How does a Sewerage System work? | Types of Sewerage System


What is a Sewerage?

A sewerage system is a setup that uses sewers to transfer surface runoff or wastewater (rainwater, meltwater, stormwater). This has parts such as rain overflow pipes, pumping stations, manholes, drains, sewers, or sanitary sewer sieve shafts. The wastewater flows into the inlet of the sewage treatment plant or is discharged into the environment.

In many cities, wastewater (or municipal wastewater) is transported to sewage treatment plants together with rainwater in a common sewer system. In some urban areas, the sewage is separately diverted to the sewer system and the street runoff is diverted to the rain drains. Sewers are typically used to access these systems for maintenance purposes. Heavy rain can lead to a mixed water overflow or sewage overflow in the sewer system, which pushes raw sewage directly into the receiving water. This can pose a serious threat to public health and the environment.

If there is no sewerage system then manure from the household is collected through pipes into sewage basins or septic tanks where it is treated or collected in a vehicle for treatment or disposal (a method known as fecal sludge handling).


MWRA supplies introductory primary and secondary treatment of the sewage stream at the Deer Island Treatment Plant. A sewerage system works in the following steps:

Collection and Pumping

Wastewater is directed by the community to various wells where bricks, logs, and other large objects are sifted. The pump pumps wastewater through a deep rock tunnel below the port to Kashima.

Preliminary Treatment

The sediment collects in a water tank called a sand tank. This material, called gravel and sieve, was then sent to landfills for environmentally friendly disposal.

Primary Treatment

The wastewater then flows into the primary clarification tank, where up to 60% of the solids in the wastewater stream are deposited as a mixture of sludge and water. This primary treatment removes very few toxic chemicals.

Secondary Treatment

In secondary treatment, O2 adds in the effluent so that the growth of the microbes can be increased. As the growth of the microorganisms completes, they eat the waste and settle at the foot of the secondary sedimentation tank. After this treatment process, 80% to 90% of the human excrement and other solids were removed. This process also removes most of the toxic chemicals.

Types of Sewerage System

#1. Combined Sewage System

Mixed water is a sewage system that absorbs rainwater and domestic sewage and treats the sewage in the STP (sewage treatment plant). This system is mainly used in narrow streets and cities with little rainfall.

#2. Separate Sewerage System  

With this system, domestic sewage and rainwater each enter two sewers. The wastewater is transported to a sewage treatment plant (WWTP), where the untreated rainwater is discharged directly into the river.

#3. Partial Separate Sewage System

A partial separation sewerage system is a variant of a separate separation system. The sewer that discharges domestic sewage and commercial waste usually contain a portion of the surface that drains from the back yard or roof of the site.

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