to know the total quantity of sewage or wastewater of sewage which would flow through a sewer must be ¢ ion of the quantity of sewage would result in a sewer of fof size much larger than required thereby unnecessarily increasing the cost, ‘ofthe following two components: w (D.W.F) is the flow through the sewers that would be available throughout as well as rainy seasons. Me flow is the additional flow through the sewers that would occur during rainy seas 2 ig rainy e fetal quantity of sewage involves the estimation of each of these two component ethods used for estimation of the quantity of each of these two components o' ow (D.W.F) includ stic or sanitary sewage Which is the sewage or wastewater derived from residential build institutional and similar public buildings such as offices, schools, cinemas, hotel “sewage which is the sewage or wastewater obtained from manufacturing plants of th idwater infiltrating into the sewers through the pipe joints and other entry points. imation of dry weather flow therefore involves the estimation of each of these compone uantity of dry weather flow is affected by several factors which are indicated below. (2) Population growth (4) Infiltration and exfiltration ter supply. It is evident that a considerable part of the water supplied to the ter supply system emerges as domestic or sanitary sewage. As such the / sewage produced depends on the rate of water supply. The quantity of ‘Scanned with CamScanner: system, assumed to be equal to ‘Water supply system. TABLE 3.1 s Variation in Rate of Water Supply and Rate of Sewage Produced with Population Population Rate of Water Supply* Rate of Sewage Produced™ (itres/capita‘day) Alitres/eapita/day) Upto 20 000 110 or) 20.000 to 50.000 11010 150 90 to 120 50 000 to 200 000 150 to 180 120 to 150 200 000 to 500 000 180 to 210 150 to 170 500 000 to 1000 000 210 0 240 170 to 190 Above 1000 000 240-10 270 190 to 200 * with maximum permissible variation of 25%. ~ (2) Population growth. Alike water supply project a sewerage project is also designed to serve not the present population but also the prospective or future population'which may occur at the end of a le period usually termed as design period. The prospective or future population for which the “Sewerage project is designed is termed as design population. For arriving at the design population various Of forecasting or estimating the prospective or future population are adopted as indicated below. (1) Arithmetical increase method (2) Geometrical increase method pe (3) Incremental increase method } @) Changing rate of increase method 4) (5) Graphical method i 6) Comparative method or Curvilinear method MY (7) Decreasing rate of growth method or Declining growth method Gv > {8) Lo ‘Scanned with CamScanner18 SEWAGE TREATMENT & DISPOSAL AND WASTEW? /ATER ENGG. “These. methods. have been discussed: in: detail in Vol (Wvaers@upely (Engineering) oF However, these methods can be used to predict the future population only ifthe population data for th Kew decades is avilable. If such information on population isnot avaiable then the dese of as suggested in the Manual on Sewerage and Sewage Treatment prepare ot Soe mn in Table 3.2. 7 Environmental Engineering Organisation may be adopted, which are sive TABLE 32 Population Densities ‘S.No | Size oftown (Population) Density of population per hectare 1 Upto 5 000 750 150 2 5.000 to 20 000 150 to 250 3 20,000 to 50 000 250 to 300 4 50.000 to 100 000 300 to 350 is Above 100 000 350 to 1000 limits are fixed by the local auith FAR is the ratio of total floor area ( as indicated a . Incities where floor space index (I the same may be used for working out all the floors) to the plot area. The densities of | by the following example. ‘Assume that a particular development plan rules provide for the following reservations for differen) land uses. Roads 20% 1 Gardens 15% Schools (including play grounds) 6% Markets M% Hospital and Dispensary 2% Total 45% FSI) oF floor area ratio (FAR) the population density. FSI or f population on this concept may be worked out + Area available for Residential Development =(100 - 45) = 55% ‘Considering an area of 1 hectare ( = 10‘ m’) ‘Area available for Residential Development ; (0.55 « 104) m? ! ‘Actual total floor area for residences = (Area available for residential development) x FSI Assuming an FSI of 0.5 ‘Actual total floor area for residence = (0.55 x 104) x05 = 2750 m? ‘Assuming a floor area requirement of 9 m? per person Number of persons per hectare or density of population per hectare Scanned with CamScannerProspective or fate population of 30 yeas Sits replacement is not during its use. Duplicating machinery within the pumping station would be f civil works Will be economical for full design period. Life of pumping machinery is generally 15 years. 30 ‘The construction may be in a phased ‘manner as initially the flows may not reach the designed levels. and it will be uneco- Romical to build the full capacity plant initially. 30. Disposal and Utilisation 30 Provision of design capacities in the stages itself is economical 1B) Type of area served. The quantity of sewage or wastewater produced from an area would depend onuiether the area to be served is residential, commercial or industrial. As indicated earlier the quantity of sewage oF wastewater produced from a residential area depends on the quantity of water supplied to the publi through public water supply system, and it may be assumed to, be equal to about 70 to 80% of the ‘quantity of water supplied to the public through public water supply system. The quantity of industrial sewage OF wastewater produced from an industrial area depends on the type of industries and the Confesponding manufacturing or industrial processes. A careful study of the processes involved in different industries is therefore required to be made to determine the quantity of industrial sewage or wastewater, A similaf study is required to be made to determine the quantity of sewage or wastewater produced from commercial undertakings. @) Infiltration and exfiltration. Some quantity of groundwater oF subsoil water may infiltrate into Sewers through defective joints, broken pipe and other similar entry points. The infiltration of groundwater Of subsoil water into sewers may take place when the head of groundwater or subsoil water surrounding the ewers is more than the head of sewage or wastewater flowing through the sewers. On the other hand is a term which indicates the leakage of sewage or wastewater from sewers into the ground: ling the sewers. Exfiltration may occur through defective joints, broken pipe, ete., when the head of ‘or wastewater flowing through sewers is more than the head of groundwater or subsoil water nding the sewers, While due to infiltration the quantity of flow through sewers increases, exfiltration in the quantity of flow through sewers. Thus exfitration is reverse of infiltration, ‘Scanned with CamScannersewer, se subsoil water into sewers may be considered in the followit on is considered per unit of arca served by the sewer a ‘on the above noted factors a flat allowance of or the amount of infiltration of groundwater into a sew 10-000 litres per day per hectare and if the sewer serves an arca that will enter the sewer through infiltration would be 10 000 « ‘of infiltration is considered per unit length of sewer a For normal conditions, the rate of infiltration of groundwater i per day per kilometre length of sewer. Thus if rate of iniltrat ‘of sewer and the length of sewer is 100 kilometres, the quant ° infiltration would be 2 000 x 100 = 200 000 litres per day the rate of infiltration is considered per unit diameter per ut e day per centimetre of diameter per kilometre. This is t nfration because larger the diameter of sewer greater wil 4 ced is generally expressed in litres per capita per dl of sanitary sewage produced may be assumed to be equal / Thus knowing the per capita rate of domestic or sanit population (Ze. at the beginning of the desi atthe end of the design period, the total quantity of domes er day at the beginning as well as at the end of r, give only the average flow of domestic or sanit tic or sanitary sewage is not constant but it varies . The rate of flow of domestic or sanit ‘day to day and from hour to hour. ‘Scanned with CamScannera un QUANTITY OF (MILLION U1 8 10 12 16 16 18 20 22 26 HOURS OF DAY —» Fig. 3.1 Hourly variation in rate of low of domestic oF sanitary sewage The maximum seasonal (or monthly), daily and hourly rates of flow of domestic or sanitary sewage Lac aoat in terms of the annual average daily rate of flow. Thus if annual average daily rate of flow , ic or sanitary sewage is taken as 100 then the maximum seasonal (or monthly) rate of flow of swage may be about 130 to 140, maximum daily rate of flow of sewage may be about 150 to 180, and ‘maximum hourly rate of flow of sewage may be about 200 to 300 ‘Maximum or Peak rate of flow of domestic or sanitary sewage and Peak factor (or Peaking factor) For the design of sewers maximum or peak rates of flow of domestic or sanitary sewage are adopted. ‘The maximum or peak rate of flow of sewage may be obtained by considering peak factor which is defined 38 the ratio of maximum to average rates of flow of sewage. The value of peak factor depends on the contributing population, and the values recommended in the Manual on Sewerage and Sewage Treatment pers by Central Public Health and Environmental, Organisation may be adopted, which are given in 34, = TABLE 3.4 é Values of Peak Factor Contributory population Peak Factor nd Upto 20 000 3.0 ae 2.0.000 to 50 000, 25 $0.00 to 750 000 225 ie mm Above 750.000 2.00 = brn ‘Scanned with CamScannerthe minimum rate of flow of domestic or sanii = ‘Sfadient that silting will not occu of flow of domestic or sanitary ‘Scanned with CamScannerabove noted values of maximum and minimum rates of groundwater infiltration are as suggested in /on Sewerage and Sewage Treatment prepared by Central Public Health and Environmental 33. ESTIMATION OF STORM WATER (OR RAIN WATER) FLOW "When rainfall takes place a part of it infiltrates into the ground surface, and the remaining part flows ‘over the ground surface. The part of rain water flowing over the ground surface is commonly known as storm water or runoff, which needs to be drained through the sewers, otherwise the entire area would be flooded. The storm water (or rain water) flow through sewers is also known as wet weather flow (W.W.F. in order to distinguish it from dry weather flow (D.W.F.) discussed earlier. For the design of sewers it is necessary to estimate the quantity of storm water (or rain water) that will reach sewers. The quantity of (or rain, water) that will reach sewers depends on intensity and duration of rainfall, Seeteeercicnent area or drainage area such as ils shape, imperviousness, topography including, ons and water pockets, and the time required for the flow to reach the sewer. For estimating the ames rain water) flow or runoff for the design of sewers the following two methods are commonly (i) Rational method (2) Empirical formulae (1) Rational method. In this method the storm water (or rain water) flow or runoff reaching a sewer is. given by the expression 4 Q-cll # 3) Qs storm water (or rain water) flow or runoff: Cis runoff coefficient: a Fis intensity of rainfall ; and He: A is catchment area or drainage area 3.3 is known as rational formula. Depending on the units of the various quantities the rational ‘may be expressed in different forms as indicated below. is expressed in cubic metre per second (m/s or cumec), Jin mitlimetre per hour (mm/h), and then the rational formula becomes @=cxix ex x4 10000 a PP Scanned with CamScannerwater) flow or runoff by rational formula three faet coffcint and intensity of rainfall are required to be de ‘area. The catchment area or drainage area to be served by as town or city. Since the coefficient of runoff depends on the: nt area or drainage area having different types of surfaces ‘coefficient represents the fraction of the total rainfall that is a ‘water) flow or runoff reaching a sewer. Its value depends 6 ent area or drainage area, and the duration of storm. The inthe imperviousness of the catchment area or drainage area, b ‘area lesser will be infiltration and hence greater will be runoff. ess of the catchment area or drainage area can be obtained from the recor of such data the following values of the percentage imperviousness| Type of area ‘Commercial and industrial area Residential area (@) High density 6010 75 __(b) Low density: 351060 _Parks and undeveloped areas 101020 ‘Scanned with CamScannersnoyaoxdust 0€ (2) ‘snoyarodunt 405 (4) IPL x p= RUDI) snoyasodus or (0) snotaiodust 209 (4) Sunrenuaouos 10129 (1) snorasog (P) snoyazoduy (0) ‘aun pares ‘uy Supenusouo smnwers0y snotaiod (P) snotasoduyy (0) ‘umn pores ut (soma) swsors fo worreanq ‘Scanned with CamScannern. A continuously long light rain but intermittent rain in the an area is significantly influ |Table 3.6. Although these values of runoff coeff bé applied to areas of other shapes also which in shape of catchment area or drainage area and of the assumptions on which it is based. also recommended different values of runoff coeffi has recommended different values of runoff coefficient area unde Table 3:8. The runoff coefficient has been designated “CA ing, imensity ¢ rmagnitud recurrenc equalled record of 0,70 to 0.95 assigned 0.85 10 0.90 highest ii 0.75 to 0.85 lowest ra its return 0,50 10 0.70 @ 0.25 100.60 0.15 10.030 0.10 100.30 0.05 100.25 0.01 100.20 0.0} t0 0.20 » 04510055 035 ‘Scanned with CamScannerI catchment area or drainage area and ' is average runoff coefficient or impermeability factor. " , be noted that the effect of duration of storm is not taken ie accous altar eee nt oF impermeabilty actor tecommended by Kuichling and Fruhling. As such the values of | ‘or impermeability factor given by Homer appear to be more rational. ahaa Intensity of rainfall. ‘The intensity of rainfall can be worked out from the rainfall records of the ndet consideration. The longer the rainfall record available the more dependable is the forecast. The. ty of rainfall, however, depends on frequency and duration of storms. 1A) Frequency of storm. The frequency of storm means the number of times a rainfall of given magnitude will be equalled or exceeded in any one year. It is usually expressed in terms of retum period” or recurrence interval 7, which is the number of years during which a rainfall of given magnitude will be equalled or exceeded once. The return period for a rainfall of given magnitude may be determined from the record of rainfall for a number of years. The rainfalls are arranged in the descending order of magnitude and assigned serial numbers. Thus the highest rainfall is placed at the top and given serial number 1, next highest is given serial number 2, and so on. Thus if rainfall record of n number of years is considered, the lowest rainfall will be at the bottom with serial number n. If a particular rainfall has a serial number m then its retum period can be found by any of the following methods. © @ California method eT a 3.8) (id Allen-Hazen method ae rt 9) (iti) Weibull method Tae between rerum period 7 and frequency of storm F expressed in percentage is given by.the following n+l Te = + B.10) —.. = Gy) Gumbel method Ss ait = m+C-1 ‘Scanned with CamScanner ews singin Linge dh, Saad aly not designed for the peak storm water (or rain water) mie ea having a return period of 10 years of ‘of very large size. However, it is necessary to provide se Moding of the catchment area or drainage area. There the design value, which has to be permitted. The frequ place to place depending on the importance of the area, ay be accepted once in a while considering the economy that @ required to be provided which would involve less cost. Sewers are to be designed depends on the importance of ti atid high priced areas should be subjected to less frequent flood ment prepared by Central Public Health and Environme following values of frequency of flooding permissible in diffe Twice a year Scanned with CamScanner Ais buil the entire The to the po of flow it under co is then gi The The duration intensity rainfall + determir of conce‘will have a larger intensity of rainfall if its retum period is large. Fig. ‘ duration of storm curves for different return periods, These curves may be prepared from the ll data, (see Ilustrative Example 3.5). From these curves the value of intensity of rainfall 1 can intensity of rainfall decreases as the duration of storm i d for a known value of time of concentration ¢., since duration of storm is taken equal to the tim n. +06 1S hoe ah lao a i : "I reer! | A=5 YEAR CURVE. 0 i t ib ori vean come | | | | ar C215 YEAR CURVE aly Ey 0:20 YEAR CURVE . | co 03! ee gs pe abe 230 se mn = 20 g z° » | ; x i / 4 0 10 20 30 40 50 60 DURATION OF STORM (minutes) —> ev ta Fig. 3.2 Rainfall intensityduration of storm curves. The intensity of rainfall may also be determined from the various empirical formulae which eet between intensity of rainfall and duration of storm. These empirical formulae are cae llowing two general forms. Tis intensity of rainfal tis duration of storm ‘Scanned with CamScanner‘Scanned with CamScanner effecti of con throug area $ lateral once! graph A rainfal Fig. 3 sewer end ol draina main | which willaeA a jt, are same as indicated above, Davis formula is based on the following Lae 7 . ¥ ‘The storm water discharge from any distriot isd x ble area contained in it. yistit is rectly proportional to the percentage of 7- rate of flow is reached when the duration of storm is equal to the time of (iv) The total volume of storm water is proportional to the maximum rate of flow. ‘Time-Area Graph In the case of large catchment areas or drainage areas the calculated time of concentration and the effective time of concentration differ from each other. Only in the case of equally distributed impervious area throughout the length of the storm sewer the calculated time of concentration practically equals the effective time of concentration and the maximum runoff is caused when the storm duration equals the time of concentration. However, the determination of the time of concentration by calculating the time of flow through the sewers produces misleading results. This is because of the irregular distribution of impervious area such that for certain sewers or drains which may be collecting only little or no flow at all from the laterals, still the time of flow taken into account for such sewers and then included in the time of concentration is bound to give wrong results. In order to obtain proper results a method based on time-area ‘graph may be adopted. A time-area graph is obtained by plotting the time in minutes after the commencement of storm or rainfall along x-axis and the impervious area in hectares contributing to the sewer along y-axis as shown in Fig, 3.3. Such a graph shows the sum total of impervious areas contributing runoff to a selected point in a Sewerage system at various periods of time after the commencement of a storm. This will continue upto the end of the calculated time of concentration. Consider a drainage area as shown in Fig. 3.3 (a) provided with drainage lines along the streets and a main sewer line in the centre. Let P be the point of observation on the ma F line, At the commencement of the storm, rain water will be arriving at P only from the area. Ay er the immediate vicinity of P. As the time passes, water from the respective areas 43, As and Ay will arrive and thus at the end of the time of concentration water from the total area (4, + A> + 43 + 44) will beariving at P. The rate of runoff will reach its maximum value and as long as the rainfall continues atthe Constant rate the rate of flow at P will remain the same. The time-area graph is represented by three straight One line start from the origin (‘e.. point of zero area and zero time) and it slopes upwards up 10 a [which represents the time of concentration (1) and total hectares of impervious area 4,. At the end of fotal area continues to contribute as long as rain continues to fall, and thus the impervious area jins constant with further increase in time, this being represented by a second line drawn parallel to x= axis. When the rain stops, the time-area graph begins to fall as represented by a third line sloping ‘Scanned with CamScanner‘Scanned with CamScannerf (hectares) —» IMPERVIOUS AREA 30 40 50 60 70 80 TIME (minutes) —» (AFTER COMMENCEMENT OF RAINFALL) Fig. 3.4 Tangent method for computing maximuim storm water runoff” “Maximum runoff 8.4104 «15.39 = cumec 3600x1000 0.3591 cumec = 359.1 litres per second. Q) Empirical Formulae. The use of rational formula for estimating the storm water (or rain water) flow or runoff for the design of sewers is usually limited to small catchment areas or drainage areas, say ‘upto about 400 hectares. This is so because for large areas the selection of suitable values of runoff coefficient and intensity of rainfall requires extreme care and judgement. For large areas empirical formulae are generally used for estimating the storm water (or rain water) flow or runoff for the design of sewers. However, the empirical formulae that are available for estimating the storm water (or rain water) flow or runoff can be used only when conditions comparable to those for which these formulae were derived initially can be assured. The various empirical formulae involve the following variables. of rainfall ; (iii) relative imperviousness ; and Scanned with CamScanner(34 ‘Scanned with CamScanner‘area or drainage area in square kilometre ; and 7 the values Of which for different regions areas follows. Value of C ee ~ 6.75 845 ed areas near hills 10.00 Fs 1 Mele ayes la is generally applicable for catchment areas or drainage areas of South India Inglis formula 124A = 14.412 oa 4 munoff in cumec (m*/s) ; and By ILLUSTRATIVE EXAMPLES : i Wa Exampl© 3.1, 4 city with a population of 20 000 has an area of 600 hectares, of w ? zoned commercial and 80 hectares are zoned industrial. The average water co 20 percent ofthis water reaches the sewers. On the ass of sever gaging he ape ial area is 750 000 ipd and the peaking factor is 1.75. Using an ir ve are/day with a peaking factor of 1.8, estimate the average ani peak waste he overall peaking facor. ‘ infiltration as 2500 litres/hectare/day, with a ail ‘Scanned with CamScannerPeak flow rate (litres per day) 1312 500 7.056 000 2.400 000 that commercial, industrial and groundwater infiltration contributions reduc ‘overall system below the domestic peaking factor. To avoid surcharging, peak 60 hectares 80 hectares: 40 hectares 20 hectares ‘Scanned with CamScannerExample 3.3. 4 ‘ith Re Sie orca ani ew I erie seficent or she sie wee SUS Time of concentration = 50 minutes ont that 75% of water supplied reaches the sewer. Use US. Minisry of Health fo the intensity of rainfall. Comment on your result. cudieadll x of water supplied reaches the sewer; = 100 000 « 200 x 0.75 = 15 « 10° litres/day _ _15x10% 24% 6060 = 173.61 litres/second WWE is tiven 2 rational formula, (equation 3.5), as litres/second ‘Scanned with CamScanner‘Scanned with CamScanneri coefficient forthe entire area equal 100. 0, Velocity ee neve foi ea ‘mis, and lic elements for circular pipes flowing full are oer in Table ( TABLE (A) Rainfall data of Duration of rainfall (minutes) oe 20 30 | gin Saab Trae Rainfall (mm) 357) 20 40 oo 288 364 382 376 285 360 ~ 36d me ABST 29.3 37.0 ‘36.7 28.0 284 349 35.2. 264 281 353 Bue Tay Aero 291 361 316 ear 287 358% b8057.0. 277 308 BS | 358 261 31 375 500 TABLE (B) Elements for a Circular Pipe Flowing Full ‘Scanned with CamScanner‘Scanned with CamScannerFrom the intensity-duration curve for duration of rainfall qual 1 the ines the ntensty of rainfall i obtained as 115 mou. ‘Using rational formula equation 35), we have o- cu a9) 03 (0.016 + 0032 + 0.024 sq. km eroginey oreigll Sater 1 = 115 mmhour By substituting these values, we Bet 103011572 0 ~ 690 litres/second. From Table (B) fora discharge of 690 lites/seeond the utfll sewer Of ope of 0.00055 may be provided an trative Example 36, 4 sewer is to carry rain water. wolf coofcient is 045. The past 10-year record of rainfall shows thal, me of concentration Determine the storm water flow for the ‘omen Solution: From Lloyd Davis formula, (equation 3.19), we have o- Zac where r= 1S mm; 4~= 30 hectares; C= 045 sand ¢,=20 minutes ‘Scanned with CamScanner= G+10)= 13 minutes cording 1 US, Ministry of Health formula intensity : ‘By Rational formula, (equation 3.4), we have CU. - 07523316 = Lim's Discharge inte sewer Area ofthe Scanned with CamScanner«asin dscharge forthe combined = (0.1043 + 0.54 = 06447 ms = 6a tired ga Aesrative Example 3.10, An orea of 25 hectares If ee con.) ot 7 onl a ttc of 30 minutes ond tha of 2 45 te Tez Yt prodice asin fom There Satin: Sorm Darton of storm ~20 mines Using US. Minty of Heal formula Ti For/=20 ines; 0= 762 and 6=10 725 i= 284 nmiout «By Rational formula (equation 3.4), we have cu oo 560 a ‘Scanned with CamScanner‘Scanned with CamScanner