In this essay we will discuss about:- 1. Introduction to the Design of Sewers 2. Estimate of Sanitary Sewage 3. Design Period 4. Population Estimate 5. Area 6. Per Capita Sewage Flow 7. Ground Water Infiltration 8. Estimate of Storm Runoff 9. Flow Assumptions 10. Determination of Velocity of Flow by Empirical Formulae 11. Velocity of Flow 12. Velocity at Minimum Flow and Other Details.
Essay # 1. Introduction to the Design of Sewers:
After the determination of the quantity of sewage, variation in its quantity, the next step is to design the sewer section, which will be economical in construction as well as can take the required discharge of the sewage at self-cleaning velocity. The design of sewer is different from that of conduit used for the conveyance of water in case of water supply.
The main difference is that in case of water supply, the water flows in pipes under pressure, whereas in case of sanitary sewers the sewage flows under gravitational force only. For most of the period the sewer is partly filled with sewage, whereas in case of water supply the pipes always remain full. Due to the above reasons all the sewers are to be designed and laid at such slopes that they can collect the sewage of the town and carry it up to point of disposal.
Essay # 2. Estimate of Sanitary Sewage:
Sanitary sewage is mostly the spent water of the community draining into the sewerage system with some quantity of ground water and a part of storm run-off from the area draining into it. The sewer should be capable of receiving the maximum expected discharge at the end of the design period.
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In the early years provision should not be made much in excess of the actual discharge, because otherwise it will cause depositions in the sewer lines. The estimate of flow, therefore, requires a very careful consideration and should be based on the contributory population and the per capita flow of the sewage, both the factors being guided by the design period.
The sewers should be designed large enough to carry the maximum discharge while flowing two-third full (for large sewers i.e. more than 75 cm diameter) and while flowing about half full in case of smaller sewers less than 75 cm diameter.
The above margin is kept as factor of safety against the following:
(i) Infiltration of underground water, storm water or illegal connections through cracks or open joints.
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(ii) Unforeseen increase in the population or water consumption due to festivals, melas, etc.
(iii) Due to rapid development of the big industries and the town or low estimation of the maximum flow.
Essay # 3. Design Period:
Sewers are usually designed for the maximum expected discharge to meet the requirements of the ultimate development of the area, because it is both difficult and uneconomical to augment the capacity of the sewerage system at a later date. That is why the population estimate is guided by the anticipated ultimate growth rates which may differ in the different zones of the same town. The sewer lines are designed for a design period of 30 years.
Essay # 4. Population Estimate:
The most suitable approach for design of sewers is to base the estimate on anticipated ultimate density of population or floor Space Index. When the desired information on population is not available in the Master Plan of the town, the flowing densities may be used.
In the towns where the flow Space Index (FSI) or Floor Area Ratio (FAR) limits are fixed by the local bodies, this approach may be used for working out the population density. FSI or FAR is the ratio of total floor area (of all the floors) to the plot area. The densities of population on this concept may be worked out, as shall be clear from example 1 below.
Example 1:
Following reservation of areas has been provided in the master plan of a town. Assuming an FSI of 0.15 and FAR of 9 m2/person, determine the density of the population.
Roads – 20%
ADVERTISEMENTS:
Gardens – 16%
Schools (including playgrounds) – 5%
Market – 2%
Hospital and Dispensary – 2%
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Solution:
Total of all the areas given above is 45% therefore area available for residential development is 55%.
Actual total floor area= Area for residential development × FSI
... No of persons or population density per hectare
Essay # 5. Area:
The tributary area for any sewer section under consideration should be marked on the key-plan. The topography of the area, lay-out of buildings, legal limitations etc. determine the tributary area draining to a sewer section. The area for designing the sewer is to be measured from the plan.
Essay # 6. Per Capita Sewage Flow:
It has been observed that a small portion of spent water is lost in evaporation, seepage in ground, leakage etc. But for the design of sewers the entire spent water or water supplied per capita may be taken into account.
In some arid areas, the sewage reaching the sewer may be as low as 40%, while for intensity developed area, it may be 90%. Usually 80% of the water supply may be expected to reach the sewers. The sewer lines should be designed for 150 c/d.
In the industrial areas, some industries often use the water other than from municipal supply, from their own tube-wells or other sources and more often discharge their industrial waste into corporation sewers. At all such places, proper calculation of extra sewage entering the sewers should be made separately. When the industrial waste is large, it should be segregated and treated suitably before discharge into sewers.
Essay # 7. Ground Water Infiltration:
The quantity of ground water entering the sewers depends on the workmanship in laying the sewers and height of the ground water table.
As the sewers are designed for the peak discharges, allowance for ground water infiltration for the worst conditions should be made as follows:
Essay # 8. Estimate of Storm Runoff:
The empirical formulae that are available for estimation of the storm water runoff can be used only when comparable conditions to those for which the equations are derived initially can be assured.
A rational approach, therefore, demands a study of the existing precipitation data of the area concerned to permit a suitable forecast. Storm sewers are not designed for the peak flow of rare occurrence such as once in 100 years. It is necessary to provide sufficient capacity to avoid too frequent flooding of the drainage area.
Essay # 9. Flow Assumptions:
The flow in the sewers varies considerable from hour to hour and season to season, but for the purpose of hydraulic design of sewers, it is the estimated peak flow, which is adopted.
The peak factor or the ratio of maximum to average flows, depends upon the contributory population and the following values are recommended for design:
Essay # 10. Determination of Velocity of Flow by Empirical Formulae:
Following formulae are generally adopted for the determination of the slopes, and the designed velocity of flow during the design of sewers:
(A) Chezy’s Formula:
(B) Crimp’s and Burge’s Formula:
Where V, m and i have the same meaning as in formula (5.1) above.
This formula has been obtained by putting n = 0.012 in Manning’s formula.
(C) Bazin’s Formula:
M and I have the same meanings as given above.
(D) Hazzen William’s Formula:
Where m and I have the same meanings as given above
C = a constant, whose values are given in table 5.5.
(E) Manning Formula:
Following solved example will illustrate the use of the above formula.
Example 2:
Determine the velocity of flow in a circular sewer of diameter 150 cm, laid on a slope of I in 750 while flowing full. The sewer is made in cast iron and is not very old.
Solution:
Essay # 11. Velocity of Flow:
Sewage of all the towns carries large amounts of organic and inorganic solid matters, which remain floating or suspended due to the flow of the sewage. If the velocity of flow is small the floating and suspended solids will get deposited on the bottom of the sewer and will go on accumulating, thereby will reduce the sectional area of the sewer and will cause obstruction in the flow of sewage.
Therefore while designing the sewer the velocity in them should be kept such that no solid gets deposited in the sewer. The minimum velocity at which no solids get deposited in the invert of the sewer is called self-cleansing velocity.
Table 5.6 gives the self-cleaning velocity for various types of suspended solids present in the sewage. These velocities are recommended by Beardmore.
Table 5.7 gives the self-Cleaning velocity for various types of suspended solids present in the sewage. These velocities are recommended by Beardmore.
The self-Cleansing velocity of the sewage depends upon the scouring action of the flowing sewage.
The minimum velocity to cause the scouring of the suspension of solids heavier than the sewage or liquid which carry them is determined by the following Shied formula:
Where V = Velocity of the flow
K = Characteristics of the solids flowing in the sewage in suspension. Its values in metric units are between 0.06 to 0.04 for organic solids.
f = Darcy’s coefficient of friction. Its common value is 0.03.
ps = Specific gravity of the solids flowing in the sewage.
Its value is between 1.2 to 2.65.
p = Specific gravity of the liquid, in case of sewage it is water having p= 1.
g = Gravitational acceleration constant.
ds = diameter of the particle of solids to be carried by the liquid.
From the above formula (5.8) it is clear that heavier and sticky solids require higher velocity for their cleaning or moving than the smaller size particles which require smaller velocities for their transport.
For the design of sewage system it is assumed that it may contain sand particles up to 1.0 mm diameter and specific gravity 2.65 as inorganic matter. The organic solids of 5 mm diameter and specific gravity 1.2 may be present in the sewage. The minimum velocity for the transportation of the above inorganic or organic solids is about 45 cm/sec.
Therefore, while designing sewers the velocity of flow should be kept between 45 cm/sec to 90 cm/sec. During the design of sewers the organic solids larger than 5 mm. diameter and sand particles larger than 1 mm diameter are not removed by the sewage flow, but they are allowed to settle in the invert of the sewer, from where they are removed by scrapping during cleaning of the sewers.
Essay # 12. Velocity at Minimum Flow:
For avoiding steeper gradients which will require deeper excavations, it is the practice to design sewers for the self-Cleansing velocity at ultimate peak flow. This is done on the assumption that although silting might occur at minimum flow, the slit would be flushed out during the peak flow.
But even the problem of silting may have to be faced in the early years, particularly for smaller sewers which are designed to flow half-full, as the actual depth of flow then is only a small fraction of the full depth. In the same way the upper reaches of laterals will pose a problem as they flow only partially full even at the ultimate design flow, because of the necessity for adopting the minimum size sewer.
It has been recorded that for sewers running partially full, for a given flow and slope, velocity is little influenced by pipe diameter. It is recommended that upto 30.l.p.s. present peak flows, table 5.8 may be used, which would ensure minimum velocity of 0.60 m.p.s. in the early years.
Essay # 13. Minimum Sewer Size:
The minimum size of public sewer should not be less than 150 mm. However, recommended practice is to provide 200 mm size. In hilly areas where the maximum slopes are available it may be 100 mm.
Essay # 14. Sewer Grades:
The sewage flows in the sewers under gravitational force only, which is obtained by laying the sewers on slope or grade. The velocity of the flow directly depends on the grade of the sewer, by hydraulic mean depth and the condition of the sewer with respect to roughness. The designer has to determine the optimum slope or grade at which the sewers are to be laid, so that self-cleaning velocity is developed in the sewage flow.
Table 5.9 gives the values of the minimum gradients for obtaining flow velocity of 75,90 and 105 cm/sec. in circular sewers of various sizes running full. These values are based on the crimps and Burges formula.
Essay # 15. Limiting Velocities:
The lowest velocity in the sewer lines should be equal to the self-cleansing velocity so that up to certain limit the suspended and floating solids should flow with the sewage. The velocity in the sewer cannot be increased up to any limit, because if the velocity is increased too much the solids carried in suspension will move in contact of the sewer material, and they will cause wear of the contact surface and will also make the sewer surface rough.
At higher velocity the flow of sewage also becomes turbulent from streamline flow. This all will cause reduction in the carrying capacity of the sewers as well as will reduce the life of the sewer. Such problem of high velocity occurs in hill areas, where the sewers are to be laid along the steep natural slopes of the region.
The high velocities in the sewers can be controlled by limiting the grades and providing drop-manholes at suitable place, in the same way as falls are provided in the irrigation canals to control the velocity of the channel. The maximum permissible velocities or limiting velocities are given in table 5.10.
The limiting velocity depends mainly upon the hardness of the materials of which the sewers are made. More the harder material more will be the limiting velocity. The limiting velocity also depends on the resistance of the sewer material to the corrosive action of the gases produced in the sewer due to decomposition of the organic matter.
Following points should be kept in mind while designing the sewers in connection with the self-cleansing and non-scouring velocities:
(i) During the design of sewers, the discharge to be carried by the sewer is first determined. The velocity of flow and the gradient of sewers are then properly determined to achieve the required results.
(ii) While designing the sewerage system for the flat country, the sewers are laid such that self-cleansing velocity is developed at the maximum discharge. But the section of sewer is designed such that even with the minimum discharge the least velocity of 0.4 m/sec should be developed because if this minimum velocity of 0.4 m/sec is not developed, flushing tanks are to be provided for flushing the sewer lines.
(iii) While designing the sewerage system for the hilly or rough country, the sewers should be designed with the maximum permissible velocity at the maximum discharge of sewage and at self-cleansing velocity with the lowest discharge. To reduce the fall, if required, drop manholes should be provided.
(iv) In case of combined system of sewerage, the section of the sewer is to be designed in such a way that self-cleaning velocity must develop at the minimum discharge.
Essay # 16. Use of Tables in Design of Sewers:
Sewerage project of a town or city involves design of large number of sewer lines. The sizes and slopes of all the various sewers are to be calculated for carrying the estimated sewage flow at the self-cleaning velocities.
This work involves large calculation work which is very tedious and laborious. The work of designing is simplified by the use of various tables or monograms and charts.
Santo Crimps Table based on the formula V = 83.45.m2.3 i are most commonly used in India for the design of sewers.
Essay # 17. Variation in Velocities:
For most of the period the sewer does not run full. When the sewer is not running full, the velocity and other characteristics of flow can be determined. Circular section is most common in use, therefore, the effect of variation of flow in it will be discussed here, and therefore, the effect of variation of flow in it will be discussed here.
Referring to Fig. 5.1, Let
D = diameter of the sewer
A = cross-sectional area
= r/4 D2
P = Perimeter = r.D
V = Velocity of flow (when flowing full)
Q = discharge = A. V.
h = depth of flow when flowing partly full
a = cross section area when flowing partly full
p = The wetted perimeter when flowing partly full
u = The velocity of flow when flowing partly full
q = Discharge when flowing partly full = av.
Since, in all above derived equations, except Q, everything is constant. Hence by giving different values to Q, all the proportionate elements can be easily calculated.
By taking proportionate depth (d/D) as reference values of other elements can be found out as shown in table 5.11:
The values given in table 5.11 can be plotted by means of smooth curves as shown in Fig. 5.2. From these curves the values of the various elements of the circular sewers can be directly noted, without doing any tedious calculation work.
The through study of the table 5.1 and Fig. 5.2 reveals the following:
(i) When the sewer is running half full, its hydraulic mean depth and the velocity shall be the same or equal to when the sewer is running full.
(ii) The maximum velocity in the sewer is developed when the proportionate depth is 0.81. The maximum velocity developed in this proportionate depth is about 12.5% more than the velocity developed when the sewer is running full. Due to this reason the maximum discharge also increases.
(iii) The sectional area of the sewage flowing in the sewer decreases more rapidly than the wetted perimeter, below the mid depth, which reduces the proportionate hydraulic mean depth less than unity. But the velocity varies with (HMD)2/3 the decline or change in the proportionate velocity is not so sharp. The discharge also reduces considerably in this case.
Essay # 18. Sewer Transitions:
Sewer transitions include change on size, slope, alignment, volume of flow, free and submerged discharge at the end of the sewer lines, passage through measuring and diversion devices and sewer junctions. While designing allowance for the head loss which will occur should also be made.
Manholes should be provided at all such transitions and a drop should be provided where the sewer is intercepted at a higher elevation, taking head loss care and to help in maintenance. The vertical drop may be provided at such places where the elevation exceeds 60 cm, below which it can be avoided by adjusting the slope in the channel in the manhole connecting the two inverts.
Following invert drops should be provided:
(i) For sewers less than 400 mm. dia. – Half the difference in dia.
(ii) For 400 mm. – 900 mm. dia sewers. – 2/3rd the difference in dia.
(iii) Above 900 mm. dia. sewers. – 4/5th the difference in dia. meter.
Transition from larger to smaller diameters should not be made. The crowns of sewers are always kept continuous. The hydraulic flow line in the larger sewers should in case be higher than the incoming sewage.
Essay # 19. Back-Water Curves:
Back-water or drawdown curves resulting from abrupt changes in sewer slopes or when there is a free fall or an obstruction to the flow may be calculated from the following formula:
Where d and hy are the changes in the water depth and velocity in a length L; Se and So being the slopes of energy grade line and the invert respectively.
The calculations work is started from a point where depth and velocity of flow are known and L is worked out for different depths of flow up to the normal depth.
Essay # 20. Force Mains:
The sewage may have to be taken to higher elevations through force mains. The size of the main is calculated by taking into account the initial cost of pipe line and operation cost of the pump for various sizes of the mains. The velocity of sewage may range from 0.8 to 3.0 mps. The frictional losses are determined by Hazen and Williams formula 5.6.
For design purpose the value of constant ‘c’ may be taken as follows:
Losses in valves, fittings etc. depend upon the velocity head V2/2g. Losses in bends and elbows depend upon the absolute friction factor to the pipe dia., besides velocity head. Loss due to sudden enlargement depends upon the ratio of diameters. However, for shorter mains with a large number of bends etc. the actual loss may be completed and expressed as equivalent length of pipes.