The procedure of lifting of wastewater or sewage is termed as Sewage Pumping. The necessity of Sewage Pumping arises under the circumstances – (i) When some area is low-lying and it can’t drain sewage by gravity into mains / submains. (ii) In flat countries, sewage pumping is adopted because it is more economical. This is because excavation for laying drains at designed grade will become more expensive. (iii) Pumping is restored when outfall sewer is at lower level than the water-body into which it is to be discharged. Also when the outfall is lower than the entrance to the treatment works. (iv) Pumping thus used to take out sewage from cellers or basement of building; when the level of celler is lower than the level of sewer which is drainage pipe.
Also know about how maintenance of sewers is done?
Problems in Sewage Pumping
The pumping of sewage is not as easy as the pumping of water. Therefore one has to face lots of problems during the pumping of sewage. Some special problems are as below –
- Sewage has foul characteristics.
- Rate of flow of sewage varies continuously and hence pumping operations needed to adjust accordingly.
- Sewage contains lots of suspended and floating materials. These generally cause frequent clogging of the pumps.
- The size of sump is finite, therefore the provision of sump is to give only a little storage space.
- Sewage contains organic and inorganic waste. These results in corrosion of parts of pump, thus reduces the life of pumps.
- Pumps should be of high order reliability. Since failure of pumps will lead to flooding which may cause unbearable nuisance.
- Sewage contains disease causing germs / bacteria / organisms. Thus it may cause health hazard to the persons working at pumping station.
Location of pumping stations –
Proper location of pumping station requires a comprehensive study of the area to be served to ensure that the entire area can be adequately drained. If the large amount of sewage is to be pumped, the site should be near the stream in which the sewage used to discharge. So that the sewage needs to be thrown directly into it, during emergencies such as break-down of pumping plant, failure of power etc. If we avoid this precaution, the flood will arise in pumping stations. Thus the site is also aesthetically satisfactory. Therefore it needs to design so that it does not gets flooded anytime.
Elements of pumping stations –
Apart from the structure of the pump house, a sewage pumping station consists of the following elements –
- Grit channel or detritus pit
- Coarse and fine screens
- Sumps or wet well
- Pump room or dry well
- Pumps with driving engine or motor
- Miscellaneous accessories – pipes, valves, float switch arrangements, flow recorders, emergency overflows, ventilation arrangements, such as extraction fans etc. etc.
Pump house structure –
The pump house structure is always design to withstand flotation forces. However the sub-structure is made up of mass-concrete or R.C.C. While super-structure can be of any material. The internal wall and floors should be designed –
- to bear the weight of machines and live load of 5 kN/m2.
- planned while keeping in view the future expansion.
- having enough ventilation so that foul gases, moisture can easily escape.
- safe against vibrations caused by pumping machinery.
- minimize humidity thus to control the corrosion.
- dust-proof, water-proof, fire-proof, explosion-proof etc.
- have stairs instead of ladders (avoid spiral-stairs) between different floors.
Types of pumps used in Sewage Pumping
1. Centrifugal pumps –
Centrifugal pumps are most widely used for sewage pumping and storm water. As it is easy to install this in pits, sumps. It can thus transfer the suspended matter present in sewage quickly without getting clog too often. However these pumps work on the principle of centrifugal force. It generally has 2 parts – (i) the casing and (ii) the impeller. The impeller rotates with high speed inside the casing. The clearance between the vane is so large thus prevents the clogging. Such pumps are therefore termed as non-clog pumps.
There are three classes of centrifugal pumps –
- dis-integrator pumps
- full way pumps
- free way pumps
Centrifugal pumps can be classified under –
- Axial flow pumps
- Mixed flow pumps
- Radial flow pumps or centrifugal pumps
|Axial flow pumps
|Mixed flow pumps
|Radial flow pumps or Centrifugal pump
|Head developed by propelling action of the impeller vanes.
|The head developed by both centrifugal action and propelling action of the impeller vanes.
|Head developed by centrifugal force only.
|Single inlet impeller.
|Single inlet impeller.
|Either single suction or double suction inlet impeller.
|Flow enters axially, discharges axially.
|In this the flow enters axially, discharges both axially and radially.
|Flow leaves the impeller radially and normal to the shaft axis.
|Usual capacity range is greater than 2000 m3/hr.
|Usual capacity range is greater than 100 m3/hr.
|Any range of capacity can be obtained.
|Head value is less than 9 m.
|Used for medium heads of 8 to 15 m.
|Any range of head can be obtained.
|Kilowatts decrease with capacity.
|KWs remains flat with the change in capacity.
|Kilowatts increase with capacity.
|Specific speed range lies between 8000 to 16000.
|Specific speed range lies between 4200 to 9000.
|Range of specific speed lies below 4200(for single suction) and 6000 (for double suction).
2. Reciprocating pumps –
The reciprocating pumps have thus become obsolete in modern sewage pumping station. Its initial cost is high and comparatively efficiency is low. Reciprocating pumps are generally of 2 types – (i) ram type and (ii) propeller type.
Diaphragm pump is an example of Ram type Reciprocating pump.
3. Propeller pumps –
The axial flow pump is sometimes called a propeller pump.
4. Air pressure pumps or ejectors –
It works on the action of compressed air. Used in following conditions –
- small quantity of sewage is to be lifted.
- pumping station construction is not possible.
- centrifugal pumps usually clogged.
Power for pumps
1. Steam engines –
Earlier, steam engines were used for operation for the operation of pumps. However Steam power is more reliable., But they are not much in favor due to the following reasons –
- Large initial cost and not economical
- High maintenance cost etc.
2. Internal combustion engines –
The internal combustion engines may run on diesel / gasoline. Initial cost of diesel engine is high and also its difficult to start. Speed is slow also. Operational cost is also very high. These are only used in emergencies, when there is electric power not available.
3. Electric motors –
Pumps are generally operated through electric motors. They are generally very convenient. They are compact, silent in operation, automatic in action and also free from nuisance of smoke.