A plastic or concrete chamber for the pumping station? A technical comparison

Key points at a glance

• Corrosion: Plastic manholes are resistant to biogenic sulphuric acid corrosion and aggressive wastewater – the most common cause of damage to concrete manholes in wastewater treatment.

• Weight & Installation: PE-HD manholes weigh a fraction of what a comparable concrete manhole weighs. This reduces transport and installation times and allows for pre-assembly at the factory.

• Tightness: Plastic manholes manufactured as a single piece or welded with a material-to-material bond significantly reduce the risk of external water infiltration.

• Structural analysis: When fitted with a load-distribution plate, plastic manholes are suitable for vehicle traffic up to load class D 400 (40 t); however, measures to prevent buoyancy must be actively incorporated into the design.

• Concrete remains a sensible choice, where high self-weight (buoyancy), fire load or established reference values are the deciding factors.

Chemical resistance and corrosion protection

The most common cause of damage to concrete manholes in the wastewater sector is the biogenic sulphuric acid corrosion (BSK). In this process, microorganisms in the wastewater convert sulphur compounds into sulphuric acid, which corrodes the cement matrix from within. This results in material loss, cracks and, in the long term, leaks.

The advantages of plastic:

• Corrosion resistance: PE-HD and PP are resistant to most substances found in municipal wastewater, as well as to biogenic sulphuric acid. An internal coating, as is required for concrete, is not necessary.

• Maintenance requirements: There is no need for recurring refurbishment of the inside of the shaft, which, in the case of concrete shafts, may be required after 15–30 years, depending on the load.

Technical term: Biogenic sulphuric acid corrosion refers to the degradation of concrete by sulphuric acid produced by bacteria from sulphur compounds in Wastewater. It is a combined chemical and biological attack on the surface.

Boundaries clearly defined: Plastic also has its chemical limitations – for example, when exposed to certain solvents, prolonged UV radiation when left exposed to the elements, or high localised temperatures. In typical municipal wastewater, however, these factors play hardly any role.

Source: Fraunhofer UMSICHT – Biogenic sulphuric acid corrosion (BSK)

A Comparison of Installation and Cost-Effectiveness

The installation of pumping stations is often carried out under time pressure and in difficult terrain. This is where a low weight really pays off – in more ways than one.

Plastic manholes weigh only a fraction of comparable concrete manholes, which is why they can be transported and installed using lighter equipment. However, prefabrication is the key factor: as fittings, pipework and pump ducts are already fitted at the factory, the amount of work on site is significantly reduced. Added to this is their watertightness – monolithically manufactured or material-bonded welded chambers have no critical joints through which external water can penetrate, which prevents operational disruptions throughout their entire service life.

Logistics and Assembly Put to the Test

Double-walled PE-HD shaft systems combine high strength with low weight. Even large pumping stations can be supplied in a single piece. They are transported horizontally on a articulated lorry directly to the construction site. There, the prefabricated station is unloaded using a mobile crane, erected and positioned in the prepared excavation pit.

The process, from the arrival of the articulated lorry to the final manoeuvre, usually takes just a few hours. The advantages:

• Shorter construction time and less disruption to traffic

• Fewer interfaces between the different trades, and consequently fewer potential sources of error

• Lower costs thanks to a shorter groundwater retention period, horizontal transport and reduced use of staff and machinery

When concrete is still the better choice

Plastic is not always the best choice. Concrete really comes into its own when:

• high groundwater level is present and the dead load is required to act as a passive buoyancy control measure,

• a very long track record and wealth of experience are required (concrete has been documented over decades),

• high mechanical point loads or fire loads are to be expected,

• regional proven supply chains and standards be the deciding factor.

The honest answer, therefore, is: it depends on the project. For most new municipal sewage pumping stations, however, plastic offers the most suitable overall solution.

Life-cycle costs rather than purchase price

Depending on the design, the purchase price of a plastic manhole is higher than that of a concrete manhole. However, what matters is the overall assessment over its service life:

• lower installation costs (time, equipment, groundwater management)

• renovation and coating costs incurred

• fewer operational disruptions caused by extraneous water

Over the entire life cycle, this means that the higher material cost is offset in many projects, or even turns into a saving.

Conclusion

For modern sewage pumping stations, plastic chambers generally offer the most technically sound overall solution: high corrosion resistance, short installation times and reliable watertightness throughout their service life. Concrete remains the right choice where buoyancy, fire load or established reference values are the deciding factors. However, anyone planning for low-maintenance and sustainable solutions should now include the plastic chamber in any assessment of options.