Rehabilitation and Maintenance of Drains and Sewers / Prof. Dr.-Ing. D. Stein, Dipl.-Ing. R. Stein (2004)

General

The cleaning of sewer systems is an important part of maintenance.

It is carried out:

  • For removing deposits within the scope of regular maintenance, in order to maintain free flow throughout the whole cross section and to prevent the appearance of smells and gases caused by fouling processes and the creation of biogenic sulphuric acid corrosion;
  • For the removal of blockage;
  • As a preparatory measure for an inspection of the sewer.

Besides the above-mentioned applications within the scope of maintenance, cleaning also serves as a preparation for rehabilitation measures. This results in additional tasks such as intensive cleaning of the inner walls, removal of corrosion products, internally projecting laterals and pipes or other artificial flow obstacles (Abschnitt 2.3.1).

In all measures taken for cleaning, the existing deposits are first loosened, conveyed to a transfer point, e.g. a manhole and there removed, possibly drained and disposed of [Stein98c] [NASSCO89a] [SIA190] [Sulli77] [TASiedl99].

The cleaning waste collected during sewer cleaning consist of:

  • Mineral substances (e.g. sand, stones);
  • Organic substances (e.g. foodstuffs remainders, plastics, paper); and
  • Other substances (e.g. cans, fragments).

This heterogeneous mixture comprises a dried solid content of about 40 %. The organic portion of the dried solid content between 10 % and 40 % .

Especially stable deposits are characterised by a mixture of mineral solids with fouled organic materials as well as by an unevenly grained structure. Tests have shown that solidified deposits can have the same mechanical behaviour as clay with good cohesive properties (difficult cleaning waste).

Sedimentation consisting only of relatively evenly grained sand can, as a rule, be easily flushed out, as the organic material that provides the internal cohesion is not present. In reverse, deposits that consist only of organic materials cannot harden, as the heavy mineral particles, which provide the density of the deposits, are missing.

According to EN 752-2 [DINEN752-7], the removal of tailings from sewer cleaning should take into account the relevant environmental effects and should be carried out in co-operation with the proper authorities (see section 12 in EN 752-4 [DINEN752-4]).

In Germany, according to [TASiedl99], "A processing and evaluation of the cleaning wastes (grit chamber and grease trap tailings, screenings, retained material from culverts, sewer systems and gully cleaning) should be attempted. Insofar as this is not possible, this waste should be sent for further disposal."

In the future, dumping will only be permissible for organic dried solid contents (measured as loss on ignition) or less than 3 or 5 mass %. All waste that does not measure up to these values must be burnt before dumping in order for the values to be below the limiting values. The transition period for the introduction of these regulations will be determined by the responsible waste department officials (see also [Stein98c]).

At present, there are no technical guidelines for carrying out sewer cleaning or which provide information for the user when cleaning is to take place, what price / performance relationship is justified and what energy or material input is necessary for the respective purposes.

As regards the cleaning intervals, [ATVA147-1] states, "The frequency of sewer and manhole cleaning depends on a number of factors, e.g. the type of drainage method, the gradient and flow behaviour, the type of deposits and the back-up behaviour. Furthermore, account must be taken of the effects on the sewage treatment plant operation or on keeping water clean. Experience has shown that cleaning intervals can lie between 2/year and 0.1/year. With advantageous conditions without deposits, cleaning can be dispensed with entirely. For small profiles up to DN 800, an area-oriented determination of cleaning intervals could be useful (System cleaning). For larger profiles, it is recommended first to measure the height of the deposits (Abschnitt 4.3.2.2) and then to establish a cleaning programme.

The following can be assumed in the system cleaning for working out the operating costs:

Frequency : 0.33/year."

The following cleaning parameters, for instance, should be considered in order also to regularly update and adapt the cleaning planning within the framework of the maintenance planning to the alterations of the limiting conditions:

  • Sewer diameter;
  • Length of section of the sewer;
  • Depth of manhole;
  • Accessibility of the manholes;
  • Flow obstacles;
  • Damage;
  • Cleaned section;
  • Time taken for cleaning;
  • Type of cleaning vehicle;
  • Number of vehicles;
  • Number of personnel;
  • Backwater conditions
  • Aims of cleaning.

"The cleaning performance must be laid down clearly and in detail in the Leistungsverzeichnis (specification sheet) when cleaning is ordered. This includes descriptions of height of deposits, type and consistency, manholes not accessible to vehicles (flushing hose length) and other expected difficulties (e.g. traffic conditions).

The cleaning specification sheet must also provide an opportunity for regulating the water source for the cleaning vehicles (streams, open waters, hydrants, etc.) as well as the disposal of the cleaning wastes. The performance of the outside company must be supervised and certified [ATVAG17397]."

Examples of the Leistungsbeschreibung (contract requirements) for cleaning drains and sewers are contained in the "Standardleistungsbuch für das Bauwesen" (standard service record of the building industry) [LB309] (Abschnitt 5.6.3.2).

Before beginning the cleaning work, information should be obtained on the structural condition of the section of the sewer in order to prevent increase in damage due to cleaning in the already damaged sewers (e.g. wear, corrosion, cracks, pipe fracture, pipe breaks) up to collapse.

During the cleaning process, the collected flushing material should be constantly monitored, if possible. Large portions of clean particles of earth and fragments of pipe are signs of strong damage, e.g. for pipe fracture, pipe break or collapse. In such cases, the cleaning process must be stopped immediately and safety measures taken in hand. A gentler method should be instituted for cleaning.

The effort expended in cleaning is dependent on the type and extent of the loose deposits (Abschnitt 2.3.2.1) and the flow obstacles as well as on the purpose of cleaning or the required degree of cleaning.

The following should additionally be considered in the choice of a suitable cleaning process or apparatus [NASSCO89a] [Sulli77] :

  • Possibility of access to the sewer;
  • Coverage height;
  • Cross sectional shape and sizes of the sewer;
  • Cross sectional changes or displacements within a section of a sewer;
  • Pipe material;
  • Structural condition;
  • Weather conditions (rain, snow, frost), especially for rainwater or combined wastewater sewers;
  • Traffic conditions.

The cleaning processes mostly used can be divided into [Führb80] :

  • Flushing processes;
  • High pressure water jetting processes (HP water jetting processes);
  • Mechanical processes
  • Other processes.

In cleaning sewer systems, the corresponding regulations must be adhered to.

Rehabilitation and Maintenance of Drains and Sewers / Prof. Dr.-Ing. D. Stein, Dipl.-Ing. R. Stein (2004)