Process of sewerage system Design

Process of sewerage system Design

Introduction

Design of sewerage system in a town is mainly based on using the gravity of the available terrain and directing the sewage flow to the desired lowest point. While doing so economy of pipe material, minimum excavation and lowering, laying labour is to be achieved. However, main constrains are minimum pipe diameter, range of slope, minimum velocity in the sewers. It is difficult to minimum velocity in the sewers until the contributory population does not increase to certain minimum number. Parameters of design of water distribution system and those of sewerage system are quite different and sewerage system has fewer options available.

Concepts behind stringent parameters

Basic criteria of proposing sewerage system is to establish 135 LPCD of water supply at consumer level. If the system attains 24x7 water supply then design peak factor is of 2.5 to 3 is attained. But still even before attaining the 24x7 water supply sewerage system can be considered. Population forecast for the city is applicable to both the designs. In both cases system designs are for the ultimate stage. With the advance of the infrastructure the development is ensured. Water supply network provides the asking rate for the development and allows the population to settle. Sewerage system network follows the developed areas. Habitations follow the road network, then it follows the water supply network and sewerage system network comes finally. New areas often are asked to develop their own network and connect to the existing sewerage network. However, these future developments are already considered in the development plan and overall sewerage system network also considers the future development. The expected load is considered and the system is proposed in stages and initially is limited to the existing development. In this case also population at the ends is having so low density that they cannot contribute sewage flow to the system which can generate the minimum flow. This minimum flow is to generate velocity 0.6 m/s, in the minimum diameter of pipe at the end. Hence, even in the existing stage there are two parts in the design. One is for the area having development potential and other is already developed. The distinctive mark is the sewage contribution to generate minimum sewage velocity in minimum diameter of pipe.  Area down below is developed part. It is this area where the sewage flow can be directly connected to the sewerage system and sewers are safe from choking possibility as minimum velocity of 0.6 m/sec will be attained at least once in a day. This velocity is called as the self-cleansing velocity.These sewers shall have minimum diameter of 150 mm dia. in smaller cities and in other cities minimum diameter is 200 mm dia. Those who satisfy these conditions are known as “Conventional Sewers.”

Sewers which do not satisfy these criteria are “Non-Conventional Sewers.” It may be due to lack of contributory population to the man-holes, pipes used are of lesser dia., velocity attained is less than minimum self-cleansing velocity. These sewers are proposed in the developing areas. They are likely to be updated as and when the contributory population is expected to increase in a due course of development. Their design period is different than the conventional sewers.As the velocity criterion is not fulfilled these sewers cannot be connected to HH systems directly. It shall be connected to the sewer systems through septic tanks only. It is expected to carry liquid waste only.

Manual refers to the concept of design period as below,

2.5 DESIGN PERIOD

The project components may be designed for the periods mentioned in Table 2-1 overleaf.

Sr. No.	Component	Design period Years (from base year)
1	Land Acquisition	30 years and more
2	Conventional sewer s (A)	30
3	Non-Conventional sewer (B)	15
4	Pumping mains	30
5	Pumping stations – Civil work	30
6	Pumping machinery	15
7	Sewage Treatment Plants	15
8	Effluent disposal	30
9	Effluent Utilization	15 or as the case may be
(A) Typical underground sewers with manholes laid in the roads. (B)  All types such as small bore, shallow sewers, pressure sewers, vacuum sewers

Concept of Project Area

For any city the project area for sewerage system design shall be developable area for the design period of 30 years. Population projection for the city as a whole shall be done for thirty years to arrive at the total waste water generation of the city after thirty years. It shall include both domestic and non-domestic waste.

It may not be expected that the whole city drains at a single point. Hence, the natural drains are traced and the area bound to drain to the particular natural drain is decided. With suitable Pumping stations, location of STP shall be decided. Choice is to weighed between a single STP and zonal STPs. For detailing purpose, wards of the cities are to be studied. Development potential remaining in the wards is to be identified and they are allowed to develop in a natural way. After reaching certain population density people will prefer to move outside and may be attracted to de-congested areas. Such a density is known as the THRESHOULD DENSITY. Noting these observations, maximum densities in other wards shall be decided. A notional sector planning is expected to be done in the developable areas and the calculated population in the city after design period shall be tallied with a justified distribution in the ward boundaries. Knowing the areas of the wards and the population in the design year gross density in the ward at design stage shall be known. It shall be modified to the net density after reducing the areas where development is not feasible. All these considerations are also affected by the drivers of the development for the city.

Having marked zones, known as drainage districts, ward areas are to be super imposed on the same map and the densities are considered in that area to operate in the design period. Knowing the present population density from the Google map as the existing density, design of flows and pipes can be proposed from the density in the design year.

These pipes in the future period will be justified whenever the population settles down in that area, but till then provisional connection pipe lines be proposed from the software. However, flows can be cumulated downstream, and allowed to attain the velocity criterion of 0.6 m/sec.

Manual also refers to the similar process as below,

2.7 PROJECT AREA

The factors that influence the determination of project area include natural topography, layout of buildings, political boundaries, economic factors, CMP, etc. For larger drainage areas, though it is desirable that the sewer capacities are designed for the total project area, sometimes the political boundaries and legal restrictions prevent construction of sewers beyond the limits of the local authority. However, when designing sewers for larger areas, there is usually an economic advantage in providing adequate capacity initially for a certain period of time and constructing additional sewers, when the pattern of growth becomes established. The need to finance projects within the available resources necessitates the design to be restricted to political boundaries. The project area under consideration should be marked on a key plan so that the area can be measured from the map.

Concepts for the layouts of sewerage

Sewerage system is to be designed for the design year of 30 year ahead and laid to the extent of the existing habitations. It will be distinguished into two categories, conventional sewers and non-conventional sewers. In many fringe areas, population density will be much less in the existing stage and also HH can be well separated from each other. Some HH can be in clusters. Roads might not have been developed fully. In such cases the main option is to adopt “on site sanitation.” Habitations are allowed to settle and other developments are also allowed to be defined. Water supply network shall be in place and LPCD levels are defined to 135 LPCD at the consumer ends. Sewers can be laid only after the roads are defined. This may cause the time difference. Conventional sewer is laid as proposed. It has the contributory population defined in the developable ward area. It can accommodate the sewage flow, if it reaches by non-conventional system. Once the settlement of population occurs, Phase II Estimates can be prepared for the conventional sewers or conversion of non-conventional sewers. Criteria mentioned above are applicable to the Conventional sewers only, but they become applicable when the conversion of non-conventional sewers is under considerations. It is for this reason software design shall be for the full area and model needs to be updated as and when the settled population increases. Prospective urban local bodies may retain the non-conventional sewer diameters to the design minimum diameters. ULB is expected to face distraction from the prospective consumers as they have already drained their waste into septic tanks and connecting to the drain sewers can cost them further. In such cases present estimates shall provide for the connecting drains to main sewers, even in the case of non-conventional sewers. ULB will become responsible for the O&M of non-conventional sewers also. Moreover, ULB shall provide facilities for the periodical cleaning of the septic tanks at reasonable charges. All connected houses whether to conventional or non-conventional sewers, are taxable at same rates.

Manual has explained it as below,

2.9 LAYOUT AND ARRANGEMENT OF SEWERAGE

The layout of collection systems shall resist the tendency to go in for underground sewerage flat out even in habitations that are only sparsely developed. The options of either time-deferred underground sewerage or incremental sewerage commensurate with the pace of development by such options as small bore, shallow sewers, twin drains, etc., to start with and eventual underground sewerage when habitations have been populated to a certain level where the revenue will be able to sustain the O&M.

The layouts by small communities shall be mandated to include the small bore sewer system / twin drain in both sides of roads, whereby the house side drain will receive the septic tank effluent and the road side drain will receive the storm water runoff. In metropolitan urban centers, decentralized sewerage shall be confined to institutional boundaries only and not culled out of habitations itself and zoning of sewerage with STPs fanning out radially outwards is to be encouraged.

A flat out choice of underground sewerage with sewers in middle of roads shall be discouraged and incremental sanitation as settled sewers, small-bore sewers, twin drain for septic tank effluents and sewers on shoulders of wide roads are to be evaluated.

2.11 Guidelines on house sewer connections

a) There is a compelling need to amend the bye-laws to make it compulsory for the population to avail house service sewer connection wherever public sewer is provided and if this is notforthcoming, the local authority shall effect the house sewer connection and institute revenue recovery proceedings.

b) Include house-service sewer connections as part of the sewerage project itself

c) Float Equated monthly installments (EMI) schemes for repayment of house service sewerconnection costs.

Concepts in planning of sewer system

Total area covered in the city limits can be far more as compared to the area occupied by the present development. Still the Design of sewerage system shall be done for design period of 30 years. Development plan is available and land use pattern is decided for the design period. Residential area development in future is also known. It is designer’s anticipation, that the sectorial development in future, will be proposed to be shaped. It shall be based on the basis of development plan reservations for particular type of development. Normally agricultural use and the residential use are dependent on the demand for the residential use. If the financial advantage is seen or anticipated in the conversion of land use from the Agricultural to NA, then the rates of land use increase and the residential settlements take place. This is a slow process and invites programmed land monetization. These developments are in clusters, nearer to the edge of present development. Infrastructure development also takes time. Initially, the onsite sanitation is the main solution. Then the connecting area drain to the main sewers becomes the ‘popular’ decision, limited to budget provisions. Over a period the incremental sewerage system upholds in this type of areas.

Manual also refers to the similar process as below,

2.14.5 Planning of Sewer System

The design principles in Chapter 3 of this manual shall be followed. In essence, it stipulates that the options of small bore sewers, shallow sewers, twin drains and underground sewers all have to be relatively evaluated to sub regions of the project site instead of blindly going in for total underground sewer flat out. Incremental sewer shall also be considered based on the phased development of the region instead of directly opting for total underground sewer system.

3.4 TRIBUTARY AREA

The natural topography, layout of buildings, political boundaries, economic factors etc., determine the tributary area. For larger drainage areas, though it is desirable that the sewer capacities be designed for the total tributary area, sometimes, political boundaries and legal restrictions prevent the sewers to be constructed beyond the limits of the local authority. However, in designing sewers for larger areas, there is usually an economic advantage in providing adequate capacity initially for a certain period of time and adding additional sewers, when the pattern of growth becomes established. The need to finance projects within the available resources necessitates the design to be restricted to political boundaries. The tributary area for any section under consideration has to be marked on a key plan and the area can be measured from the map.

Concept behind the coverage of sewerage system

Sewerage system aims at Benchmark level for coverage as 100%. It is presumed here that even though the particular consumer has constructed a septic tank for immediate disposal of waste water, he shall connect the outflow from septic tank to the available sewerage system up to 30 m from the boundary of his compound. Hence, the Govt. intends to extend the coverage of the sewerage system to 100%. Consumers shall have to co-operate. Central Govt. has given the enactment done for Goa State for users and consumers are bound to follow the Govt. intention. On the similar lines Central Govt. wants that other states shall follow the enactment Procedure, to make the consumers compulsory

2.21 MANDATORY REQUIREMENTS IN SANITATION SECTOR

These shall be as follows

1.      Each state government shall mandatorily pass a “Sewerage & Sanitation act” and notify the rules thereunder. The reason for this is to empower the ULB’s to prevail on the property owners/ occupiers to avail house service sewer connections once the sewerage system is developed by the ULB, within 30 m of the premises irrespective of whatever be the mode of existing sewage disposal system.In case the owner/occupier fails to do so the ULB by virtue of its powers can disconnect essential services like, water supply and electricity after the expiry of the notice period. An example of such a provision can be seen under rule 10(5) of the Goa Sewerage System and sanitation services management rules 2010 enacted under the Goa Sewerage system and services management act 2008 as contained in Appendix C. 2-2 of Part C Management.

2.      Similarly, each state government shall mandatorily formulate and notify appropriate act and rules for septage management.

2.17.2 Reconstruction Planning of Sewers

h) While designing the sewer system itself, trunk sewers shall be designed to be possible to be used for such diversions by temporarily using the sewer as a pumping line under low pressure. After all, these sewers are laid using long sewer pipes of 6 m length, and the load carrying capacity needs a rating of at least about 4 kg/sq. cm and this is adequate for such low head pumped diversions. Temporary pumping lines of low pressure can be laid above ground along property boundary compound walls by using double-flanged DI pipes which are easy to lay and dismantle.

PER CAPITA SEWAGE FLOW

The entire spent water of a community should normally contribute to the total flow in a sanitary sewer. However, the observed dry weather flow quantities usually are slightly less than the per capita water consumption, since some water is lost in evaporation, seepage into ground, leakage etc. In arid regions, mean sewage flows may be as little as 40% of water consumption and in well developed areas; flows may be as high as 90%. However, the conventional sewers shall be designed for a minimum sewage flow of 100 litres per capita per day or higher as the case may be. Non-conventional sewers shall be designed as the case may be.

For some areas, it is safe to assume that the future density of population for design as equal to the saturation density. It is desirable that sewers serving a small area be designed accordingly on saturation density.

For new communities, design flows can be calculated based on the design population and projected water consumption for domestic use, commercial use and industrial activity. In case a master plan containing land use pattern and zoning regulation is available, the anticipated population can be based on the ultimate densities as in Table 3.1.

From the above Table it will be clear that the Peak factors for the sewage flowing to the sewers is quite different from the water supply distribution Peak factors.

Population	Population
Up to 50000	3.0
50000 to 200000	2.5
Above 200000	2.00

Same Table is rearranged as,

Population	Population in Lakhs	Population
Up to 50000	0.5	3.0
100000	1.00	2.5
150000	1.5	2.5
200000	2.0	2

Converting into graph,

For convenience the graph is plotted for the Population in lakhs vs peak factor. It is found that the R²value is exactly unity. Hence the generated equation of the graph is applicable to be expanded in both the directions. Accordingly using the equation value of peak factors has been calculated for lower values of population up to 5000 (0.05 L) population and tabulated above.

Population mentioned in the Peak factor statement does not refer to the city population. But it is referring to population in the influence area of the concerned Storage.

Peak factor in water supply distribution

For Peak Factor concept, demand pattern for water requirement in 24 hours is used. Average hourly demand is calculated from the average daily water supply in LPCD. In case of 135 LPCD water supply at consumer end average hourly supply works out to 135/24 = 5.63 LPH per capita. With peak factor 3 in morning hours Demand in this hour will work out 5.63*3 = 16.89 LPH per capita.

Application of Peak factor in sewerage system

Use the same peak factor 3 for sewerage design. Peak factor depends on the density of population, topography of the site, hours of water supply and hence, individual cases may be further analyzed if required. Relevant figures for sewage flow will be calculated at 80% of these figures. i.e. 16.89*0.8= 13.51 LPH. This rate is supposed to be flat in the peak hour.

For minimum dia. of 150 mm and with n=0.013, for velocity to attain 0.6 m/sec and sewer to run 50% full, required flow is minimum as given below,

% full	Dia. in mm	Slope 1 in	Hz-Will. C-value	Dia. Power 0.63	Dia. Power 2.63	slope power 0.54	Velocity in m/sec	Discharge in cum/day	Discharge in cum/hr	Peak hour Discharge in cum/hr
										Peak = 3
0.5	150	170	140	23.49	528591	0.06	0.94	716.37	29.85	89.55
0.4							0.85	482.83	20.12	60.35
0.3							0.73	280.82	11.70	35.10
0.2							0.58	126.08	5.25	15.76
0.1							0.38	30.09	1.25	3.76

Manual refers to minimum dia. = 150 mm

Minimum slope = 1 in 170

Maximum c-value as applied with use of DWC pipe =140

Based on these conditions velocity attainable can be calculated.

In the initial stages the Pipe will not remain full.

Hence, change in velocity with same parameters but for change in the depth of water are Tabulated.From table it is seen that contributory population shall be such that it should create the flow of 11.70 cum/hr average and 35.10 cum/hr peak flow to attain 0.3 d in the pipe and to give 0.6 m/sec velocity.

On the other hand it is assumed that sewer in the initial stages is receiving sewage from both the sides. Construction of man-holes is normally at every 30 m on the sewer line. It is expected that at every 10 m width there is HH. On both sides there will be 6 HH in 30 m. Hence, the initial manhole will receive sewage from 6 HH. Following table will show that with number of different HH and 5 persons per HH Peak flows will be tabulated.

HH	Pop.	LPCD	Flow ave. per hour in cum/hr	Flow Peak per hour in Cum/hr
both sides	5 per HH	80%of 135		PF = 3
30	150	108	0.675	2.025
24	120	108	0.54	1.62
18	90	108	0.405	1.215
12	60	108	0.27	0.81
6	30	108	0.135	0.405

On one manhole length of 30 m and assuming about 30 HH are contributing flow is not satisfactory to give 0.6 m/sec velocity.

HH	Pop.	LPCD	Flow ave. per hour	Flow Peak per hour
both sides	5 per HH	80%of 135		PF = 3
500	2500	108	11.25	33.75
30	150	108	0.68	2.03
24	120	108	0.54	1.62
18	90	108	0.41	1.22
12	60	108	0.27	0.81
6	30	108	0.14	0.41

For 0.6 m/sec velocity with 0.3 = d/D, 500 HH with 2500 population can give required flow with the given constraints in 150 mm dia. pipe line.

The flow in sewers varies from hour to hour and seasonally. However, for the purpose of hydraulic design estimated peak flows are adopted. The peak factor or the ratio of maximum to average flows depends upon contributory population as given in Table 3.2.

Table 3.2 Peak factor for Contributory Population

Contributory Population	Peak Factor
Up to 20000	3
20001 to 50000	2.5
50001 to 750000	2.25
More than 750000	2

Source: CPHEEO, 1993Part A: Engineering 3 - 4 CHAPTER 3: DESIGN AND CONSTRUCTION OF SEWERS

The peak factor also depends upon the density of population, topography of the site, hours of water supply and hence, individual cases may be further analyzed if required. The minimum flow may vary from 1/3 to 1/2 of average flow.

Rearranging same Table data,

Contributarypopulatin	Contributarypopulatin in Lakhs	Peak factor
20000	0.2	3
50000	0.5	2.5
100000	1	2.25
150000	1.5	2.25
200000	2	2.25
250000	2.5	2.25
300000	3	2.25
350000	3.5	2.25
400000	4	2.25
450000	4.5	2.25
500000	5	2.25
550000	5.5	2.25
600000	6	2.25
650000	6.5	2.25
700000	7	2.25
750000	7.5	2.25
800000	8	2

This data is converted into graph as below,

PF	3.33	3.31	3.29	3.27	3.24	3.14	3.05	2.96
POP. In Lakhs	0.01	0.02	0.03	0.04	0.05	0.1	0.15	0.2

In case of design of sewerage system contributory population of 2500 is minimum required for 0.3=d/D.

However, manual refers to design of sewers at 0.5 depth. It requires 81 cum of flow per hour. With PF =3.

HH	Pop.	LPCD	Flow ave. per hour in cum/hr	Flow Peak per hour in Cum/hr
both sides	5 per HH	80% of 135		PF = 3
1200	6000	108	27.00	81.00

Contributory population will decrease if the PF is considered as 4.

HH	Pop.	LPCD	Flow ave. per hour in cum/hr	Flow Peak per hour in Cum/hr
both sides	5 per HH	80% of 135		PF = 4
900	4500	108	20.25	81.00

For each city PF is also dependent on the other considerations such as topography, hours of water supply, density of population etc.

Manual refers to guidelines for the Ha density assumptions when such data is not available. It is given below,

When a master plan containing land use pattern and zoning regulations is available for the town, the anticipated population can be based on the ultimate densities and permitted floor space index provided for in the master plan.

In the absence of such information on population, the following densities are suggested for adoption as in Table 3.1.

Table 3.1 Densities of Population vs. Populated areas

Size of town (Population)	Density of Population per Ha
Up to 5000	75-150
Above 5001 to 20000	150-250
Above 20001 to 50000	250-300
Above 50001 to 100000	300-350
Above 100001	350-1000

                        Source: CPHEEO, 1993

In cities where Floor Space Index (FSI) or Floor Area Ratio (FAR) limits are fixed by the local authority this approach may be used for working out the population density. The 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 in the following example for an area of one hectare (ha)

Roads 20%

Gardens 15%

Schools (including playgrounds) 5%

Markets 2%

Hospital and Dispensary 2%

Total 44%

Area available for Residential Development = 100 - 44 = 56% or 0.56

Actual total floor area = Area for residential development x FSI

Assuming an FSI of 0.5 and floor area of 9 m²/person

Number of persons or density per hectare = (0.56*10000*0.5) / 9 = 311   

Densities as mentioned of 150 persons per Ha are also not found at the fringe areas and HH are situated but with very lean density. We have seen that the at least 5000 population is required to get the adequate flow in the system at the starting points. Hence, the conventional sewers are supposed to be on inner side of the development when such flow is available. In time to come the density will increase and then no-conventional sewers will be converted into the conventional sewers.

Present cities are not processed and established for 24x7 supplies. Hence, Peak factor in this situation is considered as 4 to avoid larger no. from becoming non-conventional sewers. It has the reasoning based on the new areas are now equipped with toilet flushing system and they use flushes in the peak hours hence Peak factor is improved from 3 to 4 and even more. Flushes are capable of giving 9 lit/min flow. It gives 9*60 = 540 lit per hour flow in the system. There can be a probability of operating simultaneous flushes in the system giving rise to more peaks. Hence, in the starting region up to increase of population of 5000 persons PF of 4 is justified. Such assumptions will differ from case to case.

Water supply norms for various types of areas as per standards.

Following Table gives the utilization wise distribution of water supplied to various areas. For sewerage system to be proposed minimum level of water supply is to be attained for the future population up to 30 years.

In rural water supply and in semi-urban areas sewer systems are not proposed. It is recommended that for proper sewer system to function maximum velocity in a day shall be 0.6 m/sec. If this self-cleansing velocity is not attained then the solids in the waste shall be preferably separated and not allowed flow in the sewers.

Purpose	40 LPCD	70 LPCD	135 LPCD	150 LPCD
Drinking	3	3	3	3
Cooking	5	7	12	12
Bathing	15	25	30	30
Washing utensils and house	7	10	40	40
Ablution	10	25	40	40
Others	0	0	10	25
	40	70	135	150

Considering, 135 LPCD, as the acceptable minimum flow for sewerage system development.   Break up of 135 LPCD is given below. It can be seen that uses drinking, cooking, washing utensils and house can come at any different timings of the day. But for peak to observe Bathing and ablution use shall come in peak hour. Total consumption for these uses is 70 lit. It will mean that 65 Lit (135-70) will be utilized in 24 hours. It calculates to 65/24 = 2.70 Lit/hr.

Purpose	Drinking	Cooking	Bathing	Washing utensils and house	Ablution	Others		Maximum waste generation per hour as per table is 25.84 lit/hr. Average utilization of supplied water is 4.5 lit/hr. Hence, the working peak factor works out to be (25.84/4.5) = 5.74.
135 LPCD	3	12	30	40	40	10	135
Hours	Hour wise Dist.	Hour wise Dist.	Hour wise Dist.	Hour wise Dist.	Hour wise Dist.	Hour wise Dist.	Lit/hr	Waste/hr	Cum.lit/hr	Cum. Waste lit/hr	Cum.Ave. lit/hr	Hours
0							0	0	0	0	4.5	0
1							0	0	0	0	9	1
2							0	0	0	0	13.5	2
3	0.05	0.1		2	1	1	4.15	3.32	4.15	3.32	18	3
4	0.05	0.1		2	2		4.15	3.32	8.3	6.64	22.5	4
5	0.2	0.1		2	2	1	5.3	4.24	13.6	10.88	27	5
6	0.2	0.5	2	8		1	11.7	9.36	25.3	20.24	31.5	6
7	0.3	1	15	5	9	2	32.3	25.84	57.6	46.08	36	7
8	0.2	1	5	4	9		19.2	15.36	76.8	61.44	40.5	8
9	0.1	1	2				3.1	2.48	79.9	63.92	45	9
10	0.25	2					2.25	1.8	82.15	65.72	49.5	10
11	0.25	2			2	1	5.25	4.2	87.4	69.92	54	11
12	0.2	0.1		2	2		4.3	3.44	91.7	73.36	58.5	12
13	0.1	0.1		5		1	6.2	4.96	97.9	78.32	63	13
14	0.1	0.1					0.2	0.16	98.1	78.48	67.5	14
15	0.2	0.2					0.4	0.32	98.5	78.8	72	15
16	0.1	0.3		2	2	1	5.4	4.32	103.9	83.12	76.5	16
17	0.1	0.3			9	1	10.4	8.32	114.3	91.44	81	17
18	0.2	0.2	3				3.4	2.72	117.7	94.16	85.5	18
19	0.3	0.5	3	4	1	1	9.8	7.84	127.5	102	90	19
20	0.1	2		4	1		7.1	5.68	134.6	107.68	94.5	20
21		0.2					0.2	0.16	134.8	107.84	99	21
22		0.1					0.1	0.08	134.9	107.92	103.5	22
23		0.1					0.1	0.08	135	108	108	23
							0	0
Lit.	3	12	30	40	40	10	135	108

Graphical representation of utilization of water and generating the waste is shown below. It can be seen that the peak occurs at 7 to 8 am. Main utilization is bathing and ablution. Use of flush is expected with 135 LPCD water supplies. Compared to the average flow s per waste generation of 80% of 135 LPCD flow = 108 LPCD, is 4.5 lit/hr. Maximum hourly waste generation seems to 25.84 lit/hr. Hence, the worked out Peak factor is 5.74.

With the same data another graph is generated to show the cumulative waste generated per hour to cumulative average waste generated. It indicates that the once the peak is reached the waste generated quantity is rated above average flow.

Approaching the velocity of 0.6 m/sec. with the present setting how much population is minimum required is tried to be predicted. As per Table it works out to 800. However, the peak factor is 5.75 instead of 3 as given in the manual.

			Population wise rated utilization							Velocity in 150 mm dia.sewer with slope 1 in 170, and designed for half full
135 LPCD	135
Hours	Lit/hr	Waste/hr	30	60	90	200	400	600	800	in m/sec
0	0	0	0	0	0	0	0	0	0	0.00
1	0	0	0	0	0	0	0	0	0	0.00
2	0	0	0	0	0	0	0	0	0	0.00
3	4.15	3.32	99.6	199.2	298.8	664	1328	1992	2656	0.08
4	4.15	3.32	99.6	199.2	298.8	664	1328	1992	2656	0.08
5	5.3	4.24	127.2	254.4	381.6	848	1696	2544	3392	0.11
6	11.7	9.36	280.8	561.6	842.4	1872	3744	5616	7488	0.24
7	32.3	25.84	775.2	1550.4	2325.6	5168	10336	15504	20672	0.65
8	19.2	15.36	460.8	921.6	1382.4	3072	6144	9216	12288	0.39
9	3.1	2.48	74.4	148.8	223.2	496	992	1488	1984	0.06
10	2.25	1.8	54	108	162	360	720	1080	1440	0.05
11	5.25	4.2	126	252	378	840	1680	2520	3360	0.11
12	4.3	3.44	103.2	206.4	309.6	688	1376	2064	2752	0.09
13	6.2	4.96	148.8	297.6	446.4	992	1984	2976	3968	0.12
14	0.2	0.16	4.8	9.6	14.4	32	64	96	128	0.00
15	0.4	0.32	9.6	19.2	28.8	64	128	192	256	0.01
16	5.4	4.32	129.6	259.2	388.8	864	1728	2592	3456	0.11
17	10.4	8.32	249.6	499.2	748.8	1664	3328	4992	6656	0.21
18	3.4	2.72	81.6	163.2	244.8	544	1088	1632	2176	0.07
19	9.8	7.84	235.2	470.4	705.6	1568	3136	4704	6272	0.20
20	7.1	5.68	170.4	340.8	511.2	1136	2272	3408	4544	0.14
21	0.2	0.16	4.8	9.6	14.4	32	64	96	128	0.00
22	0.1	0.08	2.4	4.8	7.2	16	32	48	64	0.00
23	0.1	0.08	2.4	4.8	7.2	16	32	48	64	0.00
	0	0	0	0	0	0	0	0	0	0.00
Lit.	135	108	3240	6480	9720	21600	43200	64800	86400

If as per manual Peak factor is considered as 3 it does not give the cumulative population as shown above. As per manual area serving the desired population cumulating to particular manhole generating maximum velocity less than 0.6 m/sec., the pipe lines in this area cannot be termed as sewers. These are non-conventional sewers. By definition these non-conventional sewers shall not receive the sewage but it shall receive only the liquid drain.

Actually peak flows per hour flattens the velocity rating in that hour. Bath and toilet (in particular) waste has much higher rate at which it can flow in the sewer. Concerned peak factors are quite high. Manual has not referred to this possibility. It may be since the each house draining may not have the flushing system fitted to their toilets. So also Manual has also questioned about the quantity of waste water reaching to the sewers. Bathroom and kitchen waste may run to the kitchen garden. Toilets wastes have no option than to run through septic tank. Velocities are dampened substantially. However, the quantity shall be same flowing with lesser velocity. It may dampen the peak factors.

Peak factor will depend on the size of city and other amenities available in that city. Density of population also matters. Higher density will allow better peak factors. Development pattern of the city also matters as the living standards are prominent features to run more waste to drains/sewers.

Sewerage system is designed for the 30 years ahead. Apart from the increase in population development and living standards are also upgraded over a period. Other developments are functional in this period. Roads, education, Land value, area development, educational facilities, markets, will also develop in their own ways, simultaneously with the financial and infrastructural inputs. In time to come the waste generations are expected to increase and more and more conventional sewers will be added to the system.

Rationalization of peak factors is thought of by adopting flows corresponding to prospective population of 5000 to reach to the influenced manhole, with increased value of  ‘4’  instead of  ‘3’,  to account for the moderation of concern of development pattern, is proposed to be adopted. This population, however, can change according to the size of city, present development and present living standard.

As far as the living standard is concerned, it is related to the per capita income. These figures are available for each city. It can be understood from these figures that the income level at Pune and remote cities in the State from Metros are having a substantial difference. In case of Pune the per capita income is more than Rs.100000/- while that at Chandrapur is hardly Rs. 16000/-. Both are having Average LPCD levels above 135 LPCD. But quantity flowing to sewers may have a substantial difference.

Conclusion

Design of sewerage system with the use of softwares will give the quick results, but the scenarios which can be generated in waters supply distribution system, cannot be generated. Sewerage system is not proposed to be laid in advance before the population is settled in place. Hence, only well settled population is to be provided with the amenity. System is designed for the ultimate stage and the main sewers satisfy the manual criteria. In other areas sewers are proposed but they are non-conventional, (small bore, drains, twin pipes) and they need to be upgraded time to time. Non-conventional sewers have less velocity than self-cleansing velocity. Hence, septic tanks are necessary to remove solids. Peak factor in the sewerage system is dependent on the contributory population to the sewer. It shall be about 5000 population. In many cases other conditions, terrain, hours of water supply prevail. In such cases it is better to apply modified PF of more than 3 up to the desired contributory population is reached.