Quality Standards

Introduction

The objective of water works management is to ensure that the water supplied is free from pathogenic organisms, clear, palatable and free from undesirable taste and odour, of reasonable temperature, neither corrosive nor scale forming, and free from minerals which could produce undesirable physiological effects. The establishment of minimum standards of quality for public water supply is of fundamental importance in achieving this objective. Standards of quality form the yardstick against which the quality control of any public water supply has to be assessed.

Sanitary inspections are intended to provide a range of information and to locate potential problems. The inspections allow for an overall appraisal of many factors associated with a water supply system, including the water works and the distribution system. Such an appraisal may later be verified and confirmed by microbiological analysis, which will indicate the severity of the problem. Sanitary inspections thus provide a direct method of pinpointing possible problems and sources of contamination. They are also important in the prevention and control of potentially hazardous conditions, including epidemics of water-borne diseases. The data obtained may identify failures, anomalies, operator errors and any deviations from normal that may affect the production and distribution of safe drinking water. When the inspections are properly carried out at appropriate regular intervals, and where the inspector has the knowledge necessary to detect problems and suggest technical solutions, the production of good quality water is assured.

The evolution of standards for the quality control of public water supplies has to take into account the limitations imposed by local factors in the several regions of the country. The Environment Hygiene Committee (1949) recommended that the objective of a public water supply should be to supply water “that is absolutely free from risks of transmitting diseases, is pleasing to the senses and is suitable for culinary and laundering purposes”, and added that “freedom from risks is comparatively more important than physical appearance or hardness”, and that safety is an obligatory standard while physical and chemical qualities are optional within a range.

These observations are relevant in the development of country-wide programmes of protected water supply systems for communities big and small, making use of the available water resources in the different regions, with a wide variation in their physical, chemical and aesthetic qualities, that can be achieved by communities in due course within the limits of their financial resources. The immediate need is for minimum standards consistent with the safety of public water supplies. Considering the standards prescribed in the earlier Manual, further developments in international standardisation and the conditions in the country, the following guidelines are recommended.

(a) Physical and chemical quality of drinking water

The physical and chemical quality of drinking water should be in accordance with the recommended guidelines presented in Table 2.2.

Notes:

• * The figures under “Acceptable” are the limits up to which water is generally acceptable to consumers.
• ** Figures in excess of those under “Acceptable” render the water not acceptable, but it may still be tolerated in the absence of an alternative and better source, up to the limits under “Cause for rejection”, above which the source will have to be rejected.
• † If there are 250 mg/l of sulphates, the magnesium content can be increased to a maximum of 125 mg/l, with a reduction of sulphates at the rate of 1 unit for every 2.5 units of sulphates.
• ‡ Some mine and spring waters may exceed these radioactivity limits. In such cases it is necessary to analyse the individual radionuclides to assess whether the water is acceptable for public consumption.

(b) Bacteriological guidelines

The recommended guidelines for bacteriological quality are given in Table 2.3.

Source: WHO Guidelines for Drinking Water Quality, Vol. 1, 1993.

1. Immediate investigative action must be taken if either E. coli or total coliform bacteria are detected. The minimum action in the case of total coliform bacteria is repeat sampling; if these bacteria are detected in the repeat sample, the cause must be determined by immediate further investigation.
2. Although E. coli is the more precise indicator of faecal pollution, the count of thermotolerant coliform bacteria is an acceptable alternative. If necessary, proper confirmatory tests must be carried out. Total coliform bacteria are not acceptable indicators of the sanitary quality of rural water supplies, particularly in tropical areas where many bacteria of no sanitary significance occur in almost all untreated supplies.
3. It is recognised that in the great majority of rural water supplies in developing countries, faecal contamination is widespread. Under these conditions, the national surveillance agency should set medium-term targets for the progressive improvement of water supplies, as recommended in Volume 3 of the WHO Guidelines for Drinking Water Quality, 1993.

(c) Virological quality

Drinking water must essentially be free of human enteroviruses to ensure negligible risk of transmitting viral infection. Any drinking water supply subject to faecal contamination presents a risk of viral disease to consumers. Two approaches can be used to keep the risk of viral infection to a minimum: providing drinking water from a source verified free of faecal contamination, or adequately treating faecally contaminated water to reduce enteroviruses to a negligible level.

Virological studies have shown that drinking water treatment can considerably reduce the levels of viruses but may not eliminate them completely from very large volumes of water. Virological, epidemiological and risk analyses are providing important information, although it is still insufficient for deriving quantitative and direct virological criteria. Such criteria cannot be recommended for routine use because of the cost, complexity and lengthy nature of virological analysis, and because they cannot detect the most relevant viruses.

The guideline criteria shown in Table 2.4 are based upon the likely viral content of source waters and the degree of treatment necessary to ensure that even very large volumes of drinking water have negligible risk of containing viruses.

Ground water obtained from a protected source, and documented to be free from faecal contamination from its zone of influence, the well, pumps and delivery system, can be assumed to be virus free. However, when such water is distributed, it is desirable that it is disinfected and that a residual level of disinfectant is maintained in the distribution system to guard against contamination.

Notes:

1. For all sources, the median value of turbidity before terminal disinfection must not exceed 1 nephelometric turbidity unit (NTU) and must not exceed 5 NTU in a single sample.
   • Terminal disinfection must produce a residual concentration of free chlorine of ≥ 0.5 mg/l after at least 30 minutes of contact in water at pH < 8.0, or must be shown to be an equivalent disinfection process in terms of the degree of enterovirus inactivation (> 99.99%).
   • Filtration must be slow sand filtration or rapid filtration (sand, dual or mixed media) preceded by adequate coagulation–flocculation (with sedimentation or flotation). Diatomaceous earth filtration, or a filtration process demonstrated to be equivalent for virus reduction, can also be used. The degree of virus reduction must be > 90%.
   • Additional treatment may consist of slow sand filtration, ozonation with granular activated carbon adsorption, or any other process demonstrated to achieve > 99% enterovirus reduction.
2. Disinfection should be used if monitoring has shown the presence of E. coli or thermotolerant coliform bacteria.

Source: WHO Guidelines for Drinking Water Quality, 1993.

Back to Standards