Advances in Water Treatment and Enviromental Management

OPTIMISATION OF SECONDARY CHLORINATION IN UK 133disinfection, including secondary chlorination, consideration must be given to thewater quality objectives of such treatment; these are discussed below.Microbiological StandardsThe EC “drinking water” directive (EC, 1980) requires all samples of water at thepoint of use to be free of faecal coliforms (E.coli) and pathogens, and that at least95% of such samples to be free of total coliforms. The recent UK Regulationsimplement these standards although routine monitoring for pathogens is notspecified.The coliform indicator approach to microbiological quality control has been generallysuccessful in minimising water borne disease in the UK (Galbraith et al,1987).However, possible limitations of the approach are increasingly being realised: egwith respect to entero-viruses (WRc,1986) and protozoan parasites (DoE,1989).The main weakness, however, in relying solely on coliform standards for determiningdisinfection practice, is the uncertainty that is inherent in any water sampling.The confidence to be expected in a given sampling frequency can be estimatedstatistically using the properties of the binomial distribution (A E Warn, personalcommunication); such estimates indicate that there is a risk of poor water qualityfailing to be noticed at low sampling frequencies, as illustrated in Table 1.Table 1. Statistical estimates of true failure rates giving zero failed samples as afunction of sampling frequency.It must thus be realised that the minimum sampling frequencies specified by theEC directive and the UK Regulations carry risks of failing to notice poor quality,particularly for smaller supplies for which the specified minimum frequencies arelowest. The authors conclude that, in seeking to achieve high standards ofmicrobiological quality, additional approaches must also be followed.The EC directive contains guide levels for 22 and 37 C bacterial colony counts.These have been virtually ignored in the UK and the new Regulations only require“no significant increase over that normally observed ”; this approach may havesome validity to samples taken from a fixed point but is difficult to apply to randomsamples, particularly those from surface-water derived supplies in which thepotential for bacterial regrowth is often high. Greater recognition of the possiblesignificance of colony counts could be warranted in the UK for the following reasons:(i) high colony counts may be indicative of the presence of spoilage organisms(ii)of concern to hospitals and the food and drink manufacturing industrieshigh colony counts may be indicative of the presence of Aeromonashydrophilla, being of possible health concern (Burke et al,1984)(iii) high colony counts may be indicative of conditions suitable for infestationby aquatic animals (eg Nais worms)Conditions that lead to elevated colony counts include the presence of assimilibleorganic carbon (AOC), the stagnation of water and the attenuation of effective chlorineresiduals which may be exacerbated in extensive distribution networks.