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


According to [ATVM143-1:1989], the term maintenance is defined as:

Measures for maintaining the nominal condition.

The nominal condition is understood to be the determined (required) condition for the respective case of an installation, a structure or individual components (Abschnitt 2.1) (Abschnitt 5.1).

Maintenance is closely linked with the other two main groups of measures of rehabilitation and maintenance, i.e. the inspection and the rehabilitation. These linkages exist despite the clear differences in their aims, as the borders between maintenance, inspection and rehabilitation are fluid. Maintenance works, for example, are always connected with the finding of peculiarities in the condition of the object and discovering their causes (inspection) when these can be carried out with the means at the disposal of the maintenance personnel.

In the European standards, maintenance, in the sense of EN 752-7 [DINEN752-7], includes a combination of future-oriented, planned, measures and event-dependent reactions that will maintain the system in a condition that satisfactorily ensures its operational capabilities.

Examples of such measures are:

  • Local repairs or replacement of damaged pipes or other components (Abschnitt 5.2) ;
  • Removal of deposits, obstacles, etc. in order to recreate the hydraulic capabilities (Abschnitt 3.2) ;
  • Maintenance of machine equipment (e.g. pumps);
  • Combating rats and insects.

In combination with Operation, that is, the supervision, control or redistribution of the flow of sewage, the following aims are followed in maintenance [DINEN752-7] :

  • Ensuring constant operational readiness and capabilities of the whole system within the scope of the defined requirements;
  • Ensuring a safe environmentally acceptable and economical operation of the system;
  • Ensuring that, in the case of failure of a part of the system, the operational capacity of other parts is affected as little as possible.

This must also ensure that the respective drain and sewer system fulfils the requirements of EN 752-2 [DINEN752-2] such as:

  • "The pipework operates without blocking;
  • The flooding frequencies shall be limited to prescribed values (Table 3.2-1) ;
  • Public health and life shall be safeguarded;
  • The sewer surcharge frequencies should be limited to prescribed values;
  • The health and life of the operating personnel shall be safeguarded;
  • Receiving waters shall be protected from pollution within prescribed limits;
  • Drains and sewers shall not endanger existing structures and utility services;
  • The required design life and structural integrity shall be achieved;
  • Drains and sewers shall be watertight in accordance with testing requirements;
  • Odour nuisance and toxicity do not arise;
  • Appropriate access shall be provided for maintenance purposes."
Table 3.2-1: 

Recommended frequencies of design rainfall and flooding for the design of drain and sewer systems to EN 752-4 [DINEN752-4]

Design storm frequency*)
(1 in "n" years)
Location Design flooding frequency
(1 in "n" years)
1 in 1 Rural areas 1 in 10
1 in 2 Residental areas 1 in 20
City centres and industrial areas:
1 in 2 − with flooding check 1 in 30
1 in 5 − without flooding check
1 in 10 Underground railway⁄underpass 1 in 50
* No surcharge is allowed for these stroms.

A precondition for planned maintenance is the "Establishment of a binding maintenance plan adapted to the specific requirements of the respective operation or the installation" .

Furthermore there is required [DINEN752-7] :

  • Planning;
  • Access rights;
  • Sufficient and competent personnel;
  • Clear allocation of responsibilities;
  • Suitable equipment;
  • Knowledge of the system with its operational linkages and its users;.
  • Sufficient information and inspections, e.g. to [DINEN752-7] :
    • Site plans of the system including plans of the drains and sewers, manholes, pump installations, stormwater overflow, detention tanks, etc.;
    • Detail information on the permitted discharges into the system (trade effluents, dangerous materials, etc.);
    • Detail information on the permitted discharges into the receiving waters (stormwater overflow, pump installations, etc.);
    • Information on system inspections (e.g. sewer television inspection reports);
    • Information on events such as blockages, pipe breaks, failures at pump stations and pressure pipes, flooding;
    • Rainfall information;
    • Information on work carried out within the frame of maintenance;Actual reaction times in the elimination of emergencies;
    • Data on the costs of events and the maintenance for cost control and evaluation of operational efficiency;
    • Data on the hydraulic operational efficiency;
    • Information on the operational efficiency of the system (see section 8 in EN 752-2 [DINEN752-2]).

On the basis of the knowledge of the system, i.e. the sewer network with its structures and installations, as well as the operational linkages, a plan for maintenance based on [ATVA140] must be drawn up by those responsible for the operation of the installations. This will include the determination of the maintenance intervals for the various units under consideration and the establishment of priority lists for maintenance work in special operating conditions, e.g. for storm rainfalls. The duties and areas of responsibility of the personnel involved in the maintenance plan, which, as a rule, is part of a work, operation or service instruction, must be defined for all types of maintenance. The maintenance plan must be updated regularly and adapted to changing limiting conditions.

Changed limiting conditions could be, for instance:

  • Connections of new buildings to the existing sewers;
  • Changes to the quantity and composition of the sewage, e.g. due to the opening up of new industrial areas;
  • Entry of dirt and solids, e.g. in regions of large building constructions, changes of gradients, e.g. in mining subsidence areas;
  • Changes of the operational conditions, e.g. changing a gravity sewer into a pressure pipe.

While carrying out the maintenance works, observations can be made which could necessitate a detailed inspection (Abschnitt 4.1).

These comprise, for instance:

It will depend on the size and organisation of the sewer operation whether the above-mentioned local repairs will be carried out in the course of the maintenance. In any case, information on repairs that are necessary by the maintenance personnel must be passed on.

Because of its prophylactic effect and its high degree of effectiveness, maintenance is one of the most important measures for rehabilitation and maintenance. Relatively easy technical measures are consistently carried out.

Image 3.2-1:  Usefulness of maintenance, depicted with attrition curves of the wear reserve with reference to [Renke84] [Image: S&P GmbH]

The utility of properly carried out maintenance lies not only in ensuring functionality, but also, for instance, in the time displacement of the attrition curve of the wear reserve and thus the extension of the duration of utility of the sewer systems (Image 3.2-1).

The aspects of work safety and health protection for maintenance and inspection (Abschnitt 4.1) and rehabilitation work (Abschnitt 5), must be taken into special consideration as works in the sewage installations such as accessible sewers and manholes cannot be avoided. They present an increased danger potential and are thus classed, according to VBG 1 § 36 (1) [VBG1], as "Dangerous work."

Dangers, according to the safety regulations for work in enclosed spaces of sewage installations [GUVV95], are:

  • "Gases or vapours that can cause fires or explosions;
  • Lack of oxygen that can lead to suffocation;
  • Very poisonous, poisonous or less poisonous (health damaging) substances, that when coming into contact with them, can be taken up by the skin and the mouth or breathed in;
  • Introduction of stronger flows of water, e.g. as a result of heavy rain;
  • Bacteria or forms of life and their metabolic products as well as fouling that can lead to infections" [GUVV95] ; as well as
  • Danger of falling, e.g. open manholes of missing climbing irons, etc.

There are numerous safety regulations in the Federal Republic of Germany for protecting workers from the occupational dangers mentioned above. These are dealt with comprehensively in [Stein98c].

According to EN 752-7 [DINEN752-7], those responsible for work in drains and sewers, including the operators of the drain and sewer system, must ensure that the works pose no risks to the health or safety of the operating personnel or third parties.

Additionally, the employer is also responsible:

  • for work safety, which includes safe entry and exit possibilities into, or out of, the drain and sewer system;
  • for ensuring that the employees are properly acquainted, trained and supervised as regards the work and the safety measures to be carried out.

"The working conditions must take into due consideration all aspects of the work, such as work on the surface (e.g. the positions of the manholes, the traffic regulations), the access to the drain and sewer system and all the works in the system. Details of rescue and evacuation measures in the case of emergency must be available in writing. Measures must also be introduced for the recognition and prevention of sudden inflows of poisonous, easily inflammable or explosive substances, hot fluids or large quantities of water. Special precautionary measures are required for the access into an inverted siphon.

Well-trained operating personnel in sufficient numbers must be utilised in order to ensure that:

  • Help can be summoned from the surface in case of emergency;
  • There is constant connection between the operating personnel at the surface, in the entrance and exit manholes and in the sewer."

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