Purpose of this chapter is to outline basis of design against which evaluation of existing sewerage system will be made. Various consideration for design like design period, design flow, self-cleansing velocity minimum slope etc are discussed here in.

The local data available has been referred to; for this purpose.

A system of sewers is provided for collecting domestic sewage, industrial wastewater and storm water. The designed system should be adequate enough and it should fulfill all the requirements for the community.

A comprehensive plan should be designed and all the sewers should be constructed according to the given plan.

Generally 70-90% of domestic water consumption is taken as the average sewage flow.

Different criteria for the quantity of sewage flow from different agencies are given in Table 3.1:

It is the water, which seeps into the sewer pipes used to convey sewage water.

Infiltration depends upon

Infiltration is generally taken equal to one third of average domestic sewage flow but for partially separate system, some storm water allowance is also taken into account. So in our local system infiltration is taken equal to 100% of average domestic sewage flow.

Different criteria for the quantity of infiltration from different agencies are given in the tables 3.2 & 3.3.

Table # 3.2

WASA LDA

Table # 3.3

PPHED

Sewers are designed to flow under gravity. All the equations applicable for open channel flows are valid for designing. Manning’s formula commonly used is:

Where

n = Manning’s roughness co-efficient depend upon type of material of pipe.

n = 0.013 ----- 0.015 (For R.C.C. Pipes)

R = hydraulic radius = flow area / wetted perimeter

S = slope of sewer line.

The first step in design of sewers is the estimation of the flow, which they will receive.

The design flow can be calculated from the following formula.

Design flow = peak domestic flow + peak industrial waste water flow + infiltration + Storm allowance.

In our case peak industrial waste water flow is equal to zero.

Storm allowance = 100% of peak domestic flow.

It is the minimum velocity that would keep the particles in waste water in motion in the bottom of the sewers.

For domestic sewage self cleansing velocity ranger from 0.6 ----- 0.8 m/s. For storm sewage higher velocity is desired, as the specific gravity of the particles in storm run off is more. So minimum recommended velocity for storm Sewers is 1.0 m/s.

Criteria for minimum self cleansing velocity according to different agencies are given in the table 3.6.

Table # 3.6

It is the slope corresponding to the minimum velocity. The table 3.7 gives the slope for a velocity of 0.6 m/Sec for different sizes:

To avoid the erosion of pipe material there is a limit of maximum velocity, which is generally 2.4 to 3.0 m/s.

By WASA LDA:

Minimum size of sewer is 225 mm.

Minimum cover to the sewer is provided so that the house sewer may join the public server. House sewers are generally laid at 2% slope. Cover is also provided to protect the sewer from traffic load.

Usually 0.8 to 1.0 m cover is suitable for most suitable for most situations.

PPHED:

Cover = 0.8 m

The level of the inside bottom of the sewer is called invert level.

It is calculated by:

Invert level = Ground level – cover – thickness – diameter of sewer pipe

Sanitary sewers are designed to flow at depth from half to full at the design flow. In case of sanitary sewers, organic materials get decomposed evolving gases, so sanitary sewers are not designed to flow full. On the other hand, storm sewers are designed to flow full as they do not carry organic materials.

So in order to provide air – space for sanitary sewer, the design flow may be increased by a certain factor. This increased flow is used to design the sewer.

Fa = Qf/Qd

Where

Fa = flowing full factor

Qf = flowing full flow

Qd = Design flow

The general criteria being used by WASA – LDA and PPHED is given in table 3.8.

Table # 3.8

A manhole is an opening constructed in a sewer to provide a convenient access to the sewer for observation and maintenance purpose.

A typical sketch of manhole is shown in fig 4.1.

Distances in straight depend upon cleaning method. Where mechanical means can be used then large distance b/w manholes, if manual cleaning then small distance b/w manhole.

Different criteria for the spacing of manholes by different agencies are given in table 3.9 & 3.10:

Table # 3.9

WASA – LDA

Table # 3.10

PPHED

Drop manholes are provided when difference b/w the inverts of two sewers is more than 0.6 m.

The X-section of a drop manhole is shown in fig 4.2.

1. To protect workers cleaning the sewer.
2. To prevent splashing of waste water which cause objectionable odor.
3. To save excavation.

Pumping is required when basement are deep, terrain is flat, obstacles lie in the path of sewer, the receiving stream is higher then the sewer outlet, or when gravity flow is desired through an above ground treatment plant.

It consist of

Wet well receives sewage to be pumped. Ventilation is provided to wet well to prevent excessive condensation and accumulation of odour. Manholes or other means of entrance is provided.

1. Total capacity of pump = peak sewage flow.
2. More than two pumps should be provided one pump is to handle minimum flow. Second is to handle average flow and third one is to handle peak flow. The capacity of small pump should be equal to the peak flow i.e. total capacity is double of the peak flow so that there is 100% stand by capacity. So if one unit gets failed, other unit can handle the operation.

In Pakistan 50% stand by capacity is provided.

There should be alternate arrangement for power as well.

WASA recommends having two different sources of electricity.

3. Pumps should be of self – priming type foot value is not provided.

4. Each pump should have its own suction pipe.

1. Screen should be provided to remove particles of size > 50 mm.
2. Size of dry well should be adequate to accommodate all pumps.
3. Pumps should not be started and stopped frequently. A minimum cycle time should be as given below.

For small pumps = 10 min.

For large pumps = 20-30 min.

Also pump should run at least 2min.

Cycle time is defined, as the time required filling up and emptying the well.

Cycle time = to empty +time to fill.

Detention time should be less then 30 minutes otherwise sewage gets putrefied.

4. Minimum size of section and discharge lines should be 10 cm.
5. The minimum level of sewage in wet well should always be above the casing of the pump, so that there is always a positive suction head.

Two important considerations for sizing of wet well.

1. Pumps should not be stopped and started frequently i-e, size of wet well should be large enough.

Pumps should run at least for 2 minutes.

The time b/w successive starts called the cycle time, should be more than minimum time specified by the manufacturers.

Generally for small and large pimps cycle time is 10 minutes and 20-30 min respectively.

2. Sewage should not stay in the wet well for larger time, otherwise it gets putrefied. So detention time should be less then 30 min.

Formula to find out the operating volume of wet well.

Cycle time = Time to empty + Time to fill

Time to empty = V/(P-Q)

Where V = Operating Volume Of Wet well.

P= Pumping rate (pumping capacity)

Cycle Time = t= V/(P – Q) + V/Q…………………….(A)

Differentiating W.R.T Q

dt/dQ =V/(P – Q)² – V/Q²

Equating dt/dQ = 0

V/(P – Q)² –V/Q² = 0

V/(P – Q)² = V/Q²

(P – Q)² – Q² = 0

P² – Q² – 2PQ – Q² = 0

P² = 2PQ

Q = P/2

So cycle time will be minimum when wastewater flow is half of pumping rate.

Put Q = P/2 in equation (A)

tmin = V/ (P-P/2) + V/P/2

=V/P/2 + V/P/2

=2V/P/2

tmin = 4V/P

There are two extreme conditions.

i) For Q=0, t ® ¥

It implies that for Q = 0, the well dose not get filled and we do not need to start the pump.

ii) For Q=P, t ® ¥

In this Case, the level of sewage in the well remains constant and hence the pump can not be stopped.

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