Introduction:
S.W. (sewer and water) pipes are commonly used in water supply systems and underground drainage networks. However, they are generally not used for sewer mains due to several reasons. In this response, we will discuss the specific weaknesses of S.W. pipes that make them unsuitable for sewer mains.

Weakness in Compression:
One of the primary reasons S.W. pipes are not used for sewer mains is their weakness in compression. Sewer mains are subjected to significant external loads from the soil and other structures above them. S.W. pipes, which are typically made of PVC (polyvinyl chloride) or HDPE (high-density polyethylene) materials, have lower compressive strength compared to other pipe materials such as concrete or ductile iron. This weakness in compression makes them prone to deformation or collapse under the weight of the soil or heavy traffic loads, leading to sewer main failures.

Weakness in Tension:
Another weakness of S.W. pipes is their limited ability to withstand tension forces. Tension forces can occur in sewer mains due to ground movements, settlement, or external loads. S.W. pipes, especially PVC pipes, have low tensile strength and are susceptible to cracking or breaking when subjected to tension stresses. This weakness compromises the structural integrity of the sewer mains and increases the risk of leaks or pipe failures.

Hydraulic Inefficiency:
S.W. pipes are also hydraulically less efficient compared to other pipe materials commonly used for sewer mains, such as concrete or vitrified clay pipes. The internal surface of S.W. pipes is relatively smoother, which can result in higher friction losses and reduced flow capacity. Sewer mains require efficient conveyance of wastewater, and the hydraulic inefficiency of S.W. pipes can lead to increased energy consumption and reduced system performance.

Resistance to Organic Corrosion:
Sewer mains are exposed to various corrosive elements, including organic matter, chemicals, and gases present in wastewater. S.W. pipes, particularly PVC pipes, have limited resistance to organic corrosion compared to materials like vitrified clay or concrete. The organic compounds present in sewage can gradually degrade the PVC pipes, leading to material deterioration, structural weakening, and potential pipe failures.

Conclusion:
In summary, S.W. pipes are not commonly used for sewer mains due to their weaknesses in compression and tension, hydraulically less efficient nature, and limited resistance to organic corrosion. When selecting pipes for sewer mains, it is essential to choose materials that provide adequate strength, durability, hydraulic efficiency, and resistance to the corrosive environment of wastewater. Materials like concrete, vitrified clay, or ductile iron are typically preferred for sewer mains to ensure reliable and long-lasting infrastructure.