In the primary treatment process of sewage, various suspended; floating and oily substances are removed from the sewage. The units of primary treatment are:
Screens.
Grit chamber.
Detritus tank.
Skimming tank.
Primary sedimentation tank.
Flow diagram of Primary Treatment Process of Sewage
1) Screens
The screen is the first unit of the primary treatment plant.
The function of the screen is to remove all the floating matter of comparatively large size. Screens are located just before the grit chamber.
The screen may be constructed of M.S bars or rods, gratings, wire meshes or perforated plates. Screens are usually placed in an inclined position with an angle of 30° to 60° with the direction of flow.
Comminutors: Comminutors is the patented devices which break the larger sewage solids into small solids particles when the sewage is screened through them.
2) Grit Chamber
A grit chamber is an important segment in the primary treatment process of sewage. The grit chamber is an enlarged chamber or a long basin. They may be placed either before or after the screens.
The function of the grit chamberis to remove grit, sand and other inorganic matter from sewage before it enters the next treatment stage.
When sewage enters the grit chamber, it reduces the velocity of sewage movement through the chamber to allow heavy particles to settle down due to gravity.
3) Detritus Tank
A detritus tank is nothing but a grit chamber designed to flow with smaller flow velocity and a longer detention period ( 3 to 4 minutes).
The function of a detritus tank is to remove finer particles than those removed by a grit chamber.
4) Skimming Tank
In the primary treatment process of sewage skimming tanks are typically used for removing oil and grease from the sewage. They are placed before the primary sedimentation tanks.
The skimming tank consists of different compartments for completing different tasks. It works in several stages. The first stage is usually for the oil separation, in which oil and other floating pollutants are skimmed from the surface using special equipment such as oil skimmers or floating oil collectors. The second stage may include additional separation steps.
Overall we can say that the skimming tank is an effective and reliable process for the removal of oil and grease from sewage.
5) Primary Sedimentation Tank
It is also known as a primary clarifier or primary setting tank. The purpose of this unit is to remove the settleable solids present in the sewage. It is a rectangular tank constructed with brick masonry.
Baffle walls are provided in a zigzag way. Inlet and outlet pipes with valves are provided on opposite corners.
A sludge removal pipe is provided at the bottom of the tank.
The following are the physical characteristics of sewage:
l) Specific Gravity
The specific gravity of sewage is very nearly equal to that of water
2) Colour
The fresh sewage has a yellowish or grey or light brown colour. It becomes black or dark brown when the sewage attains septic stage. The colour of sewage can normally be detected by the necked eye.
3) Odour
Fresh sewage is practically odourless. But the stale sewage has an offensive odour of hydrogen sulphide and other sulphur compounds which is formed due to the decomposition of sewage. Odour is tested by osmoscope.
4) Temperature
The normal temperature of sewage is generally slightly higher than the temperature of the water. The average temperature of sewage in India is 20°C.
The temperature has an effect on the biological activity of bacteria present in sewage. Biological activities in sewage are higher at a greater temperature.
Temperature also affects the solubility of gases in sewage. In addition, temperature also affects the viscosity of sewage, which in turn affects the sedimentation process in its treatment.
5) Turbidity
Sewage is normally turbid. The turbidity directly depends upon the quantity of suspended particles. The degree of turbidity can be measured and tested by turbidity rods or by turbidimeters.
Following are the chemical characteristics of sewage:
Chemical characteristics of sewage
1) Solids
The sewage normally contains a very small amount of solids in relation to the huge quantity of water. It only contains about 0.05 to 0.1 % of total solid matters. The solid matters present the sewage may be in any of the four forms:
Suspended solids
Dissolved solids
Colloidal solids
And, settleable solids
Again the solids may be organic or inorganic.
It has been estimated that about 1000 kg of sewage contains about 0.45 kg of total solids, out of which 0.225 kg is in solution, 0.112 kg is in suspension, and 0.112 kg is settleable. Colloidal solids remain either in solution or in suspension.
Further, the solids in sewage comprise of both organic as well as inorganic solids. The organic matter is about 45% of the total solids and the remaining about 55% is the inorganic matter. Distribution solids present in 1000 kg of sewage can be present by the following diagram.
The total amount of solids present in given sewage can be determined by evaporating a known volume of sewage sample and weighing the dry residue left.
The quantity of suspended solids can be determined by passing a known volume of sewage sample through a glass-fibre filter apparatus and weighing the dry residue left.
The difference between the total solids and the suspend solids represents dissolved solids plus colloidal. The quantity of settleable solids can be determined with the help of the Imhoff cone.
2) pH Value
The pH value is defined as the logarithm of reciprocal of hydrogen-ion concentration present in water. It is used to designate the acidity and alkalinity of water.
Thus, pH value – Log[H+]
Nature of fresh sewage and treated sewage is alkaline and the septic sewage is acidic in nature. The pH value of fresh and treated sewage is generally more than 7 & the pH value of septic sewage is less than 7.
The pH value can be measured quickly and automatically with the help of a potentiometer.
3) Chloride Contents
Chlorides are derived from the kitchen wastes, human excreta, and industrial discharge. The normal chloride content of domestic sewage is 120 mg/lit.
The high chloride content of sewage indicates the presence of industrial sewage or infiltration of seawater.
The chloride content can be measured by titrating the wastewater with standard silver nitrate solution, using potassium chromate as an indicator.
4) Nitrogen Contents
The presence of nitrogen in sewage indicates the presence of organic matter. It may occur in one or more of the following forms:
a) Free ammonia
b) Albuminoidnitrogen
c) Nitrites
d) Nitrates
The presence of free ammonia indicates the first stage of decomposition of organic matter. Albuminoidnitrogen indicates the quantity of nitrogen present in sewage before the decomposition of organic matter is started.
The nitrites indicate the presence of partly decomposed organic matter. Nitrates indicate the presence of fully oxidized organic matter. The amount of free ammonia present in sewage can be easily measured by simply boiling and measuring the ammonia gas.
The amount of albuminoid nitrogen can be measured by adding a strongly alkaline solution of potassium permanganate to the already boiled sewage sample and again boiling the same.
The amount of nitrites or nitrates present in sewage sample can be measured by color matching methods.
5) Presence of fats, greases and oils
These are derived in sewage from the discharges of animals, kitchens of hotels, industries, etc.
In order to determine the amount of fats, greases, etc. a sample of sewage is first evaporated.
The residual solids left are then mixed with ether and the solution is then evaporated, leaving behind the fat, grease as a residue.
6) Presence of sulphides, sulphates and Hydrogen sulphide gas
Sulphides, sulphates and hydrogen sulphide gas are formed due to the decomposition of various sulphur-containing substances present in sewage.
Their presence in sewage reflects aerobic, and/or anaerobic decomposition. The determination of sulphides, sulphates in sewage is rarely called for.
7) Dissolved oxygen (D.C)
This represents the amount of oxygen present in sewage in a dissolved state. Sewage has generally no dissolved oxygen. Its presence in raw sewage indicates that the sewage is fresh.
Its presence in the effluent after treatment indicates that considerable oxidation has been accomplished by the sewage treatment methods.
The D.O in fresh sewage depends upon temperature. If the temperature of the sewage is more, the D.O content will be less. The D.O content of sewage is generally determined by Winkler’s method.
All the sewer pipes are generally laid starting from their outfall end towards their starting. The laying of new sewer pipes consists of the following 9 steps:
1. Marking Of The Alignment
The center line of the sewer is marked along the road with a theodolite and invert tap. It may be marked either by the reference line or with the help of the sight rail. The position of the manhole is also marked.
2. Excavation Of Trench
After marking the center line of the sewer, the excavation of the trench is started. The excavation may be carried out either by manual labor or by machines like power shovels, track excavators, etc.
The width of the trench at the bottom is generally kept 15 cm more than the dia of the sewer pipe. At the point of sewer joint, the width of the trench is made 60 cm for a length of 60 cm. The invert level is fixed by the boning rod.
3. Timbering Of The Trench
When, in ordinary soil, the depth of excavation is more than 2 m, timber bracing or sheet piling is provided on both sides of the trench so that it may not collapse. The extent of timbering required depends upon the type of soil and the depth of excavation.
4. Dewatering Of Trench
If water is met with during excavation, it is removed by pumping or any other suitable method.
5. Preparation Of Sub-grade
For soft soil, the bed of the sewer is prepared by plain concrete (1:3:6). The thickness of concrete varies from 15 to 20 cm. The bedding layer is not required in case of rocky or hard soil.
6. Laying And Joining Of Pipes
The sewers are laid along the trench very carefully. Then the joining of the sewer is done as per requirements. After joining, both sides of the pipe are finished with concrete.
8. Testing of Straightness Of Alignment And Obstruction
The straightness of the sewer pipe and the presence of any obstruction are tested by placing a mirror at one end of the sewer and a lamp at the other end. If the pipeline is straight, the full circle of light will be observed.
The presence of an obstruction in the pipe can also be tested by inserting a smooth ball at the upper end of the sewer. The dia of the ball is 13 mm less than the internal diameter of the sewer. If there is no obstruction inside the sewer, the ball shall roll down and reach the lower end of the sewer.
9. Back Filling
Lastly, the trenches are filled up with the excavated earth in layers about 15 cm thick. Each layer is properly watered and rammed.
Burkli Ziegler formula is the oldest formula used to determine the peak runoff rate. It was derived by a swiss engineer for local conditions. After that, it was followed in the whole U.S.A.
A sewer is an underground pipe used to carry wastewater. Or, we can say it is an underground system used to carry sewage. In this post, I will show you 10 different shapes of sewers with diagrammatic images as well. Mostly circular-shaped sewer is preferred for most places and is suitable for all types of sewage. Shapes of sewer can also be called types of sewer according to their shapes. Let’s begin.
1. Circular Shaped Sewer
Circular sewer is mostly used sewer and is preferred for all types of sewage. This shape of sewer is easy to construct, requires less construction materials and is economical.
Circular Shaped Sewer
2. Standard Egg-shaped Sewer
In the same flow condition, a standard egg-shaped sewer gives higher velocity as compared to a circular-shaped sewer. Therefore, it is suitable for low flow conditions. The construction process of such types of sewers is difficult and requires more amount of construction materials. Hence, it is costly. A Standard egg-shaped sewer is preferred for combined sewer.
Standard Egg-shaped Sewer
3. New Egg-shaped Sewer
The new egg-shaped sewer is also preferred for the combined sewer. The advantages and disadvantages are the same as I said for standard egg-shaped sewer. Read Also – Sewer Appurtenances.
New Egg-shaped Sewer
4. Horse Shoe Shaped Sewer
As the name suggests, its shape looks like a Horseshoe. Horse-shoe shaped sewer is a large size of sewer mostly preferred for the location where heavy discharge is required. The invert portion may be circular, parabolic or flat. The height of such types of sewer is more than its wide. As the size is large, maintenance work is also so easy.
Horse Shoe Shaped Sewer
5. Parabolic Shaped Sewer
Parabolic-shaped sewer is used for lower quantities of sewage discharge. This is small in size and the upper edge of the sewer is made like a parabola. The invert section of this sewer may be parabolic or elliptical in shape. It is economical as well. Read Also: – Brick Sewer.
Parabolic Shaped Sewer
6. Semi-Elliptical Shaped Sewer
Semi elliptical shape sewers are mostly preferred for carrying large amounts of sewage. The upper arch of this sewer forms an elliptical shape. The invert of this sewer may be parabolic or elliptical.
Semi-Elliptical Shaped Sewer
7. Rectangular Shaped Sewer
A rectangular-shaped sewer is a large sewer mostly used for carrying a large quantity of sewage, it is also used for discharging stormwater. Construction and maintenance work is very easy for this type of sewer. This section is very stable as compared to others. As they are rectangular in shape, the upper and invert both portions are flat.
Rectangular Shaped Sewer
8. U-shaped Sewer
As the name suggests, its shape is similar to the English capital letter “U”. The U-shaped sewer is used for heavy discharge of sewage. As it is large in size, it is also used for carrying stormwater as well. The invert portion for such types of sewer is semi-circular in shape. It is also used as a combined sewer. As is it large in size, its maintenance works are also easy. Read Also: – Plastic Sewer.
U-shaped Sewer
9. Semi-circular Shaped Sewer
It is suitable for the location where a large sewer is required. The upper arch of the sewer forms a semi-circular shape. It has a large section and is used for heavy sewage discharge. But, it is old and outdated. Hence, it is not preferred at the present time.
Semi-circular Shaped Sewer
10. Basket Handle Shaped Sewer
Basket Handel shaped sewer is used for the small amount of sewage discharge. Its upper portion is like a basket Handel and its lower portion got the shape of a narrow channel. But its outer surface is circular. It is also an outdated sewer.
The use of plastics for non-pressure sewer pipes is of comparatively recent origin and is still in the experimental stages.
However, some countries, such as the Netherlands, Scandinavia, France, etc., have already begun to use plastic pipes for sewers of 250 mm dia and above on a moderate to large scale (15% to 25% or so).
Their use is hardly 5-7 percent in Germany and the United Kingdom, and almost no plastic pipes are used in India for laying sewers, although, they are increasing the use of plastic pipes for internal water supply and drainage fittings.
In addition, plastic pipes of up to 1.50 mm dia have also begun to be used in house connections in India to carry sewage to the municipal sewers.
Although, the use of plastic pipes for non-pressure sewers is not very promising, however, advanced plastic technology has increased the use of plastic sewers.
The test results on uPVC pipes have shown that:
These pipes continue deformed for about two years; the deformation rate decreases over time.
Its deformation is mainly influenced by the type and method of application of the backfill material.
Traffic loading has a little effect on the final deformations, but it reduces the time to achieve equilibrium conditions; and
Due to excessive deformation, a PVC pipe failed to operate as part of an operational sewer system.
The most advanced development of a rib-reinforced solid wall uPVCpipe (ULTRA- RIB), which has been largely tested in the U.K. and Scandinavia, has shown very much satisfying results.
In ancient times mostly bricks were used as a sewer material, However, they have now almost been replaced by cement concrete sewers.
However, the brick sewer may still be used at places where the sewers are required to be constructed at the site, and if the ingredients needed for cement concrete sewers may not be easily available. They may also be preferred for constructing large-sized combined sewers, or particularly for stormwater drains.
Brick sewers are generally plastered on their outer surfaces so as to prevent the entry of tree roots and groundwater through the brick joints; are lined inside with stoneware or ceramic block so as to render them smooth and hydraulically efficient.
The stoneware or ceramic coating also helps in resisting sulfide corrosion which is not possible with ordinary cement plaster as the same is easily attacked by sewer gases like hydrogen sulfide.
Various poisonous and explosive gasses are commonly found in sewers such as hydrogen sulfide (H2S), carbon dioxide (CO2), and methane (CH4) along with petrol vapors. These gases are produced largely when the sewage becomes stale and septic, which occurs more commonly in hotter climates.
In addition to the production of these harmful gases, oxygen is consumed by the organic matter for their decomposition, resulting in a lack of oxygen inside the sewer and can, therefore, cause difficulty in breathing.
Various methods are adopted for ventilating the sewers in order to avoid the large-scale presence of these poisonous and hazardous gases inside the sewers. In addition to the ventilation of sewers, the following precautions should also be taken while allowing the workers to enter the sewers:
1. The very first precaution which should be taken before entering a sewer through a manhole is to open the covers of the manholes; at least half an hour in advance. This will help in obtaining some ventilation and exposure of the sewer to the atmospheric oxygen.
2. Tests should be conducted in order to identify the presence of any harmful gases inside the sewer. The following tests may be carried out to detect their presence.
The first test is performed to detect H2S gas present in the sewer.
The second test is performed to detect CO2 gas present in the sewer.
The third test is performed to detect CH4 gas present in the sewer.
3. If the above hazardous gases are absent, a lighted lantern may be lowered down the manhole, so as to test the presence of oxygen. If it burns brilliantly, the sewer can be safe to enter.
4. In any case, the staff who go down into the sewer for inspection must be bound to their waists with ropes and held them fast by persons on the top so that they can be pulled up quickly in case of any risk. A responsible officer should be available to handle the operations perfectly.
5. It is strictly prohibited to smoke or bring open light inside the sewers.
6. Necessary indicators of alert should be set up.
Septic tank is rectangular in plan and constructed with brick masonry over a concrete foundation. The length of the tank is usually 3 times the width. R.C.C cover with manhole is provided at the top of the tank.
How does a Septic Tank work?
Septic tank is just like a plain sedimentation tank. The sludge digestion and the sedimentation takes place in the same chamber of this tank. The biochemical reaction takes place by the anaerobic bacteria.
During the detention period, sewage is purified and the effluent is taken to soak pits for disposal. It can remove about 90% of B.O.D and about 80% of suspended solids.
Where a Septic Tank is Suitable?
In order to provide the satisfactory disposal of sewage received or obtained from isolated buildings, small institutions, hotels, and camps, etc. septic tanks may be adopted.
They are suitable for isolated or undeveloped areas where municipal sewers are not laid.
