|Cholera and other Epidemic Diarrhoeal Diseases Control. Technical cards on Environmental Sanitation (WHO, 1997, 46 p.)|
|DOSING WATER WITH HYPOCHLORITE SOLUTIONS|
· Hypochlorite solutions are used to disinfect drinking water in order to kill or inactivate pathogenic organisms. Disinfection should be constant as long as the risk of contamination exists. Hence the desire to use reliable dosing systems.
Systems for hypochlorite solution dosing have three major components:
- solution preparation.
- flow control.
· Solution preparation is simple with sodium hypochlorite (liquid bleach) which only requires dilution to 2% available chlorine and is ready for immediate use. Calcium hypochlorite comes as a white powder, needs to be dissolved in water and then allowed to settle as it contains inert components which could block flow systems if not removed. The clear solution is then decanted for use. Solutions are normally made up and left overnight before decanting.
· Flow control mechanisms vary widely and many may be constructed using simple materials. Two of the most common are shown here. The disinfection method of relying on manual dosing of storage tanks is not recommended as it is prone to human error and results in erratic chlorine levels.
· Application of hypochlorite solutions should be at a point of turbulence to ensure adequate mixing. The best site is immediately prior to a storage tank or reservoir as this allows the required contact time of one hour before water enters the distribution network. Chlorine for disinfection is added after all other treatment processes have taken place.
· It is important that there is regular monitoring of chlorine levels after dosing and also in the distribution network. The application of chlorine should be adjusted based on the data obtained from this monitoring.
· Chlorine is a dangerous substance. It should be handled with care. All installations where chlorine is used and stored should be secure, especially against children. Site security should be periodically checked.
1. Drip feed chlorinator
Used for small community water supplies, they feed a constant rate of drops of hypochlorite solution into the flow of water.
The flow of water is assumed to be constant therefore the chlorine dose is constant.
The flow of chlorine solution is controlled in this system by the depth of the regulating tube in relation to the surface level of the hypochlorite solution. With the floating bowl this remains constant regardless of the depth of hypochlorite solution in the tank.
The delivery tube is open at both ends and should allow unrestricted flow. This system can be adjusted to supply only a few litres per day of hypochlorite solution.
Adjustment is by raising the regulating tube (decreasing flow) or lowering the regulating tube (increasing flow).
2. Constant head aspirator.
A simple device which with elementary care can prove reliable for many years.
The air inlet to the aspirator is through the glass tube. As chlorine solution is released from the aspirator air is drawn down the glass tube and released into the aspirator. Atmospheric pressure is therefore at the foot of the glass tube and the flow out of the bottle is independent of the amount inside.
The flow from the aspirator is adjusted by rotating the capillary between horizontal and vertical positions as shown. Coarse adjustment is made by altering the height of the glass tube.
The capillary bore most suitable for maintaining a trouble free drip is 0.7 - 10.mm and the centre glass tube should have a diameter of 10 - 15mm.
This system can deliver as little as 1 - 2 litres per day.
Chlorine solution is allowed to flow freely to the dosing point.
The solution should be topped up to the foot of the neck of the aspirator and the bung pressed carefully into position. Air should not leak around the bung and the foot of the centre glass tube may need to be repositioned. After recharging, the capillary should be set to the vertical position and left to drip until the glass tube is full of air and begins to bubble into the aspirator. The flow rate can then be set by rotating the capillary into position.
The operator should understand the principle of the dosers so that the necessary care and adjustments are made when recharging.
Reference: WHO. 1992. Facts Sheets on Environmental Sanitation for Cholera Control. WHO/CWS/92.17
Prepared in collaboration with the Institute of Water and Sanitation Development, Zimbabwe