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Vulnerability in materials in sewage systems

This essay focuses on three central questions:

  1. What are the most common damages happening to pipes in waste water systems?
  2. By which factors are such damages caused?
  3. Which material is especially weak or strong in resistance against certain damages?

The advantages of pipe-materials and the needs of a proper sewage system are well known. Less highlighted are usually the disadvantages of materials or the most frequent damages occurring. This overview shall present these in three pinpointed questions and answers.

I. Most common damages:

  • CorrosionCorrosion counts to the most frequent damages happening in drainage systems. Corrosion is the gradual destruction of materials by chemical reaction with its environment. It can occur from in- or outside the pipe and with various consequences. According to the norm DIN EN ISO 8044 there are 37 different appearances. Most common one is rust corrosion but contact -, pitting -, erosion-, stress- or crevice corrosions are also pervasive. Also material incompatibility can cause corrosion. Results are for example instability or contaminations.
  • SealingsSealings are rarely taken into consideration when talking about damage potentials. Thus they are lacking of quality checks on the construction side although sealings are often the basic problem when others follow. Pipe joints can be mechanical and made from another/third material or they are homogenously wealded. In concrete pipes rubber seals are often used, steel and plastics can be wealded and other materials might be glued. Whatever sealing method/material is being used, it must be double checked for pressure, tightness and stability as the mecanical loading capacity is less. A common problem is additionally the durability of a sealing which is mostly shorter than the pipe itself. Many of the damages described in the following are a cause of deffected sealings.
  • Pipe burstPipe burst mostly occurs because of constant pressure such as traffic load and ground displacements but also because of strong corrosion, frost, fabrication defect or construction faults. Pipe bursts are rarely repairable but immediately in need of replacement. In case of waste water systems bursts are very dangerous because of environmental burden through its gens and pollutants. 

  • AggradationsAggradations can cause astringencies which can cause burst or cracking. A few aggradations/incrustations are normal over the years but danger depends on which substance is alluvial and to what extent. Uric stone e.g. is often found in domestic waste water pipe systems and needs a special purification. 
  • FlawsFlaws are often difficult to recognize (craze) until they slowly become longer and more obvious. Usually a flaw is found when there are brownish spots or tiny bubbles visible at the pipe. Flaws are the precursor of cracks and bursts. One distinguishes between transverse - and longitudinal flaws. Most appear while transport and installation. Depth and pathway of the flaw are important for inspectors to decide for the correct repair method. 
  • CracksCracks are caused by transport, implementation, frost or simply obsolescence. However most frequent reason is traffic and other external load, impact or ground displacements which strain the sealing. Cracks are dangerous when exfiltration and infiltration is happening. The transported liquid from inside the pipe (possibly toxic) can leak and cause environmental load. Other way round fluids and bacteria from outside can get in and cause aggradations or corrosion, which plays a greater role in drinking water systems. Therefore material consistency is a crucial factor. 
  • Material wearAbrasion is the progressive loss of material. In case of sewage systems, wear occurs on the moist inside wall. Higher roughness of the walls is a consequence and can be measured in variables. A cause is less hydraulic conductivity and reduction of wall thickness. Wear emerges by impacts, running liquids or droplet impingement. 

  • DeformationDeformation of pipes do exclusivley occur in flexible thermoplastic material. Reasons for deformation are usually elevation of the ground, earthquacks, bad compaction or external pressure such as impact, soil conditions or traffic load. The water flow will change in speed and additionally maintenance might be limited as the passways are being blocked. Deformation is not exclusivly a damage but on the contrary an advantage sometimes. It is questionable to what extend joints stand these deformations.

  • PurificationPurification inside the pipes is an indispensable service for the maintenance. It is done to clear the pipes from aggradations, pluming and/or prepare it for inspection. Residues in sewage systems are containing mineral (sand/stone), organic (food, paper, plastic) and other components. Next to normal scavenging and mechanical cleansing there is high pressure purification (GER: 120bar). This method is usually used in sewage systems but can lead to considerable damage too (→GRP). 
  • Root penetrationRoot penetration has always been a problem towards pipe systems. During the planning process architects and engineers must cooperate with the town and country planner. When the system is running, roots can still grow into the pipe/fittings or cause deformation and cracks. There are a range of methods how tree roots can be secured and pipe systems can be built without interference such as partitions or protective tubes.
  • Shifted pipesShifted pipes are pipes which drifted out of their original position as a cause of erosion, earth quarks, root penetration or other in- or external pressure. Pipes can shift up to an extent that they narrow or burst (Pipe burst). Some materials are massive and heavy that rarely anything could shift them during installation but when implemented, soil or traffic pressure will affect them too. Others are designed deliberately in a way that they deform and suit the surrounding soil and incidents (Deformation). For this distinction not only the material matters but the joint method as well. Homogenous welded joints will not break but deform, while pipes with a mechanical sealing would shift. 

II. Factors causing damage:

  • Physical (frost, heat, UV, butt, pressure, earthquakes …)
    EXMP 1: When water is freezing during very low temperatures, it expands. When the pressure cannot lighten, the pipe containing the frozen water might burst. Trouble is to detect those flaws and cracks caused by frost in time. Wet walls in domestic pipes or great amount of water loss in outside pipelines can be implications.
  • Chemical (substance of liquid, soil composition, pH value, oxygen concentration…)
    EXMP 1: Especially industrial- or sewage pipe systems contain crucial chemical impact. Factors which can cause corrosion, aggregation or abrasion are among others: fats, oils, acids, lyes, alcoholics, coolant, ketone or ester. 
  • Biochemical (biological molecules, decomposition…)
    EXMP 1: The biogenic sulfuric acid corrosion is well known in sewage/drainage systems. Constant maintenance and the removal of aggradations are indispensable processes. The warm temperature of waste water may sustain such corrosion because bacteria can receive energy.
  • Biological (animals, roots, biological waste in liquid …)
    EXMP 1: As mentioned above, tree roots can destroy pipe systems completely when they find spots to grow into or when they deform and shift the pipes. Animals such as rodents below ground level may also be a serious thread. 
  • Mechanical (density, traffic-/soil-pressure, impact, concentrated load …)
    EXMP 1: Plugging can be a cause from mechanical wear and end in deformation or burst. Outside pressure such as traffic load or concentrated/punctual load may force a pipe into deformation or even break. Impact, especially at the sealing, results often in cracks or flaws. Internal pressure can cause damage when it is too high but in sewage systems it is rarely the case. In summary, the density of a pipe plays a major role when the mechanical strength is measured.
  • Logical (defective planning, defective building construction, transport…)
    Damages do certainly not only happen underground when the system is installed. Some failures are already being done while the process of planning, choosing the correct material, the transport or on the construction side. Every single material may it be fitting, application or pipe must be checked for the requirements and density. Technical test reports help to choose correctly and stay informed about new developments. An installation guide is advisable.

III. Characteristics of Materials mattering for sewage systems

  • Concrete/ Ferroconcrete is used since ages for sewage systems and thus profits from long experiences. Concrete is utterly solid, stable and resistant against shifting while installation. Later on, pipes might definitely shift because of different pressures. Furthermore it is high temperature resistant and inflammable. On the other hand concrete has a high mass density, which makes the transport and implementing more difficult. Besides that, concrete is vulnerable towards crack initiation, aversion and thus to in- and exfiltration. Last but not least the usually very efficient and high quality surface structure is soon worn out. Cementitious materials are in sewage systems more vulnerable towards corrosion than others.
  • Steel has a high strength and can be welded. It was quite often used for coverings in sewage systems. Tremor and butt are not as harmful towards steel pipes as to other materials but still it can more easily harmed during transport than plastic for example. Steel has a low corrosion resistance and is thus laminated from inside with an additional corrosion protection.
  • Thermoplastic (PP/PE) are flexible in their structure and deform in case of pressure. These deformations happen without crack formation and are especially positive in regions of temperature variations or earthquakes. Thermoplastics have a very high corrosion resistance such as against chemical and physical factors of damage. They are easily weldable and have afterwards a high material density. On the other hand they are flammable and sensible towards oils and fats. Their light weight may, next to numerous advantages, be problematic in terms of shifting. An unchallenged advantage of thermoplastics is the durability of 100 and more years.
  • GRP is composed of strands of glass, lightweight and has good thermal insulation properties. Additionally glass fiber reinforced plastics have the ability to contain liquids with very high temperature. Their light weight is, just as for thermoplastics, a plus and a minus same time. GRP pipes have a higher impact sensibility and thus a relatively higher crack initiation. GRP pipes are very sensitive against high pressure purification.
  • Cast-iron is known for its robust chemical composition. A rust layer offers resistance against material destruction and is additionally secure because of its thick walls. Next to its weather-reliability (especially UV), it has a low electronic conduction and a good breaking stress. Problematic is, that it is only partly weldable and difficult to cut. The material is quickly destructed with aggressive waters or vagrant streams which makes them corrosion vulnerable in sewage systems. Finally cast-iron is heavy and hard to handle.

Water categories:

category Description
I hazardous to water, requiring treatment
II industrial sewage
III domestic sewage
IV Rainwater

When evaluating damages at sewage systems, it has to be taken into consideration that one is dealing with different waste water categories. One might think at first that category 4, containing rainwater, does not harm the environment in a leaking pipe and thus will not need to be considered that much. Actually that is not wise because also rainwater can accidentally contain chemical liquids. Next to poisoning, massive water loss might cause undercutting followed by traffic accidents or the collapsing of a building.

In conclusion, the water category must also be taken into consideration, when talking about sewage system damages.

Most frequent damages at sewage systems have been illustrated from the cause until the consequence. It became obvious that initially small damages can lead to dangerous results without exact planning, secure installation and constant maintenance. Some materials are nowadays optimized and developed that they are only vulnerable to very few aspects. It is highly advisable to spend more attention to currently less prominent topics such as a well functioning sealing or material durability in order to achieve a long-time satisfying result

Paulina Fröhlich, Krah AG

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