In the given case of application, the following factors influence the flushing-induced wear:

• Pipe material
• The geometrical design of the discharge cross-section
• Stress condition or strain of the pipe material
• The density of the water-solids-mixture
• Composition of the water-solids-mixture
• Type, size, form, viscosity, and distribution of the solid particles
• The contact angle between solid particles and pipe wall
• Flow velocity
• Type of flow (…

For a long time, a possibility to reduce the wear stress in drains and sewers was seen in the limitation of the flow velocity. In 1965, for example, permissible maximum flow velocities were specified as follows [Busch65]:

• Wastewater sewers of the separate system
  v_max = 2.5 m/s to 3 m/s
  
• Combined and surface water sewers (due to the relatively rare peak load)
  v_max = 3 m/s to 4 m/s (concrete pipes)
  v_max = 4 m/s to 5 m/s (spun concrete and vitrified …

The abrasion resistance proof for drain and sewer pipes is done in the laboratory utilising a mechanical rocking abrasion device. For this test, a section of pipe containing a slurry of pea gravel capped at both ends is placed in the rocking device causing the aggregate to slide back and forth within the pipe. The generally used wear indicator is the reduction of the wall thickness of the body prone to wear (sewage pipe) that is specified as the …

During their service life, stresses can occur in sewers which cause droplet erosion, particularly in the inlet areas of drains and connection sewers due to the impact of inflowing sewage onto the sewer wall on the opposite side of the connection and in drop structures.

To some extent, the droplet erosion is a “reversion” of the cavitation erosion which is described in the following [Czich2010].

How does cavitation erosion occur?

When water flows at a higher velocity in parallel to a boundary surface, each geometrical change of that area will create a change of the flow and thus, also create locally limited negative pressure areas.

If, in the process, the static negative pressure of the flowing water falls below the vapour pressure, water vapour-filled bubbles are formed. Given that these bubbles enter areas where the static pressure exceeds …

The development of cavitation and its forms of wear is primarily dependent on the flow velocity, the geometrical design of the discharge cross-section, and the properties of the pipe material.

With regard to the material properties, the degree of cavitation erosion decreases with an increase in:

• pressure resistance,

• bending tensile strength,

• E-Modulus, and/or

• adhesion between the filler and the adhesive agent.

In the given case of application, internal corrosion is defined as the corrosion of the interior surfaces of drains and sewers. It can occur both partially (e.g. in the pipe joint area) and spread across sewer sections.

For sewer systems, corrosion can be described as a reaction in sewer systems between a non-metallic or metallic construction material and its vicinity, which, due to chemical, electro-chemical, or microbiological processes, leads to …

Internal corrosion of cement-bound pipes and structures can occur in the following areas:

(Table: Type of and standard values for corrosion stresses acting on cement-bound material in sewer systems caused by municipal sewage in accordance with DWA-M 168)

(Table: Limiting values for the exposure classes XA under chemical attacks due to groundwater or bodies of water of a predominantly natural composition in accordance with DIN 4030 and Schießl et al.)

The concrete and reinforced concrete pipes for sewers produced today fully meet the requirements contained in the following table. Furthermore, an expansion attack caused by sulphate can be counteracted by using cement with a high sulphate resistance (HS cement) or a mixture of cement and fly ash [Kampe2011].

In order to avoid internal corrosion, no substances may be discharged into a public sewage system that attack structural and other materials …

The table provides an overview of the type of attack and the guideline values for corrosion stresses caused by municipal wastewater for cementitious materials in wastewater systems.

(Table: Type of and standard values for corrosion stresses acting on cement-bound material in sewer systems caused by municipal sewage in accordance with DWA-M 168)

The category “very aggressive” requires slight concrete modifications that are essentially limited to the type of cement and the concrete cover of the reinforcement.

Special corrosion protection measures, such as coatings, linings, or further concrete modifications that serve the purpose to provide the pipe with the required resistance [Stein87b] are only required in a constantly “chemically very aggressive vicinity”.

Industrial wastewater that contains …

A special type of internal corrosion due to microbiological processes is biogenic sulphuric acid corrosion (BSC) in partially filled sewers made of cement-bound materials (also called sulphide corrosion or biogenic acid corrosion).

Whereas corrosion due to direct effects of aggressive wastewater exclusively affects the lower part of the pipe (water-wetted space), the effects of biogenic sulphuric acid corrosion (i.e. microbiological processes) are …

(Image: Principle of the biogenic sulphuric acid corrosion in accordance with Bock)

The proteins contained in the wastewater as well as in the sediments (the reductive part of the pipeline) are degraded by microbial processes under anaerobic or aerobic conditions to volatile sulphur compounds (mainly hydrogen sulphide). In addition, bacterial metabolism under anaerobic conditions can reduce sulphates to hydrogen sulphide (desulphurisation).

The main …

[Biele87a] distiguishes between different types of BSC caused by biologically formed (biogenic) sulphuric acid, which can occur both individually and in combination:

• Endogenous form – The cause of the BSC lies within the sewer system. There is a distinction between:
  • the autogenous form – Sulphides develop from organic and inorganic sulphur compounds in the biofilm, sewage, and deposits.
  • the allogeneic form – Sulphides are formed due to unfavourable …

(Image: Principle of the biogenic sulphuric acid corrosion in accordance with Bock)

Since the volatile sulphur compounds in the wastewater are not oxidised fast enough, they reach the sewer atmosphere (the oxidative part of the pipeline's cross-section) and the sewer walls due to diffusion and turbulence.

There, these compounds are chemically oxidised to form elementary sulphur. Provided that there is sufficient moisture in the sewer system, this sulphur …

(Image: Principle of the biogenic sulphuric acid corrosion in accordance with Bock)

Under the assumption of ideal temperature conditions, a sulphuric acid concentration amounting to 23 % can be reached [Klose80]. Given the standard sewage temperatures measured in the Federal Republic of Germany between 10 °C and 20 °C, the acid content amounts to about 7 % to 9 %. The highest acid content of 34.4 % is achieved at 30 °C, whereas a sewage temperature …