In this article we will discuss about:- 1. Necessity of Pumping of Sewage 2. Problems in Sewage Pumping 3. Centrifugal Pumps 4. Factors to Consider for a Sewage Pumping Station 5. Components of a Sewage Pumping Station.
Contents:
- Necessity of Pumping of Sewage
- Problems in Sewage Pumping
- Centrifugal Pumps
- Factors to Consider for a Sewage Pumping Station
- Components of a Sewage Pumping Station
1. Necessity of Pumping of Sewage:
Sewage is required to be pumped under the following circumstances:
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(1) When some area of a town or city is low lying it may not be possible to drain the low lying area by gravity to discharge into a sub main or main located at a higher level, unless the entire sewerage system in the remaining higher area is laid at a correspondingly lower level. In such circumstances it is more economical to collect the sewage of the low lying area into a sump well by gravity and then pump it into the gravity main sewer of the high level area.
(2) When the land is flat the laying of sewers at the calculated gradient in order to have self-cleansing velocity would involve expensive excavation. This can be avoided by installing pumping units at intervals along the long sewer to lift the sewage from one section to the next section that can be located at a higher level and running under gravity. This process of lifting the sewage is known as boosting and the pumps used are called boosters.
(3) When an outfall sewer is lower than the entrance to treatment works or body of water or any other point of discharge, then sewage is required to be lifted by pumps either for its treatment or for its disposal into the body of water or to any other point of discharge.
(4) When a sewer is required to be taken across a high ridge then a tunnel is required to be driven for this purpose. However, instead of driving a tunnel, pumping sewage into sewers laid across the slope of the ridge at reasonable depths may be more economical.
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Such a situation may arise when a town or city is divided into two parts by a ridge line in between. The sewage in the two parts is collected separately under gravity at the lowest point and then the sewage from one part is taken to the other by crossing the ridge by means of pumping.
(5) When it is required to take out sewage from cellars or basements of buildings the floor of which is at a level lower than the invert level of sewer into which sewage is to be discharged, then the sewage is required to be pumped.
It may, however, be noted that provision of pumps in a sewerage system results in an additional expenditure because it involves capital investment, operation and maintenance costs and extra supervision charges. As such the necessity of pumping of sewage should be carefully examined before it is finally recommended.
2. Problems in Sewage Pumping
:
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The pumping of sewage is not as simple as pumping of water. Some of the special problems faced during the pumping of sewage are as follows:
(1) Sewage has foul characteristics.
(2) Sewage contains a lot of suspended and floating materials. These may make the running of pumps difficult and may cause frequent clogging of pumps.
(3) Sewage contains organic and inorganic wastes which may cause corrosion of pumping equipment and thus reduce its life.
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(4) Sewage may contain biological contents including disease producing germs and hence the pumping of sewage should not be delayed for a long time. The repairs if required, should be carried out immediately.
(5) The presence of disease producing bacteria and organisms in sewage may cause health hazards to the persons working at pumping station.
(6) The rate of flow of sewage varies continuously and hence pumping operations are to be adjusted accordingly.
(7) The size of sump or wet well is limited since large sized sumps will result in the settlement of silt and organic matter at its bottom. As such the provision of sump or wet well is made to give only a little storage space and the rate of pumping has to be adjusted with the rate of entering or incoming sewage.
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(8) The pumps should be of high order reliability because failure of pumps will lead to flooding which may cause unbearable nuisance.
3. Centrifugal Pumps:
The centrifugal pumps are generally classified as radial flow, mixed flow and axial flow pumps. The classification is usually based on the specific speed Ns of the pump. The specific speed of a centrifugal pump is defined as the speed in revolutions per minute (r.p.m.) of an imaginary pump, geometrically similar in every respect to the actual pump under consideration and of such a size that it is capable of delivering 1 litre of water per second against a head of 1 metre.
The specific speed is given by the following formula:
In which
Ns = specific speed of the pump in r.p.m.;
N = actual speed of the pump in r.p.m.;
Q = discharge or flow-rate of the pump in litres per second; and
H = head developed by the pump in metres.
In radial flow pumps the flow through the impeller is radial i.e., across a plane perpendicular to the axis of rotation. The impellers of radial flow pumps can be constructed in all the three types viz., open, semi- open and closed. The radial flow pumps can be constructed with either single suction or double suction.
In a single suction pump liquid is admitted from a suction pipe on one side of the impeller, while in a double suction pump liquid enters from both sides of the impeller thereby increasing the capacity of the pump. The radial flow pumps are used for high heads and low capacities. Further these pumps are of low specific speed.
For a radial flow pump with single suction, specific speed (as given by equation 7.1) varies from 300 to 2400. The input power required for a radial flow pump increases as the capacity, i.e., the flow rate of the pump increases. Thus in radial flow pumps the power demand is the minimum with zero flow, i.e., with the delivery valve closed. Since the pump should be started with the pump exerting the minimum load on the driver/motor, the radial flow pumps should be started with the delivery valve closed.
In mixed flow pumps the flow through the impeller is partly radial and partly axial. The impellers of mixed flow pumps can also be constructed in all the three types viz., open, semi-open and closed. However, the impellers of mixed flow pumps of higher specific speed are generally of semi-open type.
The mixed flow pumps can also be constructed with either single suction or double suction. The mixed flow pumps are used for medium heads (say up to 15 m or so) and medium capacities. Further the specific speed of mixed flow pumps (as given by equation 7.1) varies from 2400 to 5000.
The input power required for a mixed flow pump increases slightly as the capacity, i.e., the flow rate of the pump increases. As such the mixed flow pumps should also be started with the delivery valve closed.
In axial flow pumps the flow through the impeller is more or less parallel to the axis of rotation. Since the impellers of axial flow pumps resemble the propeller of a ship, these pumps are also called propeller pumps. The axial flow pumps have only the open type impellers and also these pumps are constructed with only single suction.
These pumps are used for low heads (say up to 9 m or so) and large capacities (over 2000 m3/hour). The specific speed of axial flow pumps (as given by equation 7.1) varies from 3400 to 15000. In the case of axial flow pumps the power needed to be input to the pump is maximum at zero flow and hence these pumps should be started with the delivery valve fully open.
It may, however, be indicated that it is not justified to call axial flow pumps as centrifugal pumps because there is hardly any centrifugal action in their operation. As such often axial flow pumps are considered as a separate class of pumps.
Besides ordinary centrifugal pumps another type of pumps called disintegrator pumps (or disintegrating pumps) are also used. These pumps are provided with a conical shaped impeller having sharp edged grooves, along with fixed knives set against the impeller. Due to this during pumping when sewage with solids pass between the rotating impeller and the fixed knives the solids are broken up or disintegrated.
This sewage can therefore be disposed of directly into the sea. Further by providing disintegrator pump to cut the solids present in the sewage the possibility of the settlement of the solids in rising main during the low velocity of flow can be avoided, and also the necessity of installing screens is precluded.
However, the efficiency of disintegrator pumps is low, and hence at large pumping stations the usual practice is to screen the sewage and then to pass it into ordinary centrifugal pumps of high efficiency.
4. Factors to Consider for a Sewage Pumping Station:
A sewage pumping station is a building where pumps and other accessories are installed for lifting sewage either for discharging it into a high level gravity sewer or for its treatment/disposal. For a sewage pumping station the following three factors are required to be considered.
(1) Location of pumping station;
(2) Building of pumping station; and
(3) Component parts of pumping station.
These factors are briefly discussed below:
(1) Location of Pumping Station:
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. The site for pumping station should be near a river, stream or storm water drain so that in case of emergencies, such as
breakdown of the pumping machinery, power failure, etc., it will be possible to divert the overflow.
However, pumping stations near the river, stream, etc., should be so located that they are not liable to get flooded due to high floods in the river, stream, etc. The site for pumping station should be aesthetically satisfactory. The pumping station should be so located that it is easily accessible under all weather conditions.
(2) Building of Pumping Station:
The building of a pumping station should be designed to withstand the forces to which it may be subjected. In designing the substructure of the building of pumping station due allowance must be made for earth and water pressure and uplift pressure on the bottom. The substructure of the building of pumping station may be of mass concrete or reinforced cement concrete. For superstructure of the building any material may be used.
The external walls may be either solid slab construction or panels supported on horizontal and vertical ribs. The internal walls and floors should be designed for the weight of machinery and a live load of 5 kN/m2. In case of internal water pressure the internal walls and floors should be designed to bear this load. Provision should also be made for the load due to gantry and a crane.
An adequate ventilation should be provided in the design of sewage pumping station to carry away odours, moisture and gases that may escape into the building. The ventilating equipment should have a minimum capacity of six changes of air per hour. In order to minimize corrosion it is desirable to control humidity in the building.
There should be adequate natural and artificial illumination of the interior of the building. Walls should be light coloured and washable. Dust-proof, vapour-proof and explosion-proof fixtures and luminaries should be provided as far as possible. It is advisable to provide stairs instead of ladders between different floors of the building. As far as possible the use of spiral stairs should be avoided.
In the design of pumping station wide passages with adequate lighting and ventilation and free from abrupt changes in level and other obstructions should be provided. Further the building of pumping station should be planned and designed keeping in view the future requirements and there should be enough scope for future expansion.
(3) Component Parts of Pumping Station:
The main component parts of a large pumping station are as indicated below:
(i) Grit channel or detritus pit;
(ii) Screens, coarse and fine;
(iii) Sumps or wet wells or receiving wells;
(iv) Pump room or dry well;
(v) Pumps with motors or driving units;
(vi) Inlet or low level sewer and rising main to outlet or high level sewer;
(vii) Emergency overflows or by-pass to remove sewage during emergencies;
(viii) Ventilation arrangements such as extraction fans; and
(ix) Miscellaneous accessories such as flow recorders, float operated switches, sluice valves, starters, etc.
5. Components of a Sewage Pumping Station:
A brief description of the components is given below:
(i) Grit Channel or Detritus Pit:
The sewage arriving at the pumping station contains solids of indestructible nature such as grit, gravel and sand in addition to solids in suspension such as faeces, papers, rags, etc. It is necessary to remove as much of this material as possible before pumping so as to minimise the wear and tear of the pump impeller and the rising main. Thus for removing grit from the sewage a grit channel or detritus pit is provided.
The grit channel is a long basin with an enlarged cross-section, which results in reducing the velocity of flow to 0.15 to 0.30 m/s. The bottom of the channel is kept below the invert line of the inlet sewer to allow the deposition of the grit which is removed by an endless chain to which perforated buckets are fixed.
The chain is operated by an external source of power and the grit is dredged into a closed container till it is removed and disposed of. The grit channel should have a minimum capacity of 1 per cent of the daily dry weather flow. Moreover, there should be two similar units each of which can be used, allowing the other to be cleaned. In small installations grit is removed once a week whereas in large installations the removal may be a continuous daily process.
(ii) Screens:
After the removal of grit from the sewage, it is made to pass through screens to trap the floating matter such as rags, papers, sticks, etc. It is necessary to remove these otherwise they will choke the pumps. If disintegrator pumps are used the use of screens can be avoided. However, the disintegrator pumps are not commonly used and hence screens are invariably provided at the pumping station.
Screens are of two types-coarse and fine. In large installation it is usual to provide both types of screens. The sewage is first passed through the coarse screen which is made of wrought iron bars kept parallel to each other and having a clear spacing of 50 to 100 mm in between them to intercept solids of the type scrubbing brushes, blocks of wood, etc.
Such material trapped by coarse screen is removed by hand raking. The sewage is then passed through fine screen having clear openings of about 25 mm to intercept all except very fine particles of sewage. The screenings trapped by the fine screen are removed by mechanical rakes which have fingers or teeth fixed on horizontal bars that are attached to an endless roller chain running over sprockets.
The fingers enter at the bottom of the screen and while moving upward along the screen collect the trapped screenings. The fingers then move vertically upward till they are over a collecting trough where they are tilted and screenings are deposited in sealed bins for quick disposal.
(iii) Sumps or Wet Wells:
A sump or wet well is provided to form a suction pit from which pump draws sewage through suction pipe. It also acts as an equalising basin to minimise the load fluctuations on the pump. The sump is so designed that the sewage can collect in it for some time till the rise in the level operates a float operated switch which starts the motor and the pump and the sewage is pumped out. The designed level of sewage in sump is usually kept above the pump level which avoids the necessity of priming of the pump.
The size of the sump depends on the storage capacity to be provided.
The selection of the proper storage capacity of the sump is essential because it affects:
(a) The time for which sewage will be retained in the pumping station; and
(b) The frequency of operation of the pumping unit.
If the sump of large capacity is provided sewage will be retained for a longer time resulting in the deposition of solids and also sewage turning septic. On the other hand if the capacity of the sump is small the operation of pumping unit is to be done quite frequently, with the result that the operation becomes expensive (because in the case of electrically operated unit it means increased cost of current as the starting current is greater than the full load current.
The capacity of the sump is also affected by the difference between the highest level of the sewage in the sump and the minimum level after the depletion by pumping. This should be such that the pump would run for at least 5 minutes. Thus the capacity of the sump is to be so kept that with any combination of inflow and pumping the cycle of operation for each pump will not be less than 5 minutes and the maximum detention time in the sump will not exceed 30 minutes of average flow.
On empirical basis the capacity of the sump is usually provided as 15 to 30 minutes of peak flow. Sometimes the capacity of the sump also includes the emptying of the rising main back to the sump at the time of the cleaning of the sump unless a separate washout is provided.
The sump flooring should have benching like a hopper with a minimum slope of 1:1 to avoid deposition of solids. However, there should be provision for the removal of the accumulated sludge. Further suitable provision for overflow should also be made, where feasible, as a protection against flooding, especially in the event of the breakdown of the plant or the failure of the power supply.
When two or more pumps are employed the sump storage may be suitably split into different interconnected compartments so that anyone can be shut off for cleaning or repairs.
(iv) Pump Room or Dry Well:
In the pump room or dry well pumps are installed. The end of the suction pipe of each pump is placed in the suction pit or wet well. The size of the pump room or dry well should be adequate for the number of pumps planned of such sizes as will handle the sewage-load at the desired capacity of pumping. Provision should also be made for future requirements so that additional or larger pumps can be installed. The motor room is situated above the pump room.
Provisions should be made to facilitate easy removal of pumps and motors for periodic repairs, overhauls or replacements. This shall be done by providing a gantry of suitable capacity and with suitable travelling type chain and pulley blocks. A dewatering pump of non-clog type shall be provided for the pump room.
For easy access to the pump room of the pumping stations, the pump room should have a separate entrance and suitable stairways, preferably not less than 90 cm in width shall be provided along with 90 cm high railings wherever required.
(v) Pumps:
Out of the different types of pumps described in section 7.3 only centrifugal pumps including submersible pumps and pneumatic ejectors are used at sewage pumping stations. Moreover, the pneumatic ejectors are used only in small installations where centrifugal pumps are impractical. As such at most of the sewage pumping stations centrifugal pumps including submersible pumps are used.
The selection of pumps is based on many considerations such as the type of pump, the size of pumps, the number of pumps, the capacity or flow rate of each pump, the range of throttling of each pump, the head of pumping, etc.
The capacity of the pumps should be adequate to meet the peak rate of flow with 50% stand by. To obtain the least operating cost, the pumping equipment should be selected to perform efficiently at all flows, including the peak flow. It is always desirable to have two or more pumps at the sewage pumping stations.
The size and the number of pump units for larger pumping stations should be so selected that the variations of inflow can be handled by throttling of the delivery valves of the pumps or by varying the speed of the pump by using variable speed motor, without starting and stopping the pumps too frequently or necessitating excessive storage.
The capacity of a pump is usually stated in terms of Dry Weather Flow (D.W.F.) estimated for the pumping station. The general practice is to provide 3 pumps for a small capacity pumping station comprising 1 pump of 1 D.W.F., 1 pump of 2 D.W.F., and 1 pump of 3 D.W.F. capacity. For large capacity pumping station 5 pumps are usually provided comprising 2 pumps of| D.W.F., 2 pumps of 1 D.W.F., and 1 pump of 3 D.W.F., capacity, including stand by.
(vi) Suction Pipes:
The suction pipes or pump-intakes may be of cast iron, steel or asbestos-cement pressure pipes. The velocity of flow in suction pipes should not be less than 0.75 m/s to prevent deposition of solids. In order to prevent flow-separation bell-mouths should be provided at the entrance of the suction pipes.
Diameter of the bell-mouth D should be to 1.5d to 1.8d where d is the diameter of the suction pipe. Further the clearance of the bell-mouth from the floor should be between 0.5D to 0.75D. If the clearance is less than 0.25D unsteady flow occurs in the bell-mouth. If the clearance is too much the upward flow component becomes unstable and causes swirling and vortex formation.
The distance of any wall or fillet from the lip of the bell-mouth should be between 0.25D to 0.5D. The proximity of the end and the side walls prevents swirling flow and vortex formation. The width of the sump should be between 2D and 3D. The depth of water above the lip of the bell-mouth should be greater than 1.5D.
Where multiple pumps are used, the spacing between the lips of two adjacent suction bell-mouths should be between 2D to 2.5 D. With splitters, i.e., separation walls between the suction pipes, their lengths should not be less than 4D.
(vii) Rising Mains:
The pumped sewage is led to high level gravity sewer through rising mains. The rising mains may be of cast iron, steel or asbestos-cement pressure pipes. The velocity of flow in rising mains should not be less than 0.75 m/s to prevent deposition of solids.
A reflux valve or non-return valve is fitted on the rising main just next to the pump to prevent backflow through the pump when the pump is stopped. Further sluice valves or gate valves are provided at appropriate points along the inlet or low level sewer, suction pipe and rising main to close the flow of sewage during maintenance, inspection and repair of the pumps.
(viii) Emergency Exit Pipe:
It is advisable to provide an emergency exit pipe connecting the sump or wet well with a natural stream or river or storm water drain, so that when the sump overflows due to any reason the excess sewage is diverted through the exit pipe. It is a safety measure in case of an emergency.
(ix) Flow Recorders:
The flow recorders are installed in the sewage pumping station to know the quantity of sewage pumped per unit time. These may be in the form of rectangular weir, standing wave flume, triangular weir or venturi meter. The sewage flow can also be recorded by suitable electric devices.