Public Health Engineering
Overview
Public Health Engineering, also known as sanitary engineering or wastewater engineering, is the application of engineering methods to improve sanitation of human communities, primarily by providing the removal and disposal of human waste, and in addition to the supply of safe potable water. Traditionally a branch of civil engineering and now a subset of environmental engineering, in the mid-19th century, the discipline concentrated on the reduction of disease, then thought to be caused by miasma. This was accomplished mainly by the collection and segregation of sewerage flow in London specifically, and Great Britain generally. These and later regulatory improvements were reported in the United States as early as 1865.
It is not concerned with environmental factors that do not have an immediate and clearly understood effect on public health. Areas outside the purview of sanitary engineering include traffic management, concerns about noise pollution or light pollution, aesthetic concerns such as landscaping, and environmental conservation as it pertains to plants and animals.
It is not concerned with environmental factors that do not have an immediate and clearly understood effect on public health. Areas outside the purview of sanitary engineering include traffic management, concerns about noise pollution or light pollution, aesthetic concerns such as landscaping, and environmental conservation as it pertains to plants and animals.
Skills within this field are usually employed for the primary goal of disease prevention within human beings by assuring a supply of healthy drinking water, treatment of waste water, and removal of garbage from inhabited areas.
Compared to (for example) electrical engineering or mechanical engineering which are concerned primarily with closed systems, sanitary engineering is a very interdisciplinary field which may involve such elements as hydraulics, constructive modelling, information technology, project design, microbiology, pathology and the many divisions within environmental science and environmental technology. In some cases, considerations that fall within the field of social sciences and urban planning must be factored in as well.
Although sanitary engineering may be most associated with the design of sewers, sewage treatment and wastewater treatment facilities, recycling centers, public landfills and other things which are constructed, the term applies equally to a plan of action to reverse the effects of water pollution or soil contamination in a specific area.
History
Irrigation systems were invented five to seven thousand years ago as a means of supplying water to agriculture-based societies. Aqueducts and irrigation systems were among the first forms of wastewater engineering. As population centers became more dense, they were used to remove sewage from settlements. The Romans were among the first to demonstrate the effectiveness of the aqueduct. The Dark Ages marked a period where progress in water management came to a halt.
As populations grew, the management of human waste became a growing concern and a public health threat. By the 1850s in London, more than 400,000 tons of sewage were flushed into the River Thames each day - around 150 million tons per year. Diseases such as smallpox, diphtheria, measles, scarlet fever, typhus, cholera, and typhoid were spread via the contaminated water supply. During the 19th century, major cities started building sewage systems to remove human waste out of cities and into rivers.
During the 1900s, the activated sludge process was invented. The activated sludge process is a form of water purification that uses bacteria to consume human feces. Chlorine is used later in the process to kill off the bacteria.
Over the centuries, much has changed in the field of wastewater engineering. Advancements in microbiology, chemistry, and engineering have drastically changed the field. Today, wastewater engineers also work on the collection of clean water for drinking, chemically treating it, and using UV light to kill off micro-organisms. They also treat water pollution in wastewater (blackwater and greywater) so that this water may be made safe for use without endangering the population and environment around it. Wastewater treatment and water reclamation are areas of concern in this field.
Introduction
The selection of source of water with minimum number of impurities is the most important essential of water supply system. The next thing in the water supply system is the complete layout from the source of supply to the distribution. The primary source of water is precipitation which falls on the earth's surface in the form of rain, snow, hails, dew etc. The rainfall, being the most important part of precipitation is carried off in the following four different ways:
(a) Run-off
(b) Percolation or interflow
(c) Transpiration and
(d) Evaporation
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| Fig. 1 : Hydrological Cycle |
In this way, the processes of precipitation and evaporation continue like an endless chain and thus a balance is maintained in the atmosphere. This phenomenon is known as 'Hydrological Cycle' as shown in Fig. 1.
Sources of Water
The sources of water may be classified as surface sources and sub-surface sources. The surface sources of water are the sources in which the water flows over the earth's surface. These include rivers, lakes, streams, natural ponds, storage or impounded reservoirs. The water from these sources is known as surface water. The sub-surface or ground water sources are those which supply water from below the earth's surface. These include springs, infiltration galleries, wells and porous pipe galleries.
The quantity of surface water depends mainly upon rainfall. The topography of the catchment area is important in addition to the rainfall and run-off in case of impounded reservoirs. There are two types of impurities in water supply i.e. suspended and dissolved. The surface waters have suspended impurities. The suspended matter contains the pathogenic or disease producing bacteria. Therefore, surface waters are not considered safe for water supply without the necessary treatment.
The sub-surface sources are the important sources of water supply. The water of ground water sources does not require any treatment and its temperature remains uniform throughout the year. The rain that falls on earth’s surface percolates partly through it and becomes the sub-surface water or ground water. The earth is formed of different layers of materials like sand, gravel and lime stone etc. The layers which allow the water to pass through them and contain quantities of water are known as pervious layer or aquifers or water bearing strata i.e. layers of sand, gravel etc. The layers such as limestone and sand stone which do not allow the water to pass through them are called impervious strata or aquiclude. The top surface of water in the soil is called underground water table or simply water table. The portion of the soil through which lateral movement of water takes place, is called zone of saturation.
The natural outflow of sub-surface or ground water at earth's surface is called a spring. This is also called out-cropping of the water table. Some springs discharge hot water due to the presence of sulphur and other materials. The spring may be gravity springs (surface spring and shallow spring) and artesian or deep seated spring.
When an acquifer is exposed in a valley against a vertical cut, a surface spring is formed.
In shallow springs, the ground water is collected in the form of reservoir and forces the water to overflow at the surface of the ground.
An artesian or deep seated spring is formed when pervious or water bearing stratum is closed between two impervious stratums and have too much hydraulic gradient and water flows out automatically.
The horizontal tunnels constructed at shallow depths along the banks of a river to intercept the ground water table are called infiltration galleries. The quantity of water available from infiltration gallery depends upon the yield of the source, nature of soil and size of gallery. The yield of the gallery may be about 15000 litres per metre length of the gallery. The infiltration galleries are most suitable for increasing the surface source supply in dry season.
The vertical wells provided along the banks of a river to draw ground water in dry season and percolated water in rainy season, are called infiltration wells. These are also called rainy wells.
A vertical bole dug out or drilled into the ground to get sub-surface or ground water is known as a well. The wells may be gravity wells, artesian wells, shallow wells, deep wells, open or dug wells, tube wells and driven wells.
A tube or pipe fitted at the bottom with a filter sunk into the ground to tap the underground water is known as tube well. Now-a-days, tube wells are becoming more and more popular for supplying water. The quality of tube well water is good as compared to surface water.
Yield of a Well
The rate at which water percolates into well under safe maximum critical depression head is called yield of a well. The water level goes down when the water is drawn from the well. The difference between the depressed water level and the sub-soil water table level is called depression head or infiltration head.
The yield of the well is measured either in cum/hour or litres/hour. It depends upon the permeability of the soil, position of water table, depth of well in impervious layer and rate of withdrawal.
The yield of a surface stream may be obtained by stream gauging, cross-section velocity method and chemical method. The yield of a underground source may be obtained by constant level pumping test and recuperating test.
The yield of the well is measured either in cum/hour or litres/hour. It depends upon the permeability of the soil, position of water table, depth of well in impervious layer and rate of withdrawal.
The yield of a surface stream may be obtained by stream gauging, cross-section velocity method and chemical method. The yield of a underground source may be obtained by constant level pumping test and recuperating test.
Quantity of Water
Forecasting Population
Quality of Water
Physical Examination of Water
Chemical Examination of Water
Living Organisms in Water
Biological Tests
Water Treatment
Softening of Water
Distribution of Water
Water Distribution System
Methods of Water Distribution System
Sanitary Engineering
Terms Used in Sanitary Engineering
Storm Sewage
Design of Sewers
The internal diameter of the sewer should not be less than 15 cm. The small sewers require greater care as compared to larger sewers. The velocity which causes both floating and heavy solids to he transported easily with flow is called self cleaning velocity. The slope of the sewer line should be in the direction of die slope of the ground. It may be noted that larger the size of the sewer, the higher is the velocity of flow. Therefore, larger sewers should be laid at flatter slopes. The gradient should be such that the velocity of flow should not become too high or too low and should be within permissible limits.
Methods of Sanitation
The sanitation of a town is done by the following two methods :
1. Conservancy or dry method and
2. Water carriage method
The waste products of a town are collected and carried by these methods to the treatment or disposal works. In the conservancy or dry method, different types of refuse are collected, carried and disposed off separately. In the wafer carriage method, water is used as a medium to convey the sewage to the point of treatment or disposal. This method is more hygienic, but the initial cost for the construction of the system and the maintenance cost is high. There is no smell or sight nuisance in the water carriage method and the sewers being all underground do not impair the beauty of the place.
1. Conservancy or dry method and
2. Water carriage method
The waste products of a town are collected and carried by these methods to the treatment or disposal works. In the conservancy or dry method, different types of refuse are collected, carried and disposed off separately. In the wafer carriage method, water is used as a medium to convey the sewage to the point of treatment or disposal. This method is more hygienic, but the initial cost for the construction of the system and the maintenance cost is high. There is no smell or sight nuisance in the water carriage method and the sewers being all underground do not impair the beauty of the place.
Sewerage System
We already know that the process of collecting and delivering the sewage to the disposal point is called sewerage. The following are three systems of sewerage :
1. Combined system
2. Separate system and
3. Partially separate system
In a combined system, only one set of sewers are laid for carrying sanitary sewage and storm water. This system is most suitable for the locality which has narrow streets The maintenance cost of this system is very small. It also provides automatic flushing of sewers.
In a separate system, two sets of sewers are required. One for carrying the sanitary sewage and the other for carrying the storm water. This system is most suitable for rocky areas because laying of two small sewers is easy than one large sewer. It is also suitable when the topography is flat necessitating deep excavation for combined sewers.
In a partially’ separate system, a small portion of storm water is allowed to enter in the sanitary sewage within permissible limits and the remaining storm water flows in separate set of sewers. This system is suitable by areas where storm water is small in quantity due to scarcity of rainfall.
1. Combined system
2. Separate system and
3. Partially separate system
In a combined system, only one set of sewers are laid for carrying sanitary sewage and storm water. This system is most suitable for the locality which has narrow streets The maintenance cost of this system is very small. It also provides automatic flushing of sewers.
In a separate system, two sets of sewers are required. One for carrying the sanitary sewage and the other for carrying the storm water. This system is most suitable for rocky areas because laying of two small sewers is easy than one large sewer. It is also suitable when the topography is flat necessitating deep excavation for combined sewers.
In a partially’ separate system, a small portion of storm water is allowed to enter in the sanitary sewage within permissible limits and the remaining storm water flows in separate set of sewers. This system is suitable by areas where storm water is small in quantity due to scarcity of rainfall.
Classification of Sewers
The sewers are classified as discussed below :
1. According to the construction material used, the sewers are classified as follows :
(a) Cast iron sewers
(b) Stone ware sewers
(c) Concrete or R.C.C. sewers
(d) Masonry sewers
(e) Asbestos concrete sewers and
(f) Plastic sewer pipes.
2. According to the shape of the sewers, the sewers are classified as follows :
(a) Rectangular sewer
(b) Horse-shoe type
(c) Semi-elliptical sewer
(d) Basical handle type
(e) Circular sewer
(f) Egg-shaped sewer
1. According to the construction material used, the sewers are classified as follows :
(a) Cast iron sewers
(b) Stone ware sewers
(c) Concrete or R.C.C. sewers
(d) Masonry sewers
(e) Asbestos concrete sewers and
(f) Plastic sewer pipes.
2. According to the shape of the sewers, the sewers are classified as follows :
(a) Rectangular sewer
(b) Horse-shoe type
(c) Semi-elliptical sewer
(d) Basical handle type
(e) Circular sewer
(f) Egg-shaped sewer
Sewer Joints
The joints are essential for joining various pieces of sewer pipes in order to make one continuous length of sewer line. The sewer joints are of the following types :
(a) Spigot and socket joint
(b) Collar joint
(c) Mechanical joint
(d) Bandage joint
(e) Flush joint
(f) Filled and poured type joints
For the sewers of diameters Less than 600 mm, spigot and socket joint is preferred and for sewers of diameter more than 600 mm, a collar joint is used.
(a) Spigot and socket joint
(b) Collar joint
(c) Mechanical joint
(d) Bandage joint
(e) Flush joint
(f) Filled and poured type joints
For the sewers of diameters Less than 600 mm, spigot and socket joint is preferred and for sewers of diameter more than 600 mm, a collar joint is used.
Sewerage Appurtenances
The different devices required for construction, operation and maintenance of the entire sewerage
system is known as sewage appurtenances. These are classified as follows :
1. Man hole : It is an opening in a sewer line to provide access for a man for the purpose of inspection and cleaning .of sewer. The man holes are generally located at all changes of direction, at all changes of gradients and at all junctions of main and branch sewers. The maximum distance between man holes, for large sewers, should be 300 metres. The essential parts of a man hole are a working chamber, an access shaft, cover and steps of iron. The lower portion of a man hole is called a working chamber. The minimum diameter of a man hole cover should be 50 cm.
2. Catch basin. It is a structure constructed in the form of a chamber and provided along the sewer line to admit clear rain water free from suspended silt and grit, floating rubbish etc. into the combined sewer. The catch basins are located when the sewers are laid at very small gradient and the velocity of flow of sewage is less than the self cleaning velocity.
3. Flushing tanks : These are used to hold and throw water into the sewer for the purpose of cleaning. These are usually provided at the beginning of the sewers and may be either hand operated or automatic.
4. Ventilating shaft : It is a shaft provided for the purpose of ventilation of sewers. The ventilating shafts are required to prevent the accumulation of dangerous explosive and corrosive gases.
system is known as sewage appurtenances. These are classified as follows :
1. Man hole : It is an opening in a sewer line to provide access for a man for the purpose of inspection and cleaning .of sewer. The man holes are generally located at all changes of direction, at all changes of gradients and at all junctions of main and branch sewers. The maximum distance between man holes, for large sewers, should be 300 metres. The essential parts of a man hole are a working chamber, an access shaft, cover and steps of iron. The lower portion of a man hole is called a working chamber. The minimum diameter of a man hole cover should be 50 cm.
2. Catch basin. It is a structure constructed in the form of a chamber and provided along the sewer line to admit clear rain water free from suspended silt and grit, floating rubbish etc. into the combined sewer. The catch basins are located when the sewers are laid at very small gradient and the velocity of flow of sewage is less than the self cleaning velocity.
3. Flushing tanks : These are used to hold and throw water into the sewer for the purpose of cleaning. These are usually provided at the beginning of the sewers and may be either hand operated or automatic.
4. Ventilating shaft : It is a shaft provided for the purpose of ventilation of sewers. The ventilating shafts are required to prevent the accumulation of dangerous explosive and corrosive gases.
House Drainage
The following terms are commonly used in the house drainage system :
1. Soil pipe : It is a pipe through which liquid waste carrying human excreta flows.
2. Waste pipe : It is a pipe through which liquid waste without human excreta flows.
3. Vent pipe : It is a pipe installed for ventilation purposes. This pipe should be one metre above the roof level.
4. Rain water pipe : It is a pipe which carries storm water.
5. Anti-syphonage pipe : It is a pipe installed in the house or building drainage to preserve the water seal of a trap. It maintains proper ventilation and does not allow syphonic action.
1. Soil pipe : It is a pipe through which liquid waste carrying human excreta flows.
2. Waste pipe : It is a pipe through which liquid waste without human excreta flows.
3. Vent pipe : It is a pipe installed for ventilation purposes. This pipe should be one metre above the roof level.
4. Rain water pipe : It is a pipe which carries storm water.
5. Anti-syphonage pipe : It is a pipe installed in the house or building drainage to preserve the water seal of a trap. It maintains proper ventilation and does not allow syphonic action.
Plumbing Systems
The following arc the plumbing systems for the building drainage.
1. One pipe system : The system in which only one pipe is provided to collect both the foul soil waste as well as unfoul waste from the building is known as one pipe system. In one pipe system of plumbing, waste water is carried away from kitchens, w ash basins, bathrooms, water closets etc.
2. Two pipe system : The system in which two pipes are provided, one for collecting the unfoul water from kitchen, bathrooms, house washings, rain water etc. and the other for collecting soil waste, is called two pipe system. This system is mostly used in south-asian countries like India.
1. One pipe system : The system in which only one pipe is provided to collect both the foul soil waste as well as unfoul waste from the building is known as one pipe system. In one pipe system of plumbing, waste water is carried away from kitchens, w ash basins, bathrooms, water closets etc.
2. Two pipe system : The system in which two pipes are provided, one for collecting the unfoul water from kitchen, bathrooms, house washings, rain water etc. and the other for collecting soil waste, is called two pipe system. This system is mostly used in south-asian countries like India.
Traps
A depression or bend provided in a drainage system which is always full of water to prevent the entry of foul gases in the atmosphere is known as a trap. A trap prevents the entry of foul gases in the atmosphere but it allows the sewage to flow through it. A good trap should provide an adequate water seal at all times.
It should be made of non-absorbent material and should have self-cleaning velocity.
It should be made of non-absorbent material and should have self-cleaning velocity.
Privy
The arrangement made for collection of human excreta is known as privy. There are many types of privies such as privy pit, cess pool, chemical privy, septic tank, etc.
The privy pit is the most suitable and economical disposal unit for human excreta. It should be located at least three metres away from the well or other ground water supply. A vent pipe is provided to take the foul gases.
A cess pool is an underground structure in the form of a circular or rectangular tank for the purpose of admitting sewage into it from the intercepting chamber. These are very cheap in construction. It is not recommended by the health authorities.
A chemical privy is a dry privy and most satisfactory method of disposal of human excreta without water carriage.
A septic tank is an underground sewage system with complete treatment of sewage The septic tanks followed by sub-surface disposal of effluent are economical and most suitable This method gives satisfactory results in areas having porous soil.
The privy pit is the most suitable and economical disposal unit for human excreta. It should be located at least three metres away from the well or other ground water supply. A vent pipe is provided to take the foul gases.
A cess pool is an underground structure in the form of a circular or rectangular tank for the purpose of admitting sewage into it from the intercepting chamber. These are very cheap in construction. It is not recommended by the health authorities.
A chemical privy is a dry privy and most satisfactory method of disposal of human excreta without water carriage.
A septic tank is an underground sewage system with complete treatment of sewage The septic tanks followed by sub-surface disposal of effluent are economical and most suitable This method gives satisfactory results in areas having porous soil.
Sewage Disposal
The sewage should be disposed off without causing odour and nuisance. The sewage can be disposed off without treatment or after suitable treatment either on land or in natural water courses The land disposal of sewage is suitable under the following circumstances :
1. When there is no natural courses such as streams, rivers etc. for the discharge of sewage into them.
2. When the quantity of sewage is more which will pollute lite river or stream water.
3. When the rivers run dry or have a small flow during summer.
4. When the overall rainfall is very low so that the lands can be irrigated by sewage.
The land disposal of sewage requires large urea of sandy land or alluvial sod. The sewage should be given either preliminary or primary treatment.
The disposal of sewage (raw or treated) into natural water courses such as streams, rivers, sea and lake, is known as sew age disposal by dilution. This method is only possible when large quantity of natural water is present near the town. It is suitable under the following circumstances:
1. When thorough mixing of sewage is possible.
2. When the sewage arriving at the point of disposal is fresh and non-septic.
3. When there are strong forward currents.
4. Where there are no backward currents.
When the sewage is discharged into streams, its organic matter get oxidised by the amount of dissolved oxygen present in water and this matter is converted into m offensive substances
The natural water removes their deficiency of oxygen consumed by organic matter by the absorption of atmospheric oxygen. This action is known as self-purification of streams.
Thus the self-purification of streams may be defined as “the natural process in which the oxygen of water of streams is consumed by the sewage and at the same time, it is replenished or filled again by the atmosphere."
1. When there is no natural courses such as streams, rivers etc. for the discharge of sewage into them.
2. When the quantity of sewage is more which will pollute lite river or stream water.
3. When the rivers run dry or have a small flow during summer.
4. When the overall rainfall is very low so that the lands can be irrigated by sewage.
The land disposal of sewage requires large urea of sandy land or alluvial sod. The sewage should be given either preliminary or primary treatment.
The disposal of sewage (raw or treated) into natural water courses such as streams, rivers, sea and lake, is known as sew age disposal by dilution. This method is only possible when large quantity of natural water is present near the town. It is suitable under the following circumstances:
1. When thorough mixing of sewage is possible.
2. When the sewage arriving at the point of disposal is fresh and non-septic.
3. When there are strong forward currents.
4. Where there are no backward currents.
When the sewage is discharged into streams, its organic matter get oxidised by the amount of dissolved oxygen present in water and this matter is converted into m offensive substances
The natural water removes their deficiency of oxygen consumed by organic matter by the absorption of atmospheric oxygen. This action is known as self-purification of streams.
Thus the self-purification of streams may be defined as “the natural process in which the oxygen of water of streams is consumed by the sewage and at the same time, it is replenished or filled again by the atmosphere."
Sewage Treatment
It is the artificial process of removing or changing the objectionable constituents present in the sewage in order to make it less offensive and dangerous for its disposal.
The sewage contains large quantity of bacterias (pathogens and non-pathogens). The sewage bacterias are also classified as aerobic bacterias and anaerobic bacterias. The bacterias which exist in the presence of light and take free oxygen in dissolved conditions, are known as aerobic bacterias. The bacterias which exist under dark and stagnate conditions in the absence of oxygen, are known as anaerobic bacterias.
The sewage contains large quantity of bacterias (pathogens and non-pathogens). The sewage bacterias are also classified as aerobic bacterias and anaerobic bacterias. The bacterias which exist in the presence of light and take free oxygen in dissolved conditions, are known as aerobic bacterias. The bacterias which exist under dark and stagnate conditions in the absence of oxygen, are known as anaerobic bacterias.
Notes:
1. The oxygen dissolved in the fresh sewage is known as dissolved oxygen. It depends upon the temperature. As the temperature increases, the dissolved oxygen decreases.
2. The amount of oxygen required by sewage from an oxidising agent like potassium dichromate, is known as chemical oxygen demand (C.O.D.).
3. The amount of oxygen required for the biological decomposition of dissolved organic solids to Occur under aerobic conditions and at a specified time and temperature is called bio-chemical oxygen demand (B.O.D).
Classification of Sewage Treatment
The sewage treatment may be classified as primary treatment and secondary treatment. The treatment which employs the mechanical and hydraulic separation principle for removing solids and suspended organic matter is known as primary treatment. In primary treatment of sewage, lighter materials are removed. The primary treatment units are screening, grit chamber, plain sedimentation and sedimentation followed by coagulation.
The treatment which works on the biological principles i.e. (he degradation of organic matter by the agency of living organism is known as secondary treatment. In secondary treatment of sewage, fine dissolved organic materials are removed. The secondary treatment or biological treatment of sewage is done by sewage filters and activated sludge process units, The filter mostly used is trickling filter. These filters may be low rate trickling filters or high rate trickling filters.
The activated sludge is a sludge floc produced in a raw settled sewage by the growth of bacteria and other organisms in the presence of dissolved oxygen and accumulated in sufficient concentration by returning floc previously formed. The activated sludge contains fertilising constituents, indicates the degree of aeration and high water content. The sludge treatment is mainly done in order to stabilize the organic matter, to destroy (he pathogenic bacterias and to reduce the water content.
Note:
Grit chambers are used to remove grit from sewage. The grit removal is essential before its dilution in water, and before it settles in sedimentation tank.