4.2.4 Verification of drinking water quality

Verification of drinking water quality is described in detail in Section 3.5. Testing of water in small and remote supplies can present both economic and logistic difficulties, particularly for microbial samples that need to be transported to testing laboratories within 12–24 hours of collection. Application of the Framework decreases reliance on drinking water quality testing; however, testing is still important as a means of verifying that, overall, the barriers and preventive measures implemented to protect public health are working effectively.

Small systems should be monitored on the basis that it is more effective to test for a narrow range of characteristics as frequently as possible than to analyse comprehensively less often.

Microbial quality is the most important factor in determining the ongoing safety of water supplies for human consumption. Therefore, wherever possible, a regular testing program should be instituted for the indicator E. coli. As stated in Chapters 9 and 10, a minimum of one microbial sample per week is generally recommended; however, in small systems this is not always practical. Where sampling is less frequent than recommended, sanitary inspections should be more frequent, to provide assurance on the integrity and normal operation of the system. A greater focus on operational monitoring may also be required.

In systems where disinfection is used, evidence of continuous operation is very important in providing assurance of microbial quality. Disinfection is very effective against bacterial pathogens but less so against viruses and enteric protozoa (e.g. Giardia and Cryptosporidium). The presence of viruses and protozoa can be minimised by protecting water supplies from human and livestock waste.

If chlorination is used, the presence of a free chlorine residual in the distribution system provides evidence of initial disinfection and protection against recontamination from backflow, pipeline breaks or other causes. The amount of chlorine required varies with the flow rate, the quality of the raw water and other factors. Generally, a free chlorine residual of between 0.2 and 0.5 mg/L is adequate.

• At least daily testing of chlorine residuals should be carried out to check the effectiveness of the disinfection system. This can be done using a simple diethyl-phenylenediamine (DPD) colour comparator.