Sewage Pumping

SEWAGE PUMPING

Necessity of Pumping Sewage

(i) For lifting sewage from low lying areas [sewers run at higher elevation than the sources of sewage generation].

(ii) In flat terrains, laying of sewers at designed gradient requires large excavation and is expensive. To reduce the excavation cost, pumping is done at suitable intervals. aliq ni

(iii) Pumping is resorted to when outfall sewer is lower than the entrance of treatment works or receiving water bodies.

(iv) For lifting sewage from basements of commercial buildings (since street sewers are higher than basement level of buildings).

(v) Instead of driving tunnel through ridges for sewer line, its economical to pump sewage into sewers laid across the slope of ridges.

Problems in Pumping Sewage

(i) Sewage has foul characteristics.

(ii) Suspended and floating matter in sewage clogs the pumps.

(iii) Organic and inorganic waste in sewage cause corrosion and erosion of pump parts and reduce their life.

(iv) Disease producing organisms (pathogens) in sewage cause health hazard to working personnel.

(v) Higher reliability pumps are required to prevent flooding nuisance.

Requirements of Sewage Lifting Pumps

a) Pump Capacity.

❖ Must be able to handle peak flows.

❖ Two or more pumps and power sources are required.

(b) Clogging Aspects.

❖ Special pumps which do not clog due to the floating and suspended solids in sewage are used.

❖ Pumps should be accessible for cleaning and removal of obstructions.

Types of Pumps

(1) Centrifugal Pumps.

(2) Reciprocating Pumps.

(3) Pneumatic Ejectors or Air Pressure Pumps.

 

(1) Centrifugal Pumps

Advantages:

(a) Widely used for lifting sewage.

(b) Easily installed in pits and sumps.

(c) Easily transport suspended matter present in sewage without clogging.

(d) Pre-treatment (Grit removal) of sewage is not required before pumping.

Disadvantages:

(a) Low efficiency (30 to 60%).

 

Types

(a) Horizontal Axis Flow Centrifugal Pump.

They are provided with open or closed three-vane impellers.

The clearance between vanes is large enough so as to allow the solids entering the pump to pass out with the liquid sewage.

This helps in preventing clogging of pumps and damage to rotor.

Such pumps are called non-clog pumps.

 

(b) Disintegrating Pumps.

In this pump, the solid matter present in sewage are broken up (disintegrated) as they pass through the pump impeller.

They are used when sewage is to be directly disposed off into the sea through the outfall sewer.

The centrifugal pumps are installed either submerged beneath sewage in wet well or in dry well.

In the submergible pump, the pump is set with a vertical shaft reaching to the motor placed above the sewage level in a dry space, the pump being always submerged.

In the latter case, the pump is set in dry well and sucks its inlet supplies through the suction pipe from an adjacent wet well containing

The exterior of the pump is always dry and its inspection and maintenance is easy and chances of corrosion of pump is less.

 

(2) Reciprocating Pumps

 (i) High initial cost.

(ii) Difficulty in maintenance.

(iii) Greater wear and tear.

(iv) Not used nowadays.

(v) Used for pumping sludges and where large quantity is required to be pumped against low heads.

 

Types

a) Ram Type:

❖ A piston or plunger moves inside a closed cylinder.

❖ On intake stroke, the liquid enters the cylinder through suction valve. The delivery valve remain closed.

❖ On discharge stroke, suction valve closes and liquid is forced into the delivery pipe through the delivery valve, which opens during discharge stroke.

Example: Diaphragm Pump.

 

(b) Propeller Type:

❖ In this pump, a multiple blade screw rotor or propeller moves vertically inside a pump casing, causing the sewage to lift up.

❖ The propeller draws water through the inlet guide vanes and discharges through the outlet guide vanes.

Example: Axial-flow screw pump.

 

Diaphragm Pump:

❖ Ram type reciprocating pump, which is commonly used.

❖ A piston or plunger is attached to the centre of a circular rubber diaphragm, the outer edge is bolted to flange on pump.

❖ The up and down motion of plunger increases and decreases the capacity of pump.

❖ When the piston moves upward, the liquid flows into the pump through the suction valve; the delivery valve remains closed.

❖ When the piston moves downward, the suction valve closes, and liquid is forced through delivery pipe by opening delivery valve

❖ It is a simple, durable pump that needs no priming but may wear out and need replacement.

 

(3) Pneumatic Ejectors (Air Ejectors or Air Pressure Pumps):

❖ Used for pumping smaller quantities of waste waters.

❖ Works under the action of compressed air.

❖ Consists of an air-tight tank into which waste water flows by gravity.

❖ The waste water is forced out automatically whenever sufficient waste water has accumulated to raise a float, which opens the compressed air inlet valve.

❖ A typical air ejector called Shone's ejector consists of a cast iron chamber with a spindle having an upper and a lower cup. Two check valves V₁ and V₂ are provided at the entrance and exit points. The ejector chamber rests on a seat. A lever arrangement with a counter-weight is provided, so as to open the  g compressed air inlet valve (V₃). The waste water enters the ejector chamber through V₁ when exit valve V₂ and compressed air inlet valve V₃ remain closed.

❖  As waste water rises in chamber, the air from chamber escapes through exhaust. When waste water level reaches bottom of upper cup, air inside gets entrapped and bootol ai texerts vertical pressure and lifts up the spindle closing the exhaust and opening compressed air inlet valve (V₃). The air under pressure entering the chamber from bas bon valve (V₃) forces the waste water from inside the chamber to rise up in the outlet pipe by opening the exit valve (V₂). At this stage, the valves V, and V₃remain open, but V, is closed. The waste water is thus lifted up and discharged from the outlet, till the level of waste water falls below the bottom of upper cup. At this stage, exhaust opens and compressed air inlet valve (V₃) closes.

❖ The exit valve V₂ closes and entrance valve V₁ opens to again admit the waste

Advantages:

 (i) Automatic operation and require least supervision.

(ii) Used where centrifugal pumps get clogged.

(iii) Only few parts come in contact with sewage.

(iv) Ejectors do not get clogged.

(v) Economical where smaller quantities of sewage are required to be lifted. V bus V

Disadvantage:

(i) Low efficiency - 15%.