Partially Separate System

Published: | Updated: September 4, 2020;

Definition - What does Partially Separate System mean?

A partially separate system is a type of sewer system that combines the attributes of a separate sewer system and a combined sewer system. It is a modification of the separate sewer system, where the system that would normally transport and discharge domestic and industrial sewage also carries some surface water drained from roofs or paved portions of above-ground infrastructure.

Partially separate systems attempt to offer a compromise between single and combined systems. They carry less load than their combined system counterparts, while addressing the lag of separate sewer systems.

While the separate sanitary sewer system assists the separate stormwater system, the network can be placed under burden during specific weather conditions. During heavy rainfall, excess stormwater in the sewer system may put unnecessary load on wastewater treatment plants.

Furthermore, overflows as a result of heavy rains may cause excess sewage to discharge directly into receiving water bodies, leading to environmental contamination. Roots, leaves, mud, and other debris may also make its way into the sanitary sewer system, leading to undesirable clogs and backups.

Trenchlesspedia explains Partially Separate System

To gain an appreciation for partially separate systems, it is worthwhile to understand how separate and combined sewer systems work.

Separate Sewer Systems

In these systems, domestic sanitary sewage and industrial waste are transported through one set of sewers, while surface stormwater is carried in another, separate system. The sewage and waste are then taken to a wastewater treatment plant while stormwater is discharged into natural watercourses.

Figure 1: Illustration of a separate sewer system. Sanitary waste and stormwater are carried in separate piping systems (source)

Combined Sewer System

Combined sewer systems consist of piping infrastructure that transports both stormwater and sanitary sewage. The combined flow is then taken to a sewage treatment plant.

Figure 2: Illustration of a combined sewer system. One pipe carries a combination of sewage and stormwater (source)

Partially Separate Sewer System

In the partially separate system, a portion of the stormwater is sent to a separate piping system and discharged into natural watercourses. Meanwhile, another portion of the stormwater is taken by the sanitary sewage system and sent to a wastewater treatment plant.


Figure 3: Illustration of a partially combines sewer system (source)

Why Are Partially Separate Systems Used?

Partially separate systems are a viable option in areas where combined sewer systems do not produce desirable results. For example, in some tropical countries, frequent rainfall is limited to a specific time of the year. As a result, there is considerable variation in the sewage flow throughout the year. During the dry months, sewage flow can be extremely slow due to the lack of stormwater in the partially combined system.

Additionally, partially combined systems require careful maintenance and skilled staff. This can be a problem in countries where local authorities do not give enough consideration to hiring qualified workers.

Partially separated systems draw a compromise between separated and combined systems. Local municipalities can benefit from the simplicity and cost-effectiveness of combined systems without the cost and complexity of a fully separate system.

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