Sewerage

[1] It was probably the need to get rid of foul smells rather than an understanding of the health hazards of human waste that led to the first proper sewage systems.

Most settlements grew next to natural waterways into which waste from latrines was readily channeled, but the emergence of major cities exposed the inadequacy of this approach.

Early civilizations like the Babylonians dug cesspits below floor level in their houses and created crude drainage systems for removing storm water.

But it was not until 2000 BC in the Indus valley civilization that networks of precisely made brick-lined sewage drains were constructed along the streets to convey waste from homes.

Centuries later, major cities such as Rome and Constantinople built increasingly complex networked sewer systems, some of which are still in use.

These include root intrusion, joint displacement, cracks, and hole formations that lead to a significant volume of leakage with an overall risk for the environment and public health.

After this cleaning step, the cementitious material is applied to the saturated-surface-dry substrate using either: Sewer system infrastructure often reduces the water table in areas, especially in densely populated areas where rainwater (from house roofs) is directly piped into the system, as opposed to being allowed to be absorbed by the soil.

In many low-income countries, sewage may in some cases drain directly into receiving water bodies without the existence of sewerage systems.

Some chemicals pose risks even at very low concentrations and can remain a threat for long periods of time because of bioaccumulation in animal or human tissue.