The Classification and Composition of Waste water

Sewage may be classified into different categories, according to its source, as follows:


Sanitary Sewage

This may also be known as domestic waste water or house sewage. This is the waste that comes from residences, business buildings or institutions. This type of sewage normally consists of water, liquid wastes, and the accompanying solids resulting from dish washing, washing clothing, toilet flushing, showering or bathing, or other domestic purposes. It will contain any chemicals used for household cleaning.

Its composition is generally one to two parts per thousand of solid organic matter (food wastes, faeces). Approximately half of the solids are in solution and approximately half suspended or floating in the liquid.


Industrial Waste

This is sometimes referred to as trade waste. Industrial wastes vary greatly in their composition, and this depends upon the type of industry, and the characteristics of the particular factory. The percentage of solids in the sewage from a particular plant may be very small or extremely large, depending on the processes that take place within the plant. If the factory produces mainly organic material, like food manufacturing companies, then there is mainly organic matter in waste water. Other factories manufacture chemicals, or metal parts and then waste water carries chemicals, oils, fats and heavy metals.


Storm Water

This is sometimes known as storm sewage. It is run off occurring during or immediately following rainfall. This type of sewage should be excluded from sanitary sewers. Storm sewers may carry large amounts of suspended material, as well as a considerable amount of organic material, including animal droppings, and oil, petrol and heavy metals from spills or leaking vehicles. This is especially so, if there has been a long period without rain and the first heavy rains flush considerable debris from the streets.

The organic content of sewage is important. The organic matter, in addition to being a nuisance, causes bad odours, and is a danger to health. There are other ways in which sewage can be classified:

  • Strong or weak: depending upon its organic content.

  • Fresh or stale: referring to the state of decomposition that the sewage has reached.

  • Treated or untreated: untreated sewage is also referred to as crude or raw sewage.

The above factors do not enter into the design of individual sewage lines, but they may affect the broad planning on a sewerage system through their influence on the design of the treatment plants and pump stations.

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