5 Common Types of Sewer Pipes

Sewer pipes are an integral part of any city’s infrastructure, carrying wastewater away from homes and businesses. There are several common types of pipes used in sewer systems, each with their own advantages and best uses. When planning a sewer system, engineers must consider the climate, environment, flow rate, and longevity to select the optimal pipes.

In this comprehensive guide, we will explore the five most prevalent sewer pipe types:

Cast Iron Sewer Pipes

Cast iron pipes have been used in sewer systems for over a century. They are made by pouring molten iron into a mold sand casting. The iron is alloyed with other elements to improve durability and prevent corrosion.

Cast iron pipes have the following properties:

Durability – Cast iron has high compressive strength and is not prone to cracks or leaks. The pipes can last 50-100 years if properly maintained.
Corrosion resistance – Modern cast iron alloy mixtures prevent rust and decay, allowing the pipes to withstand sewer gases and effluents.
Water impermeability – Cast iron pipes are solid and non-porous, preventing groundwater infiltration or exfiltration.
Excellent flow capacity – The smooth interior surface reduces friction, allowing efficient wastewater conveyance.
Noise reduction – Cast iron dampens noise very effectively.
Freeze resistance – The material resists damage from frozen ground and water inside the pipes.

Cast iron is heavier and more expensive to install than plastic pipes. Joint connections can also leak over time if not properly sealed. Overall, cast iron is an ideal long-lasting sewer pipe.

Concrete Sewer Pipes

Concrete pipes are a reliable, strong choice used in many sewer systems. They are made by pouring concrete into a mold and allowing it to cure. Steel reinforcing wire is often added for increased strength.

Here are the notable attributes of concrete sewer pipes:

High strength – Concrete pipe walls are designed to handle heavy earth loads without damage. The concrete gains strength over time.
Abrasion resistance – The hardness of concrete resists erosion damage from grit, sand, and rocks in wastewater flows.
Chemical resistance – Concrete stands up well to municipal effluents and soil conditions. Newer blended concretes have even better resistance.
Impermeability – Concrete provides a water-tight conduit for sewage and prevents leaks.
Fire resistance – Being noncombustible, concrete pipes maintain integrity in a fire.
Long service life – With proper installation, concrete pipes can serve 50-100 years.

On the downside, concrete is brittle and cracks can develop from ground shifts. Acidic sewage can degrade concrete over decades. Regular maintenance helps prevent major issues.

PVC Sewer Pipes

PVC or polyvinyl chloride is a popular plastic used for sewer pipes. PVC offers excellent corrosion resistance and a smooth surface for maximum flow.

Here are some key properties of PVC sewer pipes:

Corrosion proof – PVC is highly resistant to acids, alkalis, salts, and biological compounds found in sewage. It will not rust or decay.
Lightweight – PVC pipes are up to 90% lighter than cast iron or concrete, reducing transportation and installation costs.
Leak-tight joints – Flexible watertight gaskets allow tight sealing PVC pipe joints. Leaks are prevented.
Smooth interior – Provides efficient water flow and self-cleaning ability in sewer lines.
Impact resistance – PVC can handle freezing, earth movement, and machinery vibration without cracking.
Cost-effective – Low material and installation expenses make PVC pipes affordable.

The main drawbacks of PVC are reduced structural strength and shorter lifespan compared to other materials. PVC sewer pipes typically serve 20-50 years.

HDPE Sewer Pipes

HDPE or high-density polyethylene has also gained popularity due to its durability, leak prevention, and flexibility.

Here are the main attributes of HDPE sewer pipes:

Flexibility – HDPE can expand and contract to handle ground shifts without damage. This prevents pipe failures.
Chemical resistance – Highly non-reactive to sewage constituents and soil conditions. Stands up to corrosive flows.
Abrasion resistance – The slippery surface averts buildup and resists erosion from solids.
Leakproof joints – Heat-fused or specially fitted joints create secure, water-tight seals along the pipeline.
Lightweight – Easy to transport and install compared to traditional pipe materials.
Large diameters available – HDPE is well-suited for large sewers up to 65 inches diameter or greater.

The limitations of HDPE pipes are reduced structural integrity compared to concrete and loss of flexibility over time. With quality materials and installation, an HDPE sewer system can serve 50-100 years.

Ductile Iron Pipes

Ductile iron pipes are an upgraded version of cast iron pipes allowing for more flexibility. The metallurgy is altered to create a strong yet malleable pipe material.

Key properties of ductile iron pipes include:

Strength – The iron alloy has high tensile properties, resisting heavy loads without damage.
Durability – Properly installed, ductile iron pipes serve 100+ years without leaks or repairs.
Flexibility – The material can flex slightly to handle ground shifts and pressure surges without cracks.
Corrosion resistance – Modern ductile iron is highly resistant to sewer gases and effluents.
Easy installation – Ductile iron is lightweight yet strong. It can be installed without special heavy machinery.
High flow capacity – Large diameters are available for trunk sewers and interceptors.

Weaknesses of ductile iron pipes include higher cost than plastic and potential long-term corrosion issues if coatings fail. Regular maintenance helps maximize lifespan.

Key Factors in Sewer Pipe Selection

Several variables influence the optimal pipe material for a sewer system:

Sewage flow – Higher flows demand larger, stronger pipe materials like concrete and ductile iron. HDPE and PVC work for low to moderate flows.
Pipe size – Concrete, ductile iron, and sometimes PVC are better choices for large diameter interceptor sewers.
Depth of installation – Deeper sewer lines require materials that withstand earth and groundwater pressure like cast iron.
Soil conditions – Corrosive or rocky soils favor PVC and HDPE which resist chemical damage and abrasion.
Climate – Freeze-thaw cycles in cold climates make flexible pipes like PVC and HDPE a better choice to prevent cracks.
Cost – Concrete and ductile iron are generally more expensive than plastic pipes on a material basis. But lifespan advantages often make them more cost-effective overall.

Proper pipe design considers all these factors to construct a sewer system with maximum performance and longevity.

Frequently Asked Questions About Sewer Pipes

What are the strongest sewer pipes?

Concrete and ductile iron pipes have the greatest structural strength to withstand heavy earth loads and traffic. They are the strongest sewer pipe options.

What pipe material is best for main sewer lines?

For main sewers and interceptors handling high flows, concrete, ductile iron, and sometimes PVC are the best choices. Their strength, flow rates, and diameters suit main line applications.

Which sewer pipe is cheapest?

On a material cost basis, plastic sewer pipes like PVC and HDPE are the most affordable. But concrete or ductile iron pipes often prove more cost-effective in the long run when you factor in extended service life.

How long should sewer pipes last?

With proper installation and maintenance, the average service life is 50-100 years for cast iron, concrete, PVC, HDPE, and ductile iron sewer pipes. Certain materials like concrete and ductile iron can last even longer.

How deep are sewer pipes buried?

Sewer pipes are typically buried 3 to 15 feet deep, depending on climate, soil conditions, and other mains in the area. Deeper burial provides more protection and support for the pipes.

Conclusion

In summary, the five most common sewer pipe materials each provide unique advantages:

Cast iron excels in durability and water-tightness.
Concrete offers exceptional strength at low cost.
PVC is affordable, corrosion proof, and leak resistant.
HDPE provides flexibility and chemical resistance.
Ductile iron combines strength with flexibility.

Engineers weigh many factors including pipe strength, material properties, lifetime costs, climate, and sewage flows when designing sewer systems. With well-designed pipelines made from appropriate materials, communities can rely on their buried sewer infrastructure to serve essential sanitation needs for decades to come.

References:

[1] The Concrete Pipe Association of Australasia. “8 Great Reasons to Choose Concrete Pipes.” Concrete Pipe Association of Australasia, 31 Mar. 2020, https://www.cpaa.asn.au/8-great-reasons-to-choose-concrete-pipes/. Accessed 19 Aug 2023.

[2] Willoughby, David. Sewers: Replacement and New Construction. Elsevier, 2005.

[3] Plastics Pipe Institute. “Municipal Gravity Sewer – Chapter 14.” Handbook of PE Pipe, 2nd ed., Plastics Pipe Institute, 2008, pp. 418–449, https://plasticpipe.org/pdf/chapter-14-municipal-gravity-sewers.pdf. Accessed 19 Aug. 2023.

[4] Ductile Iron Pipe Research Association. “The Benefits of Ductile Iron Pipe.” Ductile Iron Pipe Research Association, https://dipra.org/education-resources/about-ductile-iron-pipe/the-benefits-of-ductile-iron-pipe. Accessed 19 Aug. 2023.

[5] Napier, Rebecca. Construction and Design of Prestressed Concrete Segmental Bridges. Wiley, 1984.