Sewage is the collective name for the liquid waste of a community. It consists of the following:
- Excreta that are faeces and urine
- Bathroom or bath waste-water
- Wash-hand basin and sink waste-water
- Kitchen waste-water
- Laundry waste-water
- Rain run-off waste-water
- Industrial waste-water.
Characterization of Sewage
As seen above, sewage contains things like faeces, urine and gray water, etc. Gray water results from washing, bathing and meal preparations. Agricultural run-off water and waste from nearby industries may also enter the system.
The important physical characteristic of waste-water is its total solid content. It includes floating, suspended colloidal and dissolved solids. Total solids are those that remain as residue upon evaporation at 105 degree centigrade. More details about these will be discussed later.
Sullage water is another name for gray water. It is domestic waste-water other than that which comes from the toilet with sources as seen above. The word gray is used to distinguish it from backwater which describes wastes containing human excreta.
Sewage or waste-water originates mainly from domestic, industrial, groundwater, and meteorological sources and these forms of waste- water are commonly referred to as domestic sewage, industrial waste, infiltration, and storm-water drainage, respectively. Domestic sewage results from people‘s day-to-day activities.
The quantity and character of industrial waste-water is highly varied, depending on the type of industry, the management of its water usage, and the degree of treatment the waste-water receives before it is discharged.
Definition and Concepts of Sewerage
By way of definition, sewerage system is the network of underground waste pipes which are employed to collect the waste-water from the community. The sewers or pipes are laid in such a way that the flow is under gravity.
A sewerage system collects waste-water and can be in the form of backwater separated from gray water, or mixed with it (sewage). Gravity is used wherever possible to convey the waste-water. It is not surprising therefore that natural storm water drainage is usually used, because this is how rainwater run-off is conveyed in nature by gravity.
The principle of using gravity as the driving force for conveying waste-water in a sewerage system should be applied wherever possible, because this will minimize the cost of pumping.
Natural storm water drainage occurs in what is usually termed a catchment basin. In a catchment basin, rainwater run-off flows to a common point of discharge, and in so doing, forms streams and rivers.
Crossing a catchment boundary may mean that the water has to be unnecessarily pumped, requiring an energy source. A waste-water sewerage system should therefore be within a storm water catchment basin.
Sewerage systems can be classified into combined sewerage and separate sewerage. Combined sewerage carries both storm water and waste-water, while separate sewerage carries storm water or waste-water separately. Recent trends have been for the development of separate sewerage systems.
The main reason for this is that storm water is generally less polluted than waste-water, and that treatment of combined waste-water and storm water is difficult during heavy rain falls, resulting in untreated overflows (commonly termed Combined Sewer Overflow, CSO).
In practice there is usually ingress of storm water into waste- water sewerage pipes, because of unsealed pipe joints, and unintentional or illegal connections of rainwater run-off. Conversely there may be unintentional or illegal waste-water connections to storm water sewerage.
Types of Sewerage
Waste-water sewerage systems can be classified into three major types:
- Conventional sewerage
- Simplified sewerage
- Settled sewerage
A Conventional Sewerage: conventional sewerage is also termed deep sewerage because the sewerage pipes are laid deep beneath the ground. Pumping is generally required at various stages of the sewer pipe network, especially if the landscape is fairly flat.
The larger the population served by the sewerage system, and the longer the planning horizon is to cope with future population increases, the larger the diameter of the final pipes becomes. The costs of the pipes, inspection manholes, pumps and pumping stations and their construction/installation are therefore high.
The costs of operation and maintenance are correspondingly high because of very conservative design assumptions.
1. Simplified Sewerage
Simplified sewerage is also known as shallow sewerage, reflecting the shallower placement of the pipes in contrast to the conventional or deep sewerage. The purpose of simplified sewerage is to reduce the cost of construction and the corresponding cost of operation and maintenance.
Simplified sewerage design is based on hydraulic theory in the same manner as for conventional sewerage but has less conservative design assumptions. Smaller diameter pipes are used when water use per person is known to be less and the minimum depth of cover of pipes can be as low as 0.2 m when there is only light traffic.
Read Also: Handling of Wastes and Treatment Methods
Manholes can be replaced by inspection cleanouts because of the shallow pipes. The design planning horizon can be shorter because the population projection may be uncertain.
In a variation of the simplified sewerage, the pipe layout passes through property lots (condominial) rather than on both sides of a street (conventional). The figures above show the sewerage layouts in conventional sewerage and in condominial sewerage systems.
The cost of construction of simplified sewerage can be 30 to 50 % less than conventional sewerage depending on local conditions.
Shallow sewerage is also conducive to local community participation because sewer pipes have to cross property boundaries. The community has to agree to this arrangement which extends after construction for maintenance (e.g. unblocking of sewer pipes).
The shallow pipe, and hence the shallow trenches, also allow members of the community to participate by, for example, providing labor for digging the trenches. This is in contrast to conventional sewerage where specialized machinery is required for the deep trenches.
Simplified sewerage was originally developed in Brazil and is increasingly being used in other parts of the world. The International Source Book on Environmentally Sound Technologies for Waste-water and Storm water Management (hereafter referred to as the Source Book), published by IWA and IETC, and provides useful case studies.
2. Settled Sewerage
Settled sewerage refers to sewerage for conveying waste-water that has been settled, for example, in a septic tank. Settled sewerage originated to convey the overflow from septic tanks where the soil cannot cope or absorb the overflow.
This usually occurs when the groundwater table is high, or where the soil permeability is low, or where there are rock outcrops. It can also be used when effluent from septic tanks pollutes groundwater and it is necessary to convey the effluent off-site and treat it.
Because there are no solids that can potentially sediment in the sewerage pipes, there is no requirement for the self-cleansing velocity.
Smaller pipes and lower gradients can be used. The cost of settled sewerage is between a third and a half of conventional sewerage. Originally developed in South Australia to overcome problems with failing septic tanks, it has been used quite widely worldwide to upgrade septic tank systems.
Where there is no existing septic tank, an interceptor box or tank can be used. It functions like a septic tank and designed in the same way as figure above. To reduce cost, the waste-water from a group of houses can be connected to one interceptor tank.
Just like in a septic tank, the accumulation of sludge has to be removed regularly from an interceptor tank.
3. Storm Water Collection
Storm water flows through the landscape‘s natural drainage system. Piped storm water collection was a development in European cities to overcome odour and improve aesthetic appearance of waste-water disposed with storm water.
The covering of ditches used for combined sewerage was an intermediate step in using natural drainage to construct sewerage for combined waste-water and storm water. Piped sewerage also allows more land area for road and footpaths.
With the separate collection of waste-water there is an opportunity to return some storm water flow path to its more natural state to improve urban amenity value.
In conclusion, sewage is a collective name for waste-water of the community which is composed of human waste (faeces and urine), kitchen waste-water, laundry, storm water, etc. These components are not friendly to man or his environment, hence are always seen as such.
It is because of this that he handles them with caution in a network of pipes (sewers) which is collectively called sewerage system. Sewerage systems are of various types as have seen.
You were made to understand that while sewage is the content which is more or less in liquid form, sewerage is the network of pipes containing the sewage.
The different types of sewerage systems which include; conventional, simplified and settled sewerage systems. Each of them has some conditions in which it works better.
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