Sewerage

For the wastewater, seeSewage.

Sewerage(orsewage system) is theinfrastructurethat conveyssewageorsurface runoff(stormwater,meltwater,rainwater) using sewers. It encompasses components such as receivingdrains,manholes,pumping stations,storm overflows, and screening chambers of thecombined sewerorsanitary sewer.Sewerage ends at the entry to asewage treatment plantor at the point of discharge into theenvironment.It is the system of pipes, chambers, manholes or inspection chamber, etc. that conveys the sewage or storm water.

Map of London sewer network, late 19th century

In many cities, sewage (municipal wastewater or municipal sewage) is carried together with stormwater, in acombined sewersystem, to a sewage treatment plant. In some urban areas, sewage is carried separately insanitary sewersand runoff from streets is carried instorm drains.Access to these systems, for maintenance purposes, is typically through amanhole.During high precipitation periods a sewer system may experience acombined sewer overflowevent or asanitary sewer overflowevent, which forces untreated sewage to flow directly to receiving waters. This can pose a serious threat topublic healthand the surrounding environment.

The system of sewers is calledsewerageorsewerage systemin British English andsewage systemorsewer systemin American English.

History

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 naturalwaterwaysinto which waste fromlatrineswas readily channeled, but the emergence of major cities exposed the inadequacy of this approach. Early civilizations like theBabyloniansdug cesspits below floor level in their houses and created crude drainage systems for removing storm water. But it was not until 2000 BC in theIndus valley civilizationthat networks of precisely madebrick-lined sewage drains were constructed along the streets to convey waste from homes.Toiletsin homes on the street side were connected directly to these street sewers and were flushed manually with clean water. Centuries later, major cities such asRomeandConstantinoplebuilt increasingly complex networked sewer systems, some of which are still in use. It was after the construction of the sewer systems that people realized the reduction of health hazards.

Components and types

Map of Seattle sewer districts, 1894

The main part of such a system is made up of large pipes (i.e. the sewers, or "sanitary sewers") that convey the sewage from the point of production to the point of treatment or discharge.

Sewers under construction inYstad,Sweden

Types of sanitary sewer systems that all usually aregravity sewersinclude:

• Combined sewer
• Simplified sewerage
• Storm drain

Sanitary sewers not relying solely ongravityinclude:

• Vacuum sewer
• Effluent sewer

Where a sewerage system has not been installed,sewagemay be collected from homes by pipes intoseptic tanksorcesspits,where it may be treated or collected in vehicles and taken for treatment or disposal (a process known asfecal sludge management).

Maintenance and rehabilitation

Severe constraints are applied to sewerage, which may result in premature deterioration. 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. For example, it is estimated that 500 million m³of contaminated water per year can leak into soil and ground-water in Germany.The rehabilitation and replacement of damaged sewers is very costly. Annual rehabilitation costs for Los Angeles County are about €400 million,and in Germany, these costs are estimated to be €100 million.

Vacuuming debris from a sewer line

Hydrogen sulfide(H₂S) is indirectly responsible forbiogenic sulfide corrosionand consequently, sewers need rehabilitation work. Various repair options are available to owners over a large range of costs and potential durability. One option is the application of a cementitious material based oncalcium aluminate cement,after a cleaning of the corroded structure to remove loose material and contaminants in order to expose a sound, rough and clean substrate. Depending on the concrete condition and contamination, the cleaning can range from simple high pressure jet water cleaning (200 bar) up to real hydro-demolition (2000 bars).

One method to ensure sound concrete is exposed is to verify that the surface pH is superior to 10.

As for any concrete repair, the state-of-the-art rules must be followed. After this cleaning step, the cementitious material is applied to the saturated-surface-dry substrate using either:

• Low pressure wet spray: this method is the more common because it does not produce dust and virtually no material is lost by rebound. It utilizes classical facade rotor pump, easily available in the market. The main drawback is the limited pumping distance that cannot exceed 75 meters.
• Spinning head wet spray: this method is similar to the first, but the manual spraying is replaced by a spinning head projecting the mortar onto the repaired surface. This method is fast and especially suited for cylindrical chambers such as manholes. When a structure is so severely corroded that human entry is a risk, spinning head application permits an “un-manned” consolidation of the manhole.
• High pressure dry spray: this method, also called “shotcrete”or “gunite” is allowing a faster rate of rehabilitation, and also to make a thicker application in a single pass. The main interest of dry shotcrete is the capacity to pump the mortar over a long distance and this is needed when the access points are distant. Perhaps the longest dryshotcretedistance is a job site inAustraliain 2014, where 100%calcium aluminatemortar was air transported over 800 meters before being sprayed. The main drawback with dry shotcrete is the generation of dust and rebound; these could be limited and controlled with appropriate means (pre-moisture ring, adapted aggregate grading, experienced nozzleman, water mist cut-off walls, etc.).

Challenges

Building a sewer in newly filled land on formertideflatsinSeattle,1910.

Water table

Sewer system infrastructure often reduces thewater tablein 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 certain areas it has resulted in a significant lowering of the water table. In the example of Belgium, a lowering of the water table by 100 meters has been the result.The freshwater that is accumulated by the system is then piped to the sea. In areas where this is a concern,vacuum sewersmay be used instead, due to the shallow excavation that is possible for them.

Lack of infrastructure

In manylow-income countries,sewage may in some cases drain directly into receiving water bodies without the existence of sewerage systems. This can causewater pollution.Pathogenscan cause a variety of illnesses. Some chemicals pose risks even at very low concentrations and can remain a threat for long periods of time because ofbioaccumulationin animal or human tissue.

Regulations

In manyEuropeancountries, citizens are obliged to connect their home sanitation to the national sewerage where possible. This has resulted in large percentages of the population being connected. For example, theNetherlandshave 99% of the population connected to the system, and 1% has an individual sewage disposal system or treatment system, e.g.,septic tank.Others have slightly lower (although still substantial) percentages; e.g., 96% forGermany.

Trends

Current approaches to sewage management may include handling surface runoff separately from sewage, handling greywater separately fromblackwater(flush toilets), and coping better with abnormal events (such as peaksstormwatervolumes fromextreme weather).

See also

• History of water supply and sanitation
• List of water supply and sanitation by country
• Sanitary sewer overflow(SSO)
• Sanitation
• Sewer mining

External links

• Media related toSewerageat Wikimedia Commons