TREATMENT AND DISINFECTION OF SMALL WATER SUPPLIES
Final Report 551-S
Report No DWI0080
Laboratory evaluations were made on 17 pumps and other dosers used for hypochlorite dosing on small water supplies and the features in their designs which made for reliability and effectiveness were identified. A specification listing detailed recommendations for the selection of dosing pumps and more general guidance on other types of doser is appended, While calcium hypochlorite tablets are attractive for use on small supplies, having long shelf life and carrying a high charge of chlorine, the simple dosers available are not as accurate or versatile as liquid dosers.
Gas chlorination using vacuum injectors is technically viable for supplies down to about 50 cu.m/d where there is a minimum water head of 2m available to drive the injector. Gas chlorination could be applied to lower flows if measurement of the gas flow could be made more accurate.
On-site generation of hypochlorite is reviewed. There are not many units available for small supplies and there is little technical or economic justification for any substantial adoption of the process.
Ultraviolet irradiation is a proven method of disinfection and a survey of users has shown it to be reliable and satisfactory in practice although not in widespread use.
Preliminary investigations reported here demonstrated the need for further developments in chlorine monitors and in the hydraulic design of contact tanks.
Many small supplies require pH elevation and this can often be achieved by letting the water pass through a contact bed of suitable material. There are several semi-manufactured materials available but, where long enough contact can be given, natural calcium carbonate has the advantage that it will saturate without raising pH excessively.
Where electricity is required to monitor or control the treatment of a supply of variable quality, all the necessary equipment can be obtained for battery operation. A range of hydraulic, wind and solar powered generators is available for keeping the batteries charged from natural energy supplies.
Sodium hypochlorite is reputed to have a short shelf life. Tests have shown 'hat exposure to light is the main agent of decomposition and that temperature is also a significant factor. Since these depend upon conditions of storage and are not under suppliers' control, a "use-by" date labelling system is not feasible. Recommendations are made for care in storage and regular turnover of stocks which should help to ensure that the loss of available chlorine up to the point of use would be negligible.Copies of this report may be available as an Acrobat pdf download under the 'Pre 2000 Reports' heading on the DWI website.