|Vector Control - Methods for Use by Individuals and Communities (WHO, 1997, 425 p.)|
|Chapter 9 - House-spraying with residual insecticides|
|Manually operated sprayers|
There are many different types of hand-operated sprayer. Most models are used for the control of agricultural pests. With some adaptations such sprayers may also be suitable for use in public health and for household pest control. The World Health Organization has produced detailed specifications of spraying equipment suitable for residual wall-spraying in order to ensure uniform and safe application of insecticides (3).
Types of hand-operated sprayer
Usually considered the standard equipment for residual spraying. However, many models are available and only a few comply with WHO specifications, which are discussed further in the text.
Widely used in agriculture, this is carried on the back. A frame or shield prevents contact between the tank and the back. It is a continuous type of sprayer with a fairly constant discharge rate. The person maintains pressure in the tank by pumping air with a lever with one hand and directs the spray lance with the other. If the sprayer is fitted with a spray control valve, continuous pumping may not be necessary. The knapsack sprayer can be used for spraying breeding sites with larvicides but should not be used for residual wall-spraying.
These are used in some vector control programmes because they are less costly than compression sprayers. The pump, mounted on a footrest or stirrup, is inserted in the spray liquid in a bucket. A hose attached to the pump leads to the spray lance. Two persons are needed, one to pump and one to direct the spray. The pressure varies with the speed of pumping, and so it is difficult to make uniform spray applications. Because of their inaccuracy and because of the risk of spilling insecticide from the open bucket inside houses, stirrup pumps are not recommended. They should not be used with hazardous pesticides.
Fig. 9.3 A stirrup pump.
Functioning and design
A hand-compression sprayer basically consists of a tank for holding a liquid insecticide formulation, which can be pressurized by means of a hand pump attached to it. The compressed air forces the liquid from the tank via a hose with a cut-off valve, a lance and a nozzle (Fig. 9.4).
Tank assembly (Fig. 9.5)
· The tank itself is usually made of stainless steel. Most tanks have four openings on top: a large one for filling, fitted with a removable cover; and openings for the air pump, discharge system and pressure gauge.
· The tank cover (Fig. 9.6) consists of: (1) a rubber gasket; (2) a handle; (3) a pressure-release valve, operated by hand or by giving the handle a quarter turn; (4) a chain to prevent the cover from being lost.
· An air pressure gauge is used to measure pressure in the tank.
· The shoulder strap should be wide enough to prevent it from cutting into the shoulder of the person using the sprayer. It is fastened to the tank with steel buckles. On large tanks it is adjustable.
· When the tank is not in use, the spray lance is held in a bracket and nozzle cup, which protects the nozzle from damage.
Fig. 9.4 Main components of a hand-compression sprayer (© WHO).
Air pump assembly (Fig. 9.7)
A piston-type pump consists of a plunger operated inside a cylinder. The plunger forces air through a valve at the base of the cylinder. The plunger seal may be made of leather, rubber or plastic, and must be resistant to the chemicals used in insecticide formulations.
The main parts are: (1) the dip tube, mounted in the tank with an O-ring gasket; if the gasket is damaged, air may leak from the tank; (2) a flexible hose of a material resistant to chemicals used in pesticide formulations; (3) a filter with housing which filters out particles too large to pass through the nozzle opening; it can be taken out for cleaning or replacement; (4) a cut-off valve that permits the person using the sprayer to close the system; (5) a lance, or extension tube, 40 - 60cm in length; some models are telescopic; (6) a nozzle assembly, comprising a nozzle tip, filter, body and cap; the tip may be of stainless steel, ceramic or plastic (Fig. 9.8). The nozzle tip is the most important part of the sprayer. It should deliver a precise amount of spray suspension per minute at a certain pressure in the tank, and maintain a uniform spray pattern and swath width. The selection of the nozzle depends on how the insecticide is to be sprayed.
Fig. 9.5 Cutaway diagram of a hand-compression sprayer (© WHO).
Fig. 9.6 Close-up of upper part of tank with tank cover removed.
Fig. 9.7 Cutaway diagram of an air pump assembly (© WHO).
Fig. 9.8 Parts of a nozzle (© WHO).
Types of nozzle (Fig. 9.9)
· The solid or jetstream nozzle is used to treat cracks and crevices for control of bedbugs, soft ticks, cockroaches and ants.
The flat-spray nozzle commonly used for wall-spraying produces a spray with an angle of 80° and 757ml per minute at a standard tank pressure of 280kPa. It is usually made of specially hardened stainless steel. The nozzle tip is designed with flat surfaces on either side of the orifice so that it can be removed easily.
Use and operation of a hand-compression sprayer
Preparation and addition of pesticide
Pesticides should be carefully handled. Water-dispersible powders should be packaged at a central point before spray operations are started. The correct amount of insecticide should be put into a suitably sized plastic or paper bag (see p. 366 for calculation of insecticide dosage). The people who do this should wear protective clothing (see p. 383). The risks of contamination of personnel, spillage and wastage in the field are thus reduced, as is the workload during spraying activities. Accurate dosing of insecticide is also made easier. Bags and insecticide containers should be disposed of safely after use (see Chapter 10, p. 385).
Before the insecticide is mixed, the sprayer should be checked and calibrated with water. In the field, a wooden paddle or stick should be used to mix water-dispersible powders with a small quantity of water to form a smooth paste. This is added to the sprayer tank. More water is used to rinse out the mixing container, the wash is poured into the sprayer tank up to the level required, and stirring is repeated. The mixing container should now be clean. House owners can assist spray personnel by providing water (Fig. 9.10).
The suspension is poured into the spray tank through a strainer or filter funnel to prevent dirt from entering (Fig. 9.11). If the suspension is not filtered the nozzle tip may become blocked during spraying.
Fig. 9.10. Mixing the insecticide solution.
Fig. 9.11. Filling the spray tank.
The tank should not be more than three-quarters full. The remaining space should be left for the compressed air. The tank usually has a mark indicating the required amount; for standard sprayers this is 8 or 10 litres.
The suspension is kept well mixed by shaking the tank before beginning to spray and from time to time during spraying. This is done by grasping the sprayer by the pump shaft and the bottom end of the tank. The tank should not be held by the strap, nor should it be swung forward and backward while on the shoulder. Formulations that meet WHO specifications should remain in suspension without extra shaking.
If preweighed pesticide sachets are used, the required amount of water should be poured directly into the sprayer. The contents of the sachet should be added, the sprayer closed, and the contents mixed by turning the sprayer upside down.
Preparation of sprayer
· To close the tank: insert the cover vertically into the tank, lift it and fit it into the tank opening; turn the handle across the width of the opening.
· To open the tank: push down the air-release valve by turning the handle on the cover; the cover will become loose once the air is at atmospheric pressure.
Pressurizing the tank
Put a foot on the foot rest (if available) and unlock the pump plunger. Pull the plunger all the way up with both hands and then push it downwards (Fig. 9.12). Use full, even strokes.
If the sprayer has a pressure gauge, keep pumping until it registers a pressure of about 380kPa (55psi). If the gauge is inaccurate, assume that a full stroke of the pump will provide 1psi, so normally use 55 full strokes when pressurizing a tank that is three-quarters full. The upper and lower limits for the working pressure are about 380kPa (55psi) and 170kPa (25psi), giving an average pressure during spraying of about 280kPa (40psi).
During spraying, pressure has to be maintained by occasional repumping. Try to get accustomed to the number of pumping strokes required to reach the maximum pressure in case the pressure gauge stops working. Keep the plunger shaft in place with the locking lever. Always release the pressure when the sprayer is not in use or when it is being transported.
Fig. 9.12. Pressurizing the spray tank.
Application of spray
The insectide suspension has to be sprayed evenly at the recommended dosage over all sprayable surfaces. The following factors determine how much insecticide is sprayed on a surface:
- the concentration of insecticide in the suspension (calculation of the dosage is discussed on p. 366);
- the air pressure in the sprayer (maintain at 170 - 380kPa (25 - 55psi));
- the nozzle tip aperture size;
- the distance from the nozzle tip to the surface being sprayed;
- the speed of application over the surface.
The two latter factors imply skill and training, and spray personnel should be trained to spray at the proper rate to cover 19m2 per minute. The wall of a building can be used for practice. Mark an area 3m high and 6.35m long, divided into nine bands, the first one 75cm wide and the remainder 70cm wide (Fig. 9.13).
The spray nozzle will produce a swath 75cm wide if kept at a distance of 45cm from the wall (Figs 9.14 and 9.15). To practise keeping the nozzle 45cm from the wall, fit a wooden stick or other extension to the lance with rubber bands or string. Make sure the length from the nozzle tip is 45cm. Extend the right arm and incline the body towards the surface while raising or lowering the right arm so that the end of the stick remains in contact with the surface.
Fig. 9.13. Training board for residual spraying which can be marked with chalk on the wall of a large building (© WHO).
Fig. 9.14. Nozzle discharge pattern (© WHO).
Fig. 9.15. When a vertical surface is sprayed, some of the insecticide particles bounce off into the air; a heavy application results in insecticide run-off down the wall (© WHO).
The spray worker stands directly in front of the wall. If the spray worker is right-handed, the sprayer is carried on the left shoulder and held in place with the left hand; the spray lance is held in the right hand (Fig. 9.16). A helmet or hat and other protective clothing should be worn.
Starting at a bottom corner of the wall and spraying at a uniform rate, move upwards to the top. Continuing to spray, take one step to the right. The next swath should overlap with the previous one by about 5cm (Fig. 9.13). Spray down to the bottom. Continue in this way until the entire area of 19m2 is covered. Each swath of 3m in height should be covered in 6.7 seconds so that nine swaths take a minute. The speed can be controlled by counting the seconds aloud or by using a stopwatch.
Fig. 9.16. Correct attitude and protective clothing for wall-spraying.
If no suitable wall surface is available, an area 1.80m high and 6.35m long (11.43m2) can be used for practice. The area should be divided into nine bands, as described above. Each swath of 1.80 m in height should be covered in 4 seconds so that nine swaths take 36 seconds.
Maintenance and repair
Most sprayers are provided with an illustrated manual giving:
· a description of the equipment;
· operating instructions;
· maintenance instructions;
· information on how to solve problems;
· a list of spare parts.
Spare parts should always be available, especially gaskets and valves. When ordering from the manufacturer or a local supplier, give the sprayer model, the part name and the identification number.
Clean the tank daily after spraying. Do not let pesticide remain in it after use. Rinse the sprayer thoroughly with water and then allow to dry. Do not discard the water in a stream, pond or place where it can be reached by humans or animals; a pit latrine or a hole in dry ground, away from water collection points, rivers, ponds or agricultural land, is the best place for disposal.
Remove, rinse and clean the filter assembly at the control valve. Remove the filter from the valve by grasping it at its base, not by its screen. Twist it slightly on pulling it out (Fig. 9.17).
Reassemble all clean parts except the nozzle. Put clean water in the tank, seal the tank and pump air into it. Open the control valve and let the water flow from the lance to flush the hose, filters, control valve and lance. Remove the tank cover and dry the inside of the tank.
Clean the nozzle tip by washing thoroughly with water (Fig. 9.18). Use a pump to blow air through the orifice, then clean and dry it. Remove any dirt from the orifice with a fine bristle from a brush or with a toothpick; never use metal wire. Dirt can also be blown out by pushing the nozzle against the pressure release valve on top of the tank cover.
Inspect the tank at regular intervals, and replace any worn or damaged part. Inspect the lip of the pump cylinder for cracks that could cause the tank to lose pressure. Check the rubber hose for cracks and weak spots. After some time the hose becomes weakened near the point of attachment to the spray-can or cut-off valve. The weak part should then be cut off and the hose remounted. Put a few drops of clean oil on the plunger cup leather to keep the pump cylinder lubricated and to ensure sufficient pressure. Replace the leather if damaged.
Nozzle tips erode during spraying. They should be replaced when worn. An eroded orifice causes an increase in the amount of pesticide delivered. The discharge rate should be measured from time to time by qualified personnel. A simple method is to spray a suspension on to a dark surface: irregularities in the swath indicate that the nozzle tip needs to be replaced.
Fig. 9.17. Removing the spray filter for cleaning (© WHO).
Fig. 9.18. Clean the nozzle tip with water (© WHO).
Check that the tank is empty, put the parts back together and store the tank upside down with the cover lying loosely inside the tank and the plunger locked (Fig. 9.19). Make sure the lance and nozzle cannot fall or be otherwise damaged. Store the cut-off valve locked open.
· Pumping does not build up pressure. Most probably the plunger cup leather is dry or damaged. Apply oil or replace.
· The sprayer does not shut off. Release pressure, and disassemble the cut-off valve as indicated in the pump manual. Clean, check and if necessary replace the valve seat, O-ring, spacer, washer and valve-pin packing.
Fig. 9.19. Store the tank dry and upside down (© WHO).