Unit - 4

Generation and collection of waste water

4.1 Generation and Collection of Waste water:

• In this all the types of waste product of the urban settlements are collected i.e. the solid (Refuse) is collected separately and the sewage (waste-water) is collected separately. This works also covers the part of transportation of the collected waste as quickly and safely as possible, to the point of treatment plants.
• The system used for collection and transportation must be having self-cleaning mechanism. It should be hydraulically tight and economical, so the urban pubic authorities can afford it.
• This work covers, the house drainage works the network of sewers to collect the waste-water from the community and also to arrange the escape of foul gases and the bacteria, within the premise of the buildings.
• The arrangement is made in such manner not to allow the growth of bacteria and the mosquitoes at the catch basins.

Treatment Works:

• This works is necessary to control the air and water and soil poliution through the waste-water before it is disposed off.
• The type of treatment to be provided to the waste-water depends upon the type of sewage, and also upon the hygienic aesthetic and economic aspects.
• If the sewage is disposed off freely without any treatment, it may lead to the following unhealthy conditions.

      A: May pollute the water supplies to be used for domestic consumption, for domestic animals and for the industrial purpose.

      B: May district, the aqua life.

      C: May pollute the bathing places and the ice-factories.

      D: May create unpleasant sights in the community area.

      E: May create the contamination of the surface water to be used for inland navigation or for recreation.

• In the remote or hilly areas the waste-water treatment can be avoided and the waste-water, can be allowed to flow into the natural stream which flows for a long distance to have self or natural purification.

Disposal Works:

• Both, the treated or untreated waste-water are disposed off there by irrigating the fields or by discharging it into a natural surface flow i.e. a stream or a river.
• These systems used for disposing untreated waste-water prove to be dangerous in the long n for the living beings in and around that region.

Key Takeaways:

In this all the types of waste product of the urban settlements are collected i.e. the solid (Refuse) is collected separately and the sewage (waste-water) is collected separately. This works also covers the part of transportation of the collected waste as quickly and safely as possible, to the point of treatment plants.

4.2 Sanitary, Storm and combined sewerage systems:

Storm Water:

• The rainwater is divided into three parts such as some of the parts get evaporated some get percolated in the sub-soil and the remaining water flow towards the slope of the ground.
• This is called as the surface flow or the ground flow or the storm-water.The rate of evaporation depends upon the temperature conditions and the rate of percolation depends upon the structure of the surface rock.
• With all the variations in the climatic conditions, seasonally and the rock structure regionally the maximum rain-water is available for the surface-flow or as the storm-water. The quantity of the storm water is very large if it is compared with the sanitary sewage.
• We already have seen that in combined system the storm water is carried together with sewage while in the separate system, it is taken through a separate set of sewers.
• Both of the systems have same advantages and have same limitations. It is total volume of storm-water which is required to be estimated before the sewage system is designed for any community. The following factors affect the total volume of the storm water.

Factors Affecting Storm-Water (Sewage)

• The total extent of the catchment area.
• The shape of the catchment area and the average slopes. (In case of steep slopes the runoff is greater while in case of gentle slope the rate of evaporation and rate of percolation is more which reduce the quantity of storm-water)
• The type of soil (If the surface soil is porus, more water will be percolated, keeping lesser for the surface flow as storm water).
• The obstructions to the surface run off like agricultural fields, plantations.
• The soil moisture conditions of the catchment area (If the surface soil is moist lesser quantity of water will be percolated to increase the quantity of surface-flow).
• The intensity and extent of the rainfall (If the intensity is high and the duration of the rainfall is long; the area will be have a huge amount of storm-water to be handled.)
• The total number and sizes of the ditches, on the surface area (through the large size ditches more water will be stored and less will be made available for the surface run off to generate the storm-water).

Combined Sewerage System:

• When only one set of sewer is laid to convey both the sanitary sew ge and the storm water, it is known as the combined system.
• This system has the following merits:

(A) The merits of combined system

• In this system, no flushing or pumping is required because due to mixing of the storm water, it gets the required self-cleaning velocity.
• Due to dilution of the sewage through the storm water i.e. the rainwater, it becomes easy to treat the sewage after it reaches the final point of treatment.
• The house plumbing becomes easy and cheap as onlyvone set of pipeline is required in this system.

(B) The demerits or the limitation of combined system

• It needs to have higher initial cost of construction.
• This system is not suitable for the areas where the rainy season is limited to a small portion of the year, as the dry-weather-sewage flow will be limited with lesser velocity which would be insufficient to develop self-cleaning velocity. This leads to blocking of the sewers due to siltation.
• It becomes non-economical if the whole sewage is required to be disposed off by pumping.
• In case of heavy rainfalls, the sewers get overflowed which create public health problems.

Key Takeaways:

The rainwater is divided into three parts such as some of the parts get evaporated some get percolated in the sub-soil and the remaining water flow towards the slope of the ground.

4.3 Quantities of sanitary waste and storm water:

• To calculate the quantity of storm-water two methods are used, which are as stated below:

The rational method:

• To measure the quantity of the storm water following equation is used.

Where in Q = The quantity of storm-water (in m3/sec)

C = The coefficient of runoff

i = The intensity of rain fall (in ram/per hrs.)

A = The total drainage area (in hectares)

• For using the above equation, we need to get the figures for the intensity of the rainfall and the coefficient of the run off.
• The following steps are taken to calculate the total area of the basin i.e. the cantonment area.
• Step 1: The plan of the settlement is prepared on which the tentative arrangement of the sewer line is given. The total area is divided into zones and the concentration points along the sewer lines are marked.
• Step 2: The intensity and the frequency of the rainfall is considered. The time of concentration of storm water includes the inlet time and the time of travel. (The inlet time is the time required for the rainwater, from the remote area to reach the inlet of sewer and the time of travel, is the time required by the storm water, in travelling from the upper most inlet and upto the point of concentration.)
• Step 3: In this, the proportion of rain-water entering into the sewers as the storm-water is calculated.

Example for Measurement of Storm Water by Rational Formula:

Determine the storm water discharge produced from a sewer district of 40 Hectors comprising different type of sub catchment as give below. The average intensity of rainfall in the area is 50 mm/hour.

Soln.:

Given: A =Catchment area in hectares = 40 Hectars:

i =Average intensity of rainfall in the area= 50 mm/hour

C=Coefficient of runoff

To load compute the area of the catchment

We know, Rational formula as;

1. Built-up area (A1)= 30%× i =

C1=0.95

2. Road surface area (A2) = 15% x i= x 40 =6 hectares

C₂ = 0.8

3.Open space area (A3)=25%×i

=×40=10hectares

C3=0.2

4. Lawns and garden area (A) = 40% x i

To compute storm water discharge by Rational formula:

Let Q = Storm water discharge of types of catchment

Q =

Q = (20.6)

=2.288 cumecs

The quantity of storm water for which the sewers of a partially system should be designed as 2.288 cumecs.

Key Takeaways:

To measure the quantity of the storm water following equation is used.

Where in Q = The quantity of storm-water (in m3/sec)

C = The coefficient of runoff

i = The intensity of rain fall (in ram/per hrs.)

A = The total drainage area (in hectares)

4.4 Design of Sewerage system:

Following are the various points which are normally considered in the design procedure of sewer.

Design Procedure of Sewer:

1. Zone formation
2. Sewers arrangement or its network
3. Quantity of sewage
4. Flow velocity of sewers
5. Section of sewer
6. Gradient of sewers

1. Zone formation

• The sewerage system or drainage system in a city areas or town areas is split up into various zones. The sewer lines are generally located along the roads. Hence the layout of sewers is marked separately in each zone.
• Various zones of sewerage system are marked on the map. In short various zones of drainage system are formed for the proper location of sewers.

2. Sewers arrangement or its network

• After zone formation, the proposed network or sewers arrangement of various zones is worked out and primly marked in the map or on the plan.
• In sewers network, the low laying areas are marked and made isolated from the main sewerage system in which pumping stations are installed.
• In the sewer design, the sewage starts from high level zones to low level and sewage lows under gravity and all the sewage from the natural flows finally to the disposal point.
• In the network system of sewers, various sowers like (i) main sewers (i) branch sewers (lit) sub-branch sowers (lv) outfall sowers (v) trunk sowers etc are properly marked and shown on the plan or map.

3. Quantity of Sewage:

• The design of sewerage system or a sower depends upon the quantity of sewage and its type. Maximum or minimum quantity of sewage which is to be carried out by each sewer line is determined.
• A suitable multiplying factor is to be applied for variations in sewage rate and finally the actual quantity of sewage is worked out for the design of sewer.

4. Flow velocity of sewers:

• After determination of quantity of sewage, a suitable velocity of flow is calculated so as to select the self cleaning velocity and limiting velocity for the design point of view.
• For the determination of flow velocity of sowers the various empirical formulae such as Chezy's formula, Manning's formula, Crimp and Bruge's formula, Hazen's and Williams formula etc are used.

5. Section of sewer:

• From the known value of quantity of sowage and its flow velocity (V) the section of sewer can be easily worked out from the following relation

A =Q/V

Where,

A= C/S area of sewer

Q = Sewage quantity

V = Flow velocity

6. Gradient of sewers:

• By adopting a suitable scale, the grade or slope of sewer line and longitudinal section of each sewer line is worked out.
• These sections consist of invert levels and various sewer appartenance.

Key Takeaways:

Design Procedure of Sewer:

1. Zone formation
2. Sewers arrangement or its network
3. Quantity of sewage
4. Flow velocity of sewers
5. Section of sewer
6. Gradient of sewers

4.5 Primary, Secondary and tertiary treatment of waste water:

Preliminary Treatment:

• It consists of screening i.e. to remove the floating material.
• Grit chambers i.e. to remove sand and grit
• Skimining i.e. to remove oil and grease.

Primary Treatments:

• In most of the cases the preliminary and primary treatments are conducted at the same place. So together with the above (given in Section) the primary treatment has one more process called as sedimentation.

1. Plain sedimentation
2. Primary clarifiers
3. Secondary clarifiers
4. Coagulation

Secondary Treatments:

• It is mainly confined to the biological treatment.

Tertiary Treatment:

• There treatments are confined will the treatment and disposal the sewage and the sludge with safe, economical and eco-friendly method.

Key Takeaways:

1. Preliminary Treatment
2. Primary Treatment
3. Secondary Treatment
4. Tertiary Treatment

4.6 Waste water disposal standards:

If and When the sewage is to be discharged in to the sea or into a tidal river, it is necessary to check the chemical and physical properties of the sewage. The tolerance level is stated in the Table (As per CPCB Norms)

Table: The standard for polluted sea or tidal-river

The following Table indicates the standards for surface water i.e. streams, rivers, lakes, etc.

Standard of Sewage Dilution

• The following Table shows the standards of dilution (1912) was formed by the Royal commission on sewage Disposal.
• The ratio of the quantity of diluting water to the sewage is called as the dilution factor.
• The following table is based on this Dilution factor only.

Table: The standards of sewage dilution as per CPCB

References:

1. Environmental Enginnering, Peavy and Rowe, MC Graw hill Publications
2. Optimal Design of Water distribution networks, P.R.Bhave, Narosa Publishing house
3. Water supply Engineering, Harold Eaton Babbit & James Joseph Doland, Tata MC Graw Hill