Advances in Water Treatment and Enviromental Management

34 WATER TREATMENTSome of the descriptive standards in the EC directive have been excluded from the WaterQuality Regulations. Dissolved oxygen is not considered meaningful for routine measurementand free carbon dioxide is not meaningful for water at the tap (Miller, 1990). The standard forhydrogen sulphide (“undetectable organoleptically” ) is considered to be included in the tasteand odour parameters. These standards apply to water samples taken from taps in thedistribution system, but the same microbiological standards are prescribed for water leavingtreatment works and in service reservoirs.4.4 Monitoring frequencies.Water leaving treatment works must be monitored for total and faecal coliforms and disinfectantresidual at frequencies depending upon the average annual output of water from the works,ranging from weekly to daily samples. Service reservoirs are monitored weekly for the samemicrobiological parameters.The majority of standards apply to water from the distribution system and the sampling ofwater is based on zones. A single source, or a blend of several sources, feeding a discrete areamust be designated as a single zone (subject to a maximum zone population of 50,000.) Areaswith water, quality variations within a zone should be subdivided. The sampling points must berepresentative of the whole zone and, for most parameters, these may be either fixed points orrandomly selected customers’ taps. At least 50% of the microbiological samples and all thesamples examined for copper, lead and zinc must be from randomly selected customers’ taps.The minimum frequencies of analysis for tap samples are generally slightly greater thanthose of the EC Drinking Water Directive (where it specifies frequencies.) Frequencies are notspecified in the Regulations for faecal streptococci, sulphite-reducing Clostridia, substancesextractable in chloroform, Kjeldahl nitrogen, dissolved hydrocarbons or phenols, althoughthe Regulations specify standards for them. This is in line with the discretion allowed the‘competent national authority’ in the EC Directive. However, the Regulations require watersuppliers to analyse for any parameter which they believe may exceed the standard. If any ofthe non-microbiological parameters exceeds the standards, increased monitoring frequenciesare specified for the zone for up to two years. If water quality in any zone is consistently withinthe standards, the frequency of monitoring can be reduced. In addition to this compliancemonitoring, water suppliers are expected to carry out sampling for operational monitoring.An example of standard monitoring frequencies is given (Table II) for selected groups ofparameters for the smallest and the largest permitted zones:Table II: Standard monitoring frequencies for the smallest and largest permitted samplingzones.5 COMPLIANCE AND ENFORCEMENT5.1 Interpretation of compliance requirements.The 1982 U.K. Government interpretation of the EC Directive (DoE 1982) considered that formost parameters the average of samples taken over a period (usually one year) should becompared with the MACs to assess compliance. This was challenged by the European commissionand now, for all the standards which derive from MACs, the Regulations require every distributiontap sample to comply with the standard (100% compliance). As in the Directive, the exceptions