1. SEWAGE : Sewage is water carried in solution or suspension, that is intended to be removed from a community . Also known as WASTE WATER Requirement of estimation of the sewage discharge is required otherwise sewers will either prove to be inadequate resulting in overflow or a size larger than required resulting in unnecessary wasteful investments. Therefore theoretically: Quantity of sewage = Water Supplied to the (domestic +industrial) contributing area

2. In actual practise, its not the precise quantity which appears as sewage due to the Additions and Subtractions as follow: ADDITIONS: Additions due to unaccounted private supplies: Private wells at times are also used as source of water. Large scale private industries also have their private sources of water. The unaccounted water quantity can be estimated by actual field surveys Additions due to infiltration: Sewer systems are usually infiltrated with water seeping

3. SUBTRACTIONS: Subtractions due to water lossess: The water loss due to leakages in water system therefore not reaching consumers only. Subtractions due to water not entering the sewerage system: The amount of water used which is used by the public for purposes such as sprinkling the streets, lawns, steam boilers; water consumed in industrial products such as beverage etc. is the water which doesnt enter the sewerage system. hence , Total water entering = Total water supplied + additions - subtractions

4. DESIGN PERIODS AND FUTURE FORECASTS A sewerage scheme involves the laying down of costly treatment units, which cannot be replaced easily in order to suffice the growing population and accordingly growing sewage discharge. There for a future period is taken for which the provision made in designing the capacities of the various components of the sewerage scheme is done. This period is known as Design Period. The design period is driven by the following considerations: Should not exceed the usable life period of the components. Ease and difficulty that is likely to be faced if expansion

5. The rate of interest, if the rate is small a higher value of design period maybe economically justified and therefore adopted. Additional investments likely to be incurred for providing additional provisions. Anticipated growth of population, of change in landuse of the area. FUTURE FORECASTS AND ESTIMATING DESIGN SEWAGE DISCHARGE The quantity of sewage that is likely to pass through a sewer (Q`) at the end of the design period can be computed by : Percapita production of sewage(q`) * expected population at the end of the design period the population can be forecasted using the population from the census department and extrapolating using various methods such as :

6. The flow of sewage depends on the per capita water supplied. The flow in these sanitary sewers fluctuates seasonally, monthly , daily as well as hourly. The peak flow occurs will depend upon the flow time in sewers. The peak flows will be much larger for smaller lateral sewers than large sewers ( long distance sewers). The sewer greatly depends on the population. The peak is investigated using :

7. DRAINAGE: The natural or artificial removal of surface and subsurface water from an area is known as drainage. When the rain falls on a certain area, a part of it is intercepted a part of it is evaporated and the remaining water flows overland towards the valleys as storm water. To design a drainage system, frequency of rain is taken. The frequency of rainfall adopted in design should neither be so large as to cause too heavy investments nor should be so small as to cause very frequent overflowing drains.

8. ESTIMATING PEAK RUN OFF: The peak run- off that is produced from a catchment depends upon a numerous factors such as: The rain run off produced during a monsoon season is generally very high say 20-25 times the time of sewage Discharge. Also called as DRY WEATHER FLOW. To determine the Peak run off / drainage discharge RATIONAL METHOD Rational Method is used mostly to Calculate the peak run off for areas less than 50 hectares. For areas larger than that empirical formulas are used. Type of precipitati on Intensity of rainfall Durati on of rainfall Rainfall distribut ionSoil moisture deficiency Direction of prevailing storm Climatic conditio ns Shape and size of catchment basin

9. RATIONAL METHOD if a rainfall is applied to an impervious surface at a constant rate, the resultant run off from the surface would finally reach a rate equal to the rainfall. This will happen after a certain gap of time. Time of concentration: The time period after which the entire area will start contributing to the run off . Further it has been established that the maximum run off will be obtained from the rain having a duration equal to the time of concentration and this is called the critical rainfall duration. The formula was evolved by the efforts of Fruhling of Germany, Kuichling of America and Llyod davis of

10. Qp=(1/36) K.Pc.A Qp= peak rate of runoff in cumecs K= coeff of run off A= the catch ment area contributing to runoff is considered (taken in hectares) Pc= critical rainfall intensity during critical rainfall duration. COEFFICIENT OF RUNOFF (K): The impervious factor of run off , representing the ratio of precipitation. Different surfaces have different Coefficients: For eg: K=1 for perfectly impervious surface K= .9 for paved areas and 0.15 for lawns

11. TIME OF CONCENTRATION: may be defined as the time taken for the water to reach the outlet point from the most remote point of the drainage area. The time of concentration for a given storm water drain generally consists of two parts: Inlet or overland flow time : the time taken by the water to flow over the land from the critical point up to the point where it enters the drain mouth. Estimated by: Ti= {0.855 L3/H} Ti= inlet time (in hours) L= Length of overland flow in kilometres from the critical point to the mouth of the drain H= Total fall of level from the critical point to the mouth of the drain in metres

12. The channel flow time or gutter flow time (Tf): The time taken by the water to flow in the drain channel from the mouth to the considered point . This is attained using: Tf= length of the drain/ velocity in the drain Hence forth,: Tc= Tf + Tic The intensity of rain expressed in cm/hr is the rate at which the rain falls, this rate keeps on changing continuously throughout the storm period. The intensity of rain can be determined with the help of automatic rain gauges. These calculate the cumulative amount of rainfall with time on a graph paper. The rain fall intervals such as 5 minutes , 10 minutes , 15 minutes etc are worked out and analysed and converted in to intensity

13. The rain fall intervals such as 5 minutes , 10 minutes , 15 minutes etc are worked out and analysed and converted in to intensity duration curves. An example of intensity duration curve

14. PEAK DRAINAGE USING EMPIRICAL FORMULAS 1) Burkli- Ziegler Formula- the oldest empirical formula used for determining peak run off rate. Followed in Sweden and the USA 2) Dickens formula: this formula is generally used for Indian catchments , particularly north India. The value of C in this formula is ascertained for each catchment and depends upon the nature of the catchment and intensity of rainfall. The value of C is generally taken as 11.5 as has to be increased for hilly catchments and vice versa 3) Ryves Formula: similar to dickens formula. Used majorly in south Indian states. Average C1 o be taken is 6.8. 4) Inglis Formula: Applicable to fan shaped catchments in Old Bombay state of India.

15. Question: Assuming that the surface on which the rain falls is in a district is classified as follows: Total area of the district is 36 hectares and maximum rain intensity is taken as 5 cm/hr. What is the total Run off Rate? TYPE OF SURFACE % AREA RUN OFF RATIO ROOF 20% 0.9 PAVEMENTS 20% 0.85 PAVED YARDS 5% 0.80 MACADAM ROADS 15% 0.40 LAWNS, GARDENS 35% 0.10 WOODED AREA 5% 0.05

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