The following points highlight the six effective methods that has been adopted for the development of tube-wells: 1. Development by Pumping 2. Development by Compressed Air 3. Development by Surging 4. Development by Back-Washing 5. Development by High Velocity Jetting 6. Development by Using Chemicals.
Method # 1. Development by Pumping:
It is the simplest and most common method of removing fine particles. In this method water is ultimately pumped from the well at a discharge equal to or higher than the design discharge. So it is a case of over pumping. A variable speed pump of large capacity is used. Water is withdrawn at very slow rate in the beginning. Then the rate of withdrawal is increased in steps.
In between the steps the rate of withdrawal is kept constant until no further sand particles are removed. Pumping should be continued until the maximum discharge is reached and no further sand particles are withdrawn. The water withdrawal is then stopped and the water level is allowed to rise to its normal position. The procedure is again repeated until no further sand particles are removed.
In the initial stages of pumping if the speed is kept high fine particles will be sucked in with great force and may clog the perforations in the pipe or the filter media. It may cause failure of tube-well.
Method # 2. Development by Compressed Air:
The main components of this assembly are an air pipe (airline) of smaller diameter and a drop pipe of bigger diameter. The drop pipe is also called discharge pipe because the assembly is similar to that of air lift pump assembly. An air compressor is directly connected to an air tank which in turn is connected to the air pipe through a quick opening valve.
In this method an assembly of air pipe of smaller diameter and drop or discharge pipe surrounding the air pipe is introduced into the well till it reaches nearly bottom of the first strainer pipe. The air pipe is so adjusted that its bottom end is about 30 cm above the lower end of discharge pipe. It is called pumping position of the air pipe.
The air is then compressed into the well to start pumping. The pumping is continued till the pumped water is free from sand. At this point air entry is cut off by closing the valve. The tank is brought to full pressure by keeping the compressor on. In the meantime the air pipe is so lowered that now it emerges below the lower end of discharge pipe by 30 cm. It can be called back-washing position of the air pipe.
Now the valve is quickly opened to allow sudden rushing entry of compressed air into the well. Because of heavy rush of air a surge of water is created. It forces the well water into the aquifer through the screen. The surge agitates the aquifer and dislodges the fine sand particles.
The air pipe is again raised inside the discharge pipe i.e., to a pumping position and as the pumping starts the direction of flow is reversed and now water enters the well through the soil.
The entering water brings with it dislodged fine sand particles. The process of alternate air lift pumping and surging is continued till the aquifer is fully developed and sand flow is stopped. In this manner full aquifer is developed taking 1 to 2 m length of the screen at a time.
Method # 3. Development by Surging:
A surge is formed by the reciprocating movement of a plunger in the well. The water moves alternately into the soil and comes out in the well during downward and backward stroke respectively. The speed of the plunger is slowly increased. The plunger is operated in the casing pipe provided above the screened portion of the well.
The repeated application of surging force draws the fine particles into the well, leaving coarser particles intact in the aquifer. Alternate surging and bailing is continued to draw sand from aquifer and to remove that water from the well respectively till no sand is drawn into the well.
Method # 4. Development by Back-Washing:
As the name suggests it is a process in which the water is made to flow into the aquifer formation from the well through the screen. The back-washing causes agitation of the formation and breaks down the bridging of sand particles. Back-washing thus helps in effective removal of fine particles. Various methods can be used for creating back-wash or causing reverse flow.
The main methods are:
(a) Intermittent pumping method;
(b) Back-washing with bailers; and
(c) Back-washing with air pressure.
Method # 5. Development by High Velocity Jetting:
It is one of the most effective methods of development. In this method high velocity jets released through the jetting tool pass through the screen and the formation behind the screen gets agitated. It loosens the fine particles which can be removed from the well by pumping or bailing the well water.
This method has following advantages:
(i) The energy is concentrated over a small area and is therefore more effective.
(ii) Each portion of the screen can be covered selectively and completely.
(iii) The process is simple and does not require elaborate arrangement or special equipment.
(iv) There is little chance of over development by this method.
Method # 6. Development by Using Chemicals:
Dispersing agents are many times added to the water used for back-washing or jetting. The dispersing agents counteract the property of clay to stick to sand particles.
The common dispersing agents which are quite effective are various polyphosphates like Tetrasodium pyrophosphate, Sodium tripolyphosphate, sodium hexameta phosphate (Calgon) and sodium depta phosphate. Once the dispersing agent neutralises colloidal property of clay it can be easily removed by surging and back-washing.