laboratory and in-situ trials run by Stein, as well as according to the experience of numerous sewer network operators, their performance capacity is very limited [SteinR08].

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.